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Biomedical subjects

Thomas E Mallouk

Publications and source records attributed to Thomas E Mallouk.

At least 19 recordsLinked to original sources
PubMed2006

Anodic electrodeposition of highly oriented zirconium phosphate and polyaniline-intercalated zirconium phosphate films.

Films of highly oriented alpha-zirconium phosphate and polyaniline-intercalated zirconium phosphate with controllable thickness in the micrometer range were grown anodically on Pt electrodes. To optimize the electrodeposition conditions, the exfoliation of alpha-zirconium phosphate by tetrabutylammonium (TBA) salts was investigated in several nonaqueous solvents. Acetonitrile was found to be the best solvent for making crack-free, oriented films because of its high vapor pressure, low viscosity, and relatively high permittivity. With TBA salts of neutral or weakly acidic anions (TBACl, TBABr, TBAI, TBA(HSO4), or TBA(H2PO4)), full exfoliation did not occur and alpha-zirconium phosphate and/or polyaniline were deposited as rough films. With basic anions (TBAF or TBAOH), dense, adherent films were obtained. X-ray diffraction patterns of the films showed that they were highly oriented along the stacking axis. The thickness could be controlled, up to about 40 microm, by limiting the time of the electrodeposition reaction. At monomer concentrations below 1.0 x 10(-2) mol/dm3, the emeraldine form of the intercalated polymer was obtained. Electrodeposition thus provides a thick film alternative to layer-by-layer assembly for intercalation compounds of alpha-zirconium phosphate with a conducting polymer.

Journal Article↗
PubMed2006

Catalytically driven colloidal patterning and transport.

We recently reported the convection and pattern formation of tracers caused by a catalytically generated electric field. The electric field arises due to the heterogeneous electrochemical reduction and oxidation of hydrogen peroxide (H2O2) on silver (Ag) and gold (Au), respectively.1 Here we describe an electrokinetic model, developed in conjunction with experiments, that explains the details of the convection and pattern formation phenomenon. The model also enables the measurement of reaction kinetic parameters that are otherwise difficult to obtain. This quantitative model serves as a platform for the modeling of other catalytic redox systems and systems with broken symmetries.

Journal Article↗
PubMed2006

Bipolar electrochemical mechanism for the propulsion of catalytic nanomotors in hydrogen peroxide solutions.

Bimetallic nanorods are propelled in aqueous solutions by the catalytic decomposition of hydrogen peroxide to oxygen and water. Several mechanisms (interfacial tension gradients, bubble recoil, viscous Brownian ratchet, self-electrophoresis) have been proposed for the transduction of chemical to mechanical energy in this system. From Tafel plots of anodic and cathodic hydrogen peroxide reactions at various metal (Au, Pt, Rh, Ni, Ru, and Pd) ultramicroelectrodes, we determine the potential at which the anodic and cathodic reaction rates are equal for each metal. These measurements allow one to predict the direction of motion of all possible bimetallic combinations according to the bipolar electrochemical (or self-electrophoretic) mechanism. These predictions are consistent with the observed direction of motion in all cases studied, providing strong support for the mechanism. We also find that segmented nanorods with one Au end and one poly(pyrrole) end containing catalase, an enzyme that decomposes hydrogen peroxide nonelectrochemically, perform the overall catalytic reaction at a rate similar to that of nanorods containing Au and Pt segments. However, in this case there is no observed axial movement, again supporting the bipolar electrochemical propulsion mechanism for bimetallic nanorods.

Catalysis↗
PubMed2006

Catalytically induced electrokinetics for motors and micropumps.

We have explored the role of electrokinetics in the spontaneous motion of platinum-gold nanorods suspended in hydrogen peroxide (H2O2) solutions that may arise from the bimetallic electrochemical decomposition of H2O2. The electrochemical decomposition pathway was confirmed by measuring the steady-state short-circuit current between platinum and gold interdigitated microelectrodes (IMEs) in the presence of H2O2. The resulting ion flux from platinum to gold implies an electric field in the surrounding solution that can be estimated from Ohm's Law. This catalytically generated electric field could in principle bring about electrokinetic effects that scale with the Helmholtz-Smoluchowski equation. Accordingly, we observed a linear relationship between bimetallic rod speed and the resistivity of the bulk solution. Previous observations relating a decrease in speed to an increase in ethanol concentration can be explained in terms of a decrease in current density caused by the presence of ethanol. Furthermore, we found that the catalytically generated electric field in the solution near a Pt/Au IME in the presence of H2O2 is capable of inducing electroosmotic fluid flow that can be switched on and off externally. We demonstrate that the velocity of the fluid flow in the plane of the IME is a function of the electric field, whether catalytically generated or applied from an external current source. Our findings indicate that the motion of PtAu nanorods in H2O2 is primarily due to a catalytically induced electrokinetic phenomenon and that other mechanisms, such as those related to interfacial tension gradients, play at best a minor role.

Catalysis↗
PubMed2006

Template-grown metal nanowires.

The growth of metal nanowires using membranes as hard templates is reviewed. The method provides access to arrays of single-crystal metal nanowires and to quasi-one-dimensional metal nanostructures with controlled compositional variation along their length. Recent applications of these kinds of nanowires to problems in superconductivity, optical spectroscopy and sensing, and catalytic conversion of chemical to mechanical energy are reviewed.

Journal Article↗
PubMed2006

Chemical locomotion.

Research into the autonomous motion of artificial nano- and microscale objects provides basic principles to explore possible applications, such as self-assembly of superstructures, roving sensors, and drug delivery. Although the systems described have unique propulsion mechanisms, motility in each case is made possible by the conversion of locally available chemical energy into mechanical energy. The use of catalysts onboard can afford nondissipative systems that are capable of directed motion. Key to the design of nano- and micromotors is the asymmetric placement of the catalyst: its placement in an environment containing a suitable substrate translates into non-uniform consumption of the substrate and distribution of reaction products, which results in the motility of the object. These same principles are exploited in nature to effect autonomous motion.

Journal Article↗
PubMed2006

Observation of superconductivity in granular bi nanowires fabricated by electrodeposition.

Bulk rhombohedral Bi at ambient pressure is a well-known semimetal, and its transition to a superconductor has not been observed, at least down to 50 mK. We report that, unlike bulk rhombohedral Bi, granular Bi nanowires with well-defined rhombohedral grains of approximately 10 nm diameter, fabricated by electrochemically depositing Bi into porous polycarbonate membranes at ambient pressure, are superconducting with two transition temperatures, Tc, of 7.2 and 8.3 K. These Tc values coincide with Tc values of the high-pressure phases Bi-III and Bi-V, respectively. Analysis of our structural and transport data indicates that the superconductivity in granular Bi nanowires probably arises from grain boundary areas where there are structural reconstructions between the grains showing a preferred orientation within a small angular distribution.

Journal Article↗
PubMed2005

Catalytic micropumps: microscopic convective fluid flow and pattern formation.

As innovations continue to be made in the fields of microfluidics and the colloidal assembly, new strategies for moving particles and fluids may be needed. Heterogeneous catalysis provides means of locally converting the stored chemical energy of fuels to mechanical energy. We report an ambient temperature stationary "pump" that generates a proton concentration gradient through the bipolar electrochemical decomposition of hydrogen peroxide on patterned silver-gold surfaces. The resulting electric field drives convective fluid flow and pattern formation of colloidal tracer particles at the microscopic level by a combination of electroosmotic and electrophoretic forces.

Journal Article↗
PubMed2005

Motility of catalytic nanoparticles through self-generated forces.

Small-scale synthetic motors capable of generating their own motive forces by exploiting the chemical free energy of their environment represent an important step in developing practical nanomachines. Catalytic particles are capable of generating concentration and other gradients that can be used to self-propel small objects. However, the autonomous movement of catalytic nanoparticles by self-generated forces is a relatively unexplored area in colloid and interfacial chemistry. This paper explores the potential of catalytically self-generated forces for propulsion of small objects through fluids.

Catalysis↗
PubMed2005

Light-to-chemical energy conversion in lamellar solids and thin films.

Photocatalytic water splitting by visible light is one of the most difficult and persistent challenges in chemistry. This Forum Article reviews progress in this field, focusing on efforts made in our laboratory to control electron- and energy-transfer reactions, and catalytic hydrogen and oxygen evolution, in lamellar solids and thin films.

Journal Article↗
PubMed2005

Increasing the conversion efficiency of dye-sensitized TiO2 photoelectrochemical cells by coupling to photonic crystals.

The mechanism of enhancing the light harvesting efficiency of dye-sensitized TiO(2) solar cells by coupling TiO(2) inverse opals or disordered scattering layers to conventional nanocrystalline TiO(2) films has been investigated. Monochromatic incident photon-to-current conversion efficiency (IPCE) at dye-sensitized TiO(2) inverse opals of varying stop band wavelengths and at disordered titania films was compared to the IPCE at bilayers of these structures coupled to nanocrystalline TiO(2) films and to the IPCE at nanocrystalline TiO(2) electrodes. The results showed that the bilayer architecture, rather than enhanced light harvesting within the inverse opal structures, is responsible for the bulk of the gain in IPCE. Several mechanisms of light interaction in these structures, including localization of heavy photons near the edges of a photonic gap, Bragg diffraction in the periodic lattice, and multiple scattering events at disordered regions in the photonic crystal or at disordered films, lead ultimately to enhanced backscattering. This largely accounts for the enhanced light conversion efficiency in the red spectral range (600-750 nm), where the sensitizer is a poor absorber.

Journal Article↗
PubMed2005

Ultrathin anisotropic films assembled from individual single-walled carbon nanotubes and amine polymers.

Oxidized individual single-walled carbon nanotubes and amine polymers have been assembled into 11-32-nm-thick well-ordered conductive films. The films show highly anisotropic electrical conductivity, which is dominated by the nanotubes in the horizontal plane and by polymer-mediated tunneling in the vertical direction. The ratio of the "along" to "across" conductivity is approximately 10(3). The subnanometer thick polymer layers interleaved with monolayers of nanotubes show conductivity several orders of magnitude higher than films of pristine polymers.

Journal Article↗
PubMed2005

Penetrating the oxide barrier in situ and separating freestanding porous anodic alumina films in one step.

A simple method for penetrating the barrier layer of an anodic aluminum oxide (AAO) film and for detaching the AAO film from residual Al foil was developed by reversing the bias voltage in situ after the anodization process is completed. With this technique, we have been able to obtain large pieces of free-standing AAO membranes with regular pore sizes of sub-10 nm. By combining Ar ion milling and wetting enhancement processes, Au nanowires were grown in the sub-10 nm pores of the AAO films. Further scaling down of the pore size and extension to the deposition of nanowires and nanotubes of materials other than Au should be possible by further optimizing this procedure.

Journal Article↗
PubMed2005

Controllable template synthesis of superconducting Zn nanowires with different microstructures by electrochemical deposition.

A systematic study was conducted on the fabrication, structural characterization, and transport properties of Zn nanowires with diameters between 40 and 100 nm. Zinc nanowires were fabricated by electrodepositing Zn into commercially available polycarbonate (PC) or anodic aluminum oxide (AAO) membranes. By controlling the electrodeposition process, we found that the nanowires can be single-crystal, polycrystalline Zn, crystalline Zn/nanocrystalline ZnO composites, or entirely ZnO. The microstructure and chemistry was characterized by using transmission electron microscopy. Transport studies on single-crystal or polycrystalline Zn nanowire arrays embedded inside the membrane showed that the superconducting transition temperature, Tc, is insensitive to the nanowire diameter and morphology. The superconductivity shows a clear crossover from bulklike to quasi-1D behavior, as evidenced by residual low-temperature resistance, when the diameter of the wires is reduced to 70 nm (20 times smaller than the bulk coherence length).

Crystallization↗
PubMed2004

Catalytic nanomotors: autonomous movement of striped nanorods.

Rod-shaped particles, 370 nm in diameter and consisting of 1 microm long Pt and Au segments, move autonomously in aqueous hydrogen peroxide solutions by catalyzing the formation of oxygen at the Pt end. In 2-3% hydrogen peroxide solution, these rods move predominantly along their axis in the direction of the Pt end at speeds of up to 10 body lengths per second. The dimensions of the rods and their speeds are similar to those of multiflagellar bacteria. The force along the rod axis, which is on the order of 10(-14) N, is generated by the oxygen concentration gradient, which in turn produces an interfacial tension force that balances the drag force at steady state. By solving the convection-diffusion equation in the frame of the moving rod, it was found that the interfacial tension force scales approximately as SR(2)gamma/muDL, where S is the area-normalized oxygen evolution rate, gamma is the liquid-vapor interfacial tension, R is the rod radius, mu is the viscosity, D is the diffusion coefficient of oxygen, and L is the length of the rod. Experiments in ethanol-water solutions confirmed that the velocity depends linearly with the product Sgamma, and scaling experiments showed a strong dependence of the velocity on R and L. The direction of motion implies that the gold surface is hydrophobic under the conditions of the experiment. Tapping-mode AFM images of rods in air-saturated water show soft features that are not apparent in images acquired in air. These features are postulated to be nanobubbles, which if present in hydrogen peroxide solutions, would account for the observed direction of motion.

Journal Article↗
PubMed2004

Coaxially gated in-wire thin-film transistors made by template assembly.

Nanowire field effect transistors were prepared by a wet chemical template replication method using anodic aluminum oxide membranes. The membrane pores were first lined with a thin SiO2 layer by the surface sol-gel method. Au, CdS (or CdSe), and Au wire segments were then sequentially electrodeposited within the pores, and the resulting nanowires were released by dissolution of the membrane. Electrofluidic alignment of these nanowires between source and drain leads and evaporation of gold over the central CdS (CdSe) stripe affords a "wrap-around gate" structure. At VDS = -2 V, the Au/CdS/Au devices had an ON/OFF current ratio of 103, a threshold voltage of 2.4 V, and a subthreshold slope of 2.2 V/decade. A 3-fold decrease in the subthreshold slope relative to that of planar nanocrystalline CdSe devices can be attributed to coaxial gating. The control of dimensions afforded by template synthesis should make it possible to reduce the gate dielectric thickness, channel length, and diameter of the semiconductor segment to sublithographic dimensions while retaining the simplicity of the wet chemical synthetic method.

Journal Article↗
PubMed2004

Ordered SBA-15 nanorod arrays inside a porous alumina membrane.

The growth of ordered nanorods of mesoporous SBA-15 inside a porous alumina membrane has been achieved for the first time by a simple sol-gel method. The obtained SBA-15 nanorods themselves have ordered hexagonal mesochannels with a size of about 6 nm and have been arranged to form hexagonal arrays by the limitation of pores of the alumina membrane. The synthesized alumina membrane with mesoporous SBA-15 inside combines the advantages of porous alumina membranes and mesoporous SBA-15 and provides fine and vertical mesochannels, which may serve as a new and efficient mold and lead to extensive applications in nanodevice fabrication, biomacromolecule separations, etc.

Journal Article↗