Array-based vapor sensing using chemically sensitive, polymer composite resistors

Abstract

(8 18) 395-6335 ABS TRACTWe describe herein the construction of simple, low-power, broadly responsive vapor sensors. Insulating polymer-conductor composites have been shown to swell reversibly upon exposure to vapors. Thin films of polymer composites have been deposited across two metallic leads, with swelling-induced resistance changes of the films signaling the presence of vapors. To identify and classify vapors, arrays of such vapor-sensing elements have been constructed, with each element containing either carbon black or poly(pyrro1e) as the conducting phase mixed with one of several different organic polymers as the insulating phase. A convenient chemical polymerization of poly(pyrro1e) which allows a high degree of processibility is also described. The differing gas-solid partition coefficients for the various polymers of the sensor array produce a pattern of resistance changes that can be used to classify vapors and vapor mixtures. This type of sensor array has been shown to resolve common organic solvents, including molecules of different classes (such as aromatics from alcohols) as well as those within a particular class (such as benzene from toluene and methanol from ethanol). The response of an individual composite to varying concentrations of solvent is shown to be consistent with the predictions of percolation theory. Accordingly, significant increases in the signals of array elements have been observed for carbon black-polymer composites that were operated near their percolation thresholds. *Present address: Department of Chemistry, University of Oregon-Ehgene Present address: Department of Chemistry, Lehigh University, 6 East Packer Avenue, Bethlehem, PA 18015-3172 'Author to whom correspondence should be addressed. ** I . INTRODUC'TIONConventional approaches to chemical sensors have traditionally malde use of a "lock-andkey" design, wherein a specific receptor is synthesized in order to bind strongly and highly selectively to the analyte of interest. A related approach involves exploiting a general physicochemical effect selectively toward a single analyte, such as the use of the ionic effect in the construction of a pH electrode.With both of these approaches, selectivity is achieved through precise chemical design of the receptor site. Such approaches are appropriate when a specific target compound is 583 to be identified in the presence of controlled backgrounds and interferences. However, this type of approach requires the synthesis of a separate, highly selective sensor for each analyte to be detected. In addition, this type of approach is not particularly useful for analyzing, classifying, or assigning human value judgments to the composition of complex vapor mixtures such as perfumes, beers, foods, mixtures of solvents, etc.An alternative approach to chemical sensing is closer conceptually to a design widely proposed for the mammalian sense of In such an approach, the strict "lock-and-key" design criterion of traditional sensing devices is abandoned. Instead, in this alternative sensor a...

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