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Showing papers by "Nathan S. Lewis published in 2004"


Journal ArticleDOI
TL;DR: Evaluation of the performance of the man-made array provides suggestions regarding some of the fundamental odor detection principles of the mammalian olfactory system.
Abstract: Arrays of broadly cross-reactive vapor sensors provide a man-made implementation of an olfactory system, in which an analyte elicits a response from many receptors and each receptor responds to a variety of analytes. Pattern recognition methods are then used to detect analytes based on the collective response of the sensor array. With the use of this architecture, arrays of chemically sensitive resistors made from composites of conductors and insulating organic polymers have been shown to robustly classify, identify, and quantify a diverse collection of organic vapors, even though no individual sensor responds selectively to a particular analyte. The properties and functioning of these arrays are inspired by advances in the understanding of biological olfaction, and in turn, evaluation of the performance of the man-made array provides suggestions regarding some of the fundamental odor detection principles of the mammalian olfactory system.

221 citations


Journal ArticleDOI
TL;DR: In this article, the thermodynamics and kinetics of binding to nanocrystalline TiO_2 were investigated for five ruthenium complexes that differed structurally in the number of possible anchoring carboxy groups (one, two, four, or six) attached to coordinated bipyridyl ligands (bipyridine, CN-, or SCN-).
Abstract: The thermodynamics and kinetics of binding to nanocrystalline TiO_2 were investigated for five ruthenium complexes that differed structurally in the number of possible anchoring carboxy groups (one, two, four, or six) attached to coordinated bipyridyl ligands and in the number of auxiliary ligands (bipyridine, CN-, or SCN-). Diffuse reflectance infrared spectroscopic data indicated that the dyes predominantly bound to TiO_2 in a bridging mode in which the oxygen atoms of an attached carboxy group were bound to separate titanium atoms on the TiO_2 surface. Furthermore, in the dry state, complexes with only one monocarboxy or dicarboxy ligand used essentially all of their available carboxy groups to bind to the surface. However, complexes having two or three dicarboxy ligands used on average two carboxylato groups in binding to TiO_2. The structural differences between the complexes were manifested chemically in that the five dyes yielded similar maximum coverages (>100 nmol cm^(-2)) on nanocrystalline TiO_2 electrodes, but exhibited different binding constants (10^3−10^5 M^(-1)) and different adsorption and desorption kinetics (3−11) × 10^3 M^(-1) h^(-1) and 1−100 h, respectively). The binding constant for the monocarboxy dye was significantly lower than the binding constants for dyes with dicarboxy ligands, correlating primarily with an increase in the desorption rate of the monocarboxy complex. The adsorption rate constants were similar for all of the dyes, suggesting that formation of the first bond to TiO_2 was rate limiting. Binding of the dyes from an ethanolic solution that contained pyridine and pyridinium as an acidic proton activity buffer yielded lower coverages than binding from a nonbuffered ethanol solution, even though the binding constants were up to 100 times greater under buffered conditions. The lower equilibrium dye coverage in buffered ethanol did not correlate with changes in the protonation state of the dyes but rather indicated competition for, and/or deactivation of, TiO_2 active sites in buffered ethanol. The more weakly bound monocarboxy dye displayed the lowest short-circuit current density and open-circuit voltage under simulated solar illumination in a photoelectrochemical cell containing 0.50 M LiI, 0.040 M I_2, 0.020 M pyridine, and 0.020 M pyridinium triflate in acetonitrile. Additionally, even at constant coverage, the integrated quantum yield for photocurrent flow was lowest for TiO_2 sensitized with the monocarboxy dye. The potential required to drive 0.1 mA cm^(-2) of cathodic current density in the dark on dye-sensitized TiO_2 photoelectrodes was least negative for the monocarboxy dye, indicating more facile electron transfer between reduced TiO_2 and the solution redox couple. Hence, in this series of ruthenium carboxy-bipyridyl dyes, the most weakly bound species (i.e., the monocarboxy dye) yielded inferior photoelectrode properties, whereas differences between the dyes that contained at least one dicarboxy ligand resulted primarily from differences in the light absorption and energetic properties of the metal complexes. These observations suggest an important role for the linkage to the TiO_2 surface in achieving temporal stability as well as in tuning both the steady-state quantum yield and the magnitude of the predominant back-reaction rate in dye-sensitized TiO_2-based photoelectrochemical solar cells.

116 citations


Journal ArticleDOI
TL;DR: In this paper, a photoluminescence (PL) blueshift was observed in SiO2-embedded silicon nanocrystals with an initial diameters of 2.9-3.4 nm.
Abstract: Silicon nanocrystals embedded in SiO2 were isolated with a selective etching procedure, and the isolated nanocrystals' excitonic emission energy was studied during controlled oxidation. Nanocrystals having initial diameters, d(0), of similar to 2.9-3.4 nm showed a photoluminescence (PL) blueshift upon oxidatively induced size reduction, as expected from models of quantum confinement. Oxidation of smaller Si nanocrystals (d(0)similar to 2.5-2.8 nm) also initially resulted in a PL blueshift, but a redshift in the PL was then observed after growth of similar to 0.3 monolayers of native oxide. This decrease in excitonic emission energy during oxidation is consistent with the theoretically predicted formation of an oxygen-related excitonic recombination state.

88 citations


Journal ArticleDOI
TL;DR: In this paper, a closed-form analytical model is developed to describe the steady-state current density−potential (J−E) characteristics of dye-sensitized nanostructured semiconductor photoelectrodes.
Abstract: A closed-form analytical model is developed to describe the steady-state current density−potential (J−E) characteristics of dye-sensitized nanostructured semiconductor photoelectrodes. The basic components of the model are a set of differential equations that describe the generation, recombination, and transport of charge carriers in mesoporous semiconductor electrode systems. Charge-carrier transport is treated as a diffusion process, and semiclassical Marcus theory is used to describe the kinetics at the interfaces between the semiconductor and the contacting phase as well as the kinetics at the interfaces with adsorbed dye. The model relates explicitly, within a single formalism, the rate constants for charge transfer of the mesoporous membrane electrode system to conventional intramolecular and intermolecular electron-transfer rate constant expressions and to interfacial electron-transfer processes at planar metal or semiconductor electrodes. The near-equilibrium situation is considered by including t...

66 citations


Journal ArticleDOI
TL;DR: Porous alumina films with controllable pore sizes and having submicrometer film thicknesses were fabricated by the anodization of Al overlayers as mentioned in this paper, where the Al was deposited by sputtering onto either glass or onto silicon that had been coated with a layer of silicon nitride.
Abstract: Porous alumina films with controllable pore sizes and having submicrometer film thicknesses were fabricated by the anodization of Al overlayers. The Al was deposited by sputtering onto either glass or onto silicon that had been coated with a layer of silicon nitride. Alumina membranes having thicknesses between 300 and 1000 nm were prepared analogously using a lithographic process to produce free-standing porous alumina films that were peripherally supported on a 500-μm-thick silicon substrate.

57 citations


Journal ArticleDOI
TL;DR: In this paper, the impact of changes in various parameters on the steady-state current density−potential (J−E) characteristics of dye-sensitized nanostructured semiconductor photoelectrodes has been evaluated through a series of simulations.
Abstract: The impact of changes in various parameters on the steady-state current density−potential (J−E) characteristics of dye-sensitized nanostructured semiconductor photoelectrodes has been evaluated through a series of simulations. The model parameters can be divided into three classes, designated as type I, type II, and type III, respectively. Type I parameters primarily affect the open-circuit potential without changing the overall shape of the J−E curves. Type II parameters primarily affect the limiting quantum yield for photocurrent production. Rate constants for injection and quenching of the excited state of the dye by the contacting phase are type II parameters, whereas the rate constant for the direct electrolyte reduction reaction, the reduced equilibrium constant for iodine formation, and the rate constant for recombination are type I parameters. The rate constant for regeneration affects both the shape of the J−E curves and the limiting quantum yield for photocurrent flow, and it is therefore designated as a type III parameter. Variation of the diffusion coefficient parameter for electrons in the semiconducting membrane produces a mixed response having both type I and II characteristics. Comparisons between simulations and experimental data have delineated the factors that should be manipulated to increase the energy conversion efficiency of photoelectrochemical cells that utilize nanostructured TiO_2 electrodes.

46 citations


Journal ArticleDOI
20 Jan 2004-Langmuir
TL;DR: In this article, the amine-capped Au nanocrystal films exhibited a much larger, irreversible decrease in resistance upon exposure to vapors possessing the thiol (−SH) functionality, including H_2S, CH_3SH, and propanethiol.
Abstract: Alkylamine-capped gold nanocrystals have been used as chemically sensitive resistors for the detection of volatile organic mercaptan vapors. Thin (<1 × 10^(-4) cm) films of dodecylamine-capped, 6−7 nm diameter Au nanocrystals produced dc electrical resistances of 10 kΩ to 10 MΩ when deposited onto interdigitated Au/Cr electrodes. These chemiresistive vapor detectors displayed a reversible increase in dc electrical resistance when exposed to non-thiol-containing vapors such as water, acetone, or toluene delivered at a constant fraction (0.05) of their vapor pressure, with relative differential resistance responses to these vapors of ≈5% under such conditions. In contrast, the amine-capped Au nanocrystal films exhibited a much larger, irreversible decrease in resistance upon exposure to vapors possessing the thiol (−SH) functionality, including H_2S, CH_3SH, and propanethiol. Optical spectroscopic and transmission electron microscopy data indicated that the thiols displaced the amine caps, allowing the gold cores to move closer together and in some cases enter into contact, thereby lowering the film resistance. Consistently, the resistance of such films did not decrease upon exposure to octanethiol. For CH_3SH, the rate of resistance change under repeated experimental conditions allowed extraction of the concentration of analyte over the range 4 ppb to 1.5 ppm in air.

43 citations


Journal ArticleDOI
06 Jan 2004-Langmuir
TL;DR: The use of nanosphere lithography to construct two-dimensional arrays of polystyrene particles coated with multilayered polyelectrolyte (PE) shells and truncated eggshell structures composed of PE thin layers is reported.
Abstract: The use of nanosphere lithography to construct two-dimensional arrays of polystyrene (PS) particles coated with multilayered polyelectrolyte (PE) shells and truncated eggshell structures composed of PE thin layers is reported. The truncated eggshell PE structures were produced by extraction of the PS particle cores with toluene. The core-extraction process ruptures the apex of the PE coating and causes a slight expansion of the PE thin layers. Aniline hydrochloride was infiltrated into the PE shells and subsequently electropolymerized to yield an array of a composite containing polyaniline (PAni) and PE thin shells. Voltammetric, quartz crystal microbalance, and reflectance Fourier transform infrared spectroscopic measurements indicate that aniline monomers were confined within the thin PE shells and the electropolymerization occurred in the interior of the PE shell. The PE thickness governs the amount of infiltrated monomer and the ultimate loading of the PAni in the truncated eggshell structure. Surface-structure imaging by atomic force microscopy and scanning electron microscopy, carried out after each step of the fabrication process, shows the influence of the PE thickness on the organization and dimensions of the arrays. Thus, the PE thin shells composed of different layers can function as nanometer-sized vessels for the entrapment of charged species for further construction of composite materials and surface modifications. This approach affords a new avenue for the synthesis of new materials that combine the unique properties of conductive polymers and the controllability of template-directed surface reactions.

41 citations


Patent
15 Jan 2004
TL;DR: In this article, a method for detecting the presence of an analyte indicative of various medical conditions, including halitosis, periodontal disease and other diseases are also disclosed.
Abstract: The present invention provides methods for detecting the presence of an analyte indicative of various medical conditions, including halitosis, periodontal disease and other diseases are also disclosed.

41 citations


Journal ArticleDOI
TL;DR: In this article, a man-made implementation of an olfactory system is presented, in which an analyte elicits a response from many receptors and each receptor responds to a variety of analytes.
Abstract: Arrays of broadly cross-reactive vapor sensors provide a man-made implementation of an olfactory system, in which an analyte elicits a response from many receptors and each receptor responds to a variety of analytes. Pattern recognition methods are then used to detect analytes based on the collective response of the sensor array. With the use of this architecture, arrays of chemically sensitive resistors made from composites of conductors and insulating organic polymers have been shown to robustly classify, identify, and quantify a diverse collection of organic vapors, even though no individual sensor responds selectively to a particular analyte. The properties and functioning of these arrays are inspired by advances in the understanding of biological olfaction, and in turn, evaluation of the performance of the man-made array provides suggestions regarding some of the fundamental odor detection principles of the mammalian olfactory system.

13 citations


Journal ArticleDOI
TL;DR: In this paper, differential capacitance versus potential and current density versus potential techniques were used to measure the energetics and kinetics of interfacial electron-transfer reactions in contact with a 70:30 (v:v) mixture of CH3CN−tetrahydrofuran that contained either 1,1‘-dimethylferrrocene+/0 or decamethylferrocene +/0.
Abstract: The electrochemical behavior of freshly etched (111)B-oriented InP surfaces was compared to that of (111)B-oriented InP surfaces that had been chemically modified by reaction with p-BrCH2C6H4CF3. Differential capacitance versus potential and current density versus potential techniques were used to measure the energetics and kinetics of interfacial electron-transfer reactions in contact with a 70:30 (v:v) mixture of CH3CN−tetrahydrofuran that contained either 1,1‘-dimethylferrrocene+/0 or decamethylferrocene+/0. For both the etched and modified (111)B InP contacts, plots of differential capacitance versus potential measurements indicated a linear dependence of the equilibrium voltage drop (Vbi) in the semiconductor space-charge region, as a function of the redox potential (E(A/A-)) of the solution, with the slope of Vbi vs E(A/A-) ≈ 1.0, as expected for ideal behavior of a semiconductor/liquid junction. The barrier heights calculated for the chemically modified InP/liquid junctions were 100 ± 20 mV higher ...

Patent
08 Dec 2004
TL;DR: In this paper, a sensor array for detecting an analyte in a fluid, comprising sensors having regions of nonconductive material and of conductive material compositionally different than said nonconductively material, such that said sensors have a predetermined inter-sensor variation in the structure and/or composition of the non-conductive materials, thereby providing sensor diversity.
Abstract: A sensor array for detecting an analyte in a fluid, comprising sensors having regions of nonconductive material and of conductive material compositionally different than said nonconductive material, such that said sensors have a predetermined inter-sensor variation in the structure and/or composition of the nonconductive material, thereby providing sensor diversity.