Hu, Lanhua, Phillip M. Flanders, Penney L. Miller, Timothy J. Strathmann, "Oxidation of Sulfamethoxazole and Related Antimicrobial Agents by TiO2 Photocatalysis." Water Research 41 pp. 2612-2626 2007.
The widespread detection of pharmaceutically active compounds, including many synthetic antimicrobial agents, in aquatic environments is raising public health concerns. As a result, there is growing interest in the development of innovative technologies to efficiently transform these compounds to non-toxic and pharmaceutically inactive byproducts. This work examines the photocatalytic degradation of sulfamethoxazole (SMX) and related sulfonamide antimicrobial agents in aqueous suspensions of nanophase titanium dioxide (TiO2). Experimental results demonstrate that SMX is mineralized by TiO2 irradiated with ultraviolet-A light (UVA wavelength: 324-400 nm). Rates of UVA-TiO2 photocatalyzed SMX degradation are dependent upon several variables, including the initial SMX concentration, catalyst phase identity and concentration, electron acceptor identity and concentration, and the presence of non-target water constituents. In contrast, reaction rates are not sensitive to changes in sulfonamide structure. Although pH has little direct effect on reaction rates, the presence of natural organic matter (NOM) inhibits photocatalytic degradation of SMX to a much greater extent at pH 5 than pH 9. In addition, the presence of bicarbonate leads to enhanced SMX photocatalysis at pH 9. Kinetic trends are consistent with a mechanism involving sulfonamide oxidation by hydroxyl radicals (dOH) generated via TiO2 band gap excitation by UVA radiation. Identified transformation intermediates and products are consistent with SMX mineralization initiated by dOH attack on either the aromatic or heterocyclic rings or the sulfonamide bond. Results demonstrate that UVA-TiO2 photocatalysis can be a very effective approach for degrading sulfonamide micropollutants, particularly in natural waters exhibiting either alkaline pH or low concentrations of NOM, or both conditions.
Mi, Baoxia, Benito J. Marinas, and David G. Cahill, "RBS Characterization of Arsenic (III) Partitioning from Aqueous Phase into the Active Layers of Thin-Film Composite NF/RO Membranes." Environmental Science and Technology 41 pp. 3290-3295 2007.
The main objective of this study was to apply Rutherford backscattering spectrometry (RBS) for characterizing the partitioning of arsenic(III) from aqueous phase into the active layer of NF/RO membranes. NF/RO membranes with active layer materials including polyamide (PA), PA-polyvinyl alcohol derivative (PVA), and sulfonatedpolyethersulfone (SPES) were investigated. The partition coefficient was found to be constant in the investigated As(III) concentration range of 0.005-0.02 M at each pH investigated. The partitioning of As(III) when predominantly present as H3AsO3 (pH 3.5-8.0) was not affected by pH. In contrast, the partition coefficient of As(III) at pH 10.5, when it was predominantly present as H2AsO3-, was found to be approximately 33-49% lower than that of H3AsO3. The partition coefficients of H3AsO3 and H2AsO3- for membranes containing PA in their active layers were within the respective ranges of 6.2-8.1 and 3.6-5.4, while the corresponding values (4.8 and 3.0, respectively) for the membrane with SPES active layer were approximately 30% lower than the average values for the PA membranes.
Padmasiri, Sudini I., Jiangzhao Zhang, Mark Fitch, Birgir Norddahl, Eberhard Morgenroth, and Lutgarde Raskin, "Methanogenic Population Dynamics and Performance of an Anaerobic Membrane Bioreactor (AnMBR) Treating Swine Manure under High Shear Conditions." Water Research 41 pp. 134-144 2007.
A 6-L, completely mixed anaerobic bioreactor with an external ultrafiltration membrane module was operated for 300 days to evaluate the startup and performance of an anaerobic membrane bioreactor (AnMBR) treating swine manure. The reactor had a successful startup at the initial loading rate of 1 g volatile solids (VS)/L/day. After a two-fold increase in loading rate followed by a sudden, two-fold increase in flow velocity through the membrane module on day 75, the performance of the AnMBR deteriorated as measured by volatile fatty acid (VFA) accumulation, decrease in pH, and decrease in biogas production. The methanogenic population dynamics in the reactor were monitored with terminal restriction fragment length polymorphism (T-RFLP). Changes in the relative levels of Methanosarcinaceae and Methanosaetaceae were consistent with changes in VFA concentrations, i.e., high and low levels of acetate corresponded to a high abundance of Methanosarcinaceae and Methanosaetaceae, respectively. The levels of hydrogenotrophic methanogens of the order of Methanomicrobiales increased during decreased reactor performance suggesting that syntrophic interactions involving hydrogenotrophic methanogens remained intact regardless of the degree of shear in the AnMBR.
Asatekin, Ayse, Adrienne L. Menniti, Seoktae Kang, Menachem Elimelech, Eberhard Morgenroth, and Ann M. Mayes, "Antifouling Nanofiltration Membranes for Membrane Bioreactors from Self-Assembling Graft Copolymers." Journal of Membrane Science 285 pp. 81-89 2006.
Ultrafiltration (UF) membranes, often employed in membrane bioreactors (MBRs), exhibit high susceptibility to fouling by extracellular polymeric substances (EPS). As potential alternatives, commercial polyvinylidene fluoride (PVDF) UF membranes were coated with the amphiphilic graft copolymer poly(vinylidene fluoride)-graft-poly(oxyethylene) methacrylate, PVDF-g-POEM, to create thin film composite (TFC) nanofiltration membranes. Pure water permeabilities up to 56 L/m2 hMPa were obtained at pressures of 0.21MPa (30 psi). The new TFC NF membranes exhibited no irreversible fouling in 10-day dead-end filtration studies of model organic foulants bovine serum albumin, sodium alginate and humic acid at concentrations of 1000 mg/L and above. Dead-end filtration of activated sludge from an MBR (1750 mg/L volatile suspended solids, VSS) resulted in constant flux throughout the 16 h filtration period. Fouling performance of the TFC NF membrane and effluent water quality were substantially improved in all cases over that for the base PVDF UF membrane. Utilizing the atomic force microscope (AFM) colloid probe technique, the measured interaction force profiles indicated the presence of repulsive steric interactions, which likely prevent the attachment of foulants to the TFC NF membrane. Similarly, the adhesion (pull-off) curves reveal the absence of foulant adhesion to the TFC NF membrane surface, even in the presence of divalent calcium ions. In contrast, when such force measurements are carried out with the base PVDF UF membrane, substantial adhesion forces are registered.
Chaplin, Brian P., Eric Roundy, Kathryn A. Guy, John R. Shapley, and Charles J. Werth, "The Effects of Natural Water Ions and Humic Acid on Nitrate Reduction Using an Alumina Supported Pd-Cu Catalyst." Environmental Science and Technology 30 pp. 3075-3081 2006.
Catalytic nitrate reduction was evaluated for the purpose of drinking water treatment. Common anions present in natural waters and humic acid were evaluated for their effects on NO3- hydrogenation over a bimetallic supported catalyst (Pd-Cu/gamma-Al2O3). Groundwater samples, with and without powder activated carbon (PAC) pretreatment, were also evaluated. In the absence of inhibitors the NO3- reduction rate was 2.4 x 10-01 L/min g cat. However, the addition of constituents (SO42-, SO32-, HS-, Cl-, HCO3-, OH-, and humic acid) on the order of representative concentrations for drinking water decreased the NO3- reduction rate. Sulfite, sulfide, and elevated chloride decreased the NO3- reduction rate by over 2 orders of magnitude. Preferential adsorption of Cl- inhibited NO3- reduction to a greater extent than NO2- reduction. Partial regeneration of catalysts exposed to SO32- was achieved by using a dilute hypochlorite solution, however Cu dissolution occurred. Dissolved constituents in the groundwater sample decreased the NO3- reduction rate to 3.7 x 10-03 L/min g cat and increased ammonia production. Removal of dissolved organic matter from the groundwater using PAC increased the NO3- reduction rate to 5.06 x 10-02 L/min g cat and decreased ammonia production. Elemental analyses of catalysts exposed to the natural groundwater suggest that mineral precipitation may also contribute to catalyst fouling.
Collart, David, Sharifeh Mehrabi, Liah Robinson, Bryan Kepner, and Eric A. Mintz, "Efficacy of Oligodynamic Metals in the Control of Bacteria Growth in Humidifier Water Tanks and Mist Droplets." Journal of Water and Health 4 pp. 149-156 2006.
Antimicrobial capsules were evaluated for their effectiveness to control bacterial contamination of cool mist humidifiers. These capsules contain a mixture of silver and copper promoted alumina beads designed to release low concentrations of these oligodynamic metals into the reservoir water for bacteria control. The reservoir water and mist droplets from the humidifier units were tested for the presence of bacteria over a three-week period. A control unit (without capsule) showed significant bacterial contamination by day three, which increased throughout the three-week test period, in both the reservoir and mist droplets, whereas the antimicrobial capsules reduced contamination during the first week, and minimized the presence of bacteria, in both the reservoir water and mist droplets, to less than 2% of the control unit throughout the three-week test period. It was also observed that, after each inactive weekend, the initial discharge of bacteria via the mist droplets in the control unit was significantly higher than during daily use. However, initial bacterial discharge from the test unit following weekend inactivity never exceeded 0.5% of the control unit. In conclusion, these capsules containing oligodynamic metals are effective in controlling bacteria growth in humidifier water tanks and mist droplets.
Ding, Li, Benito J. Mariñas, Lance C. Schideman, Vernon L. Snoeyink, and Qilin Li, "Competitive Effects of Natural Organic Matter: Parameterization and Verification of the Three-Component Adsorption Model COMPSORB." Environmental Science and Technology 40 pp. 350-356 2006.
Natural organic matter (NOM) hinders adsorption of trace organic compounds on powdered activated carbon (PAC) via two dominant mechanisms: direct site competition and pore blockage. COMPSORB, a three-component model that incorporates these two competitive mechanisms, was developed in a previous study to describe the removal of trace contaminants in continuous-flow, hybrid PAC adsorption/membrane filtration systems. Synthetic solutions containing two model compounds as surrogates for NOM were used in the original study to elucidate competitive effects and verify the model. In the present study, a quantitative method to characterize the components of NOM that are responsible for competitive adsorption effects in natural water was developed to extend the application of COMPSORB to natural water systems. Using batch adsorption data, NOM was differentiated into two fictive fractions, representing the strongly competing and pore blocking components, each treated as a single compound. The equilibrium and kinetic parameters for these fictive compounds were calculated using simplified adsorption models. This parameterization procedure was carried out on two different natural waters, and the model was verified with experimental data obtained for atrazine removal from natural water in a PAC/membrane system. The model predicted the system performance reasonably well and highlighted the importance of considering both direct site competition and pore blockage effects of NOM in modeling these systems.
Flachsbart, Bruce R., Kachuen Wong, Jamie M. Iannacone, Edward N. Abante, Robert L. Vlach, Peter A. Rauchfuss, Paul W. Bohn, Jonathon V. Sweedler, and Mark A. Shannon, "Design and Fabrication of a Multilayered Polymer Microfluidic Chip with Nanofluidic Interconnects via Adhesive Contact Printing." Lab on a Chip 6 pp. 667-674 2006.
The design and fabrication of a multilayered polymer micro-nanofluidic chip is described that consists of poly(methylmethacrylate) (PMMA) layers that contain microfluidic channels separated in the vertical direction by polycarbonate (PC) membranes that incorporate an array of nanometre diameter cylindrical pores. The materials are optically transparent to allow inspection of the fluids within the channels in the near UV and visible spectrum. The design architecture enables nanofluidic interconnections to be placed in the vertical direction between microfluidic channels. Such an architecture allows microchannel separations within the chip, as well as allowing unique operations that utilize nanocapillary interconnects: the separation of analytes based on molecular size, channel isolation, enhanced mixing, and sample concentration. Device fabrication is made possible by a transfer process of labile membranes and the development of a contact printing method for a thermally curable epoxy based adhesive. This adhesive is shown to have bond strengths that prevent leakage and delamination and channel rupture tests exceed 6 atm (0.6 MPa) under applied pressure. Channels 100 micrometers in width and 20 micrometers in depth are contact printed without the adhesive entering the microchannel. The chip is characterized in terms of resistivity measurements along the microfluidic channels, electroosmotic flow (EOF) measurements at different pH values and laser-induced-fluorescence (LIF) detection of green fluorescent protein (GFP) plugs injected across the nanocapillary membrane and into a microfluidic channel. The results indicate that the mixed polymer micro–nanofluidic multilayer chip has electrical characteristics needed for use in microanalytical systems.
Ge, Zhenbin, David G. Cahill, and Paul V. Braun, "Thermal Conductance of Hydrophobic and Hydrophilic Interfaces." Physical Review Letters 96 p. 186101 2006.
Using time-domain thermoreflectance, we have measured the transport of thermally excited vibrational energy across planar interfaces between water and solids that have been chemically functionalized with a self-assembled monolayer (SAM). The Kapitza length - i.e. the thermal conductivity of water divided by the thermal conductance per unit area of the interface - is analogous to the `slip length` for water flowing tangentially past a solid surface. We find the Kapitza length at hydrophobic interfaces (10-12 nm) is a factor of 2-3 times larger than the Kapitza length at hydrophilic interfaces (3-6 nm). If a vapor layer is present at the hydrophobic interface, and this vapor layer has a thermal conductivity that is comparable to bulk water vapor, then our experimental results constrain the thickness of the vapor layer to be less than 0.25 nm.
Long, Timothy M., Shaurya Prakash, Mark A. Shannon, and Jeffrey S. Moore, "Water-Vapor Plasma-Based Surface Activation for Trichlorosilane Modification of PMMA." Langmuir 22 pp. 4104-4109 2006.
Separation rates and resolutions within capillary electrophoretic (CE) systems can be enhanced when surface zeta potentials are uniform with minimum deviations from ideal pluglike flow. Microfluidic CE devices based on poly-(methyl methacrylate) (PMMA) are being developed due to the optical clarity, availability, stability, and reproducible electroosmotic flow (EOF) rates displayed by this polymer. Control of EOF in polymer-based CE systems can be achieved by surface zeta potential alteration through chemical modification. Herein, a method will be presented for the surface functionalization of PMMA with chemistry analogous to formation of trichlorosilane self-assembled monolayers on SiO2. The current method involves two separate steps. First, surface activation with water-vapor plasma introduces surface hydroxylation. Second, treatment of the plasma-treated PMMA with a substituted trichlorosilane solution forms the functional surface layer. The modified surfaces were characterized using several analytical techniques, including water contact angle, X-ray photoelectron spectroscopy, Fourier transform infrared-attenuated total reflection, secondary ion mass spectroscopy, and measurement of EOF velocities within PMMA microchannels.
Mi, Baoxia, Orlando Coronell, Benito J. Marinas, Fumiya Watanabe, David G. Cahill, and Ivan Petrov, "Physico-chemical Characterization of NF/RO Membrane Active Layers by Rutherford Backscattering Spectrometry." Journal of Membrane Science 282 pp. 71-81 2006.
The physico-chemical properties of the active layer of nanofiltration/reverse osmosis (NF/RO) membranes were characterized by Rutherford backscattering spectrometry (RBS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). NF/RO membranes with active layer materials including polyamide (PA), polyvinyl alcohol derivative (PVA), PA–PVA, and sulfonated-polyethersulfone (SPES) were investigated. SEM and TEM images of membrane cross-sections confirmed the presence of a denser active layer supported by an asymmetric porous polysulfone structure but could only provide a rough estimate of membrane active layer average thickness. RBS provided an accurate tool to determine the elemental composition of NF/RO membrane active and support layers, and the thickness and roughness of the membrane active layer. The oxygen-to-nitrogen molecular ratio obtained for PA membranes was in the range of 1.1–3.0, which is consistent with the reported presence of unreacted carboxyl groups in the active layer surface of this type of membranes.
Qiao, R., John G. Georgiadis, and Narayana Aluru, "Differential Ion Transport Induced Electroosmosis and Internal Recirculation in Heterogeneous Osmosis Membranes." Nano Letters 6 pp. 995-999 2006.
Water and ion transport through a heterogeneous membrane separating two electrolyte solutions at different concentrations is investigated by using molecular dynamics simulations. The membrane features pairs of oppositely charged pores with identical diameters. Simulation results indicate that the differential transport of K+ and Cl- ions through the membrane pores creates an electrical potential difference across the membrane, which then induces an electroosmotic water flux. The induced electroosmosis creates an internal recirculation loop of water between adjacent pores. The implications of these new observations are discussed.
Wernette, Daryl P., Carla B. Swearingen, Donald M. Cropek, Yi Lu, Jonathan V. Sweedler, and Paul W. Bohn, "Incorporation of a DNAzyme into Au-Coated Nanocapillary Array Membranes with an Internal Standard for Pb(II) Sensing." The Analyst 131 pp. 41-47 2006.
A Pb(II)-specific DNAzyme has been successfully incorporated into Au-coated polycarbonate track-etched (PCTE) nanocapillary array membranes (NCAMs) by thiol–gold immobilization. Incorporation of the DNAzyme into the membrane provides a substrate-bound sensor using a novel internal control methodology for fluorescence-based detection of Pb(II). A non-cleavable substrate strand, identical to the cleavable DNAzyme substrate strand except the RNA-base is replaced by the corresponding DNA-base, is used for ratiometric comparison of intensities. The cleavable substrate strand is labeled with fluorescein, and the non-cleavable strand is labeled with a red fluorophore (Cy5 or Alexa 546) for detection after release from the membrane surface. This internal standard based ratiometric method allows for real-time monitoring of Pb(II)-induced cleavage, as well as standardizing variations in substrate size, solution detection volume, and monolayer density. The result is a Pb(II)-sensing structure that can be stored in a prepared state for 30 days, regenerated after reaction, and detect Pb(II) concentrations as low as 17 nM (3.5 ppb).
Chang, In-Hyoung, Joseph J. Tulock, Juewen Liu, Won-Suk Kim, Donald M. Cannon Jr., Yi Lu, Paul W. Bohn, Jonathan V. Sweedler, and Donald M. Cropek, "Miniaturized Lead Sensor Based on Lead-Specific DNAzyme in a Nanocapillary Interconnected Microfluidic Device." Environmental Science and Technology 39 pp. 3756-3761 2005.
A miniaturized lead sensor has been developed by combining a lead-specific DNAzyme with a microfabricated device containing a network of microfluidic channels that are fluidically coupled via a nanocapillary array interconnect. A DNAzyme construct, selective for cleavage in the presence of Pb2+ and derivatized with fluorophore (quencher) at the 5-prime (3-prime) end of the substrate and enzyme strands, respectively, forms a molecular beacon that is used as the recognition element. The nanocapillary array membrane interconnect is used to manipulate fluid flows and deliver the small-volume sample to the beacon in a spatially confined detection window where the DNAzyme is interrogated using laser-induced fluorescence detection. A transformed log plot of the fluorescent signal exhibits a linear response (r2 = 0.982) over a Pb2+ concentration range of 0.1-100 uM, and a detection limit of 11 nM. The sensor has been applied to the determination of Pb2+ in an electroplating sludge reference material, the result agreeing with the certified value within 4.9%. Quantitative measurement of Pb2+ in this complex sample demonstrates the selectivity of this sensor scheme and points favorably to the application of such technologies to analysis of environmental samples. The unique combination of a DNAzyme with a microfluidic-nanofluidic hybrid device makes it possible to change the DNAzyme to select for other compounds of interest, and to incorporate multiple sensing systems within a single device for greater flexibility. This work represents the initial steps toward creation of a robust field sensor for lead in groundwater or drinking water.
Chatterjee, Aveek N., Donald M. Cannon Jr., Enid N. Gatimu, Jonathan V. Sweedler, Narayana R. Aluru, and Paul W. Bohn, "Modeling and Simulation of Ionic Currents in Three-Dimensional Microfluidic Devices with Nanofluidic Interconnects." Journal of Nanoparticle Research 7 pp. 507-516 2005.
Electrokinetic fluid flow in nanocapillary array (NCA) membranes between vertically separated micro-fluidic channels offers an attractive alternative to using mechanical action to achieve fluidic communication between different regions of lab-on-a-chip devices. By adjusting the channel diameter, a, and the inverse Debye length, K, and applying the appropriate external potential, the nanochannel arrays, can be made to behave like digital fluidic switches, and the movement of molecules from one side of the array to the other side can be controlled. However, inherent differences in ionic mobility lead to non-equilibrium ion populations on the downstream side, which, in turn, shows up through transient changes in the microchannel conductance. Here we describe coupled calculations and experiments in which the electrical properties of a microfluidic–nanofluidic hybrid architecture are simulated by a combination of a compact model for the bulk electrical properties and iterative self-consistent solutions of the coupled Poisson, Nernst–Planck, and Navier–Stokes equations to recover the detailed ion motion in the nanopores. The transient electrical conductivity in the microchannel, after application of a forward bias pulse to the NCA membrane, is recovered in quantitative detail. The surface charge density of the nanopores and the capacitance of the membrane, which are critical determinants of electrokinetic flow through NCA, fall out of the analysis in a natural way, providing a clear mechanism to determine these critically important parameters.
Sanders, Lori K., Camilo Guáqueta, Thomas E. Angelini, Jae-Wook Lee, Scott C. Slimmer, Erik Luijten, and Gerard C.L. Wong, "Structure and Stability of Self-Assembled Actin-Lysozyme Complexes in Salty Water." Physical Review Letters 95 p. 108302 2005.
Interactions between actin, an anionic polyelectrolyte, and lysozyme, a cationic globular protein, have been examined using a combination of synchrotron small-angle x-ray scattering and molecular dynamics simulations. Lysozyme initially bridges pairs of actin filaments, which relax into hexagonally coordinated columnar complexes comprised of actin held together by incommensurate one-dimensional close-packed arrays of lysozyme macroions. These complexes are found to be stable even in the presence of significant concentrations of monovalent salt, which is quantitatively explained from a redistribution of salt between the condensed and the aqueous phases.
Swearingen, Carla B. Daryl P. Wernette, Donald M. Cropek, Yi Lu, Jonathan V. Sweedler, and Paul W. Bohn, "Immobilization of a Catalytic DNA Molecular Beacon on Au for Pb(II) Detection." Analytical Chemistry 77 pp. 442-448 2005.
A Pb(II)-specific DNAzyme fluorescent sensor has been modified with a thiol moiety in order to immobilize it on a Au surface. Self-assembly of the DNAzyme is accomplished by first adsorbing the single-thiolated enzyme strand (HS-17E-Dy) followed by adsorption of mercaptohexanol, which serves to displace any Au-N interactions and ensure that DNA is bound only through the S-headgroup. The preformed self-assembled monolayer is then hybridized with the complementary fluorophore-containing substrate strand (17DS-Fl). Upon reaction with Pb(II), the substrate strand is cleaved, releasing a fluorescent fragment for detection. Fluorescence intensity may be correlated with original Pb(II) concentration, and a linear calibration was obtained over nearly four decades: 10 uM is greater than or equal to [Pb(II)] is greater than or equal to 1 nM. The immobilized DNAzyme is a robust system; it may be regenerated after cleavage, allowing multiple sensing cycles. In addition, drying of fully assembled DNAzyme before reaction with Pb(II) does not significantly affect analytical performance. These results demonstrate that, in comparison with solution-based schemes, immobilization of the DNAzyme sensor onto a Au surface lowers the detection limit (from 10 to 1 nM), maintains activity and specificity, and allows sensor regeneration and long-term storage. Realization of Pb(II) detection through an immobilized DNAzyme is the first important step toward creation of a stand-alone, portable Pb(II) detection device such as those immobilizing DNAzyme recognition motifs in the nanofluidic pores of a microfluidic-nanofluidic hybrid multilayer device.
Wang, Jianwei, Andrey G. Kalinichev, R. James Kirkpatrick, and Randall T. Cygan, "Structure, Energetics, and Dynamics of Water Adsorbed on the Muscovite (001) Surface: A Molecular Dynamics Simulation." Journal Physical Chemistry B 109 pp. 15893-15905 2005.
Molecular dynamics (MD) computer simulations of liquid water adsorbed on the muscovite (001) surface provide a greatly increased, atomistically detailed understanding of surface-related effects on the spatial variation in the structural and orientational ordering, hydrogen bond (H-bond) organization, and local density of H2O molecules at this important model phyllosilicate surface. MD simulations at constant temperature and volume (statistical NVT ensemble) were performed for a series of model systems consisting of a two-layer muscovite slab (representing 8 crystallographic surface unit cells of the substrate) and 0 to 319 adsorbed H2O molecules, probing the atomistic structure and dynamics of surface aqueous films up to 3 nm in thickness. The results do not demonstrate a completely liquid-like behavior, as otherwise suggested from the interpretation of X-ray reflectivity measurements and earlier Monte Carlo simulations. Instead, a more structurally and orientationally restricted behavior of surface H2O molecules is observed, and this structural ordering extends to larger distances from the surface than previously expected. Even at the largest surface water coverage studied, over 20% of H2O molecules are associated with specific adsorption sites, and another 50% maintain strongly preferred orientations relative to the surface. This partially ordered structure is also different from the well-ordered 2-dimensional ice-like structure predicted by ab initio MD simulations for a system with a complete monolayer water coverage. However, consistent with these ab initio results, our simulations do predict that a full molecular monolayer surface water coverage represents a relatively stable surface structure in terms of the lowest diffusional mobility of H2O molecules along the surface. Calculated energies of water adsorption are in good agreement with available experimental data.
Cannon, Jr., Donald M., Bruce R. Flachsbart, Mark A. Shannon, Jonathan V. Sweedler, and Paul W. Bohn, "Fabrication of Single Nanofluidic Channels in Poly(methylmethacrylate) Films via Focused-Ion Beam Milling for Use as Molecular Gates." Applied Physics Lettters 85 pp. 1241-1243 2004.
Focused-ion beam (FIB) milling provides rapid fabrication of individual cylindrical submicrometer channels with reproducible dimensions (±5% diameters) through 8- um thick poly(methylmethacrylate) (PMMA) films. PMMA films are spincast on sacrificial Si carriers and sputter-coated with Au before the 30-kV gallium FIB milling process. By adding a trace amount of poly(ethyleneoxide) and poly(dimethylsiloxane) to the PMMA solution before casting, the films can be released for subsequent mounting in microfluidic devices to create hybrid microfluidic-nanofluidic multilevel architectures. In situ FIB sectioning demonstrates the smooth cylindrical surface within the pore. Placing a milled film in contact with an aqueous fluorescein solution fills the channel by capillary action, as verified by confocal fluorescence microscopy. Confocal fluorescence of dyed films reveals that the pores span the thickness of the PMMA film. Small arrays of channels with a defined number and density and arbitrary in-plane spatial arrangement are fabricated with this process, allowing a unique testbed for high aspect ratio nanofluidic devices.
Ostroverkhov, Victor, Glenn A. Waychunas, and Y. Ron Shen, "Vibrational Spectra of Water at Water/alpha-Quartz (0001) Interface." Chemical Physics Letters 386 pp. 144-148 2004.
Selected beam geometry allows us to suppress the bulk contribution to sum-frequency generation from crystalline quartz and use sum-frequency vibrational spectroscopy to study water/[alpha]-quartz interfaces with different bulk pH values. The spectra are qualitatively similar to those of water/fused-quartz interfaces, but display an ice-like peak that resembles very closely that of a real ice surface, providing the first evidence to the belief that water molecules at a crystalline oxide surface form a more ordered hydrogen bonding network.
Liu, Chunqing, Nathaniel Naismith, Lei Fu, and James Economy, "Novel Nanoporous Hybrid Organic-Inorganic Silica Containing Iminodiethanol Chelating Groups Inside the Channel Pores." Chemical Communications 2003 pp. 1920-1921 2003.
Novel nanoporous hybrid organic–inorganic silica with covalently bound iminodiethanol chelating groups inside the channel pores has been synthesized by template-directed co-condensation of tetraethoxysilane (TEOS) and organotrimethoxysilane (CH3O)3SiR [IDES, R = (HOCH2CH2)2NCH2CH(OH)CH2O(CH2)3], and is shown to be very efficient in recovery of germanium and antimony oxides from water.
Frierdich, Andrew J., John R. Shapley, and Timothy J. Strathmann, "Rapid Reduction of <i>N</i>-Nitrosamine Disinfection Byproducts in Water with Hydrogen and Porous Nickel Catalysts." Environmental Science and Technology 42 pp. 262-269 2008.
There is a need for new technologies to rapidly and economically treat water contaminated with <i>N</i>-nitrosodimethylamine(NDMA) and related compounds because of their high toxicity and recent detection in drinking water sources as a consequence of industrial releases and chlorine disinfection of wastewater effluent. Treatment of <i>N</i>-nitrosamines with H<sub>2</sub> in conjunction with a high surface area porous nickel material, a model nonprecious metal catalyst, has been evaluated. Experiments show that NDMA is reduced rapidly and catalytically to dimethylamine and N<sub>2</sub> (e.g., t<sub>1/2</sub> = 1.5 min for 500 mg/L catalyst and P<sub>H<sub>2</sub></sub> = 1 atm), and kinetic trends are consistent with a surface-mediated mechanism involving scission of the <i>N</i>-nitrosamine N-N bond and subsequent reactions with adsorbed atomic hydrogen. The metal-loading-normalized pseudo-first-order rate constant (77.9 +/- 13.1 L g<sub>Ni</sub><sup>-1</sup> h<sup>-1</sup>) exceeds values reported for Pd-based catalysts. Several related N-nitrosamines react at rates similar to those of NDMA, indicating a weak dependence on structure. The reaction rates for NDMA reduction are not significantly affected by changing pH, and the presence of high concentrations of many common water constituents (Na<sup>+</sup>, Ca<sup>2+</sup>, Mg<sup>2+</sup>, Cl<sup>-</sup>, SO<sub>4</sub><sup>2-</sup>, HCO<sub>3</sub><sup>-</sup>, and NOM) exerts only a small effect on reaction rates. Nitrate is also reduced by the Ni catalyst, and high nitrate concentrations competitively inhibit the reduction of NDMA. (Bi)sulfide poisons the catalyst by strong chemisorption to the Ni surface. Cost-normalized rate constants for the Ni catalyst are highly favorable compared to Pd-based catalysts, indicating that, with further development, Ni-based catalysts may become attractive alternatives to precious metal catalysts.
Asatekin, Ayse, Seoktae Kang, Menachem Elimelech, and Ann M. Mayes, "Anti-Fouling Ultrafiltration Membranes Containing Polyacrylonitrile-graft-poly(ethylene oxide) Comb Copolymer Additives." Journal of Membrane Science 298 pp. 136-146 2007.
Membrane fouling is one of the most important challenges faced in membrane ultrafiltration (UF) operations. In this study, polyacrylonitrile-graft-poly(ethylene oxide) (PAN-g-PEO), an amphiphilic comb copolymer with a water-insoluble polyacrylonitrile (PAN) backbone and hydrophilic poly(ethylene oxide) (PEO) side chains, was used as an additive in the manufacture of novel PAN UF membranes. During casting, the PAN-g-PEO additive segregates to form a PEO brush layer on all membrane surfaces, including internal pores. Wettability, purewater permeability, and resistance to irreversible fouling increased when either the amount of PAN-g-PEO added to the membrane or the PEO content of the comb copolymer was increased. These trends were consistent with measured adhesion forces between the membranes and a carboxylated latex particle probe in an atomic force microscopy (AFM) analysis, and with the near-surface PEO coverage as determined by X-ray photoelectron spectroscopy (XPS). Scanning electron microscopy (SEM) revealed further effects of additive incorporation on membrane morphology. In 24-h dead-end filtration studies, blend membranes prepared with 20 wt% PAN-g-PEO (comb PEO content: 39 wt%) were found to resist irreversible fouling by 1000 ppm solutions of bovine serum albumin (BSA), sodium alginate, and humic acid, recovering the initial pure water flux completely by a pure water rinse, or a backwash in the case of humic acid. This exceptional anti-fouling performance holds promise for extending UF membrane lifetimes without need for aggressive cleaning procedures.
Briones, Aurelio M., Becky J. Daugherty, Largus T. Angenent, Kent D. Rausch, Mike E. Tumbleson, and Lutgarde Raskin, "Microbial Diversity and Dynamics in Multi- and Single-Compartment Anaerobic Bioreactors Processing Sulfate-rich Waste Streams." Environmental Microbiology 9 pp. 93-106 2007.
We investigated bacterial and archaeal community structures and population dynamics in two anaerobic bioreactors processing a carbohydrate- and sulfate-rich synthetic wastewater. A five-compartment anaerobic migrating blanket reactor (AMBR) was designed to promote biomass and substrate staging, which partially separates the processes of methanogenesis and sulfidogenesis in the middle and outer compartment(s) respectively. The second reactor was a conventional, single-compartment upflow anaerobic sludge blanket (UASB) reactor. Both reactors, which were seeded with the same inoculum, performed well when the influent chemical oxygen demand (COD)/SO42– mass ratio was 24.4. The AMBR performed worse than the UASB reactor when the influent COD/SO42– mass ratio was decreased to 5.0 by raising the sulfate load. Terminal restriction fragment length polymorphism analyses of bacterial 16S rRNA genes showed that the increase in sulfate load had a greater impact on bacterial diversity and community structure for the five AMBR compartments than for the UASB reactor. Moreover, bacterial community profiles across AMBR compartments became more similar through time, indicating a converging, rather than a staged community. While similar populations were abundant in both reactors at the beginning of the experiment, fermenting bacteria (clostridia, streptococci), and sulfate-reducing bacteria became more abundant in the AMBR, after shifting to a higher sulfate load, while a novel Thermotogales-like population eventually became predominant in the UASB reactor. A similar shift in the community structure of the hydrogenotrophic methanogens in the AMBR occurred: representatives of the Methanobacteriaceae out-competed the Methanospirillaceae after increasing the sulfate load in the AMBR, while the archaeal community structure was maintained in the UASB.
Chandrasekharan, Ramesh, Richard I. Masel, and Mark A. Shannon, "Experimental Technique Using FTIR to Estimate IR Optical Properties at Variable Temperatures: Application to PMDA-ODA Polyimide Thin Films from 100 to 380oC." Review of Scientific Instruments 78 p. 53105 2007.
An experimental technique is presented to measure reflectance at high sample temperature with respect to room temperature in the infrared using Fourier transform infrared fitted with a reflectometer. Sample temperature artifacts are accounted for by sequential measurements taken with the lamp source on and with the lamp source off. The sequential measurements are shown mathematically to correct for the modulation of sample and detector thermal emissions. Further, the technique is applied to a polyimide (PMDA-ODA) film on a layer of gold deposited on a thermally oxidized Si wafer. It is shown that the optical properties (index of refraction and extinction coefficient) remain relatively constant with temperature (from room temperature to 380oC) in the 4000–6000 cm-1 spectral region. The significant changes that occur with temperature are the change in thickness of the film and also the spectral properties in the 2000–4000 cm-1 region. Also, by using a Lorentz oscillator model, it is shown that this method is able to discern that spectral features corresponding to the OH stretching bands at 3630 and 3470 cm-1 show significant variation with increasing temperature.
Chandrasekharan, Ramesh, Shaurya Prakash, and Mark A. Shannon, "Change in Radiative Optical Properties of Ta2O5 Thin Films due to High Temperature Heat Treatment." Journal of Heat Transfer 129 pp. 27-36 2007.
Thin films (0.85 um, 3 um) of Ta2O5 deposited on Si and SiO2 were heated to 900 °C. Their reflectance in the infrared was measured using a Fourier transform infrared spectrometer equipped with a multiple angle reflectometer before and after exposure to the high-temperature heat treatment. An interfacial layer (TaSixOy) formed by the diffusion of Si from the substrate into the deposited film was observed using Auger depth profiling, and the effect of this interfacial layer on the reflectance was measured. Using a least squares optimization technique coupled with an optical admittance algorithm, the multiple angle reflectance data were used to calculate the optical constants of the as deposited Ta2O5 film, crystalline Ta2O5, and the interfacial layer in the 1.6 to 10 um range. The interfacial layer formed due to exposure to high temperature was found to be more absorptive than the crystalline Ta2O5.
de Kerchove, Alexis J., and Menachem Elimelech, "Formation of Polysaccharide Gel Layers in the Presence of Ca2+ and K+ Ions: Measurements and Mechanisms." Biomacromolecules 8 pp. 113-121 2007.
Understanding the adsorption properties of polysaccharides in terms of substrate affinity, kinetics, and layer structure is of paramount importance in numerous industrial and natural systems. The structural growth of the layers of two model polysaccharides--sodium alginate and polygalacturonic acid (PGA)--was characterized by quartz crystal microbalance with dissipation and atomic force microscopy. Monitoring the variations in frequency and dissipation energy provides insights into both the average adsorbed mass and the viscoelastic properties of the adsorbed layer of polyelectrolytes along with the associated ions and water molecules. Both polysaccharides had similar adsorption patterns with increasing ionic strengths and showed significant complexation of calcium ions. In the presence of calcium, the alginate gel layer exhibited significant swelling with an increasing concentration of monovalent salt that the PGA gel layer did not manifest. Basing our discussion on the egg-box model, we interpreted these different swelling behaviors as resulting from differences in the complexation modes of the two polysaccharides. The dimerization of the polymers by cross-linking and the weaker dimer-dimer associations play major roles in the sensitivity of the polysaccharide gel matrix to high salt concentration environments.
Dong, C.X., A.P. Xian, E.H. Han, and Jian-Ku Shang, "C-Doped TiO2 with Visible-Light Photocatalytic Activity." Solid State Phenomena 121-123 pp. 939-942 2007.
Carbon doped titanium dioxide photocatalysts have been synthesized by a precipitation method. Anatase TiO2 was obtained when the precipitate was heat-treated at 400oC when rutile phase appeared at 600oC. The absorption spectra of the powders showed strong absorption in visible region. For the sample calcined at 400oC, the absorption extended up to 750 nm. X-ray photoelectron spectrum (XPS) confirmed that carbon atoms were incorporated into the TiO2 crystal lattice. Photodegradation experiments showed that the powders had visible-light degradation activity.
Hurley, Keith D., and John R. Shapley, "Efficient Heterogeneous Catalytic Reduction of Perchlorate in Water." Environmental Science and Technology 41 pp. 2044-2049 2007.
A new heterogeneous catalyst that promotes the reduction by hydrogen of perchlorate ion in water under mild conditions has been developed. The catalyst is prepared by adsorption of a rhenium(VII) precursor (either ammonium perrhenate or methylrhenium trioxide) onto carbon powder containing 5% palladium by weight. Under standard batch conditions of room temperature, 1 bar of hydrogen, and 200 ppm perchlorate (as HClO4), reduction proceeded to less than 1 ppm in as little as 5 h. Extended reaction times led to residual perchlorate at low parts per billion levels. Chloride was the only observed product, with good material balance. Catalytic materials ranging from 3% to 13% Re showed (pseudo) first-order rates linearly dependent on the Re content. Representative normalized rate constants for catalysts with 5-9% Re were in the range 0.1-0.3 L h-1 (g of cat.)-1. Inhibition by chloride was not significant, with little change in perchlorate reduction rate in the presence of excess chloride to 1000 ppm. However, optimal activity occurred in acidic solutions (pH ca. 3), and both the rate and extent of reaction decreased at higher values of pH. In its current form the catalyst might be best applied to destroy perchlorate in the acidic regeneration stream from selective ion exchange columns.
Ingram, Conrad W., Yohannes Ghirmazion, and Tesfamariam Mereteab, "Mesoporous Aluminosilicate Catalysts from FAU Precursor under Mild Acidic Conditions and with Al in Totally Tetrahedral Coordination." Journal of Porous Materials 14 pp. 7-17 2007.
Al-SBA-15 mesoporous catalyst with strong Bronsted acid sites and Al stabilized in a totally tetrahedral coordination was synthesized from the addition of hydrothermally aged zeolite Y precursor to SBA-15 synthesis mixture under mildly acidic condition of pH 5.5. The materials possessed surface areas between 690 and 850 m2/g, pore sizes ranging from 5.6 to 7.5 nm and pore volumes up to 1.03 cm3, which were comparable to parent SBA-15 synthesized under similar conditions. As much as 2 wt.% Al was present in the most aluminated sample that was investigated, and the Al remained stable in totally tetrahedral coordination, even after calcination at 550oC. Calcined Al-SBA-15 showed high hydrothermal stability when treated with steam (20% v/v in nitrogen) at 650oC for 2 h. Textural characteristics are maintained on steam treatment, and very little or no conversion of Al from tetrahedral to octahedral coordination resulted. The Al-SBA-15 mesoporous catalyst showed significant catalytic activity for cumene dealkylation, and activity increased as the amount of zeolite precursor added to the SBA-15 mixture was increased. The catalyst's activity was not affected by the aging time of the precursor for up to the 24 h aging time investigated. This method of introducing Al and maintaining it in a total tetrahedral coordination is very effective, in comparison to other direct and post synthesis alumination methods reported.
Jung, Y.J., B.S. Oh, J.W. Kang, Martin A. Page, M.J. Phillips, and Benito J. Marinas, "Control of Disinfection and Halogenated Disinfection Byproducts by the Electrochemical Process." Water Science and Technology 55 pp. 213-219 2007.
The aim of this study was to investigate some aspects of the performance of electrochemical process as an alternative disinfection strategy, while minimising DBPs, for water purification. The study of electrochemical processes has shown free chlorine to be produced, but smaller amounts of stronger oxidants, such as ozone, hydrogen peroxide and OH radicals (.OH), were also generated. The formation of mixed oxidants increased with increasing electric conductivity, but was limited at conductivities greater than 0.6 mS/cm. Using several microorganisms, such as E. coli and MS2 bacteriophage, inactivation kinetic studies were performed. With the exception of free chlorine, the role of mixed oxidants, especially OH radicals, was investigated for enhancement of the inactivation rate. Additionally, the formation and reduction of DBPs was studied by monitoring the concentration of haloacetic acids (HAAs) during the process.
Kang, Seoktae, Ayse Asatekin, Ann M. Mayes, and Menachem Elimelech, "Protein Antifouling Mechanisms of PAN UF Membranes Incorporating PAN-g-PEO Additive." Journal of Membrane Science 296 pp. 42-50 2007.
The antifouling mechanism of a novel polyacrylonitrile (PAN) ultrafiltration (UF) membrane incorporating the amphiphilic comb copolymer additive, polyacrylonitrile-graft-polyethylene oxide (PAN-g-PEO), has been investigated using a laboratory-scale cross-flow test unit and atomic force microscopy (AFM). In fouling tests with a bovine serum albumin (BSA) solution, PAN UF membranes incorporating 20% PAN-g-PEO possessed excellent antifouling characteristics, whereas a commercial PAN UF membrane showed severe pore blocking that followed with surface cake formation. The PAN-g-PEO/PAN blend membranes also exhibited fouling reversibility and constant BSA retention, which indicate their ability to separate macromolecules without altering the molecular weight cut-off. An AFM analysis with a model carboxylated colloid-probe measured no adhesion force with the blend membrane, while the commercial PAN membrane exhibited strong adhesion to the probe, consistent with the fouling properties of the membranes. The antifouling character of the blend membrane is attributed to the surface segregation and local orientation of PAN-g-PEO molecules at the membrane surface and pore walls during membrane casting, creating a dense PEO brush layer that provides a steric barrier to protein adsorption.
Li, Qi, Rongcai Xie, Jian-Ku Shang, and Eric A. Mintz, "Effect of Precursor Ratio on Synthesis and Optical Absorption of TiON Photocatalytic Nanoparticles." Journal of the American Ceramics Society 90 pp. 1045-1050 2007.
A simple sol–gel process was developed to synthesize a nitrogen-doped titanium oxide (TiON) nanoparticle photocatalyst. Our work shows that the precursor ratio (tetramethyl ammonium hydroxide (TMA)/titanium tetraisopropoxide (TTIP)) has significant effects on the structure, composition, and optical properties of TiON nanoparticles. At high initial TMA/TTIP ratios, crystallization of the sol–gel TiON nanoparticle powders was sluggish, requiring a higher calcination temperature. As a high calcination temperature caused greater nitrogen loss, a high initial TMA/TTIP ratio in the TiON precursor did not ensure a high N/Ti atomic ratio in the TiON powders. By optimizing the initial TMA/TTIP ratio and the calcination temperature, crystalline powders were obtained with high nitrogen concentrations. These sol–gel TiON nanoparticles showed strong visible light absorbance.
Li, Qi, Wei Liang, and Jian-Ku Shang, "Enhanced Visible Light Absorption from PdO Nanoparticles in Nitrogen Doped Titanium Oxide Thin Films." Applied Physics Letters 90 p. 63109 2007.
Composite films were made by embedding nanoparticles of p-type PdO semiconductor in a n-type titanium oxide matrix. With the assistance of nitrogen ion beam, these PdO nanoparticles were confined to less than a couple of nanometers. The resulting nanocomposite films demonstrated an unusual redshift in the optical absorption spectrum, contrary to the well-known quantum size effect from semiconductor nanoparticles. The absorption in the visible-light range is shown to result from absorption by metallic-like nanoparticles as palladium changed its valence state from Pd2+ to Pd in nitrogen-doped titanium oxide matrix under visible-light illumination.
Liu, Juewen, Andrea K. Brown, Xiangli Meng, Donald M. Cropek, Jonathan D. Istok, David B. Watson, and Yi Lu, "A Catalytic Beacon Sensor for Uranium with Parts-per-Trillion Sensitivity and Million-Fold Selectivity." Proceedings of the National Academy of Sciences USA 104 pp. 2056-2061 2007.
Here, we report a catalytic beacon sensor for uranyl (UO22+) based on an in vitro-selected UO22+-specific DNAzyme. The sensor consists of a DNA enzyme strand with a 3-prime quencher and a DNA substrate with a ribonucleotide adenosine (rA) in the middle and a fluorophore and a quencher at the 5-prime and 3-prime ends, respectively. The presence of UO22+ causes catalytic cleavage of the DNA substrate strand at the rA position and release of the fluorophore and thus dramatic increase of fluorescence intensity. The sensor has a detection limit of 11 parts per trillion (45 pM), a dynamic range up to 400 nM, and selectivity of greater than 1-million-fold over other metal ions. The most interfering metal ion, Th(IV), interacts with the fluorescein fluorophore, causing slightly enhanced fluorescence intensity, with an apparent dissociation constant of ~230 uM. This sensor rivals the most sensitive analytical instruments for uranium detection, and its application in detecting uranium in contaminated soil samples is also demonstrated. This work shows that simple, cost-effective, and portable metal sensors can be obtained with similar sensitivity and selectivity as much more expensive and sophisticated analytical instruments. Such a sensor will play an important role in environmental remediation of radionuclides such as uranium.
Lu, Yunyi, Tasuma Suzuki, Wei Zhang, Jeffrey S. Moore, and Benito J. Marinas, "Nanofiltration Membranes Based on Rigid Star Amphiphiles." Chemistry of Materials 19 pp. 3194-3204 2007.
Rigid star amphiphiles (RSAs) of nanoscale dimension were synthesized and used to fabricate a new generation of nanofiltration (NF) membranes. NF membranes were prepared by direct percolation of methanol solutions of the RSAs through an asymmetric polyethersulfone (PES) support film that had been previously conditioned with methanol and cross-linked polyvinyl alcohol (PES-MeOH-PVA support). The resulting RSA membranes (RSAMs) have been shown to exhibit significantly enhanced water permeability while maintaining high rejection of water contaminants compared to commercial NF membranes. The RSAMs were characterized with the goal of elucidating the structural changes brought about by deposition of RSA. Characterization techniques used included attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectrometry, field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), confocal microscopy, Rutherford backscattering spectrometry (RBS), and gas adsorption/desorption analyses. Results suggest that the RSAs produce a uniform, ultrathin active layer atop the PES-MeOH-PVA support after lining its nanopores with sizes similar to those of the RSAs. Such active layer constitution was found crucial for rejecting organic contaminants and achieving high water flux. These findings encourage further exploration of NF membrane preparation by molecular deposition as an attractive approach for constructing ultrathin membrane active layers to remove challenging contaminants with high water permeability.
Luh, Jeanne, and Benito J. Marinas, "Inactivation of Mycobacterium avium with Free Chlorine." Environmental Science and Technology 41 pp. 5096-5102 2007.
The inactivation kinetics of Mycobacterium avium with free chlorine was characterized by two stages: an initial phase at a relatively fast rate followed by a slower second stage of pseudo first-order kinetics. The inactivation rate of each stage was approximately the same for all experiments performed at a certain condition of pH and temperature; however, variability was observed for the disinfectant exposure at which the transition between the two stages occurred. This variability was not a function of the initial disinfectant concentration, the initial bacterial density, or the bacterial stock. However, the transition to the second stage varied more significantly at high temperatures (30oC), while lower variability was observed at lower temperatures (5 and 20oC). Experiments conducted at pH values in the range of 6-9 revealed that the inactivation of M. avium was primarily due to hypochlorous acid, with little contribution from hypochlorite ion within this pH range. The inactivation kinetics was represented with a two-population model. The activation energies for the resulting pseudo first-order rate constants for the populations with fast and slow kinetics were 100.3 and 96.5 kJ/mol, respectively. The magnitude of these values suggested that for waters of relatively high pH and low temperatures, little inactivation of M. avium would be achieved within treatment plants, providing a seeding source for distribution systems.
Mi, Baoxia, Benito J. Marinas, and David G. Cahill, "Physico-Chemical Integrity of Nanofiltration/Reverse Osmosis Membranes during Characterization by Rutherford Backscattering Spectrometry." Journal of Membrane Science 291 pp. 77-85 2007.
The suitability of using RBS for the characterization of polymeric composite RO/NF membranes was investigated with the specific goal of determining helium ion fluence thresholds resulting in measurable changes in membrane elemental composition and the thickness/roughness of membrane active layers. Two commercial membranes with polyamide (PA) and sulfonated-polyethersulfone (SPES) active layers on PSf support, and the PSf support without active layer were investigated. Changes in physico-chemical properties of the polymers were assessed by model fitting of the RBS spectra, and analyses with scanning electron microscopy (SEM) and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. Oxygen was found to be the most unstable element in all three materials investigated (PSf, PA, SPES) thus setting the helium ion fluence thresholds at 3×1014 He+/cm2 for PSf, 5×1014 He+/cm2 for PA, and 9×1014 He+/cm2 for SPES for accurate determination of elemental composition of both the active and support layers. If RBS is used to determine only the thickness and roughness of the membrane active layer, then the helium ion beam fluence could be set below higher thresholds of 2×1015 and 4×1015 He+/cm2 for membranes with PA and SPES active layers, respectively.
Milferstedt, K., M.-N. Pons, and Eberhard Morgenroth, "Texture Analysis of Spatial Biofilm Development." Water Science and Technology 55 pp. 481-488 2007.
The quantitative evaluation of images taken during biofilm experiments is an important step in determining the relation between biofilm performance and biofilm architecture. Whereas areal descriptors are used by some researchers, descriptors of biofilm texture have received limited attention. In our research, the texture of images documenting long-term biofilm experiments was evaluated using a spatial grey level dependence matrices (SGLDM) approach. By calculating SGLDM for a wide range of position operators (angle–distance combinations), the discriminatory power of this approach was extended. For some descriptors, surface plots allowed the direct spatial interpretation of texture. Using principal component analysis (PCA) a subset of independent textural descriptors was identified. It is suggested to determine textural fingerprints of stages during biofilm development by making use of PCA and biplots.
Paul, Tias, Penney L. Miller, and Timothy J. Strathmann, "Visible Light-Mediated TiO2 Photocatalysis of Fluoroquinolone Antibacterial Agents." Environmental Science and Technology 41 pp. 4720-4727 2007.
This study reports on the photocatalytic transformation of fluoroquinolone antibacterial agents (ciprofloxacin, enrofloxacin, norfloxacin, and flumequine) in aqueous titanium dioxide (TiO2) suspensions irradiated with ultraviolet (UV; [lambda] greater than 324 nm) or visible light ([lambda] greater than 400, 420, or 450 nm). Visible-light-mediated fluoroquinolone degradation is unexpected from direct photolysis or established TiO2 band gap photoexcitation mechanisms, which both require UV light. Visible-light-mediated photocatalysis requires an appropriate conduction band electron acceptor (e.g., O2, BrO3-), but is not dependent upon hydroxyl radical, superoxide, or other reactive oxygen species generated upon TiO2 band gap excitation. The process slows considerably when fluoroquinolone adsorption is inhibited. Whereas fluoroquinolone decomposition in UV-irradiated TiO2 suspensions is accompanied by mineralization, no changes in dissolved organic carbon occur during visible-light-photocatalyzed degradation. Results are consistent with a proposed charge-transfer mechanism initiated by photoexcitation of surface-complexed fluoroquinolone molecules. Complexation to the TiO2 surface causes a red shift in the fluoroquinolone absorption spectrum (via ligand-to-metal charge transfer), enabling photoexcitation by visible light. Fluoroquinolone oxidation then occurs by electron transfer into the TiO2 conduction band, which delivers the electron to an adsorbed electron acceptor. The lack of organic carbon mineralization indicates formation of stable organic byproducts that are resistant to further degradation by visible light. In UV-irradiated TiO2 suspensions, the charge-transfer mechanism acts in parallel with the semiconductor band gap photoexcitation mechanism.
Prakash, Shaurya, Junghoon Yeom, Niu Jin, I. Adesida, and Mark A. Shannon, "Characterization of Ionic Transport at the Nanoscale." Journal of Nanoengineering and Nanosystems 220 pp. 45-52 2007.
This paper reports on the development of a multi-layer microscale impedance measurement system with integrated working, counter, and reference electrodes that can be used to probe transport at the nanoscale. System fabrication and testing are carried out to demonstrate the feasibility of such a system for characterizing transport through nanocapillary array membranes (NCAMs). Results indicate that transport through NCAMs is a complex phenomenon, and impedance does not scale linearly with either pore diameter or ionic concentration. Use of a microscale construct for probing ionic transport at the nanoscale appears to be a promising path forward with further development.
Prakash, Shaurya, Timothy M. Long, John C. Selby, Jeffrey S. Moore, and Mark A. Shannon, "Click Modification of Silica Surfaces and Glass Microfluidic Channels." Analytical Chemistry 79 pp. 1661-1667 2007.
This paper demonstrates a chemical surface modification method for covalent attachment of various polymers by using silane-based click chemistry on silica surfaces and within glass microchannels suitable for CE systems. Modified surfaces are characterized by contact angle measurements, X-ray photoelectron spectroscopy, and Fourier transform infrared-attenuated total reflection spectroscopy. Electroosmotic flow (EOF) measurements in modified and unmodified channels are provided. Spectroscopic and transport data show that various polymers can be covalently attached to glass surfaces with a measurable change in EOF.
Qi, Shaoying, Lance Schideman, Benito J. Marinas, Vernon L. Snoeyink, and Carlos Campos, "Simplification of the IAST for Activated Carbon Adsorption of Trace Organic Compounds from Natural Water." Water Research 41 pp. 440-448 2007.
Recent studies have shown that the ideal adsorbed solution theory (IAST) coupled with the concept of equivalent background compound (EBC) can be simplified for describing trace organic compound adsorption from natural water, provided that the adsorbent surface loading is dominated by competing natural organic matter. The resulting simplified IAST has been used to reduce the complexity of kinetic models for various dynamic adsorption processes. In order to be correctly applied, however, the simplified IAST requires some additional clarification and a quantitative evaluation of the deviation caused by the simplifying assumption. In this study, we derive a simple equation that relates the relative deviation of the simplified IAST directly to the molar ratio of EBC and trace organic compound surface loadings and their Freundlich isotherm exponents. We then verify the simplified IAST using the original IAST and experimental isotherm data from the literature for trace organic compounds at various initial concentrations in natural water. By further assuming that the adsorbed amount of the EBC is substantially greater than what remains in solution, a new pseudo single-solute isotherm equation is derived and a simple relation is subsequently established between the carbon dose and the remaining trace compound concentration. The results show that the adsorption capacity and relative removal of a trace organic compound at any carbon dose can be estimated directly with the simple equations developed here and data from a single isotherm experiment for the target compound conducted in the natural water of interest.
Raguin, L. Guy, and Luisa Ciobanu, "Multiple Echo NMR Velocimetry: Fast and Localized Measurements of Steady and Pulsatile Flows in Small Channels." Journal of Magnetic Resonance 184 pp. 337-343 2007.
The understanding of fluid transport in miniaturized flow devices is an important component in the design of flow cells, micromixers, and microreactors. In this manuscript, we employ NMR in the form of a voxel-selective multiple modulation multiple echo sequence (MMMEV) to monitor average velocities in individual microchannels inside a six-channel network. The technique produces average velocities which are consistent with the imposed flow rates. In addition, we take advantage of the short acquisition time (32 ms per velocity component) of the technique to quantitatively track the time evolution of the fluid velocity in a pulsatile flow phantom.
Schideman, Lance C., Vernon L. Snoeyink, Benito J. Marinas, Li Ding, and Carlos Campos, "Application of a Three-Component Competitive Adsorption Model to Evaluate and Optimize Granular Activated Carbon Systems." Water Research 41 pp. 3289-3298 2007.
A recently developed kinetic model for granular activated carbon (GAC) adsorbers (COMPSORB-GAC) that quantitatively describes the adsorption of trace organic contaminant in the presence of competing natural organic matter (NOM) was applied to evaluate the performance of different GAC system configurations: conventional fixed-bed adsorbers, layered upflow carbon adsorbers (LUCA), and moving-bed adsorbers (with few or many bed sections). COMPSORB-GAC separately tracks the adsorption of three components: a trace compound, a strongly competing NOM fraction that reduces trace compound equilibrium capacity, and a pore-blocking NOM fraction that reduces kinetics. Performance was simulated for various design criteria and with model parameters derived for two natural waters with significantly different NOM concentrations. For the range of simulated conditions and with baseline performance defined by a fixed-bed adsorber, LUCA generally reduced carbon usage rates (CURs) by 15–35%. A 2-section and a 16-section moving-bed reactor reduced baseline CURs by 20–30% and 45–55%, respectively. Projected CURs for the water source with a relatively high NOM concentration were 2–3 times higher for all reactor configurations and indicated that NOM preloading would cause performance deterioration in deep GAC beds, which highlights the importance of source water quality. These results show how COMPSORB-GAC can be used in a comprehensive, site-specific optimization of GAC systems to ensure robust system performance and to balance capital and operating costs.
Zhang, J., Sudini I. Padmasiri, Mark Fitch, Birgir Norddahl, Lutgarde Raskin, and Eberhard Morgenroth, "Influence of Cleaning Frequency and History on Membrane Fouling in an Anaerobic Membrane Bioreactor." Desalination 207 pp. 153-166 2007.
Membrane fouling was evaluated in a side stream anaerobic membrane bioreactor (AnMBR) operated for the treatment of swine manure. The AnMBR consisted of an external tubular polyethersulphone ultrafiltration membrane module (diameter = 12 mm) connected to a six-liter mixed bioreactor. The system was operated for 135 days without chemical membrane cleaning resulting in a membrane flux of 5–10 L/m2 h. Membrane fouling was dominated by a loosely attached fouling layer, which could be removed by flushing the tubular membrane. Intensive chemical cleaning after the 135 days of continuous operation resulted in an irreversible resistance of 3×1012 1/m, equivalent to 1.3 times the resistance of the new membrane. More frequent chemical membrane cleaning using HNO3 could not prevent the development of irreversible fouling. Equilibrium calculations and scanning electron microscopy with energy dispersive spectroscopy demonstrated that inorganic precipitation contributed to fouling of the membrane surface and in the membrane pores.
Zhang, Luning, Weitao Liu, Y. Ron Shen, and David G. Cahill, "Competitive Molecular Adsorption at Liquid/Solid Interfaces: A Study by Sum-Frequency Vibrational Spectroscopy." Journal of Physical Chemistry C 111 pp. 2069-2076 2007.
We used sum-frequency vibrational spectroscopy to study competitive adsorption of water-alcohol binary liquid mixtures at hydrophilic fused silica and hydrophobic alkyl silane-covered substrates. Monitoring the strength of the methyl stretching modes of alcohols in the mixtures allowed deduction of the surface coverage of alcohols and their adsorption isotherms. It was found that for both types of substrates, alcohol adsorbs preferentially than water at liquid/solid interfaces. The driving force behind preferential alcohol adsorption appears to be the strong hydrogen-bonding interaction among water molecules as they would like to maintain the three-dimensional hydrogen-bonding network and minimize loss of hydrogen bonds in the interfacial layer. The mechanism is believed to be generally true if interaction among molecules of one species is significantly stronger than other intermolecular interactions in a mixture and the interactions of different molecular species with the substrate are about the same.
Bondarenko, Galina V., Yuri E. Gorbaty, A.V. Okhulkov, and Andrey G. Kalinichev, "Structure and Hydrogen Bonding in Liquid and Supercritical Aqueous NaCl Solutions at a Pressure of 1000 Bar and Temperatures up to 500oC: A Comprehensive Experimental and Computational Study." Journal of Physical Chemistry A 110 pp. 4042-4052 2006.
The behavior of aqueous 1.1 M NaCl solution at a constant pressure of 1000 bar in the temperature range 25-500oC has been studied with the use of IR absorption, Raman scattering, X-ray diffraction, and molecular dynamics (MD) simulations. The results are compared with the data for pure water under identical external conditions. The main purpose of the experimental and theoretical studies was to understand in what way an electrolyte dissolved in water influences the hydrogen bonding and structural features of water. As was found, the vibrational spectra show no essential difference between the properties of solution and pure water. However, the experimental pair correlation functions and the results of MD simulations present an evidence for very different nature of these substances. A characteristic feature of the structure of NaCl solution is a considerable contribution of strong O-H - - - Cl- bonds. As the temperature increases, the number of such bonds decreases partially due to a phenomenon of ion pairing, so that at high temperatures the properties of the solution become closer to the properties of water.
Davie, Matthew G., Martin Reinhard, and John R. Shapley, "Metal-Catalyzed Reduction of N-Nitrosodimethylamine with Hydrogen in Water." Environmental Science and Technology 40 pp. 7329-7335 2006.
There is considerable need for the rapid destruction of N-nitrosodimethylamine (NDMA) in water because current alternative treatment methods are relatively inefficient. Powdered metal catalysts in conjunction with hydrogen gas showed notable potential for rapid destruction of N-nitrosodimethylamine (NDMA) in water. Palladium, copper-enhanced palladium, and nickel catalysts showed significant efficacy for NDMA reduction, with observed half-lives on the order of hours using 10 mg L-1 catalyst metal. Other catalysts were screened because of their well-documented efficacy for reduction of halogenated hydrocarbons, including zerovalent iron, nickel-enhanced iron, nickel, and manganese. Starting with 100 ug L-1 NDMA, a level observed at multiple field sites, pseudo-first-order kinetics were observed for all catalysts tested. No reaction intermediates were observed in any experiment; the amine group of NDMA was cleaved and reduced to dimethylamine with carbon balance in excess of 97%. Reductive catalysis may prove an efficient technology for mitigating the health risk posed by NDMA; this study provides the foundation for mechanistic and longevity research.
de Kerchove, Alexis J., and Menachem Elimelech, "Structural Growth and Viscoelastic Properties of Adsorbed Alginate Layers in Mono- and Divalent Salts." Macromolecules 39 pp. 6558-6564 2006.
The growth and evolution of the viscoelastic properties of an alginate layer as a function of ionic strength and the presence of divalent calcium ions are monitored by quartz crystal microbalance with dissipation and atomic force microscopy. The properties of the alginate layer are investigated by combining results of incremental layer thickness or adsorbed areal mass, viscoelastic properties of the film, adhesion forces between adsorbing polysaccharides, and pull-off distances of the adsorbed molecules. In the absence of calcium, alginate adsorption behavior is strongly influenced by charge screening of the negatively charged carboxyl moieties of the alginate molecules. Reduction in alginate layer swelling is observed as ionic strength is increased, most likely due to Donnan equilibrium effects. The presence of calcium ions induces the formation of a thick and fluid gel layer via the complexation of alginate molecules. As the monovalent salt concentration is increased, a displacement of the ion-exchange equilibrium takes place in response to the competition between monovalent (potassium) and divalent (calcium) cations. The resulting impact of this phenomenon on the structure and viscoelastic properties of the alginate layer is discussed.
Dong, C.X., A.P. Xian, E.H. Han, and Jian-Ku Shang, "Acid-Mediated Sol-Gel Synthesis of Visible-Light Active Photocatalysts." Journal of Materials Science 41 pp. 6168-6170 2006.
Flachsbart, Bruce R., K. Wong, J.M. Iannacone, E.N. Abante, R.L. Vlach, P.A. Rauchfuss, T.W. Yasunaga, Paul W. Bohn, Jonathon V. Sweedler, and Mark A. Shannon, "Design and Fabrication of a Multilayered Polymer Microfluidic Chip with Nanofluidic Interconnects via Adhesive Contact Printing." Lab on a Chip 6 pp. 667-674 2006.
Gatimu, Enid N., Jonathan V. Sweedler, and Paul W. Bohn, "Nanofluidics and Mass-Limited Chemical Analysis." The Analyst 131 pp. 705-709 2006.
Integrated microfluidic structures, comprised of three-dimensional assemblies of microfluidic channels, can effect sequentially-linked analytical operations with mass-limited samples. This three-dimensional operation is enabled by electrically switchable nanocapillary array membranes with novel transport properties.
Guaqueta, Camilio, Lori K. Sanders, Gerard C.L. Wong, and Erik Luijten, "The Effect of Salt on Self-Assembled Actin–Lysozyme Complexes." Biophysical Journal 90 pp. 4630-4638 2006.
We present a combined experimental and computational study of the bundling of F-actin filaments induced by lysozyme proteins. Synchrotron small-angle x-ray scattering results show that these bundles consist of close-packed columnar complexes in which the actin is held together by incommensurate, one-dimensional arrays of lysozyme macroions. Molecular dynamics simulations of a coarse-grained model confirm the arrangement of the lysozyme and the stability of this structure. In addition, we find that these complexes remain stable even in the presence of significant concentrations of monovalent salt. The simulations show that this arises from partitioning of the salt between the aqueous and the condensed phases. The osmotic pressure resulting from the excess concentration of the salt in the aqueous phase balances the osmotic pressure increase in the bundle. These results are relevant for a variety of biological and biomedical problems in which electrostatic complexation between anionic polyelectrolytes and cationic globular proteins takes place, such as the pathological self-assembly of endogenous antibiotic polypeptides and inflammatory polymers in cystic fibrosis.
Joseph, Sony, and N.R. Aluru, "Hierarchical Multiscale Simulation of Electrokinetic Transport in Silica Nanochannels at the Point of Zero Charge." Langmuir 22 pp. 9041-9051 2006.
Effects of nanoscale confinement and partial charges that stem from quantum calculations are investigated in silica slit channels filled with 1M KCl at the point of zero charge by using a hierarchical multiscale simulation methodology. Partial charges of both bulk and surface atoms from ab initio quantum calculations that take into account bond polarization and electronegativity are used in molecular dynamics (MD) simulations to obtain ion and water concentration profiles for channel widths of 1.1, 2.1, 2.75, and 4.1 nm. The interfacial electron density profiles of simulations matched well with that of recent X-ray reflectivity experiments. By simulating corresponding channels with no partial charges, it was observed that the partial charges affect the concentration profiles and transport properties such as diffusion coefficients and mobilities up to a distance of about 3 [sigma]O-O from the surface. Both in uncharged and partially charged cases, oscillations in concentration profiles of K+ and Cl- ions give rise to an electro-osmotic flow in the presence of an external electric field, indicating the presence of an electric double layer at net zero surface charge, contrary to the expectations from classical continuum theory. I-V curves in a channel-bath system using ionic mobilities from MD simulations were significantly different for channels with and without partial charges for channel widths less than 4.1 nm.
Liu, Juewen, and Yi Lu, "Fast Colorimetric Sensing of Adenosine and Cocaine Based on a General Sensor Design Involving Aptamers and Nanoparticles." Angewandte Chemie, International Edition 45 pp. 90-94 2006.
Liu, Juewen, and Yi Lu, "Preparation of Aptamer-linked Gold Nanoparticle Purple Aggregates for Colorimetric Sensing of Analytes." Nature Protocols 1 pp. 246-252 2006.
Aptamers are single-stranded DNA or RNA molecules that can bind target molecules with high affinity and specificity. The conformation of an aptamer usually changes upon binding to its target analyte, and this property has been used in a wide variety of sensing applications, including detection based on fluorescence intensity, polarization, energy transfer, electrochemistry or color change. Colorimetric sensors are particularly important because they minimize or eliminate the necessity of using expensive and complicated instruments. Among the many colorimetric sensing strategies, metallic nanoparticle-based detection is desirable because of the high extinction coefficients and strong distance-dependent optical properties of the nanoparticles. Here, we describe a protocol for the preparation of aptamer-linked gold nanoparticle purple aggregates that undergo fast disassembly into red dispersed nanoparticles upon binding of target analytes. This method has proved to be generally applicable for colorimetric sensing of a broad range of analytes. The time range for the entire protocol is ~5 d, including synthesis and functionalization of nanoparticles, preparation of nanoparticle aggregates and sensing.
Liu, Weitao, Luning Zhang, and Yuen-Ron Shen, "Interfacial Structures of Methanol:Water Mixtures at a Hydrophobic Interface Probed by Sum-Frequency Vibrational Spectroscopy." Journal of Chemical Physics 125 p. 144711 2006.
Sum-frequency vibrational spectroscopy was used to study interfacial structure of methanol:water mixtures at an octyltrichlorosilane-covered hydrophobic surface. Methanol was found to adsorb preferentially than water at the interface with its methyl group tilted from the surface normal by ~35o for all methanol concentrations. Redshift of the methanol symmetric stretch mode, gradual disappearance of the water dangling-OH mode, and blueshifts of the dangling and liquid-like bonded-OH modes were also observed as the methanol concentration increased. They could be understood from the change of the interfacial hydrogen-bonding network associated with the change of surface composition.
Luijten, Erik, "Fluid Simulation with the Geometric Cluster Algorithm." Computing in Science and Engineering 8:2 pp. 20-29 2006.
I discuss the recently developed Geometric Cluster Algorithm for the simulation of fluids. Building upon earlier approaches, this Monte Carlo method permits the efficient simulation of cases where conventional simulation methods fail, such as mixtures of particles of different sizes. This is possible because entire clusters of particles are moved via geometric operations. Starting from the celebrated Metropolis Monte Carlo algorithm, I describe the development of cluster methods for magnetic systems and their connection with the geometric cluster algorithm.
Milferstedt, K., M.-N. Pons, and Eberhard Morgenroth, "Optical Method for Long-Term and Large-Scale Monitoring of Spatial Biofilm Development." Biotechnology and Bioengineering 94 pp. 773-782 2006.
A method was developed that allows biofilm monitoring on the square centimeter scale over extended periods of time. The method is based on image acquisition using a desktop scanner and subsequent image analysis. It was shown that results from grey level analysis are highly correlated with physical properties of the biofilm like average biomass and biofilm thickness. The scanner method was applied to monitor overall biofilm growth, detachment, and surface roughness during two 3 and 4 week long experiments. Two significantly different growth dynamics during the biofilm development could be identified, depending on the biofilm history. Surface roughness on transects in flow direction was always higher than on transects perpendicular to the flow, reflecting the anisotropic characteristics of biofilms growing in a flow field.
Musk, Dinty J., and Paul J. Hergenrother, "Chemical Countermeasures for the Control of Bacterial Biofilms: Effective Compounds and Promising Targets." Current Medicinal Chemistry 13 pp. 2163-2177 2006.
The pathogenic nature of many infectious bacteria is enhanced by their ability to form surface-associated, protected communities known as biofilms. Due to various factors, bacteria in biofilm communities display significantly greater resistance to traditional antimicrobial therapies than their planktonic brethren. This resistance complicates many common bacterial infections, resulting in recurrent ear infections, bacterial endocarditis, chronic lung infection in cystic fibrosis, infectious kidney stones, and surface infection of implanted medical devices. Owing to the serious nature of many biofilm-mediated infections and the near-complete dearth of effective strategies for treating them, efforts are underway to further understand the nature of bacterial infections involving biofilms and to discover and develop effective therapies to combat them. Particularly, several classes of chemical compounds have shown promise in combating biofilms when used in conjunction with traditional antimicrobials. The vast majority of these compounds exert their anti-biofilm properties through disruption of quorum sensing, a common means of intercellular communication in bacterial communities that allows coordinated expression of virulence factors and facilitates formation of the oft-complex architecture of mature bacterial biofilms. Other new chemical entities are effective against biofilms without necessarily affecting quorum sensing. This review summarizes salient research in the development of effective chemical countermeasures for Gram-negative and Gram-positive bacterial infections involving biofilms.
Pons, M.N., K. Milferstedt, and Eberhard Morgenroth, "Modeling of Chord Length Distributions." Chemical Engineering Science 61 pp. 3962-3973 2006.
A procedure for the calculation of chord length distributions (CLD) of populations of rigid and opaque particles of any size and shape distribution is given. It combines the capabilities of a virtual reality renderer to create 2D projections of particles and of an image analysis software which determine their chord lengths. The procedure has been validated on simple shapes (spheres, ellipsoids, parallelepipeds, cuboids, uniform polyhedra) that can be combined to simulate agglomerates or twinned crystals. The procedure has been used to discuss the experimental results obtained on gibbsite particles in different size ranges and to compare the mean chord length to the average particle size.
Poynor, Adele, Liang Hong, Ian K. Robinson, and Steve Granick, "How Water Meets a Hydrophobic Surfaces." Physical Review Letters 97 p. 266101 2006.
Synchrotron x-ray reflectivity measurements of the interface between water and methyl-terminated octadecylsilane monolayers with stable contact angle greater than 100o conclusively show a depletion layer, whether or not the water is degassed. The thickness is of order one water molecule: 2–4 angstroms with electron density less than 40% that of bulk water. Considerations of coherent and incoherent averaging of lateral inhomogeneities show that the data cannot be explained by nanobubbles. When the contact angle is lower, unstable in time, or when monolayers fail to be sufficiently smooth over the footprint of the x-ray beam, there is no recognizable depletion.
Putnam, Shawn A., David G. Cahill, Paul V. Braun, Zhenbin Ge, and Robert G. Shimmin, "Thermal Conductivity of Nanoparticle Suspensions." Journal of Applied Physics 99 p. 84308 2006.
We describe an optical beam deflection technique for measurements of the thermal diffusivity of fluid mixtures and suspensions of nanoparticles with a precision of better than 1%. Our approach is tested using the thermal conductivity of ethanol-water mixtures; in nearly pure ethanol, the increase in thermal conductivity with water concentration is a factor of 2 larger than predicted by effective medium theory. Solutions of C60–C70 fullerenes in toluene and suspensions of alkanethiolate-protected Au nanoparticles were measured to maximum volume fractions of 0.6% and 0.35 vol %, respectively. We do not observe anomalous enhancements of the thermal conductivity that have been reported in previous studies of nanofluids; the largest increase in thermal conductivity we have observed is 1.3% ± .8% for 4 nm diam Au particles suspended in ethanol.
Raghunathan, Anjan V., and N.R. Aluru, "Molecular Understanding of Osmosis in Semipermeable Membranes." Physical Review Letters 97 p. 24501 2006.
We investigate single-file osmosis of water through a semipermeable membrane with an uncharged, a positively and a negatively charged nanopore. Molecular dynamics simulations indicate that the osmotic flux through a negatively charged pore (J-) is higher compared to the osmotic flux in a positively charged pore (J+) followed by the osmotic flux in the uncharged pore (J0), i.e., J- is greater than J+ is greater than J0. The molecular mechanisms governing osmosis, steady state osmosis, and the observed osmotic flux dependence on the nanopore charge are explained by computing all the molecular interactions involved and identifying the molecular interactions that play an important role during and after osmosis. This study helps in a fundamental understanding of osmosis and in the design of advanced nanoporous membranes for various applications of osmosis.
Raghunathan, Anjan, and N.R. Aluru, "Effect of Size-Asymmetric Electrolyte on Single-File Osmosis." Applied Physics Letters 89 p. 64107 2006.
Single-file osmosis through a 0.9 nm diameter semipermeable carbon membrane using various size-asymmetric electrolyte solutions is investigated using molecular dynamics simulations. In an uncharged pore, the osmotic flux with a KCl solution is found to be higher than that of in a NaCl solution, i.e., JKCl is greater than JNaCl, for the same concentration gradient of solute. Mean force and hydration analysis indicate that distinct ion-water and ion-pore molecular interactions cause a differential cation and anion affinity to the pore, leading to the observed size-asymmetric electrolyte dependence on osmotic flux. The water orientation in the pore during osmosis is also related to the ion affinity and exhibits a size-asymmetric electrolyte dependence.
Schideman, Lance C., Benito J. Marinas, Vernon L. Snoeyink, and Carlos Campos, "Three-Component Competitive Adsorption Model for Granular Activated Carbon Columns - Part I: Model Development." Environmental Science and Technology 40 pp. 6805-6811 2006.
Heterogeneous natural organic matter (NOM) present in all natural waters impedes trace organic contaminant adsorption, and predictive modeling of granular activated carbon (GAC) adsorber performance is often compromised by inadequate accounting for these competitive effects. Thus, a 3-component adsorption model, COMPSORB-GAC, is developed that separately tracks NOM adsorption and its competitive effects as a function of NOM surface loading. In this model, NOM is simplified into two fictive fractions with distinct competitive effects on trace compound adsorption: a smaller, strongly competing fraction that reduces equilibrium capacity and a larger pore-blocking fraction that reduces adsorption kinetics (both external film mass transfer and surface diffusion). COMPSORB-GAC tracks these two NOM fractions, along with the trace compound, and changes adsorption parameters according to the local surface loading of the two NOM fractions. Model parameters are allowed to vary both temporally and spatially to reflect differences in the NOM preloading conditions that occur in GAC columns. This dual-resistance model is based on homogeneous surface diffusion with external film mass-transfer limitations. The governing equations are expressed in a moving-grid finite-difference formulation to accommodate the modeling of spatially varying parameters and moving-bed reactors with counter-current adsorbent flow. A series of short-term adsorption tests with fresh and preloaded GAC is proposed to determine the necessary model input parameters. The accompanying manuscript demonstrates the parameterization procedure and verifies the model with experimental data.
Schideman, Lance C., Benito J. Marinas, Vernon L. Snoeyink, and Carlos Campos, "Three-Component Competitive Adsorption Model for Granular Activated Carbon Columns - Part II: Model Parameterization and Verification." Environmental Science and Technology 40 pp. 6812-6817 2006.
COMPSORB-GAC is a 3-component competitive adsorption kinetic model for granular activated carbon (GAC) adsorbers that was developed in Part I of this study, including a proposed procedure for determining model parameters in natural water applications with background natural organic matter (NOM). Part II of this study demonstrates the proposed parameterization procedure and validates the modeling approach by comparing predictions with experimental breakthrough curves at multiple empty-bed contact times for both fixed-bed and moving-bed reactors. The parameterization procedure consists of a set of independent, short-term experimental tests with fresh and batch preloaded adsorbents and then data fitting using both classic and recently developed theoretical expressions. The model and parameterization procedure simplifies NOM into two fictive fractions (pore-blocking and strongly competing) and incorporates three competitive effects that vary both temporally and axially in a GAC column (direct competition for sites, intraparticle pore blockage, and external surface pore blockage). With all three competitive mechanisms accounted for, the model could accurately predict breakthrough profiles for column lengths and durations that were much longer than those used for model parameterization. Model predictions that ignored one or more of the competitive mechanisms showed that each mechanism was important for different regions of the breakthrough curve. The external surface pore-blockage effect was predominant for the prediction of early breakthrough data, whereas direct competition for sites and intraparticle pore blockage were prevalent when predicting higher breakthrough levels and data later in the column run.
Shen, Y. Ron, and Victor Ostroverkhov, "Sum-Frequency Vibrational Spectroscopy on Water Interfaces: Polar Orientation of Water Molecules at Interfaces." Chemical Reviews 106 pp. 1140-1154 2006.
Wang, Jianwei, Andrey G. Kalinichev, and R. James Kirkpatrick, "Effects of Substrate Structure and Composition on the Structure, Dynamics, and Energetics of Water at Mineral Surfaces: A Molecular Dynamics Modeling Study." Geochimica et Cosmochimica Acta 70 pp. 562-582 2006.
Molecular dynamics computer simulations of the molecular structure, diffusive dynamics and hydration energetics of water adsorbed on (001) surfaces of brucite Mg(OH)2, gibbsite Al(OH)3, hydrotalcite Mg2Al(OH)6Cl•2H2O, muscovite KAl2(Si3Al)O10(OH)2, and talc Mg3Si4O(OH)2 provide new insight into the relationships between the substrate structure and composition and the molecular-scale structure and properties of the interfacial water. For the three hydroxide phases studied here, the differences in the structural charge on the octahedral sheet, cation occupancies and distributions, and the orientations of OH groups all affect the surface water structure. The density profiles of water molecules perpendicular to the surface are very similar, due to the prevalent importance of H-bonding between the surface and the water and to their similar layered crystal structures. However, the predominant orientations of the surface water molecules and the detailed 2-dimensional near-surface structure are quite different. The atomic density profiles and other structural characteristics of water at the two sheet silicate surfaces are very different, because the talc (001) surface is hydrophobic whereas the muscovite (001) surface is hydrophilic. At the hydrophilic and electrostatically neutral brucite and gibbsite (001) surfaces, both donating and accepting H-bonds from the H2O molecules are important for the development of a continuous hydrogen bonding network across the interfacial region. For the hydrophilic but charged hydrotalcite and muscovite (001) surfaces, only accepting or donating H-bonds from the water molecules contribute to the formation of the H-bonding network at the negatively and positively charged interfaces, respectively. For the hydrophobic talc (001) surface, H-bonds between water molecules and the surface sites are very weak, and the H-bonds among H2O molecules dominate the interfacial H-bonding network. For all the systems studied, the orientation of the interfacial water molecules in the first few layers is influenced by both the substrate surface charge and the ability by the surfaces to facilitate H-bond formation. The first layer of water molecules at all surfaces is well ordered in the xy plane (parallel to the surface), and the atomic density distributions reflect the substrate crystal structure. The enhanced ordering of water molecules at the interfaces indicates reduced orientational and translational entropy. In thin films, water molecules are more mobile parallel to the surface than perpendicular to it due to spatial constraints. At neutral, hydrophilic substrates, single-monolayer surface coverage stabilizes the adsorbed water molecules and results in a minimum of the surface hydration energy. In contrast, at the charged and hydrophilic muscovite surface, the hydration energy increases monotonically with increasing water coverage over the range of coverages studied. At the neutral and hydrophobic talc surface, the adsorption of H2O is unfavorable at all surface coverages, and the hydration energy decreases monotonically with increasing coverage.
Wang, Jinwen, Zhongren Yue, Jeffrey S. Ince, and James Economy, "Preparation of Nanofiltration Membranes from Polyacrylonitrile Ultrafiltration Membranes." Journal of Membrane Science 286 pp. 333-341 2006.
Polyacrylonitrile (PAN) membranes display some unusual features for ultrafiltration (UF). The meso-macropores of PAN UF membranes can be easily reduced into the range of micro-mesopores by taking advantage of surface tension forces within the capillary pores during heat treatment in the presence of ZnCl2. Asymmetric PAN nanofiltration (NF) membranes with controlled highly dense pore surface functional groups were prepared by hydrolysis of the nitrile groups with NaOH. The combined effects of heat treatment and the presence of ZnCl2 on the formation of nanofiltration membranes were investigated. In addition, membrane post-treatment with NaOH was studied. The effect of counterion species on the membrane performance was also investigated. A simple new method, which utilized univalent alkaline ions as probes, was developed to derive the average pore size of cationic nanofiltration PAN membranes from pure water permeability coefficients.
Won, Chang Y., Sony Joseph, and N.R. Aluru, "Effect of Quantum Partial Charges on the Structure and Dynamics of Water in Single-Walled Carbon Nanotubes." Journal of Chemical Physics 125 p. 114701 2006.
In this work, using quantum partial charges, computed from 6–31G** /B3LYP density functional theory, in molecular dynamics simulations, we found that water inside (6,6) and (10,0) single-walled carbon nanotubes with similar diameters but with different chiralities has remarkably different structural and dynamical properties. Density functional calculations indicate that tubes with different chiralities have significantly different partial charges at the ends of tubes. The partial charges at the ends of a (10,0) tube are around 4.5 times higher than those of a (6,6) tube. Molecular dynamics simulations with the partial charges show different water dipole orientations. In the (10,0) tube, dipole vectors of water molecules at the end of the tube point towards the water reservoir resulting in the formation of an L defect in the center region. This is not observed in the (6,6) tube where dipole vectors of all the water molecules inside the tube point towards either the top or the bottom water reservoir. The water diffusion coefficient is found to increase in the presence of the partial charges. Water in the partially charged (10,0) tube has a lower diffusion coefficient compared to that of in the partially charged (6,6) tube.
Wu, Yan, and Mark A. Shannon, "An AC Driving Amplitude Dependent Systematic Error in Scanning Kelvin Probe Microscope Measurements: Detection and Correction." Review of Scientific Instruments 77 p. 43711 2006.
The dependence of the contact potential difference (CPD) reading on the ac driving amplitude in scanning Kelvin probe microscope (SKPM) hinders researchers from quantifying true material properties. We show theoretically and demonstrate experimentally that an ac driving amplitude dependence in the SKPM measurement can come from a systematic error, and it is common for all tip sample systems as long as there is a nonzero tracking error in the feedback control loop of the instrument. We further propose a methodology to detect and to correct the ac driving amplitude dependent systematic error in SKPM measurements. The true contact potential difference can be found by applying a linear regression to the measured CPD versus one over ac driving amplitude data. Two scenarios are studied: (a) when the surface being scanned by SKPM is not semiconducting and there is an ac driving amplitude dependent systematic error; (b) when a semiconductor surface is probed and asymmetric band bending occurs when the systematic error is present. Experiments are conducted using a commercial SKPM and CPD measurement results of two systems: platinum-iridium/gap/gold and platinum-iridium/gap/thermal oxide/silicon are discussed.
Xu, Xiang, and R. James Kirkpatrick, "NaCl Interaction with Interfacially Polymerized Polyamide Films of Reverse Osmosis Membranes: A Solid-State 23Na NMR Study." Journal of Membrane Science 280 pp. 226-233 2006.
23Na nuclear magnetic resonance (NMR) spectroscopy of NaCl-exchanged polyamide (PA) films comparable to those of the active skin layer of many reverse osmosis (RO) membranes provides novel insight into the structural environments and dynamical behavior of Na+ in such films. Unsupported PA films were synthesized via interfacial polymerization of trimesoyl chloride in hexane and m-phenylenediamine in aqueous solution, and SEM, FT-IR, and 13C NMR data demonstrate successful thin film polymerization. Compositional data confirm this conclusion and demonstrate equal Na and Cl incorporation during NaCl exchange from aqueous solution. The 23Na NMR spectra for freshly made polymer samples exchanged in 1 M NaCl solution show significant relative humidity (RH) dependence. At near 0% RH, there are resonances for crystalline NaCl and rigidly held Na+ in the PA. With increasing RH, a resonance for solution-like dynamically averaged Na+ appears and above 51% RH is the only signal observed. The slightly negative chemical shift of this resonance suggests a dominantly hydrous environment with some atomic-scale coordination by atoms of the polymer. The greatly reduced 23Na T1 relaxation rates for this resonance relative to bulk solution and crystalline NaCl confirm close association with the polymer. Variable temperature 23Na NMR spectra for a sample equilibrated at 97% RH obtained from −80 to 20oC show the presence of rigidly held Na+ in a hydrated environment at low temperatures and replacement of this resonance by the dynamically averaged signal at temperatures above about −20oC. The results provide support for the solution–diffusion model for RO membranes transport and demonstrate the capabilities of multi-nuclear NMR methods to investigate molecular-scale structure and dynamics of the interactions between dissolved species and RO membranes.
Xu, Xiang, Andrey G. Kalinichev, and R. James Kirkpatrick, "133Cs and 35Cl NMR Spectroscopy and Molecular Dynamics Modeling of Cs+ and Cl- Complexation with Natural Organic Matter." Geochimica et Cosmochimica Acta 70 pp. 4319-4331 2006.
Interaction of dissolved aqueous species with natural organic matter (NOM) is thought to be important in sequestering some species and enhancing the transport of others, but little is known about these interactions on a molecular scale. This paper describes a combined experimental 133Cs and 35Cl nuclear magnetic resonance (NMR) and computational molecular dynamics (MD) modeling study of the interaction of Cs+ and Cl- with Suwannee River NOM. The results provide a detailed picture of the molecular-scale structure and dynamics of these interactions. Individual NOM molecules are typically hundreds to thousands of Daltons in weight, and on the molecular scale their interaction with small dissolved species can be investigated in ways similar to those used to study the interaction of dissolved aqueous species with mineral surfaces. As for such surface interactions, understanding both the structural environments and the dynamics over a wide range of frequencies is essential. The NMR results show that Cs+ is associated with NOM at pH values from 3.4 ± 0.5 (unbuffered Suwannee River NOM solution) to 9.0 ± 0.5. The extent of interaction increases with decreasing CsCl concentration at constant pH. It also decreases with increasing pH at constant CsCl concentration due to pH-dependent negative structural charge development on the NOM caused by progressive deprotonation of carboxylic and phenolic groups. The presence of NOM has little effect on the 133Cs chemical shifts, demonstrating that its local coordination environment does not change significantly due to interaction with the NOM. Narrow, solution-like line widths indicate rapid exchange of Cs+ between the NOM and bulk solution at frequencies of greater than 102 Hz. The MD simulations support these results and show that Cs+ is associated with the NOM principally as outer sphere complexes and that this interaction does not reduce the Cs+ diffusion coefficient sufficiently to cause NMR line broadening. The 35Cl NMR data and the MD results are consistent in demonstrating that there is no significant complexation between Cl- and NOM in the pH range investigated, consistent with negative structural charge on the NOM.
Yue, Zhongren, and James Economy, "Synthesis of Highly Mesoporous Carbon Pellets from Carbon Black and Polymer Binder by Chemical Activation." Microporous and Mesoporous Materials 96 pp. 314-320 2006.
High surface area, highly mesoporous carbon granules in the form of pellets were prepared by first mixing a solution of polymer (phenolic resin, cellulose, or PAN) with water and a chemical reagent (NaOH or ZnCl2). Carbon black powder was then added to form a mixture, followed by molding into a pellet, drying and activating at a given temperature. The pore structures of the porous carbon binder and the resulting carbon pellets were characterized using N2 adsorption/desorption at 77 K and SEM. Chemical activation increases the BET surface area as well as the microporous and mesoporous volumes of the resulting carbon pellets. The effects of the weight ratio of the mixture and activation temperatures on the formation of pore structures of the carbon pellets were also investigated. The mesopore size and mesopore size distribution of the carbon pellets can also be adjusted by the molding pressure. SEM showed that highly mesoporous structures were formed by the free space in the aggregates and agglomerates of bonded carbon black particles.
Yue, Zhongren, James Economy, and Gary Bordson, "Preparation and Characterization of NaOH-Activated Carbons from Phenolic Resin." Journal of Materials Chemistry 16 pp. 1456-1461 2006.
Chemically (NaOH) activated carbons (CACs) from phenolic resin were prepared at temperatures from 600 to 800 deg C in N2. The effect of the NaOH/phenolic resin ratio and activation temperature on the BET surface area and carbon yield of the resulting carbons was investigated. The optimum NaOH/phenolic ratio is from 1.5 to 2.0 to produce high surface area and carbon yield at 600 deg C. With increasing activation temperature from 600 deg C to 800 deg C, the BET surface area increased but the carbon yield decreased. NaOH-activated carbons have a higher concentration of narrow micropores (<10 angstroms) and a lower concentration of mesopores than a commercially available activated carbon, GAC-F400. Lower temperature (600 deg C) NaOH-activated CAC-600 has a higher concentration of narrow micropores (<7 angstroms) than an 800 deg C NaOH-activated carbon, CAC-800. Much higher adsorption capacity for CO2 and H2 is obtained with a NaOH-activated carbon as compared to GAC-F400. A higher content of oxygen-containing groups but without any hydrogen in CAC-600 is observed by EA and XPS. The adsorption isotherms of methyl tert-butyl ether (MTBE) show that CAC-800 has a higher adsorption capacity than GAC-F400 and CAC-600.
Yue, Zhongren, James Economy, Kishore Rajagoplanan, Gary Bordson, Marv Piwoni, Li Ding, Vernon L. Snoeyink, and Benito J. Marinas, "Chemically Activated Carbon on a Fiberglass Substrate for Removal of Trace Atrazine from Water." Journal of Materials Chemistry 16 pp. 3375-3380 2006.
Chemically activated fiber (CAF) for removal of trace atrazine from water was prepared by coating fiberglass assemblies with a phenolic resin along with a chemical activation agent of ZnCl2, then stabilization and heat treatment in N2 at 500oC. The carbon on the CAF shows similar BET surface area and volumes of narrow micropores (less than 10 angstroms), higher volumes of wide micropores (10–20 angstroms) and narrow mesopores (20–50 angstroms), as compared with a commercially available GAC F-400. Adsorption isotherm data show that the CAF has a higher adsorption capacity for atrazine than the GAC, primarily because the CAF has an increased pore (10–50 angstroms) volume. The breakthrough tests show that the CAF filter is ten times more effective over the GAC filter in removing the atrazine to below the current USEPA standard of 3 ppb. The CAF filter also shows a better competitive adsorption of atrazine over the GAC filter in the presence of 50 times higher concentration of humic acid. Such a filter can be regenerated to 90% of its original activity by heating at 350oC in air.
Zhang, Liangfang, Liang Hong, Yan Yu, Sung Chul Bae, and Steve Granick, "Nanoparticle-Assisted Surface Immobilization of Phospholipid Liposomes." Journal of the American Chemical Society 128 pp. 9026-9027 2006.
Zhang, Xijing, and David G. Cahill, "Measurements of Interface Stress of Silicon Dioxide in Contact with Water—Phenol Mixtures by Bending of Microcantilevers." Langmuir 22 pp. 9062-9066 2006.
We use the bending of silicon microcantilevers to measure changes in mechanical stress at interfaces between phenol-water mixtures and SiO2. The curvature of the microcantilever is measured by an optical system that combines a rapidly scanning laser beam, a position-sensitive detector, and lock-in detection to achieve a long-time stability on the order of 6 mN m-1 over 4 h and a short-time sensitivity of better than 1 mN m-1. Thermally oxidized Si shows the smallest changes in interface stress as a function of phenol concentration in water. For hydrophilic SiO2 prepared by chemical treatment, the change in interface stress at 5 wt % phenol in water is larger than that of thermally oxidized Si by -60 mN m-1; for SiO2 formed by exposure of the silicon microcantilever to ozone, the change in surface stress is larger than that of thermally oxidized Si by -330 mN m-1.
Zribi, Olena V., Hee Kyung, Ramin Golestanian, Tanniemola B. Liverpool, and Gerard C.L. Wong, "Condensation of DNA-Actin Polyelectrolyte Mixtures Driven by Ions of Different Valences." Physical Review E 73 p. 31911 2006.
Multivalent ions can induce condensation of like-charged polyelectrolytes into compact states, a process that requires different ion valence for different polyelectrolyte species. In this work we examine the condensation behavior in binary anionic polyelectrolyte mixtures consisting of DNA coils and F-actin rods in the presence of monovalent, divalent, and trivalent ions. As expected, monovalent ions do not condense either component, and divalent ions selectively condense F-actin rods out of the polyelectrolyte mixture. For trivalent ions, however, we observe a micro-phase separation between the two polyelectrolytes into coexisting finite-sized F-actin bundles and DNA toroids. Further, by increasing the DNA volume fraction in the mixture, condensed F-actin bundles can be completely destabilized, leading to only DNA condensation within the mixture. We examine a number of possible causes and propose a model based on polyelectrolyte competition for ions.
Angelini, Thomas E., Hongjun Liang, Willy Wriggers, and Gerard C.L. Wong, "Direct Observation of Counterion Organization in F-Actin Polyelectrolyte Bundles." European Journal of Physics E 16 pp. 389-400 2005.
Attractions between like-charged polyelectrolytes have been observed in a variety of systems (W.M. Gelbart, R.F. Bruinsma, P.A. Pincus, V.A. Parsegian, Phys. Today 53, September issue, 38 (2000)). Recent biological examples include DNA, filamentous viruses, and F-actin. Theoretical investigations on idealized systems indicate that counterion correlations play a central role, but no experiments that specifically probe such correlations have been performed. Using synchrotron X-ray diffraction, we have directly observed the organization of multivalent ions on cytoskeletal filamentous actin (a well-defined biological polyelectrolyte) and found an unanticipated symmetry-breaking collective counterion mechanism for generating attractions. Surprisingly, the counterions do not form a lattice that simply follows actin’s helical symmetry; rather, the counterions organize into “frozen” ripples parallel to the actin filaments and form structures reminiscent of charge density waves. Moreover, these 1D counterion charge density waves form a coupled mode with twist deformations of the oppositely charged actin filaments. This counterion organization is not sensitive to thermal fluctuations in temperature range accessible to protein-based polyelectrolyte systems. Moreover, the counterion density waves are “pinned” to the spatial periodicity of charges on the actin filament even if the global filament charge density is varied, indicating the importance of charge periodicity on the polyelectrolyte substrate.
Angelini, Thomas E., Lori K. Sanders, Hongjun Liang, Willy Wriggers, Jay X. Tang, and Gerard C.L. Wong, "Structure and Dynamics of Condensed Multivalent Ions within Polyelectrolyte Bundles: A Combined X-Ray Diffraction and Solid-State NMR Study." Journal of Physics: Condensed Matter 17 pp. S1123-S1135 2005.
Like-charged polyelectrolytes can attract and condense into compact ordered states via counterion-mediated interactions (Gelbart et al 2000 Phys. Today 53, 38–44). Recent examples include DNA toroids and F-actin bundles. We have investigated the structure and dynamics of condensed divalent ions within F-actin polyelectrolyte bundles. Using synchrotron x-ray diffraction, the structural organization of Ba2+ ions on F-actin has been directly observed. The Ba2+ ions organize into counterion charge density waves (CDWs) parallel to the actin filaments. Moreover, these 1-D counterion charge density waves couple to twist deformations of the oppositely charged actin filaments, and mediate attractions by effecting a ‘zipper-like’ charge alignment between the counterions and the polyelectrolyte charge distribution. We have also examined condensed divalent 25Mg ions within F-actin bundles using solid-state NMR. Preliminary measurements indicate that the longitudinal relaxation time T1 of Mg2+ ions decreases by approximately an order of magnitude as they organize into the CDW state within condensed F-actin bundles. The measured value of T1 for Mg2+ ions in the CDW is intermediate between typical liquid-like and solid-like values.
Berkowski, Kimberly L., Kyle N. Plunkett, Qing Yu, and Jeffrey S. Moore, "Introduction to Photolithography: Preparation of Microscale Polymer Silhouettes." Journal of Chemical Education 82 pp. 1365-1369 2005.
Much of today’s information technology relies on microchips developed by the semiconductor industry. These microchips are the brains behind most electronic devices such as personal computers, cell phones, and CD players. Although microchip function is typically described in terms of physics and engineering, the underlying process for the fabrication of these complex electronic devices, photolithography, is rooted in polymer chemistry. Developed in 1959, photolithography is a process that uses high-intensity light and a photomask to prepare a polymer network on a silicon wafer. The polymer network is formed through a physical change to a photoresist, which contains a light-sensitive compound and a mixture of polymers that becomes soluble or insoluble when exposed to UV light. The patterned polymer network acts as a guide for the chemical etching of the silicon wafer, much as a canyon channels a river’s flow and causes it to carve away the underlying soil. The terms given to the different types of photoresists, positive and negative, depict the resulting polymer “image” that is displayed. A positive photoresist is composed of an insoluble polymer that degrades into a soluble polymer when exposed to UV light, while a negative photoresist is composed of monomers or polymers that polymerize or crosslink to form insoluble polymers upon UV exposure. Previously, Christenson and coworkers reported an experiment for undergraduates that mimics the process of microchip construction using a positive photoresist. This article describes an easy procedure based on a negative photoresist process that is designed for junior high or high school students, which will introduce them to the key terms and concepts of photolithography. The experiment allows the students to visualize the fundamental process behind microchip fabrication, observe the rapid prototyping enabled by such a technique, and appreciate its versatility by fabricating polymer thin film objects in virtually any shape they choose, including their own polymer silhouettes, all within minutes.
Chandrasekharan, Ramesh, Inkyu Park, R.I. Masel, and Mark A. Shannon, "Thermal Oxidation of Tantalum Films at Various Oxidation States from 300 - 700oC." Journal of Applied Physics 98 p. 114908 2005.
This paper presents the combined use of mathematical modeling and Auger depth profiling to study and quantify the oxidation of Ta films over a wide range of temperatures. The thermal oxidation of tantalum films (~700 nm) is studied using direct measurements of species concentration by means of Auger depth profiling. The oxidation temperature range of this study extends from 300 to 700oC and the oxidation period varies from 5 s to 12.5 h. The Auger depth profiles revealed that the metallic film oxidizes to first form low valence oxides of Ta that progressively convert to tantalum pentoxide with increasing temperature and time. A first-order reaction diffusion model is used to quantify the diffusion of oxygen through a film that is evolving in composition. The Auger depth profiling and reaction-diffusion model are used to estimate the actual diffusivity values for oxygen in the evolving Ta/Ta-oxide thin-film matrix, rather than more conventional techniques that estimate either the initial diffusion of oxygen through a semi-infinite metal or give a depth- and time-integrated value for the diffusivity. A comparison between the actual diffusivity values estimated in this work and the depth- and time-integrated version using the same model revealed that the integrated values are higher than the actual diffusion values by greater than 300% for the temperature range tested. Moreover, these depth- and time-integrated values for diffusivity values match over the applicable temperature ranges the diffusivity values given in the literature, which are essentially integrated average values for Ta/Ta oxide matrix. Furthermore, using the Auger depth profiles, the oxide growth rates are quantified as a function of temperature and compared with available literature. The growth rate of the oxide that is observed to be logarithmic at 300oC is seen to have a parabolic growth at 500oC and then a multistep growth behavior (a combination of parabolic and linear growth) at 700oC. These growth rates and the transition from one growth type to another strongly correlate to the change in surface and film morphology and also the transition from amorphous to crystalline Ta2O5.
Clark, Mark M., Won-Young Ahn, Xiang Li, Neal Sternisha, and Robert L. Riley, "Formation of Polysulfone Colloids for Adsorption of Natural Organic Foulants." Languir 21 pp. 7207-7213 2005.
An ever-present problem in the use of commercial membranes for treatment of drinking water is fouling of the membranes by natural organic matter (NOM). This work describes a new approach to elimination or minimization of membrane fouling by NOM. When a 2% solution of polysulfone in NMP and propionic acid is slowly injected into water, ~50 nm polysulfone particles are spontaneously formed, and these hydrophobic particles quickly coagulate into ~12-um diameter aggregates; the formed material has a surface area of ~100 m2/g and an equivalent “pore” size of 25 nm. When 50 mg/L of the new material is equilibrated with a local drinking water supply, virtually all adsorptive fouling of a 20-kDa molecular weight cut-off ultrafiltration membrane is eliminated. Interestingly, although only a very small percentage of the NOM is removed by adsorption on the polysulfone aggregates, it appears that exactly this small NOM component is responsible for nearly all of the membrane fouling. This paper describes the fabrication and characterization of the new polysulfone adsorbent and offers a hypothesis for the formation of the product via spontaneous emulsification and spinodal decomposition.
Georgiadis, John G., and Mahadevan Ramaswamy, "Magnetic Resonance Imaging of Water Freezing in Packed Beds Cooled from Below." International Journal of Heat and Mass Transfer 48 pp. 1064-1075 2005.
Full-field quantitative visualization of freezing interfaces requires the introduction of high resolution noninvasive methods. Magnetic resonance imaging (MRI) is a versatile tool for mapping the distribution of liquids (primarily water) in three-dimensional space, and is the only practical solution in systems that are strongly refracting or opaque to visible light. MRI is employed to visualize ice formation in water-saturated packed beds consisting of spherical beads packed in a cylindrical cavity and cooled from below. Imaging of the stagnant interstitial water is accomplished by exploiting the strong contrast in proton spin density signal between interstitial ice and liquid water. Our implementation of MRI allows fully three-dimensional reconstruction of the solidification front and adequate time resolution to quantify the freezing of pore water. The effect of pore space heterogeneity near the lateral walls of the cavity, as expressed by the ratio of bed to bead diameter, is examined with respect to the shape and propagation rate of the freezing interface. A modification of the test section also allows the study of freezing in pure water which is used for comparison. The present work demonstrates the kind of extra provisions in terms of design and choice of materials of the test section that are necessary in order to accommodate the special environment of the MRI scanner in heat transfer applications.
He, D., Mark A. Shannon, and N.R. Miller, "Micromachined Silicon Electrolytic Conductivity Probes with Integrated Temperature Sensor." IEEE Sensors Journal 5 pp. 1185-1196 2005.
Electrolytic conductivity measurements of fluids currently require sample volumes greater than a milliliter. Many applications would benefit from accurate measurements of nano- to microliter sample volumes. However, polarization and nonlinear electrode impedance effects, along with stray impedance and temperature effects, strongly affect measurements of the solution conductance for microliter and smaller sample volumes. MEMS-based silicon electrolytic conductivity probes, down to 100-um wide, with integrated temperature sensors, have been designed and fabricated to overcome these effects. Several electrode configurations were tested: plain electrode pairs, electrode pairs plated with platinum black, plain four electrode sets, and four electrode sets plated with platinum black were investigated. The same accuracy as normal scale probes has been achieved with these sensors over almost three orders of magnitude in solution concentration and electrolytic conductivity ranges.
Kirkpatrick, R. James, Andrey G. Kalinichev, and Jianwei Wang, "Molecular Dynamics Modelling of Hydrated Mineral Interlayers and Surfaces: Structure and Dynamics." Mineralogical Magazine 69 pp. 289-308 2005.
This paper reviews the results of recent molecular dynamics (MD) modelling studies of the interaction of water and solute species with mineral surfaces and their behaviour in mineral interlayers. Emphasis is on results for single and double hydroxide phases. Computational results are presented for water and anions in the interlayers of the Ca2Al, Mg2Al, and LiAl2 layered double hydroxides and on the surfaces of the Ca2Al phase. Detailed results for water on the (001) surface of brucite (Mg(OH)2) are presented and compared to published results for other phases. In all these cases, hydrogen bonding and the development of a hydrogen-bond network involving the H2O molecules and the solid substrate play very significant roles. The MD methods are especially effective for investigating the structure and dynamics of mineral-fluid interfaces and mineral interlayers, because they can be applied to systems containing hundreds to thousands of atoms and for extended durations of the order of nanoseconds.
Koh, Melvin, Mark M. Clark, and Kerry J. Howe, "Filtration of Lake Natural Organic Matter: Adsorption Capacity of a Polypropylene Microfilter." Journal of Membrane Science 256 pp. 169-175 2005.
Although natural organic matter (NOM) is known to adsorb on filtration membranes, little is known about whether membranes possess an adsorption capacity for NOM. In this research, 4000 mL of particle-free lake water containing dissolved NOM was filtered through a 0.2-μm polypropylene (PP) microfiltration membrane. Permeate was collected over 10 batches of 400 mL each. A 100-mL sample from each batch of permeate was then filtered through a fresh 20,000 Da molecular weight cut-off (MWCO) polyethersulfone (PES) ultrafiltration membrane. It was observed that later batches of microfiltration permeate caused more ultrafiltration membrane fouling than earlier batches of permeate. This suggests an adsorption capacity for the PP microfilter, and this was determined to be about 14 mg of DOC adsorbed per gram of PP membrane. A mass balance across the microfilter revealed that about 85% of DOC in the lake water did not cause fouling. Used PES ultrafilters were analyzed using scanning electron microscopy, and while there was a slight increase in material deposited on the ultrafilter for later batches of microfiltration permeate, a clean membrane surface was apparent in all micrographs. This suggests that the NOM responsible for membrane fouling was smaller than the pores of the 20,000 Da PES ultrafiltration membrane. These findings provide useful insights into membrane fouling, and suggest that adsorptive fouling should be considered in the design of membrane processes used to filter natural waters.
Liu, Jiwen, and Erik Luijten, "Colloidal Stabilization via Nanoparticle Halo Formation." Physical Review E 72 p. 61401 2005.
We present a detailed numerical study of effective interactions between micrometer-sized silica spheres, induced by highly charged zirconia nanoparticles. It is demonstrated that the effective interactions are consistent with a recently discovered mechanism for colloidal stabilization. In accordance with the experimental observations, small nanoparticle concentrations induce an effective repulsion that counteracts the intrinsic van der Waals attraction between the colloids and thus stabilizes the suspension. At higher nanoparticle concentrations an attractive potential is recovered, resulting in reentrant gelation. Monte Carlo simulations of this highly size-asymmetric mixture are made possible by means of a geometric cluster Monte Carlo algorithm. A comparison is made to results obtained from the Ornstein-Zernike equations with the hypernetted-chain closure.
Liu, Jiwen, and Erik Luijten, "Generalized Geometric Cluster Algorithm for Fluid Simulation." Physical Review E 71 p. 66701 2005.
We present a detailed description of the generalized geometric cluster algorithm for the efficient simulation of continuum fluids. The connection with well-known cluster algorithms for lattice spin models is discussed, and an explicit full cluster decomposition is derived for a particle configuration in a fluid. We investigate a number of basic properties of the geometric cluster algorithm, including the dependence of the cluster-size distribution on density and temperature. Practical aspects of its implementation and possible extensions are discussed. The capabilities and efficiency of our approach are illustrated by means of two example studies.
Liu, Juewen, and Yi Lu, "Stimuli-Responsive Disassembly of Nanoparticle Aggregates for Light-up Colorimetric Sensing." Journal of the American Chemical Society pp. 12677-12683 2005.
Controlled assembly of nanomaterials has been the focus of much research. In contrast, controlled disassembly has not received much attention, even though both processes have been shown to be important in biology. By using a Pb2+-dependent RNA-cleaving DNAzyme, we demonstrate here control of the disassembly of gold nanoparticle aggregates in response to Pb2+. In the process, we show that nanoparticle alignment plays an important role in the disassembly process, with the tail-to-tail configuration being the most optimal, probably because of the large steric hindrance of other configurations. The rate of disassembly is significantly accelerated by using small pieces of DNA to invade the cleaved substrate of the DNAzyme. Investigation of such a controlled disassembly process allows the transformation of previously designed “light-down” colorimetric Pb2+ sensors into “light-up” sensors.
Liu, Weitao, Luning Zhang, and Y. Ron Shen, "Interfacial Layer Structure at Alcohol/Silica Interfaces Probed by Sum-Frequency Vibrational Spectroscopy." Chemical Physics Letters 412 pp. 206-209 2005.
Sum-frequency vibrational spectroscopy was used to probe the adsorption of C1–C4 1-alcohol molecules at both vapor/silica and liquid/silica interfaces. The observed CH stretching spectra show that alcohol molecules adsorbed on silica with methyl groups pointing away from the interface. Reduction of the symmetric modes and enhancement of the antisymmetric modes of alcohol at liquid/silica interfaces are understood from a model of oppositely oriented molecules forming a bilayer at the interfaces.
McCauley, Kevin M., Derek A. Pratt, Scott R. Wilson, Justin Shey, Theodore J. Burkey, and Wilfred A. van der Donk, "Properties and Reactivity of Chlorovinylcobalamin and Vinylcobalamin and Their Implications for Vitamin B12-Catalyzed Reductive Dechlorination of Chlorinated Alkenes." Journal of the American Chemical Society 127 pp. 1126-1136 2005.
Vitamin B12-catalyzed reductive dechlorination of perchloroethylene (PCE) and trichloroethylene (TCE) is a potential strategy for cleanup of polluted environments. Presented are crystal structures of vinylcobalamin 2 and cis-chlorovinylcobalamin 1. They show a strong resistance toward photolysis. Reduction of 2 is difficult, but reduction of 1 occurs readily and produces 2. The mechanism of this latter reaction involves acetylene as an intermediate. These and other findings are discussed in the context of environmental studies on B12-catalyzed dechlorination of PCE and TCE and investigations of the haloalkene reductive dehalogenases that catalyze similar reactions.
Musk, Dinty J., David A. Banko and Paul J. Hergenrother, "Iron Salts Perturb Biofilm Formation and Disrupt Existing Biofilms of Pseudomonas aeruginosa." Chemistry & Biology 12 pp. 786-796 2005.
Bacterial biofilms are thought to aid in the survivability of a variety of intractable infections in humans. Specifically, biofilm production in Pseudomonas aeruginosa has been shown to play a significant role in chronic infection of cystic fibrosis (CF) patients. Unfortunately, no clinically effective inhibitors of biofilm formation are available. A rapid screen of 4509 compounds for nonantibiotic biofilm inhibitors in Pseudomonas aeruginosa PA14 was executed in 384-well plates. Among those compounds, ferric ammonium citrate inhibited biofilm formation in a dose-dependent manner; other iron salts functioned similarly. In addition to biofilm inhibition in static culture, pregrown biofilms could be disrupted and cleared by switching to iron-rich media in flow-chamber experiments. Furthermore, P. aeruginosa strains taken from the sputum of 20 CF patients showed a similar response to elevated iron levels. Previous expression-profiling analyses demonstrated that high levels of iron repress the expression of genes whose products are essential for scavenging iron and that expression of these genes is critical for virulence. Our results, combined with existing transcriptional-profiling data, now indicate that elevated iron concentrations repress the expression of certain genes essential for biofilm production in P. aeruginosa.
Nelson, Kevin E., Peter J. Bruesehoff, and Yi Lu, "In Vitro Selection of High Temperature Zn2+-Dependent DNAzymes." Journal of Molecular Evolution 61 pp. 216-225 2005.
In vitro selection of Zn2+-dependent RNA-cleaving DNAzymes with activity at 90°C has yielded a diverse spool of selected sequences. The RNA cleavage efficiency was found in all cases to be specific for Zn2+ over Pb2+, Ca2+, Cd2+, Co2+, Hg2+, and Mg2+. The Zn2+-dependent activity assay of the most active sequence showed that the DNAzyme possesses an apparent Zn2+-binding dissociation constant of 234 μM and that its activity increases with increasing temperatures from 50–90°C. A fit of the Arrhenius plot data gave Ea = 15.3 kcal mol-1. Surprisingly, the selected Zn2+-dependent DNAzymes showed only a modest (~3-fold) activity enhancement over the background rate of cleavage of random sequences containing a single embedded ribonucleotide within an otherwise DNA oligonucleotide. The result is attributable to the ability of DNA to sustain cleavage activity at high temperature with minimal secondary structure when Zn2+ is present. Since this effect is highly specific for Zn2+, this metal ion may play a special role in molecular evolution of nucleic acids at high temperature.
Ostroverkhov, Victor, Glenn A. Waychunas, and Y. Ron Shen, "New information on water interfacial structure revealed by phase-sensitive surface spectroscopy." Physical Review Letters 94 p. 46102 2005.
A phase-sensitive sum-frequency vibrational spectroscopic technique is developed to study interfacial water structure of water/quartz interfaces. Measurements allow deduction of both real and imaginary parts of the surface nonlinear spectral response, revealing an unprecedentedly detailed picture of the net polar orientations of the water species at the interface. The orientations of the icelike and liquidlike species appear to respond very differently to the bulk pH change indicating the existence of different surface sites on quartz with different deprotonation pK values.
Putnam, Shawn A., and David G. Cahill, "Transport of Nanoscale Latex Spheres in a Temperature Gradient." Langmuir 21 pp. 5317-5323 2005.
We use a micrometer-scale optical beam deflection technique to measure the thermodiffusion coefficient DT at room temperature (≈24 °C) of dilute aqueous suspensions of charged polystyrene spheres with different surface functionalities. In solutions with large concentrations of monovalent salts, ≥ 100 mM, the thermodiffusion coefficients for 26 nm spheres with carboxyl functionality can be varied within the range -0.9 x 10-7 cm2 s-1 K-1 < DT < 1.5 x 10-7 cm2 s-1 K-1 by changing the ionic species in solution; in this case, DT is the product of the electrophoretic mobility μE and the Seebeck coefficient of the electrolyte, Se = (Q*C – Q*A)/2eT, DT = -Se μE, where Q*C and Q*A are the single ion heats of transport of the cationic and anionic species, respectively. In low ionic strength solutions of LiCl, ≤5 mM, and particle concentrations ≤2 wt %, DT is negative, independent of particle concentration and independent of the Debye length; DT = -0.73 ± 0.05 x 10-7 cm2 s-1 K-1.
Qiao, R., and Narayana R. Aluru, "Atomistic simulation of KCl Transport in Charged Silicon Nanochannels: Interfacial Effects." Colloids and Surfaces A 267 pp. 103-109 2005.
Electroosmotic flow is an important fluid transport mechanism in nanofluidic systems. In this paper, we investigate the ion distribution and velocity profiles of KCl solution in two oppositely charged silicon nanochannels by using molecular dynamics simulations. The continuum theories, based on the Poisson–Boltzmann equation and the Navier–Stokes equations, predict that the distribution of the counter-ions, water flux and ionic conductivity in the two oppositely charged channels are the same. However, molecular dynamics simulations show very different results. First, the counter-ion distributions are substantially different in the two channels. Second, the water flux and ionic conductivity in the two channels differ by a factor of more than three. Third, the co-ion fluxes are in the opposite direction. The different counter-ion distributions in the two channels are attributed to the different size of the K+ and Cl- ions and the discreteness of the water molecules, and the asymmetric dependence of the water and ion transport is attributed to the asymmetric dependence of the hydrogen bonding of water near the charged silicon surface, which influences the dynamic behavior of interfacial water significantly.
Qiao, R., and Narayana R. Aluru, "Scaling of Electrokinetic Transport in Nanometer Channels." Langmuir 21 pp. 8972-8977 2005.
Electrokinetic transport is a popular transportmechanismused in nanofluidic systems, and understanding its scaling behavior is important for the design and optimization of nanofluidic devices. In this article, we report on the scaling of electroosmotic flow and ionic conductivity in positively charged slit nanochannels by using continuum and atomistic simulations. The effects of confinement and surface charge are discussed in detail. In particular, we found that the viscosity of the interfacial water increases substantially as the surface charge density increases and the electrophoretic mobility of the interfacial ions decreases. We show that such effects can influence the scaling of the electrokinetic transport in confined nanochannels significantly.
Qiao, R., and Narayana R. Alurua, "Surface-Charge-Induced Asymmetric Electrokinetic Transport in Confined Silicon Nanochannels." Applied Physics Letters 86 p. 143105 2005.
Molecular dynamics simulations of a NaF solution transport in a confined silicon nanochannel indicated that the water flux and the ionic conductivity through two oppositely charged silicon channels, that are otherwise similar, differ by a factor of more than three, and the co-ion fluxes are in the opposite direction. Such a behavior cannot be predicted by the classical electrokinetic transport theory, and is found to originate from the asymmetric dependence of the transport properties of water near the charged silicon surface.
Thormann, Kai M., Renée M. Saville, Soni Shukla, and Alfred M. Spormann, "Induction of Rapid Detachment in Shewanella oneidensis MR-1 Biofilms." Journal of Bacteriology 187 pp. 1014-1021 2005.
Active detachment of cells from microbial biofilms is a critical yet poorly understood step in biofilm development. We discovered that detachment of cells from biofilms of Shewanella oneidensis MR-1 can be induced by arresting the medium flow in a hydrodynamic biofilm system. Induction of detachment was rapid, and substantial biofilm dispersal started as soon as 5 min after the stop of flow. We developed a confocal laser scanning microscopy-based assay to quantify detachment. The extent of biomass loss was found to be dependent on the time interval of flow stop and on the thickness of the biofilm. Up to 80% of the biomass of 16-h-old biofilms could be induced to detach. High-resolution microscopy studies revealed that detachment was associated with an overall loosening of the biofilm structure and a release of individual cells or small cell clusters. Swimming motility was not required for detachment. Although the loosening of cells from the biofilm structure was observed evenly throughout thin biofilms, the most pronounced detachment in thicker biofilms occurred in regions exposed to the flow of medium, suggesting a metabolic control of detachability. Deconvolution of the factors associated with the stop of medium flow revealed that a sudden decrease in oxygen tension is the predominant trigger for initiating detachment of individual cells. In contrast, carbon limitation did not trigger any substantial detachment, suggesting a physiological link between oxygen sensing or metabolism and detachment. In-frame deletions were introduced into genes encoding the known and putative global transcriptional regulators ArcA, CRP, and EtrA (FNR), which respond to changes in oxygen tension in S. oneidensis MR-1. Biofilms of null mutants in arcA and crp were severely impacted in the stop-of-flow-induced detachment response, suggesting a role for these genes in regulation of detachment. In contrast, an ΔetrA mutant displayed a variable detachment phenotype. From this genetic evidence we conclude that detachment is a biologically controlled process and that a rapid change in oxygen concentration is a critical factor in detachment and, consequently, in dispersal of S. oneidensis cells from biofilms. Similar mechanisms might also operate in other bacteria.
Wang, Jianwei, Andrey G. Kalinichev, and R. James Kirkpatrick, "Structure and Decompression Melting of a Novel, High-Pressure Nanoconfined 2-D Ice." Journal of Physical Chemistry B 109 pp. 14308-14313 2005.
Molecular dynamics (MD) simulations of water confined in nanospaces between layers of talc (system composition Mg3Si4O10(OH)2 + 2H2O) at 300 K and pressures of approximately 0.45 GPa show the presence of a novel 2-D ice structure, and the simulation results at lower pressures provide insight into the mechanisms of its decompression melting. Talc is hydrophobic at ambient pressure and temperature, but weak hydrogen bonding between the talc surface and the water molecules plays an important role in stabilizing the hydrated structure at high pressure. The simulation results suggest that experimentally accessible elevated pressures may cause formation of a wide range of previously unknown water structures in nanoconfinement. In the talc 2-D ice, each water molecule is coordinated by six Ob atoms of one basal siloxane sheet and three water molecules. The water molecules are arranged in a buckled hexagonal array in the a-b crystallographic plane with two sublayers along . Each H2O molecule has four H-bonds, accepting one from the talc OH group and one from another water molecule and donating one to an Ob and one to another water molecule. In plan view, the molecules are arranged in six-member rings reflecting the substrate talc structure. Decompression melting occurs by migration of water molecules to interstitial sites in the centers of six-member rings and eventual formation of separate empty and water-filled regions.
Yeom, Junghoon, Yan Wu, John C. Selby, and Mark A. Shannon, "Maximum Achievable Aspect Ratio in Deep Reactive Ion Etching of Silicon due to Aspect Ratio Dependent Transport and the Microloading Effect." The Journal of Vacuum Science and Technology B 23 pp. 2319-2329 2005.
When etching high-aspect-ratio silicon features using deep reactive ion etching (DRIE), researchers find that there is a maximum achievable aspect ratio, which we define as the critical aspect ratio, of an etched silicon trench using a DRIE process. At this critical aspect ratio, the apparent etch rate (defined as the total depth etched divided by the total elapsed time) no longer monotonically decreases as the aspect ratio increases, but abruptly drops to zero. In this paper, we propose a theoretical model to predict the critical aspect ratio and reveal its causal mechanism. The model considers aspect ratio dependent transport mechanisms specific to each of the reactant species in the three subprocesses of a time-multiplexed etch cycle: deposition of a fluorocarbon passivation layer, etching of the fluorocarbon polymer at the bottom of the trench, and the subsequent etching of the underlying silicon. The model predicts that the critical aspect ratio is defined by the aspect ratio at which the polymer etch rate equals the product of the deposition rate and the set time ratio between the deposition and etching phases for the time-multiplexed process. Several DRIE experiments were performed to qualitatively validate the model. Both model simulations and experimental results demonstrate that the magnitude of the critical aspect ratio primarily depends on (i) the relative flux of neutral species at the trench opening, i.e., the microloading effect, and (ii) aspect ratio dependent transport of ions during the polymer etching subprocess of a DRIE cycle.
Yue, Zhongren, and James Economy, "Nanoparticle and Nanoporous Carbon Adsorbents for Removal of Trace Organic Contaminants from Water." Journal of Nanoparticle Research 7 pp. 477-487 2005.
Removal of a wide range of trace organic contaminants from water to concentrations below USEPA Maximum Contaminant Levels (MCL) remains an important goal for the water industry. Design of advanced carbon based adsorption systems represents a unique approach to solving these problems. A number of successful examples are cited in this paper and are briefly summarized in the following section.
(1) Removal of foulants such as humic acid using nanoparticle carbon blacks and chemically activated nanoporous fibers;
(2) Removal of trace organic contaminants such as benzene, toluene, ethylbenzene and p-xylene (BTEX) to levels below USEPA MCL using nanoporous carbon fibers;
(3) Removal of trace chemical warfare simulants such as diisopropylmethyl phosponate and chloroethylethylsulfide using enlarged nanoporous carbon fibers;
(4) Removal of trace chlorinated solvents such as trichloroethylene (TCE) and chloroform using tailored nanoporous carbon fibers;
(5) Removal of the trace herbicide, atrazine, to below USEPA MCL level using nanoporous chemically activated fibers. In this paper the enormous improvement of the above systems over commercially available products in static and dynamic adsorption evaluation is described.
Yue, Zhongren, Kelly R. Benak, Jinwen Wang, Christian L. Mangun, and James Economy, "Elucidating the Porous and Chemical Structures of ZnCl2 - Activated Polyacrylonitrile on a Fiberglass Substrate." Journal of Materials Chemistry 15 pp. 3142-3148 2005.
ZnCl2-activated polyacrylonitrile (PAN) coated onto a fiberglass substrate was prepared in N2 at 450 ºC. The porous and chemical structures were characterized by using N2 adsorption at 77 K, XPS, FTIR, competitive adsorption of CO2/CH4 and the adsorption of Cs+, Sr2+ and Ag+. The activated PAN displays a high BET surface area up to 1012 m2 g-1, a broad mesopore size distribution as well as a major micropore size distribution. Up to 19.3 wt% of nitrogen, which is probably in the form of pyridinic and pyrrolic structures, is incorporated in the chemically activated PAN. HCl uptake results show that such a material has a higher amount of weakly basic functional groups compared to a commercially available ACF15. The activated PAN also exhibits a higher selectivity coefficient for CO2/CH4 at STP and higher adsorption amounts for Cs+, Sr2+ and Ag+ than ACF15. It is proposed that nitrogen-containing, weakly basic functional groups positioned on the suitable-sized pore walls improve the adsorption ability of the activated PAN toward CO2, Cs+, Sr2+ and Ag+.
Zhou, Guangchang, Issifu, I. Harruna, and Conrad W. Igram, "Ruthenium-Centered Thermosensitive Polymers." Polymer 46 pp. 10672-10677 2005.
Bipyridine-centered poly(N-isopropylacrylamide) polymers with controlled molecular weight and low polydispersity were synthesized by RAFT polymerization using a novel bipyridine-functionalized dithioester as a RAFT agent, and were further complexed with ruthenium ion to produce ruthenium-centered thermosensitive polymers with well-defined structure. Results from UV–vis, fluorescence and DSC characterizations of the thermosensitive polymeric ruthenium complexes indicated that tris(2,20-bipyridine)ruthenium(II) ion was successfully grafted onto the center of polymer chains.
Zribi, Olena V., Hee Kyung, Ramin Golestanian, Tanniemola B. Liverpool, and Gerard C.L. Wong, "Salt-Induced Condensation in Actin-DNA Mixtures." Europhysics Letters 70 pp. 541-547 2005.
Multivalent ions can induce condensation of like-charged polyelectrolytes into compact states, a process that requires different ion valence for different polyelectrolyte species. We have examined the trivalent ion-induced condensation behavior in binary anionic polyelectrolyte mixtures consisting of DNA coils and F-actin rods, and observe a micro-phase separation between the two polyelectrolytes into coexisting finite-sized F-actin bundles and DNA toroids. Further, by increasing the DNA volume fraction in the mixture, condensed F-actin bundles can be completely destabilized, leading to only DNA condensation within the mixture. We examine a number of possible causes and propose a model based on polyelectrolyte competition for ions.
Biesheuvel, P.M., and Henk Verweij, "Comment on `Electrophoretic Deposition-Mechanisms, Myths and Materials` by Y. Fukuda, N. Nagarajan, W. Mekky, Y. Bao, H.-S. Kim and P. S. Nicholson." Journal of Materials Science 39 pp. 7081-7083 2004.
Bishop, Ann Peterson, Bertram C. Bruce, Karen J. Lunsford, M. Cameron Jones, Muzhgan Nazarova, David Linderman, Mihye Won, P. Bryan Heidorn, Rajeev Ramprakash, and André Brock, "Supporting Community Inquiry with Digital Resourses." Journal of Digital Information 5 p. 308 2004.
Today there are a number of fields that address the need to develop better means of employing information and communication technologies (ICTs) to help communities achieve their goals. Digital infrastructure and repositories are widely created to support the activities of educational, workplace, and scientific communities, as well as virtual communities of interest that may center on topics as diverse as entertainment, crisis management, and health. However, the research and development of ICTs faces numerous challenges. Community inquiry theory can help address some of these challenges. The Inquiry Page project supports a set of ICTs that have been developed by a community of inquiry in order to support communities of inquiry. The paper presents the theory of community inquiry and illustrates how inquiry theory can influence the research and development of ICTs and their adoption and use within real communities.
Boddu, Veera M., Krishnaiah Abburi, Jonathon L. Talbott, and Edgar D. Smith, "Removal of Hexavalent Chromium from Wastewater Using a New Composite Chitosan Biosorbent." Environmental Science & Technology 37 pp. 4449-4456 2004.
A new composite chitosan biosorbent was prepared by coating chitosan, a glucosamine biopolymer, onto ceramic alumina. The composite bioadsorbent was characterized by high-temperature pyrolysis, porosimetry, scanning electron microscopy, and X-ray photoelectron spectroscopy. Batch isothermal equilibrium and continuous column adsorption experiments were conducted at 25oC to evaluate the biosorbent for the removal of hexavalent chromium from synthetic as well as field samples obtained from chrome plating facilities. The effect of pH, sulfate, and chloride ion on adsorption was also investigated. The biosorbent loaded with Cr(VI) was regenerated using 0.1 M sodium hydroxide solution. A comparison of the results of the present investigation with those reported in the literature showed that chitosan coated on alumina exhibits greater adsorption capacity for chromium(VI). Further, experimental equilibrium data were fitted to Langmuir and Freundlich adsorption isotherms, and values of the parameters of the isotherms are reported. The ultimate capacity obtained from the Langmuir model is 153.85 mg/g chitosan.
Hoke, J.L., John G. Georgiadis, and A.M. Jacobi, "The Effect of Substrate Wettability on Frost Properties." Journal of Thermophysics & Heat Transfer 18 pp. 228-235 2004.
Microscopic observations of frost deposition on clean glass (hydrophilic) and polytetrafluoroethylene (PTFE) (hydrophobic) substrates allow the quantification of frost growth and characterization of structure. In contrast to early growth behavior, the thickness of the frost layer during the mature growth increases faster on substrates that have lower contact angles, and the frost density is less than that measured for high-contact angle substrates. This behavior is explained in terms of the effects of substrate wettability and its impact on condensate distribution, the initial condition for frost growth. A higher conductivity layer is formed on the hydrophilic than on the hydrophobic substrate. Modeling that is based on a relation between thermal conductivity and frost structure is used to predict the growth rate and density on the two different substrates. These predictions agree with the experimental data and support the explanation that substrate wettability affects mature frost growth through its effect on condensate distribution at frosting incipience.
Holdych, David J., David R. Noble, John G. Georgiadis, and Richard O. Buckius, "Truncation Error Analysis of Lattice Boltzmann Methods." Journal of Computational Physics 193 pp. 595-619 2004.
A truncation error analysis is performed for models based on the lattice Boltzmann (LB) equation. This analysis involves two steps: the recursive application of the LB equation and a Taylor series expansion. Unlike previous analytical studies of LB methods, the present work does not assume an asymptotic relationship between the temporal and spatial discretization parameters or between the probability distribution function, f, and its equilibrium distribution, feq. Effective finite difference stencils are derived for both the distribution function and the primitive variables, i.e., density and velocity. The governing partial differential equations are also recovered. The associated truncation errors are derived and the results are validated by numerical simulation of analytic flows. Analysis of the truncation errors elucidates the roles of the kinetic theory relaxation parameter, [tau], and the discretization parameters, [delta]x and [delta]t. The effects of initial and boundary conditions are also addressed and are shown to significantly affect the overall accuracy of the method.
Holdych, David J., John G. Georgiadis, and Richard O. Buckius, "Hydrodynamic Instabilities of Near-Critical CO2 Flow in Microchannels: Lattice Boltzmann Simulation." Physics of Fluids 16 pp. 1791-1802 2004.
Motivated by systematic CO2 evaporation experiments which recently became available (J. Pettersen, "Flow vaporization of CO2 in microchannel tubes," Doctor technicae thesis, Norwegian University of Science and Technology, 2002), the present work constitutes an exploratory investigation of isothermal flow of CO2 near its liquid–vapor critical point through a long 5 um diameter microchannel. A modified van der Waals constitutive model—with properties closely approximating those of "real" near-critical CO2—is incorporated in a two-dimensional lattice Boltzmann hydrodynamics model by embedding a dimensionless parameter X, with X [arrow] 1 denoting the "real" fluid. The hydrodynamic phenomena resulting by imposing a constant pressure gradient along a periodic channel are investigated by considering two regimes in tandem: (1) transition from bubbly to annular flow with a liquid film formed at the channel walls and (2) destabilization of the liquid film by the Kelvin–Helmholtz instability. Due to numerical constraints, intrinsic modeling errors are introduced and are shown to be associated with discrepancies in the relative vapor–liquid interfacial thickness, which is expressed by X. The effects of these errors are investigated both theoretically and numerically in the physical limit X [arrow] 1. Numerically determined flow patterns compare qualitatively well with direct visualization results obtained by Pettersen. Overall, the characteristics of isothermal near-critical two-phase flow in microchannels can be reproduced by the appropriate modification of the thermophysical properties of CO2.
Liu, Chunqing, Jinfeng Wang, and James Economy, "Synthesis and Characterization of a Novel, Mesoporous Organosilica with Additional Cyclodextrin-Based Micropores." Macromolecular Rapid Communications 25 pp. 863-866 2004.
A novel mesoporous organosilica with additional cyclodextrin-based micropores has been synthesized from tetraethoxysilane (TEOS) and cyclodextrin-based silane monomer precursors and triblock PEO-PPO-PEO (poly(ethylene oxide)-poly(propylene oxide)- poly(ethylene oxide)) copolymer P123 as the structure-directing template with the aid of sodium chloride and the supramolecular assembly of cyclodextrins with P123.
Liu, Chunqing, Lei Fu, and James Economy, "A Simple, Template-Free Route for the Synthesis of Mesoporous Titanium Dioxide Materials." Journal of Materials Chemistry 14 pp. 1187-1189 2004.
Mesoporous titanium dioxide (TiO2) materials have been synthesized by a novel simple, template-free synthetic approach, which was carried out via nitric acid-catalyzed hydrolysis and polycondensation reactions of titanium(IV) n-butoxide. The as-prepared and calcinated mesoporous TiO2 materials are characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), and N2 adsorption/desorption measurements. The as-prepared amorphous TiO2 possesses a high BET surface area of 470 m2 g-1 and a pore volume of 0.28 cm3 g-1. After calcinations at 450oC, anatase mesoporous TiO2 is formed with a BET surface area of 106 m2 g-1 and an average pore size of 4.8 nm. Avoidance of the structure-directing template during the synthesis of mesoporous TiO2 is of significant importance for application from both environmental and industrial points of view.
Liu, Chunqing, Lei Fu, and James Economy, "Synthesis and Characterization of a Novel Calixarene-Based Periodic Mesoporous Organosilica." Macromolecular Rapid Communications 25 pp. 804-807 2004.
A new calixarene-based periodic mesoporous organosilica has been synthesized using tetraethoxysilane (TEOS) and a calixarene-based silane monomer as the precursors and cetyltrimethylammonium bromide (CTAB) surfactant as the structure-directing template, and is shown to be capable of visual detection and entrapment of NO2.
Liu, Chunqing, Nathaniel Naismith, and James Economy, "Advanced Mesoporous Organosilica Material Containing Microporous beta-Cyclodextrins for the Removal of Humic Acid from Water." Journal of Chromatography A 1036 pp. 113-118 2004.
A new mesoporous organosilica material ([beta]-CD-Silica-4%) containing microporous [beta]-cyclodextrins ([beta]-CDs) has been prepared by the co-polymerization of a silylated [beta]-CD monomer with tetraethoxysilane in the presence of a structure-directing template, cetyltrimethylammonium bromide. Solid-state 13C and 29Si NMR studies provided evidence for the presence of covalently attached [beta]-CDs in the mesoporous material. Nitrogen adsorption experiments showed that [beta]-CD-Silica-4% material had a BET surface area of 460 m2/g and an average mesopore diameter of 2.52 nm. Small-angle powder X-ray diffraction pattern of [beta]-CD-Silica-4% material revealed the lack of highly ordered mesoporous structure. Adsorption experiments showed that [beta]-CD-Silica-4% material removed up to 99% of humic acid from an aqueous solution containing 50 ppm of humic acid at a solution-to-solid ratio of 100 mL/g.
Liu, Jiwen, and Erik Luijten, "Rejection-Free Geometric Cluster Algorithm for Complex Fluids." Physical Review Letters 92 p. 35504 2004.
We present a novel, generally applicable Monte Carlo algorithm for the simulation of fluid systems. Geometric transformations are used to identify clusters of particles in such a manner that every cluster move is accepted, irrespective of the nature of the pair interactions. The rejection-free and non-local nature of the algorithm make it particularly suitable for the efficient simulation of complex fluids with components of widely varying size, such as colloidal mixtures. Compared to conventional simulation algorithms, typical efficiency improvements amount to several orders of magnitude.
Liu, Jiwen, and Erik Luijten, "Stabilization of Colloidal Suspensions by Means of Highly Charged Nanoparticles." Physical Review Letters 93 p. 247802 2004.
We employ a novel Monte Carlo simulation scheme to elucidate the stabilization of neutral colloidal microspheres by means of highly charged nanoparticles [V. Tohver et al., Proc. Natl. Acad. Sci. U.S.A. 98, 8950 (2001)]. In accordance with the experimental observations, we find that small nanoparticle concentrations induce an effective repulsion that prevents gelation caused by the intrinsic van derWaals attraction between colloids. Higher nanoparticle concentrations induce an attractive potential which is, however, qualitatively different from the regular depletion attraction. We also show how colloid-nanoparticle size asymmetry and nanoparticle charge can be used to manipulate the effective interactions.
Liu, Juewen, and Yi Lu, "Accelerated Color Change of Gold Nanoparticles Assembled by DNAzymes for Simple and Fast Colorimetric Pb2+ Detection." Journal of the American Chemical Society 126 pp. 12298-12305 2004.
The combination of high metal selectivity of DNAzymes with the strong distance-dependent optical properties of metallic nanoparticles has presented considerable opportunities for designing colorimetric sensors for metal ions. We previously communicated a design for a colorimetric lead sensor based on the assembly of gold nanoparticles by a Pb2+-dependent DNAzyme. However, heating to 50 °C followed by a cooling process of ~2 h was required to observe the color change. Herein we report a new improved design that allows fast (<10 min) detection of Pb2+ at ambient temperature. This improvement of sensor performance is a result of detailed studies of the DNAzyme and nanoparticles, which identified “tail-to-tail” nanoparticle alignment, and large (42 nm diameter) nanoparticle size as the major determining factors in allowing fast color changes. The optimal conditions for other factors such as temperature (35 °C) and concentrations of the DNAzyme (2 μM), its substrate (3 nM), and NaCl (300 mM) have also been determined. These results demonstrate that fundamental understanding of the DNAzyme biochemistry and nanoparticle science can lead to dramatically improved colorimetric sensors.
Liu, Juewen, and Yi Lu, "Adenosine-Dependent Assembly of Aptazyme-Functionalized Gold Nanoparticles and Its Application as a Colorimetric Biosensor." Analytical Chemistry 76 pp. 1627-1632 2004.
Previous work has shown that DNAzyme-directed assembly of gold nanoparticles can be utilized to make effective colorimetric biosensors. However, the method is restricted to analytes that are directly involved in phosphodiester cleavage. To expand the methodology to a broader range of analytes, a colorimetric adenosine biosensor based on the aptazyme-directed assembly of gold nanoparticles is reported here. The aptazyme is based on the 8-17 DNAzyme with an adenosine aptamer motif that can modulate the DNAzyme activity through allosteric interactions depending on the presence of adenosine. In the absence of adenosine, the aptazyme is inactive and the substrate strands can serve as linkers to assemble DNA-functionalized 13-nm-diameter gold nanoparticles, resulting in a blue color. However, the presence of adenosine activates the aptazyme, which cleaves the substrate strand, disrupting the formation of nanoparticle aggregates. A red color of separated gold nanoparticles is observed. Concentrations of adenosine of up to 1 mM can be measured semiquantitatively by the degree of blue to red color changes or quantitatively by the extinction ratio at 520 and 700 nm. Under the same conditions, 5 mM guanosine, cytidine, or uridine resulted in a blue color only, indicating good selectivity of the sensor. The color difference can be clearly observed by the naked eye by spotting the resulting sensor solution onto an alumina TLC plate. Since aptamers that can target many classes of important analytes have already been selected, they can be adapted into aptazyme systems through rational design or further selection. Thus, colorimetric biosensors for many analytes of interest can be designed using the method presented here, regardless of whether the analytes are directly involved in the cleavage reaction or not.
Liu, Juewen, and Yi Lu, "Colorimetric Biosensors Based on DNAzyme-Assembled Gold Nanoparticles." Journal of Fluorescence 14 pp. 343-354 2004.
Taking advantage of recent developments in the field of metallic nanoparticle-based colorimetric DNA detection and in the field of in vitro selection of functional DNA/RNA that can recognize a wide range of analytes, we have designed highly sensitive and selective colorimetric biosensors for many analytes of choice. As an example of the sensor design strategy, a highly sensitive and selective colorimetric lead biosensor based on DNAzyme-directed assembly of gold nanoparticles is reviewed. The DNAzyme consists of an enzyme and a substrate strand, which can be used to assemble DNA-functionalized gold nanoparticles. The aggregation brings gold nanoparticles together, resulting in a blue-colored nanoparticle assembly. In the presence of lead, the DNAzyme catalyzes specific hydrolytic cleavage of the substrate strand, which disrupts the formation of the nanoparticle assembly, resulting in red-colored individual nanoparticles. The application of the sensor in lead detection in leaded paint is also demonstrated. In perspective, the use of allosteric DNA/RNAzymes to expand the range of the nanoparticle-based sensor design method is described.
Liu, Juewen, and Yi Lu, "Optimization of a Pb2+-Directed Gold Nanoparticle/DNAzyme Assembly and Its Application as a Colorimetric Biosensor for Pb2+." Chemistry of Materials 16 pp. 3231-3238 2004.
We previously communicated a method for directed assembly of gold nanoparticles using
a Pb2+-dependent DNAzyme and demonstrated the application of this system as a colorimetric biosensor. The sensor shows high sensitivity and selectivity toward Pb2+ and undergoes a blue-to-red color transition in the presence of Pb2+. To gain a deeper insight into the analyte-directed nanomaterials assembly and sensing processes, a detailed characterization of the system has been performed. First, we found that the presence of gold nanoparticles had no effect on the Pb2+-dependent activity of the DNAzyme and the presence of DNAzyme has little effect on the melting properties of the DNA-functionalized nanoparticle aggregates, suggesting that the performance of the nanoparticle and DNAzyme systems can be optimized independently. Second, the optimal length of the DNAzyme and the alignment of the DNA-functionalized gold nanoparticles for the assembly and sensing processes have been determined to be 9 base pairs on each end for the DNAzyme, and “head-to-tail” alignment for the DNA-functionalized gold nanoparticles. Third, the optimal stoichiometry of the enzyme to the substrate strands of the DNAzyme was shown to be one to one in nanoparticle aggregates. Finally, the most favorable temperature and pH conditions for the system have also been established, with a temperature of 37 °C and pH of 6.4 to 9.2 as the best operating conditions. The study also revealed that, for most efficient assembly of nanoparticles, the DNA backbone should be rigidified by formation of a double helix with other DNA molecules. These findings allow optimization of the processes for directed assembly of nanomaterials and for colorimetric sensing.
Mitchell, Mark B., Viktor N. Sheinker, Woodrow W. Cox, Jr., Enid N. Gatimu, and Aron B. Tesfamichael, "The Room Temperature Decomposition Mechanism of Dimethyl Methylphosphonate (DMMP) on Alumina-Supported Cerium Oxide." Journal of Physical Chemistry B 108 pp. 1634-1645 2004.
The adsorption and decomposition reactions of dimethyl methylphosphonate (DMMP) on cerium oxide supported on aluminum oxide have been examined at 25 °C. Experiments were carried out that involved dosing the reactive adsorbent with small doses of DMMP, followed by quantitative determination of the decomposition products. The results suggest that the formation reactions of methanol and dimethyl ether are competitive processes involving the same surface intermediate, which is most likely a surface methoxy species. Based on the observed results, it is proposed that the formation of dimethyl ether is due to the combination of two surface methoxy groups, while an important, if not the dominant, reaction producing methanol involves a surface methoxy group interacting with a vapor phase or physisorbed DMMP molecule. The presence of significant amounts of methoxy fragments formed upon DMMP adsorption is supported by results from diffuse reflectance spectroscopy, which also show that those groups are primarily associated with the cerium oxide domains. FT-Raman spectroscopy shows that the most active cerium oxide domains are highly dispersed two-dimensional domains or very small (<1 nm) crystallites. Somewhat larger (<6 nm) three-dimensional crystallites add to the decomposition yield, but less strongly. The FT-Raman evidence also supports the formation of a relatively narrow particle size distribution of cerium oxide crystallites on the alumina support surface from the sample preparation method. The alumina-supported cerium oxide reactive adsorbents developed as part of this study are the most active that have been reported in the literature for ambient temperature applications, decomposing approximately 775 μmol of DMMP per gram of adsorbent at 25 °C, and strongly or irreversibly adsorbing an additional 400 μmol, for a total capacity at room temperature of 1.1-1.2 mmol of DMMP per gram.
Moser, Kevin W., and John G. Georgiadis, "Extraction and Validation of Correlation Lengths from Interstitial Velocity Fields Using Diffusion-Weighted MRI." Magnetic Resonance Imaging 22 pp. 257-268 2004.
Magnetic Resonance Imaging methods sensitive to individual molecular displacements (q-space MRI) provide a convenient means of measuring dispersion in complex interstitial spaces. Pressure-driven flow experiments through a water-saturated packed bed phantom have been conducted to prove the feasibility of using q-space MRI to measure the coherence length associated with the interstitial velocity field. The method involves measuring the dependence of the apparent dispersion coefficient on the distance along the mean flow by repeating a small number of pulsed-gradient stimulated-echo experiments with increasing gradient pulse separation times. Assuming homogeneous interstitial flow statistics inside the averaging volume, an integral spatial scale characterizing the Eulerian velocity auto-correlation coefficient is extracted via a stochastic convective model. The validity of the a priori statistical description of interstitial flow is verified by comparing with an independent MRI measurement of the Eulerian velocity field using phase contrast methods in the same phantom with pore-level resolution. The integral length scale obtained via q-space MRI agrees with the mean pore size in the present as well as in similar phantoms found in the literature. This method has direct applicability in the quantification of the interstitial morphology of fluid-saturated porous media with resolution independent of voxel size, assuming “perfectly reflecting pore walls” (no surface relaxation) and no contribution to the MR signal from outside the pore space.
Putnam, Shawn A., and David G. Cahill, "Micron-Scale Apparatus for Measurements of Thermodiffusion in Liquids." Review of Scientific Instruments 75 pp. 2368-2372 2004.
An ac beam deflection technique for measurements of thermodiffusion in liquid mixtures is described. In comparison to conventional beam deflection methods, the micron-scale geometry of our apparatus speeds up equilibration by a factor of ≈300. Our apparatus and analysis methods are validated with measurements on molecular polystyrene dissolved in toluene. Thermodiffusion experiments on 26 nm polystyrene colloids with carboxyl functionality (COOH) in water yield Soret coefficients of ST = -0.28±0.025 K-1.
Raguin, L. Guy, and John G. Georgiadis, "Kinematics of the Stationary Helical Vortex Mode in Taylor-Couette-Poiseuille Flow." Journal of Fluid Mechanics 516 pp. 125-154 2004.
We reconstruct a kinematically admissible (volume-preserving) three-dimensional velocity field corresponding to the stationary helical vortex (SHV) mode which is observed in the Taylor–Couette–Poiseuille (TCP) system with a ratio of inner to outer cylinder radii of 0.5 and a length to annulus gap ratio of 16, starting from experimental data obtained via magnetic resonance imaging (MRI) for Re =11.14 and Ta1/2 =170 in water. The goal of the present work is to provide a complete kinematic representation of a strongly nonlinear duct .ow that is of importance in the fields of mixing and segregation, as well as in the study of the kinematic structure of three-dimensional flows. By a judicious choice of a set of global basis functions that exploit the helical symmetry of SHV, an analytical approximation of the streamfunction is obtained despite the coarse MRI data and the non-uniform distribution of measurement error. This approximation is given in terms of a truncated series of smooth functions that converges weakly in L2, and the reconstruction method is directly applicable to three-dimensional incompressible flows that possess a continuous volume-preserving symmetry. The SHV flow structure consists of a pair of asymmetric counter-rotating helical cells in a double helix structure, foliated with invariant helically symmetric surfaces containing fibre-like fluid particle orbits wrapped around the inner cylinder. Imposing general topological constraints, juxtaposing SHV with neighbouring hydrodynamic modes such as the propagating Taylor vortex flow and direct numerical simulation help corroborate the validity of the reconstruction of the SHV flow field. The kinematically admissible flow field obeys the Navier–Stokes equations with 10% accuracy, which is consistent with experimental error, and has the same flow portrait as the numerically computed flow. Global analysis of the SHV mode indicates that it corresponds to a minimum in dissipation and mixing in comparison with a wide class of perturbed neighbouring modes; hence it is a candidate for the study of particle segregation. To our knowledge, the present study reports the first synthesis of a physically realizable complex open flow that can be represented by an integrable Hamiltonian system starting from point-wise experimental data and using solely kinematic constraints.
Thormann, Kai M., Renée M. Saville, Soni Shukla, Dale A. Pelletier, and Alfred M. Spormann, "Initial Phases of Biofilm Formation in Shewanella oneidensis MR-1." Journal of Bacteriology 186 pp. 8096-8104 2004.
Shewanella oneidensis MR-1 is a facultative Fe(III)- and Mn(IV)-reducing micro-organism and serves as a model for studying microbially induced dissolution of Fe or Mn oxide minerals as well as biogeochemical cycles. In soil and sediment environments, S. oneidensis biofilms form on mineral surfaces and are critical for mediating the metabolic interaction between this microbe and insoluble metal oxide phases. In order to develop an understanding of the molecular basis of biofilm formation, we investigated S. oneidensis biofilms developing on glass surfaces in a hydrodynamic flow chamber system. After initial attachment, growth of microcolonies and lateral spreading of biofilm cells on the surface occurred simultaneously within the first 24 h. Once surface coverage was almost complete, biofilm development proceeded with extensive vertical growth, resulting in formation of towering structures giving rise to pronounced three-dimensional architecture. Biofilm development was found to be dependent on the nutrient conditions, suggesting a metabolic control. In global transposon mutagenesis, 173 insertion mutants out of 15,000 mutants screened were identified carrying defects in initial attachment and/or early stages in biofilm formation. Seventy-one of those mutants exhibited a nonswimming phenotype, suggesting a role of swimming motility or motility elements in biofilm formation. Disruption mutations in motility genes (flhB, fliK, and pomA), however, did not affect initial attachment but affected progression of biofilm development into pronounced three-dimensional architecture. In contrast, mutants defective in mannose-sensitive hemagglutinin type IV pilus biosynthesis and in pilus retraction (pilT) showed severe defects in adhesion to abiotic surfaces and biofilm formation, respectively. The results provide a basis for understanding microbe-mineral interactions in natural environments.
Wang, Jianwei, Andrey G. Kalinichev, and R. James Kirkpatrick, "Molecular Modeling of Water Structure in Nano-Pores between Brucite (001) Surfaces." Geochimica et Cosmochimica Acta 68 pp. 3351-3365 2004.
Molecular dynamics (MD) computer simulations of liquid water held in one-dimensional nanoconfinement by two parallel, electrostatically neutral but hydrophilic surfaces of brucite, Mg(OH)2, provide greatly increased, atomistically detailed understanding of surface-related effects on the spatial variation in the structural ordering, hydrogen bond (H-bond) organization, and local density of H2O molecules at this important model hydroxide surface. NVT-ensemble MD simulations (i.e., at constant number of atoms, volume and temperature) were performed for a series of model systems consisting of 3 to 30 Å-thick water layers (containing 35 to 360 H2O molecules) confined between two 19 Å-thick brucite substrate layers. The results show that the hydrophilic substrate significantly influences the near-surface water structure, with both H-bond donation to the surface oxygen atoms and H-bond acceptance from the surface hydrogen atoms in the first surface layer of H2O molecules playing key roles. Profiles of oxygen and hydrogen atomic density and H2O dipole orientation show significant deviation from the corresponding structural properties of bulk water to distances as large as 15 Å (~5 molecular water layers) from the surface, with the local structural environment varying significantly with the distance from the surface. The water molecules in the first layer at about 2.45 Å from the surface have a two-dimensional hexagonal arrangement parallel to brucite layers, reflecting the brucite surface structure, have total nearest neighbor coordinations of 5 or 6, and are significantly limited in their position and orientation. The greatest degree of the tetrahedral (ice-like) ordering occurs at about 4 Å from the surface. The translational and orientational ordering of H2O molecules in layers further from the surface become progressively more similar to those of bulk liquid water. A quantitative statistical analysis of the MD-generated instantaneous molecular configurations in terms of local density, molecular orientation, nearest neighbor coordination, and the structural details of the H-bonding network shows that the local structure of interfacial water at the brucite surface results from a combination of “hard wall” (geometric and confinement) effects, highly directional H-bonding, and thermal motion. This structure does not resemble that of bulk water at ambient conditions or at elevated or reduced temperature, but shares some similarities with that of water under higher pressure.
Wu, Ping-Gui, C.H. Ma, and Jian-Ku Shang, "Effects of Nitrogen Doping on Optical Properties of TiO2 Thin Films." Applied Physics A: Materials Science & Processing 81 pp. 1411-1417 2004.
The potential for extending the optical absorption range of TiO2 by doping with nonmetallic elements was examined in nitrogen-containing TiO2 thin films. Thin films of TiO2-xNx were synthesized on glass and silicon substrates by ion-beam-assisted deposition to obtain a wide range of nitrogen concentrations. The compositions of the films were determined by Rutherford backscattering spectrometry and X-ray photoelectron spectroscopy. The structures of the films were analyzed by X-ray diffraction, transmission electron microscopy, and atomic force microscopy. The optical properties of the films were measured by UV-Vis spectroscopy and ellipsometry. A characteristic decreasing trend in band-gap values of the films was observed within a certain range of increasing dopant concentrations. As the nitrogen concentration increased, the structure of the films evolved from a well-defined anatase to deformed
anatase. The reduced band gaps are associated with the N 2p orbital in the TiO2-xNx films.
Wu, Yan, and Mark A. Shannon, "Theoretical Analysis of the Effect of Static Charges in Silicon-Based Dielectric Thin Films on Micro- to Nanoscale Electrostatic Actuation." Journal of Micromechanics and Microengineering 14 pp. 989-998 2004.
Silicon-based dielectric thin films such as SiO2 and Si3N4 are commonly used as insulation layers in electrostatic microactuators to protect the device from short circuiting if the electrodes are in contact. However, dielectric films can store bulk and/or surface static charges. In this paper, the effect that these static charges have on the force applied by an electrostatic actuator is analyzed. Based on a one-dimensional model, the electric field within the gap of an electrostatic actuator for a metal/gap/dielectric/doped silicon-layered configuration is calculated for three assumed charge distributions in the dielectric layer. A characteristic voltage is defined for the metal/gap/dielectric/doped silicon system with static charges, which is found to govern the interaction. It is found that when the applied voltage to the actuator is within one order of magnitude of the characteristic voltage, the real electric field within the gap can differ by many orders of magnitude from the electric field that is predicted without considering the static charges.
Yue, Zhongren, Christian L. Mangun, and James Economy, "Characterization of Surface Chemistry and Pore Structure of H3PO4 - Activated Poly(vinyl alcohol) Coated Fiberglass." Carbon 42 pp. 1973-1982 2004.
H3PO4-activated poly(vinyl alcohol) coated fiberglass mats (PAPCF) were prepared in air at temperatures below 400 °C. The surface chemistry and pore structure of PAPCF were characterized by using N2 adsorption at 77 K, XPS, DRIFTS, TGA, NaOH uptake and Ag+ adsorption. PAPCF prepared at 300 °C yields a high BET surface area (up to 1745 m2/g of coating) with a structure composed mainly of micropores. Some oxygen-containing functional groups, such as carboxylic, phenolic hydroxyl groups, and phosphorus species, are incorporated into the PVA-based char coating, with higher activation temperatures leading to increased formation of these groups. Post-treatment with sulfuric acid decreases the content of phosphorus species, but increases that of oxygen-containing groups in the coating. NaOH uptake and Ag+ adsorption show substantial cationic exchange capacity with the PAPCF or acid-treated PAPCF. TGA data show that PAPCF is more thermally stable in air than acid-treated PAPCF. Furthermore, PAPCF activated at 300 °C can be regenerated at least 5 times with no change in ion exchange capacity.
Angelini, Thomas E., Hongjun Liang, Willy Wriggers, and Gerard C.L. Wong, "Like-Charge Attraction Between Polyelectrolytes Induced by Counterion Charge Censity Waves." Proceedings of the National Academy of Sciences 100 pp. 8634-8637 2003.
Electrostatics in aqueous media is commonly understood in terms of screened Coulomb interactions, where like-charged objects, such as polyelectrolytes, always repel. These intuitive expectations are based on mean field theories, such as the Poisson–Boltzmann formalism, which are routinely used in colloid science and computational biology [Israelachvili, J. (1992) Intermolecular and Surface Forces (Academic, London), 2nd ed.]. Like-charge attractions, however, have been observed in a variety of systems [Gelbart, W. M., Bruinsma, R. F., Pincus, P. A. and Parsegian, V. A. (2000) Phys. Today 53, 38–44]. Intense theoretical scrutiny over the last 30 years suggests that counterions play a central role, but no consensus exists for the precise mechanism. We have directly observed the organization of multivalent ions on cytoskeletal filamentous actin (a well defined biological polyelectrolyte) by using synchrotron x-ray diffraction and discovered an unanticipated symmetry-breaking collective counterion mechanism for generating attractions. Surprisingly, the counterions do not form a lattice that simply follows actin’s helical symmetry; rather, the counterions organize into “frozen” ripples parallel to the actin filaments and form 1D charge density waves. Moreover, this 1D counterion charge density wave couples to twist distortions of the oppositely charged actin filaments. This general cooperative molecular mechanism is analogous to the formation of polarons in ionic solids and mediates attractions by facilitating a “zipper-like” charge alignment between the counterions and the polyelectrolyte charge distribution. We believe these results can fundamentally impinge on our general understanding of electrostatics in aqueous media and are relevant to a wide range of colloidal and biomedical processes.
Briones, Aurelio, and Lutgarde Raskin, "Diversity and Dynamics of Microbial Communities in Engineered Environments and Their Implications for Process Stability." Current Opinion in Biotechnology 14 pp. 270-276 2003.
The availability of molecular biological tools for studying microbial communities in bioreactors and other engineered systems has resulted in remarkable insights linking diversity and dynamics to process stability. As engineered systems are often more manageable than large-scale ecosystems, and because parallels between engineered environments and other ecosystems exist, the former can be used to elucidate some unresolved ecological issues. For example, the process stability of methanogenic bioreactors containing well-defined trophic groups appears to depend on the diversity of the functional groups within each trophic level as well as on how these functional groups complement each other. In addition to using engineered systems to study general ecological questions, microbial ecologists and environmental engineers need to investigate conditions, processes, and interactions in engineered environments in order to make the ecological engineering of bioreactor design and operation more practicable.
Brown, Andrea K., Jing Li, Caroline M.-B. Pavot, and Yi Lu, "A Lead-Dependent DNAzyme with a Two-Step Mechanism." Biochemistry 42 pp. 7152-7161 2003.
A detailed biochemical and mechanistic study of in vitro selected variants of 8-17 DNAzymes is presented. Even though the 8-17 DNAzyme motif has been obtained through in vitro selection under three different conditions involving 10 mM Mg2+ (called 8-17), 0.5 mM Mg2+/50 mM histidine (called Mg5), or 100 uM Zn2+ (called 17E), all variants are shown to be the most active with Pb2+ (8-17: kobs ~0.5 min-1; Mg5: kobs ~2 min-1; 17E: kobs ~1 min-1 with 200 uM Pb2+ at pH 5.0). For the 17E variant of the 8-17 DNAzyme, the single-turnover rate constants followed the order of Pb2+ is much greater than Zn2+ is much greater than Mn2+ is approximately equal to Co2+ is greater than Ni2+ is greater than Mg2+ is approximately equal to Ca2+ is greater than Sr2+ is approximately equal to Ba2+. The catalytic rate is half-maximal at 13.5 uM Pb2+, 0.97 mM Zn2+, or 10.5 mM Mg2+, suggesting that the metal-binding affinity of the DNAzymes is in the order of Pb2+ > Zn2+ > Mg2+. The Pb2+-dependent activity increases linearly with pH and the slope of the plot of log kobs versus pH is ~1, suggesting a single deprotonation in the rate-limiting step of the reaction. Sequence variations of the DNAzyme confirm the importance of the G•T wobble pair, the two loops and the intervening stem in maintaining the active conformation of the system. While Mg2+ and Zn2+ catalyze only a transesterification reaction with formation of a product containing a 2-prime,3-prime-cyclic phosphate, Pb2+ catalyzes a transesterification reaction followed by hydrolysis of the 2-prime,3-prime--cyclic phosphate. Although this two-step mechanism has shown to be operative in protein ribonucleases and in the leadzyme RNAzyme, it is now demonstrated for the first time that this DNAzyme may also use the same mechanism. Therefore, the two-step mechanism is observed in metalloenzymes of all classes, and this 8-17 DNAzyme provides a simple, stable, and cost-effective model system for understanding the structure of Pb2+-binding sites and their roles in the two-step mechanism.
Bruce, Bertram, Ann P. Bishop, P. Bryan Heidorn, Karen J. Lunsford, S. Poulakos, and Mihye Won, "The Inquiry Page: Bridging Digital Libraries to Learners." Knowledge Quest 31 pp. 15-17 2003.
Much advice regarding digital collections focuses on how to build and maintain resources on behalf of users. However, no matter how appropriate, comprehensive, well-maintained, and organized a digital collection is, it must be both accessible and meaningful to learners. Educational resources must make sense in terms of the interests, backgrounds, and abilities of the users. As John Dewey insisted, the basis of learning is not the curriculum map, but the learner's world.
Digital library development risks promoting learning in which teachers and students are passively retrieving information. The Inquiry Page focuses on building a constructivist environment using Web resources, collaborative processes, and knowledge that bridges digital libraries with users in K-12 schools, museums, community groups, nature and science centers, homes, or other organizations, especially those who live on the economic, cultural, or linguistic margins.
A national science digital library has the potential to span boundaries and build capacities for integrating digital resources into curricula, promoting appropriate pedagogy, and facilitating communication. Moreover, it can pay attention to groups normally excluded by technological progress. At the Inquiry Page, our digital library research work draws on the tenets of participatory action research and the notion of "community of inquiry' to create a democratic and collaborative process in vhich school library media specialists (SLMS), teachers, and students can learn from one another.
Butler, John C., Thomas Angelini, Jay X. Tang, and Gerard C.L.Wong, "Ion Multivalence and Like-Charge Polyelectrolyte Attraction." Physical Review Letters 91 p. 28301 2003.
It is known empirically that multivalent ions generate attractions between like-charged polyelectrolytes, with different valence requirements for different systems. How multivalent must an ion be before it can condense a given polyelectrolyte? Using charge-tunable M13 virus rods and a family of artificial homologous “dumbbell” divalent ions of different sizes, we have constructed a multivalent ion-polyelectrolyte phase diagram, and find an experimentally motivated general criterion for like-charged attraction based on the ion valence, ion size, and the Gouy-Chapman length.
Liang, Hongjun, Thomas E. Angelini, James Ho, Paul V. Braun, and Gerard C.L. Wong, "Molecular Imprinting of Biomineralized CdS Nanostructures: Crystallographic Control Using Self-Assembled DNA-Membrane Templates." Journal of the American Chemical Society 125 pp. 11786-11787 2003.
A wide range of biomineralization and templating methods exist for organizing inorganic materials at a wide range of lengthscales. Here, we show that crystallographic control of the inorganic nanostructures is possible using synthetic biomolecular templates comprised of anionic DNA and cationic membranes, which self-assemble into a multilamellar structure where a periodic one-dimensional (1D) lattice of parallel DNA chains is confined between stacked two-dimensional (2D) lipid sheets. We have organized Cd2+ ions within the interhelical pores between DNA strands and subsequently reacted them with H2S to form CdS nanorods of controllable widths and crystallographic orientation. The strong electrostatic interactions align the templated CdS (002) polar planes parallel to the negatively charged sugar-phosphate DNA backbone, which indicates that molecular details of the DNA molecule are imprinted onto the inorganic crystal structure. The resultant nanorods have (002) planes tilted by 60° with respect to the rod axis, in contrast to all known II-VI semiconductor nanorods.
Liu, Chunqing, Nathaniel Naismith, Lei Fu, and James Economy, "Ordered Mesoporous Organic-Inorganic Hybrid Materials Containing Microporous Functional Calixarene Amides." Chemical Communications 2003 pp. 2472-2473 2003.
Ordered mesoporous organic–inorganic hybrid materials containing microporous functional calixarene amides have been synthesized and characterized for the first time, and are shown to be effective in removal of trace humic acid contaminant from water.
Liu, Chunqing, Nathaniel Naismith, Yong Qing Huang, and James Economy, "Synthesis and Characterization of Novel Hyperbranched Poly(imide-silsesquioxane) Membranes." Journal of Polymer Science Part A: Polymer Chemistry 31 pp. 3736-3743 2003.
A new family of hyperbranched polymers with chemical bonds between the hyperbranched polyimide and polysilsesquioxane network was synthesized by the reaction of an amine-terminated aromatic hyperbranched polyimide with 3-glycidoxypropyl trimethoxysilane, followed by hydrolysis and polycondensation in the presence of an acid catalyst. The hyperbranched poly(imide silsesquioxane) membranes were fabricated by the casting the aforementioned polymer solution onto a NaCl optical flat, which was followed by heating at 80oC for 24 h. The membranes were characterized by Fourier transform infrared, X-ray diffraction, thermogravimetric analysis, scanning electron microscopy, N2 adsorption and desorption, and CO2 adsorption and desorption. The presence of covalent bonds between the hyperbranched polyimide and polysilsesquioxane segments had a significant effect on the properties of the membranes. N2 adsorption–desorption isotherms for these membranes showed surface areas of 6–16 m2/g, whereas CO2 adsorption–desorption isotherms showed much higher surface areas in the range of 106–127 m2/g.
Liu, Chunqing, Nathaniel Naismith, Yong Qing Huang, James Economy, and Jonathan Talbott, "Novel Polymeric Chelating Fibers for Selective Removal of Mercury and Cesium from Water." Environmental Science and Technology 37 pp. 4261-4268 2003.
We report here the synthesis and characterization of two new classes of chelating fibers, namely, (1) polymercaptopropylsilsesquioxane (PMPS) and (2) copper(II) ferrocyanide complexed with poly[1-(2-aminoethyl)-3-aminopropyl]silsesquioxane (Cu-FC-PAEAPS) fibers. These fibers were evaluated for selective removal of trace amount of mercury and cesium ions respectively in the presence of competing metal ions from water. The PMPS and Cu-FC-PAEAPS fibers were prepared by coating their corresponding soluble prepolymers, which are derived from mercaptopropyltrimethoxysilane and [1-(2-aminoethyl)-3-aminopropyl] trimethoxysilane monomers, respectively, on a glass fiber substrate, followed by a cross-linking step at 120 °C. The fibers were characterized through infrared spectroscopy, scanning electron microscopy (SEM), and thermogravimetric analysis (TGA). These novel materials are extremely efficient in removing low concentrations of mercury and cesium ions from water in the presence of high concentrations of sodium or potassium ions. They were shown to remove trace mercury and cesium contaminants effectively to well below parts per billion concentrations under a variety of conditions.
Liu, Juewen, and Yi Lu, "A Colorimetric Lead Biosensor Using DNAzyme-Directed Assembly of Gold Nanoparticles." Journal of the American Chemical Society 125 pp. 6642-6643 2003.
The design of metal ion sensors has long been a focus of research as it can provide on-site, real-time detection and quantification of beneficial and toxic metal ions in applications such as household and environmental monitoring, developmental biology, or clinical toxicology. While significant progress has been made in making fluorescent sensors, including Pb(II) sensors, few colorimetric metal sensors have been reported. A simple colorimetric sensor could eliminate or minimize most costs associated with instrumentation and operation in fluorescence detection and thus can make on-site, real-time detection easier. Furthermore, a metal ion sensor with a tunable dynamic range is desirable for applications in widely different concentration ranges. However, few general strategies have been reported to vary the dynamic range without having to design new sensors. Here we describe a design of colorimetric metal sensors based on DNAzyme-directed assembly of gold nanoparticles and their use for sensitive and selective detection and quantification of metal ions, particularly lead in leaded paint. A novel strategy for tuning the dynamic range of the sensor using an inactive variant of the DNAzyme is also demonstrated.
Liu, Juewen, and Yi Lu, "Improving Fluorescent DNAzyme Biosensors by Combining Inter- and Intramolecular Quenchers." Analytical Chemistry 75 pp. 6666-6672 2003.
A previously reported DNAzyme-based biosensor for Pb2+ has shown high sensitivity and selectivity at 4 °C. In the system, the substrate and the enzyme strand of the DNAzyme are labeled with a fluorophore and a quencher, respectively. In the presence of Pb2+, the substrate strand is cleaved by the enzyme strand, and the release of the cleaved fragment results in significant fluorescence increase. However, the performance of the sensor decreases considerably if the temperature is raised to room temperature because of high background fluorescence. A careful analysis of the sensor system, including measurement of the melting curve and fluorescence resonance energy-transfer (FRET) study of the free substrate, suggests that a fraction of the fluorophore-labeled substrate strand is dissociated from the enzyme strand, resulting in elevated background fluorescence signals at room temperature. To overcome this problem, we designed a new sensor system by introducing both inter- and intramolecular quenchers. The design was aided by the FRET study that showed the dissociated substrate maintained a random coil conformation with an end-to-end distance of ~39 Å, which is much shorter than that of the fully extended DNA. With this new design, the background fluorescence was significantly suppressed, with 660% increase of fluorescence intensity as compared to 60% increase for the previous design. This suppression of background fluorescence signals was achieved without losing selectivity of the sensor. The new design makes it possible to use the sensor for practical applications in a wide temperature range. The design principle presented here should be applicable to other nucleic acid-based biosensors to decrease background fluorescence.
Lu, Yi, Juewen Liu, Jing Li, Peter J. Bruesehoff, Caroline M.-B. Pavot, and Andrea K. Brown, "New Highly Sensitive and Selective Catalytic DNA Biosensors for Metal Ions." Biosensors & Bioelectronics 18 pp. 529-540 2003.
While remarkable progress has been made in developing sensors for metal ions such as Ca(II) and Zn(II), designing and synthesizing sensitive and selective metal ion sensors remains a significant challenge. Perhaps the biggest challenge is the design and synthesis of a sensor capable of specific and strong metal binding. Since our knowledge about the construction of metal-binding sites in general is limited, searching for sensors in a combinatorial way is of significant value. Therefore, we have been able to use a combinatorial method called in vitro selection to obtain catalytic DNA that can bind a metal ion of choice strongly and specifically. The metal ion selectivity of the catalytic DNA was further improved using a ‘negative selection’ strategy where catalytic DNA that are selective for competing metal ions are discarded in the in vitro selection processes. By labeling the resulting catalytic DNA with a fluorophore/quencher pair, we have made a new class of metal ion fluorescent sensors that are the first examples of catalytic DNA biosensors for metal ions. The sensors combine the high selectivity of catalytic DNA with the high sensitivity of fluorescent detection, and can be applied to the quantitative detection of metal ions over a wide concentration range and with high selectivity. The use of DNA sensors in detection and quantification of lead ions in environmental samples such as water from Lake Michigan has been demonstrated. DNA is stable, cost-effective, environmentally benign, and easily adaptable to optical fiber and microarray technology for device manufacture. Thus, the DNA sensors explained here hold great promise for on-site and real-time monitoring of metal ions in the fields of environmental monitoring, developmental biology, clinical toxicology, wastewater treatment, and industrial process monitoring.
Moser, Kevin W., L. Guy Raguin, and John G. Georgiadis, "Synchronized EPI Phase Contrast Velocimetry in a Mixing Reactor." Magnetic Resonance Imaging 21 pp. 127-133 2003.
Notwithstanding its widespread use in cardiovascular and functional MRI studies, Echo Planar Imaging (EPI) has only recently been subjected to systematic validation studies. Most velocity measurement studies employing such ultrafast MRI methods involve the use of phantoms characterized by rigid or deformable solid motion. The current implementation involves a rotating phantom (angular velocity up to 10.5 rpm) with a superimposed swirling liquid flow (with axial velocities ranging between 0.145 and 0.27 cm/s) of water doped with copper sulfate. The standard implementation of single-shot EPI with phase contrast velocity encoding allows the complete mapping of the Eulerian velocity field in slices perpendicular to the rotation axis following a subtractive procedure requiring the synchronized acquisition of each velocity component on each selected transverse slice during two revolutions of the rotor. The image acquisition time is 100 ms (per velocity component) at each 64 x 64 slice. In addition to acquiring full-field velocity data for future direct comparisons with other techniques, EPI is employed here for the first time to reconstruct the three-dimensional flow field between the blades of a partitioned pipe mixer.
Reinhard, Martin, John Montgomery-Brown, Jennifer S. Louie, and Birgit Gross, "From Effluent to New Water: Performance Evaluation and Quality Assurance." Chimia 57 pp. 561-566 2003.
As water reuse becomes increasingly important to satisfy water demand, ensuring the quality of recycled wastewater becomes ever more vital. Pharmaceuticals (PhACs) and alkylphenol polyethoxylates (APEOs) metabolites are two groups of chemicals that are commonly present in treated effluent and have received attention for their demonstrated or potential biological effects. In this paper we present data on the effects of river transport, wetland treatment, and groundwater recharge on the attenuation of these emerging chemicals. Using data from three advanced water treatment plants, we also report on the efficiency of microfiltration, reverse osmosis and ultraviolet oxidation in removing these compounds from advanced treated effluents. With respect to natural attenuation processes, decreases in pharmaceutical concentrations during river transport were likely attributed to sediment sorption and chemical and biological degradation or transformation. Wetland treatment was less efficient when compared to river transport. Groundwater recharge appeared to be an effective removal process (> 99% attenuation) for PhACs and APEO metabolites, although trace levels of the latter can travel substantial distances in the subsurface. With regards to the engineered treatment options, reverse osmosis was capable of almost complete rejection of all PhACs and APEO metabolites analyzed, whereas the performances of microfiltration and UV treatment were much less efficient and consistent.
Verweij, Henk, "Ceramic Membranes: Morphology and Transport." Journal of Materials Science 38 pp. 4677-4695 2003.
Ceramic membranes generally consist of permselective material as standalone disks or tubes or as thin films on porous supports. Applications are often energy/environment related in H2, CO2 and O2 separation, H2O pervaporation, hydrocarbon separation/partial oxidation and liquid treatment such as water purification. Thin membrane films can be applied on porous supports by particulate, wet-chemical or vapor phase deposition techniques. Examples of permselective inorganic membrane compositions are dense Pd alloys and various perovskites, micro-porous (Ø < 2 nm) amorphous silica and zeolites and meso-porous (2 < Ø < 50 nm) alumina, silica and titania. The latter membranes may act as intermediate supporting layers for micro-porous membranes. Transport descriptions for meso- and macro-porous (Ø > 50 nm) membranes are based on the concepts of Knudsen diffusion (gases only), viscous flow or Maxwell-Stephan (MS) multi-component transport (Dusty Gas Model for gasses). Transport in dense membranes is described by Onsager irreversible thermodynamics and often worked out in terms of concentration- and/or field-driven diffusion. The transport descriptions as mentioned are near-equilibrium approaches that incorporate semi-empirical expressions for the chemical potential (µl) of transporting species, l. The limited definition of state-of-the-art membranes justifies the use of ideal gas thermodynamics for gases, empirical Davies µl’s for ions in liquids and Langmuir thermodynamics for surface adsorption and for most mobile species in 3-D lattices. Mobile electrons in cobaltates and metals form an exception to the latter, being better described as a Fermi liquid correlated electron system. Onsager cross-terms are seldom considered and are likely to be most relevant for molecular diffusion in gas mixtures and mixed electron-ion conductors; in both cases when different species have a significant energetic interaction. Differences in mobility of charged species may lead to the development of diffusion fields that can be incorporated in the chemical potential of that species. Single- and multi-component diffusion in liquids and in micro-porous and dense membranes can be described with chemical-, field- or MS diffusion coefficients. In solid state transport these can be related to mechanical mobilities for vacancy or interstitial mechanisms. Non-equilibrium correlation effects in diffusion can generally be ignored, except for the case of multi-component diffusion of species on a host lattice at high concentrations and with large differences in mobilities. Attempts to increase fluxes with thinner membranes have resulted in support transport resistances becoming comparable to membrane resistances. Complete descriptions of multi-component transport in supported membrane structures generally requires a numerical treatment with increasing importance of multi-scale methods. Those descriptions are needed to design fully optimized membrane structures and processes. The supports can be made at a reasonable cost by conventional ceramic pressing and extrusion techniques. Modern colloidal consolidation techniques enable very homogenous structures for accurate transport measurements and design of optimized graded porosity structures. For practical applications more attention must be paid to membrane adhesion, surface functionalization (hydrophobicity), thermochemical stability, mechanical and dimensional properties and sealing.
Wong, Gerard C.L., Alison Lin, Jay X. Tang, Youli Li, Paul A. Janmey, and Cyrus R. Safinya, "Lamellar Phase of Stacked Two-Dimensional Rafts of Actin Filaments." Physical Review Letters 91 p. 18103 2003.
We examined liquid crystalline phases of the cytoskeletal polyelectrolyte filamentous (F-) actin in the presence of multivalent counterions. As a function of increasing ion concentration, the F-actin rods in either an isotropic or a nematic phase will transform into a new and unexpected lamellar phase of crosslinked rafts (LXR phase), before condensing into a bundled phase of parallel, close-packed rods. This behavior is generic for alkali earth divalent ions Mg2+, Ca2+, Sr2+, and Ba2+, and the structural transitions are achieved without any architecture-specific actin-binding linker proteins.
Yue, Zhongren, James Economy, and Christian L. Mangun, "Preparation of Fibrous Porous Materials by Chemical Activation 2. H3PO4 Activation of Polymer Coated Fibers." Carbon 41 pp. 1809-1817 2003.
Fibrous porous materials (FPMs) have been prepared by coating a glass fiber with an aqueous solution of poly(vinyl alcohol) (PVA) and H3PO4, followed by stabilization and heat treatment in air. The H3PO4 was then removed by washing with deionised water and NaOH. The results show that H3PO4 acts as a dehydration agent to promote pyrolytic and thermal crosslinking of PVA at a much lower temperature of 170 ºC, leading to FPMs having much higher char yields and surface areas. The activation in air is of benefit to achieve higher surface areas as compared to using N2. Utilizing a fiberglass mat to support coatings of PVA activated with H3PO4 results in much higher specific surface areas. The activation temperature, activation time and concentration of H3PO4 have strong effects on the surface area, pore size distribution and coating content of FPMs.
de Lint, W.B. Samuel, Nieck E. Benes, Arnoud P. Higler, and Hank Verweij, "Derivation of Adsorption Parameters for Nanofiltration Membranes Using a 1-pK Basic Stern Model." Desalination 145 pp. 87-95 2002.
The ion retention and flux of nanofiltration (NF) membranes are to a large extent determined by the membrane surface charge. This surface charge is in turn strongly influenced by adsorption of ions from the solution onto the membrane material. A I-pK adsorption model with a Basic Stern electrostatic double layer model is used to describe ion adsorption, and the sensitivity of this model for various parameters is discussed. From a non-linear regression analysis of literature data regarding the surface charge and the zeta-potential, adsorption parameters for the I-pK model are obtained for sodium chloride on [gamma]-alumina. The I-pK Basic Stern model could predict the surface charge well, except for the highest concentration of 1000 mol/m3. Reasonable agreement is found between the measured zeta-potentials and the model predictions.
de Lint, W.B. Samuel, P. Maarten Biesheuvel, and Hank Verweij, "Application of the Charge Regulation Model to Transport of Ions through Hydrophilic Membranes: One-Dimensional Transport Model for Narrow Pores (Nanofiltration)." Journal of Colloid and Interface Science 251 pp. 131-142 2002.
The charge regulation concept is combined with the Navier-Stokes and Nernst-Planck equations to describe the ion retention of nanofiltration membranes consisting of narrow cylindrical pores. The charge regulation approach replaces the assumption of a constant charge or a constant potential at the membrane pore surface, and accounts for the influence of pH, salt concentration, and type of electrolyte on ion retention. In the current model, radial concentration and potential gradients are considered to be negligibly small (valid for narrow enough pores), resulting in a one-dimensional transport description. The model describes typical experimental data for nanofiltration membranes, such as the change of ion retention with pore radius, ion concentration, pH, and pressure both for monovalent and multivalent ions. For a constant solvent velocity, the model in some cases predicts an optimum pore size for retention. Nonequal retentions for anions and cations are predicted at low and high pH values, as well as a minimum solvent velocity for very low salt concentrations. For higher salt concentrations, and at a fixed pressure difference, an increase in solvent velocity with increasing ion concentrations is predicted, in agreement with other one-dimensional transport descriptions found in the literature, but in contrast to some experimental data.
Liu, Juewen, and Yi Lu, "FRET Study of a Trifluorophore-Labeled DNAzyme." Journal of the American Chemical Society 124 pp. 15208-15216 2002.
A fluorescence resonance energy transfer (FRET) study of biomolecules typically employs two fluorophores. The increasing number of branches and complexity of biomolecules call for simultaneously monitoring structures and dynamics of several branches in a single system. Furthermore, despite recent studies that show DNAzymes can be a stable and cost-effective alternative to protein and ribozymes for pharmaceutical and biotechnological applications, no FRET study of DNAzymes has been reported. Here, we describe the FRET study of a trifluorophore-labeled “8-17” DNAzyme, in which each of the three branches is labeled with a different fluorophore. From the study, we found that the (ratio)A method that has been commonly used in dual-fluorophore-labeled systems is also applicable to trifluorophore-labeled systems. However, while both FRET efficiency and fluorophore-to-fluorophore distance can be used to measure FRET in dual-fluorophore-labeled systems, only the average distance should be used in trifluorophore-labeled systems. The ability to monitor all three branches in a single system allowed us to reveal new metal-ion-dependent conformational changes in the DNAzyme. The trifluorophore-labeled “8-17” DNAzyme has been found to adopt a two-step folding process in the presence of Zn2+. Each step is induced by one Zn2+ binding, with apparent dissociation constants of 19 μM and 260 μM for binding the first and second Zn2+, respectively. The trifluorophore FRET results are verified by a dual-labeled control experiment. The results demonstrated that the trifluorophore-labeled system is simple and yet powerful in studying complicated biomolecular structure and dynamics and is capable of revealing new sophisticated structural changes that may have functional implications.
Lu, Yi, "New Transition-Metal-Dependent DNAzymes and Their Applications as Efficient Endonucleases and as Selective Metal Biosensors." Chemistry: A European Journal 8 pp. 4588-4596 2002.
Like proteins and RNA molecules, many DNA molecules have now been shown to catalyze a variety of reactions and are thus called DNAzymes. With limited building blocks, DNAzymes need to recruit other co-factors in order to match other enzymes in terms of reaction diversity and catalytic efficiency. Several unique properties make transition-metal ions an ideal choice of co-factor for DNAzymes. Indeed, new DNAzymes that bind transition-metal ions with high affinity and selectivity have been obtained through the use of a powerful combinatorial biology tool called in vitro selection. This accomplishment now makes it possible to obtain different classes of metallo-DNAzymes in the laboratory within a short period of time. It also offers a rare opportunity to compare and contrast structural and functional properties of metal-binding sites in proteins and in DNAzymes. The resulting transition-metal-dependent DNAzymes have displayed high activity toward cleavage of DNA and RNA and thus hold promise for their biochemical and pharmaceutical applications. Finally, the use of DNAzymes as a new class of highly sensitive and selective biosensors for metal ions has been demonstrated recently.
Sun, Y.Q., and Tianming Gao, "The Optimum Wetting Angle for the Stabilization of Liquid-Metal Foams by Ceramic Particles: Experimental Simulations." Metallurgical and Materials Transactions 33 pp. 3285-3292 2002.
The stabilization of liquid-metal foams by ceramic particles is studied by experimental simulations. The objective is to determine the optimum wetting property for liquid-foam stability. Ceramic particles are mimicked by inert plastic particles. The liquid metal is mimicked by a continuous, surfactant-free ethanol-water solution. The wetting property of the plastic particles in the liquid solution is changed continuously by varying the liquid composition. The experimental simulation shows that the liquid-foam stabilization by the solid particles depends strongly on the wetting property. An optimum wetting-angle range of 75 to 85 deg is determined from the experiments. The foam stability is shown to be unrelated to liquid viscosity, which remains unchanged with the wetting angle. Foams formed in the optimum wetting condition exhibit a slow decay, a stable foam volume that persists for a long time, and a fine cell structure in the micrometer range. The selection of ceramic particles for optimal stabilization of liquid-metal foams and the foam-processing procedures are discussed in the light of these experimental simulation results.