Carbon Nanotube Chemiresistors Coated with Hygroscopic Aqueous Film for the Selective Detection of Hydrolysable Toxic Compounds

A large class of toxic compounds, including organophosphates, react with water. Although detection of such hydrolysable toxic compounds are important, gas sensors that utilize hydrolysis as a detection mechanism have not been developed, mainly because water quickly evaporates and thus cannot be employed in the sensor that operates in the open-air environment. In this work, we demonstrate that carbon nanotube (CNT)-based chemiresistor array coated with aqueous solution of hygroscopic salts selectively detects water-soluble and hydrolysable toxic compounds. The aqueous solutions of LiCl, LiBr, and H₃PO₄ in this study did not dry out and remained as liquid film indefinitely, owing to their extremely low (<15%) deliquescence relative humidity (DRH) above which the salts become aqueous solution by absorbing moisture from the air. Upon exposure to diphenyl chlorophosphate (DPCP) and 2-chloroethyl phenyl sulfide (CEPS), both of which react with water, the resistance of the sensors showed a large decrease, and the amount of resistance decrease was much greater than that of dry CNT chemiresistors without the aqueous film. We also tested the sensor response to compounds that are not hydrolyzed and confirmed that our sensors coated with aqueous film show high sensitivity and selectivity toward hydrolysable compounds. The amount and direction of the resistance change varied with both the type of salts dissolved and the analytes, suggesting that our sensors potentially apply to discrimination of a wide range of water soluble and/or hydrolysable chemical compounds.