Surfactant Additives for Enhanced Baseline Stability of Chemiresistor Films
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Chemiresistors have the potential for being effective, low powered, low cost sensors designed to detect volatile organic compounds across the entire solubility spectrum, from non-polar analytes like isooctane to very polar solvents like water. Using dilute solutions of dissolved polymers and submicron particles of graphitized carbon, films were cast onto silicon platforms with parallel, partitioned, platinum electrodes that allow current to flow through the film and then measure any resistance. When a solvent is present at the interface of the film, the polymers adsorb the analyte and swell causing higher resistance, hence the term chemiresistor. Each polymer has varying degrees of affinity toward analytes throughout the solubility spectrum. The selective affinity characteristic of the film can be applied in an array of chemiresistors thus determining specific compounds across the entire spectrum. This technology promises inexpensive and reliable chemical sensors, but long term baseline drift has ultimately degraded their precision and performance. To increase the stability of these systems, surfactants were added to the carbon-polymer solutions. Particularly attractive is the addition of non-ionic surfactants to help form steric barriers against aggregation with other carbon particles. These steric barriers would then form a stable matrix of carbon within the film and therefore reduce baseline drift. Size analysis conducted on the carbon particles in the colloidal solution has shown that surfactant addition can prevent aggregation in several polymer-carbonsolvent solutions. However, solvent selection was observed to be the main determining factor in carbon particle size. The resulting smaller, dispersed carbon particles were used as chemiresistor films, however, an increase in stability of the resistance baselines was not observed. Rather, an unimagined attraction to the solvent TCE was detected for most polymer-carbon-surfactant films.