Showing papers by "Nathan S. Lewis published in 1998"

Quantitative Study of the Resolving Power of Arrays of Carbon Black−Polymer Composites in Various Vapor-Sensing Tasks

Abstract: A statistical metric, based on the magnitude and standard deviations along linear projections of clustered array response data, was utilized to facilitate an evaluation of the performance of detector arrays in various vapor classification tasks. This approach allowed quantification of the ability of a 14-element array of carbon black−insulating polymer composite chemiresistors to distinguish between members of a set of 19 solvent vapors, some of which vary widely in chemical properties (e.g., methanol and benzene) and others of which are very similar (e.g., n-pentane and n-heptane). The data also facilitated evaluation of questions such as the optimal number of detectors required for a specific task, whether improved performance is obtained by increasing the number of detectors in a detector array, and how to assess statistically the diversity of a collection of detectors in order to understand more fully which properties are underrepresented in a particular set of array elements. In addition, the resolvi...

Progress in Understanding Electron-Transfer Reactions at Semiconductor/Liquid Interfaces

Abstract: This article describes theoretical treatments and experimental data focused on the rates of interfacial electron-transfer processes at semiconductor/liquid contacts. These systems are of practical interest because such electron transfers are a critical factor in understanding the behavior of photoelectrochemical cells as energy conversion devices. These processes are of theoretical interest because the description of how a delocalized charge carrier in a semiconducting solid reacts with a localized redox acceptor that is dissolved in the liquid electrolyte is a relatively undeveloped area of electron-transfer theory. The general principles of these processes, a discussion of past and present experimental data, and a comparison between theoretical expectations and experimental observations on a variety of semiconducting electrode systems are the main focus of this article.

Electrochemical properties of (111)-oriented n-si surfaces derivatized with covalently-attached alkyl chains

Abstract: The electrochemical properties of alkyl-terminated, (111)-oriented, n-type Si surfaces, prepared via a two-step halogenation/alkylation procedure, were analyzed in contact with CH3OH−1,1‘-dimethylferrocene+/0 (Me2Fc+/0) solutions. Current density−potential and differential capacitance−potential measurements of these surfaces in contact with CH3OH−Me2Fc+/0 indicated that the electrochemical properties of the alkyl-terminated surfaces were very similar to those of the H-terminated Si surface. The alkyl overlayers did not shift the Si band edges or induce significant surface recombination, but they did provide an additional electrical series resistance to charge transfer across the Si/liquid interface. The efficacy of alkyl overlayers in preventing photooxidation and photocorrosion of n-silicon surfaces was measured in contact with CH3OH−Me2Fc+/0 solutions to which a known amount of water had been added. Under these conditions, the alkyl-terminated surfaces consistently showed excellent current density−poten...

Techniques and systems for analyte detection

Abstract: Techniques are used to detect and identify analytes. Techniques are used to fabricate and manufacture sensors to detect analytes. An analyte (810) is sensed by sensors (820) that output electrical signals in response to the analyte. The electrical signals may be preprocessed (830) by filtering and amplification. In one embodiment, a plurality of sensors are formed on a single integrated circuit. The sensors may have diverse compositions.

Trends in odor intensity for human and electronic noses: Relative roles of odorant vapor pressure vs. molecularly specific odorant binding

Abstract: Response data were collected for a carbon black-polymer composite electronic nose array during exposure to homologous series of alkanes and alcohols. The mean response intensity of the electronic nose detectors and the response intensity of the most strongly driven set of electronic nose detectors were essentially constant for members of a chemically homologous odorant series when the concentration of each odorant in the gas phase was maintained at a constant fraction of the odorant’s vapor pressure. A similar trend is observed in human odor detection threshold values for these same homologous series of odorants. Because the thermodynamic activity of an odorant at equilibrium in a sorbent phase is equal to the partial pressure of the odorant in the gas phase divided by the vapor pressure of the odorant and because the activity coefficients are similar within these homologous series of odorants for sorption of the vapors into specific polymer films, the data imply that the trends in detector response can be understood based on the thermodynamic tendency to establish a relatively constant concentration of sorbed odorant into each of the polymeric films of the electronic nose at a constant fraction of the odorant’s vapor pressure. Similarly, the data are consistent with the hypothesis that the odor detection thresholds observed in human psychophysical experiments for the odorants studied herein are driven predominantly by the similarity in odorant concentrations sorbed into the olfactory epithelium at a constant fraction of the odorant’s vapor pressure.

Use of compatible polymer blends to fabricate arrays of carbon black-polymer composite vapor detectors

Abstract: Compatible blends of poly(vinyl acetate) and poly(methyl methacrylate) have been used to produce a series of electrically conducting carbon black composites whose resistance is sensitive to the nature and concentration of an analyte in the vapor phase. The dc electrical resistance response of the composites was found to be a nonlinear function of the mole fraction of poly(vinyl acetate) in the blend. These compatible blend composite detectors provided additional analyte discrimination information relative to a reference detector array that only contained composites formed using the pure polymer phases. The added discrimination power provided by the compatible blend detectors, and thus the added diversity of the enhanced detector array, was quantified through use of a statistical metric to assess the performance of detector arrays in various vapor classification tasks.

Differential detection of enantiomeric gaseous analytes using carbon black-chiral polymer composite, chemically sensitive resistors.

Abstract: Carbon black−chiral polymer composites were used to provide diagnostic differential resistance responses in the presence of enantiomers of chiral gaseous analytes. Vapors of (+)-2-butanol and (−)-2-butanol, (+)-α-pinene and (−)-α-pinene, (+)-epichlorohydrin and (−)-epichlorohydrin, and methyl (+)-2-chloropropionate and methyl (−)-2-chloropropionate were generated and passed over a chemically sensitive carbon black−poly((R)-3-hydroxybutyrate-co-(R)-3-hydroxyvalerate) (77% butyrate) composite resistor. Each enantiomer of a pair produced a distinct relative differential resistance change on the chiral detector, whereas both enantiomers of a set produced identical signals on achiral carbon black−poly(ethylene-co-vinyl acetate) (82% ethylene) detectors.

Compositionally different polymer-based sensor elements and methods for preparing same

Abstract: The present invention provides a combinatorial approach for preparing arrays of chemically sensitive polymer-based sensors which are capable of detecting the presence of a chemical analyte in a fluid in contact therewith. The described methods and devices comprise combining varying ratios of at least first and second organic materials which, when combined, form a polymer or polymer blend that is capable of absorbing a chemical analyte, thereby providing a detectable response. The detectable response of the sensors prepared by this method is not linearly related to the mole fraction of at least one of the polymer-based components of the sensors, thereby making arrays of these sensors useful for a variety of sensing tasks.

Sensor arrays for detecting microorganisms

Abstract: A sensor array for detecting a microorganism comprising first and second sensors electrically connected to an electrical measuring apparatus, wherein the sensors comprise a region of nonconducting organic material and a region of conducting material compositionally that is different than the nonconducting organic material and an electrical path through the regions of nonconducting organic material and the conducting material. A system for identifying microorganisms using the sensor array, a computer and a pattern recognition algorithm, such as a neural net are also disclosed.

Cyclic voltammetry of semiconductor photoelectrodes III: A comparison of experiment and theory for n-Si and p-Si electrodes

Abstract: Cyclic voltammograms have been obtained under a variety of conditions using a semiconducting photoelectrode or a circuit containing a diode in series with a metallic electrode Simulations of the voltammetry of both types of systems were performed using a model circuit in which electrode nonideality, double-layer capacitance, and parallel resistance effects were accounted for quantitatively The simulated voltammograms were in excellent agreement with the experimental data for a diode/electrode circuit, yielding a reliable description of the shapes of the voltammograms as well as of the voltage dropped across the diode element as a function of the total potential dropped across the circuit The digital simulations were in good agreement with the voltammetry of p-Si/CH3OH−CoCp2+/0 contacts at high light intensities, but could not quantitatively describe the shapes of the voltammograms at low light intensities

Conditions Under Which Heterogeneous Charge-Transfer Rate Constants Can Be Extracted from Transient Photoluminescence Decay Data of Semiconductor/Liquid Contacts As Determined by Two-Dimensional Transport Modeling

Abstract: An extensive series of digital simulations of the decay dynamics of photoexcited charge carriers at a semiconductor/liquid interface has been performed using the two-dimensional simulation code ToSCA. ToSCA treats majority and minority carrier capture processes separately and incorporates field-dependent carrier mobility terms. These features produce dramatic differences in the output parameters obtained when fitting experimental data with ToSCA relative to those obtained by fitting such data with prior, less complete, simulations. The simulations revealed that for a typical (n-type in our example) InP electrode in contact with outer-sphere redox reagents dissolved in the liquid phase the photoluminescence decays were generally insensitive to the value of the minority carrier charge-transfer rate constant, kht. Instead, diffusion and drift-induced separation of photogenerated carriers in the space-charge layer of the semiconductor dominated the time decay of the observed luminescence signal under most exp...

Progress in Understanding Electron-Transfer Reactions at Semiconductor/Liquid Interfaces

Abstract: This article describes theoretical treatments and experimental data focused on the rates of interfacial electron-transfer processes at semiconductor/liquid contacts. These systems are of practical interest because such electron transfers are a critical factor in understanding the behavior of photoelectrochemical cells as energy conversion devices. These processes are of theoretical interest because the description of how a delocalized charge carrier in a semiconducting solid reacts with a localized redox acceptor that is dissolved in the liquid electrolyte is a relatively undeveloped area of electron-transfer theory. The general principles of these processes, a discussion of past and present experimental data, and a comparison between theoretical expectations and experimental observations on a variety of semiconducting electrode systems are the main focus of this article.

Array of chemically sensitive capacitors

Abstract: In an array of sensors (1310,1320), the capacitance of the sensor material is measured to determine the presence of an analyte. Each sensor in the array may include a different type of sensor material.

Progress in the development of an electronic nose using arrays of chemically sensitive carbon black-polymer resistors

Abstract: Response data were collected for a carbon black-polymer composite electronic nose array during exposure to homologous series of alkanes and alcohols. At a fixed partial pressure of odorant in the vapor phase, the mean response intensity of the electronic nose signals varied significantly for members of each series of odorants. However, the mean response intensity of the electronic nose detectors, and the response intensity of the most strongly-driven set of electronic nose detectors, was essentially constant for members of a chemically homologous odorant series when the concentration of each odorant in the gas phase was maintained at a constant fraction of the odorant's vapor pressure. Because the thermodynamic activity of an odorant at equilibrium in a sorbent phase is equal to the partial pressure of the odorant in the gas phase divided by the vapor pressure of the odorant, and because the activity coefficients are similar within these homologous series of odorants for sorption of the vapors into specific polymer films, the data imply that the trends in detector response can be understood based on the thermodynamic tendency to establish a relatively constant concentration of sorbed odorant into each of the polymeric films of the electronic nose at a constant fraction of the odorant's vapor pressure. This phenomenon provides a natural mechanism for enhanced sensitivity to low vapor pressure compounds, like TNT, in the presence of high vapor pressure analytes, such as diesel fuel. In a related study to evaluate the target recognition properties of the electronic nose, a statistical metric based on the magnitudes and standard deviations along Euclidean projections of clustered array response data, was utilized to facilitate an evaluation of the performance of detector arrays in various vapor classification tasks. This approach allowed quantification of the ability of a fourteen-element array of carbon black-insulating polymer composite chemiresistors to distinguish between members of a set of nineteen solvent vapors, some of which vary widely in chemical properties (e.g. methanol and benzene) and others of which are very similar (e.g. n-pentane and n-heptane). The data also facilitated evaluation of questions such as array performance as a function of the number of detectors in the system.

Trends in odor intensity for human and electronic noses: Relative roles of odorant vapor pressure vs. molecularly specific odorant binding (carbon black compositesyolfactionyvapor sensoryodor threshold)

Abstract: Response data were collected for a carbon black-polymer composite electronic nose array during expo- sure to homologous series of alkanes and alcohols. The mean response intensity of the electronic nose detectors and the response intensity of the most strongly driven set of electronic nose detectors were essentially constant for members of a chemically homologous odorant series when the concentration of each odorant in the gas phase was maintained at a constant fraction of the odorant's vapor pressure. A similar trend is observed in human odor detection threshold values for these same homologous series of odorants. Because the thermody- namic activity of an odorant at equilibrium in a sorbent phase is equal to the partial pressure of the odorant in the gas phase divided by the vapor pressure of the odorant and because the activity coefficients are similar within these homologous series of odorants for sorption of the vapors into specific polymer films, the data imply that the trends in detector response can be understood based on the thermodynamic tendency to establish a relatively constant concentration of sorbed odorant into each of the polymeric films of the electronic nose at a constant fraction of the odorant's vapor pressure. Similarly, the data are consistent with the hypoth- esis that the odor detection thresholds observed in human psychophysical experiments for the odorants studied herein are driven predominantly by the similarity in odorant con- centrations sorbed into the olfactory epithelium at a constant fraction of the odorant's vapor pressure. Numerous attempts have been made to understand the trends in odor detection thresholds that are displayed by the human olfactory sense. High odor detection thresholds are observed for most odorants that are gases under standard pressure and temperature conditions, whereas odorants with low vapor pressures generally have low odor detection thresholds (1). Quantitative structure-activity relationships have been for- mulated in an attempt to correlate trends in olfactory odor intensity with specific microscopic and macroscopic properties of various odorants. For example, many workers have pro- posed that trends in odor detection thresholds arise from the presence of important steric and functional group features in certain olfactory receptors (2, 3). Such receptors could then primarily respond to chemically specific features such as odorant molecular length and polarity (3-6). Other workers have empirically correlated trends in human odor detection thresholds with macroscopic properties of the odorant, such as the boiling point of the liquid phase of the odorant species (7-9). Some workers have noted the correlation between odor thresholds and the vapor pressure of the odorant (10-14). In this work, we have measured the response intensities of an electronic nose (15), based on an array of carbon black- polymer composite detectors, to straight chain alkanes and alcohols. We propose a fundamental first-order explanation for the observed trends in response intensity of the detectors in the electronic nose, based on the thermodynamic tendency for odorants to partition into sorbent phases as a function of the odorant's vapor pressure. A striking resemblance has been observed in the odor intensity trends for the human and electronic olfactory systems for these series of odorants. This similarity in odor intensity behavior occurs even though the detectors in the electronic nose array have no specific receptor sites and even though the electronic nose array is not a structural model for the human olfactory system.