Nerve agent simulant detection using oligoaniline-based chemiresistors

We have developed an e-nose chemiresistor array to detect simulants for nerve agents Sarin and Soman in vapor phase using oligoaniline based chemiresistors. A six-node sensor array was fabricated using thin films of tetraaniline, octaaniline, hexadecaaniline and polyaniline in a chemiresistor format. Nerve agent simulants for Sarin i.e., dimethyl methylphosphonate (DMMP) and for Soman i.e., diisopropyl methylphosphonate (DIMP) are detected in the concentration range of 25 to 1620 ppm.;Oligoanilines viz., tetramer and octamer were synthesized using a new oxidant i.e., 1, 4-benzoquinone (BQ) which is a safer alternative to ammonium peroxydisulphate (APS). A new synthesis method was used for aniline hexadecamer synthesis starting from aniline dimer which makes it a one pot synthesis.;Chemiresistor sensors were fabricated by drop-casting and inkjet-printing oligoanilines or polyaniline on flexible, light weight substrates like plastic and paper. The inkjet-printing method provided improved thickness control, low sensor-to-sensor variability and ease in fabrication. Unlike drop-casting where the nascent nanofiber morphology is lost after fabrication, inkjet-printing is more robust, reliable where the original morphology is preserved.;We report inkjet printing of as synthesized aniline tetramer and polyaniline on paper for the first time. An unexpected response is observed for films printed on paper (resistance decreases) vs. other substrates like plastic (resistance increases). This suggests that something fundamentally different is going on in oligoaniline films on paper when exposed to vapors. We believe sensor response in drop-casted films on plastic is dominated by adsorption driven swelling of the plastic and/or oligoaniline films resulting in increased barrier to charge transport and hence an increase in resistance. In contrast, inkjet-printed films on paper are very thin (100-300 nm), so comparatively with less swelling of polymer than drop-casted films, we believe that inkjet-printed films deposited on paper are prone to change in crystallinity when exposed to vapors. Unlike amorphous plastics, cellulose is highly crystalline which confers some degree of order in the oligoaniline films. Our working hypothesis is that the crystallinity increases reversibly upon vapor exposure. The obtained resistance change data was analyzed using the Flory-Huggins interaction parameter in case of nerve agent simulants and various solvent polarity scales, such as, ET 30, dipole moment and solubility parameter values for common organic solvents to further support our proposed hypothesis. The magnitude of resistance changes in drop-casted sensors seems to correlate to proximity of the Flory-Huggins and solubility parameter values of vapors to that of oligoanilines and substrate, with closest value resulting in highest resistance change. In case of inkjet-printed sensors, the magnitude of resistance changes seems to better correlate with dipole moments and proximity of the polar bond solubility values of vapors with oligoaniline or polyaniline.;The observed selectivity in resistance change between drop-casted and inkjet-printed sensors presents an excellent opportunity to fabricate an array using both plastic and paper substrates in an effort to identify unique response signature for individual vapors. An identification of unique response signature was performed with radar plots and Principal Component Analysis (PCA). Radar plots show n unique fingerprint response for each vapor while separate clusters were seen for every vapor in PCA plots.;Keywords: Polyaniline, aniline oligomers, nerve agent simulants, inkjet printing.