A chemically diverse conducting polymer-based "electronic nose".

Abstract

We describe a method for generating a variety of chemically diverse broadly responsive low-power vapor sen-sors. The chemical polymerization of pyrrole in the presence of plasticizers has yielded conducting organic polymer films whose resistivities are sensitive to the identity and concentration of various vapors in air. An array of such sensing elements produced a chemically reversible diagnostic pattern of electrical resistance changes upon exposure to different odorants. Principal component analysis has demonstrated that such sensors can identify and quantity different airborne organic solvents and can yield information on the components of gas mixtures.There is considerable interest in developing sensors that act as analogs of the mammalian olfactory system (1, 2). This system is thought to utilize probabilistic repertoires of many different receptors to recognize a single odorant (3, 4). In such a configuration, the burden of recognition is not on highly specific receptors, as in the traditional "lock-and-key" molecular recognition approach to chemical sensing, but lies instead on the distributed pattern processing of the olfactory bulb and the brain (5, 6). We describe herein the construction and characterization of a broadly responsive vapor detection array based on conducting polymer "chemiresistor" elements. Such conducting polymer elements are simply prepared and are readily modified chemically to respond to a broad range of analytes. In addition, these sensors yield a fairly rapid lowpower dc electrical signal in response to the vapor of interest, and their signals are readily integrated with software-or hardware-based neural networks for purposes of analyte identification.Prior attempts to produce a broadly responsive sensor array have exploited heated metal oxide thin film resistors (7-9), polymer sorption layers on the surfaces of acoustic wave resonators (10, 11), arrays of electrochemical detectors (12-14), or conductive polymers (15, 16). Arrays of metal oxide thin film resistors, typically based on SnO2 films that have been coated with various catalysts, yield distinct diagnostic responses for several vapors (7-9). However, due to the lack of understanding of catalyst function, SnO2 arrays do not allow deliberate chemical control of the response of elements in the arrays nor reproducibility of response from array to array. Surface acoustic wave resonators are extremely sensitive to both mass and acoustic impedance changes of the coatings in array elements, but the signal transduction mechanism involves somewhat complicated electronics, requiring frequency measurement to 1 Hz while sustaining a 100-MHz Rayleigh wave in the crystal (10, 11). Electrically conductive organic polymer elements are well-suited for such an array, because swelling of the polymer upon exposure to an analyte will induce changes in the resistivity of the polymer film (17,18). This enables a direct low-power electrical signal readout (the film resistance) to be used as the sensing signal. Some prior work has been pe...

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