Carbon nanotube chemiresistors coated with hygroscopic aqueous film for the selective detection oftoxic compounds

Abstract

A large class of toxic compounds, including organophosphates, react with water. Although thedetection of such hydrolyzable toxic compounds is important, gas sensors that utilize hydrolysis as a detectionmechanism have not been developed, mainly because water quickly evaporates and thus cannot be employedin a sensor that operates in an open-air environment. In this work, we demonstrate that a carbon nanotube(CNT)-based chemiresistor coated with an aqueous solution of hygroscopic salts selectively detects water-dissolvable and hydrolyzable toxic compounds. The aqueous solutions of LiCl, LiBr, and H PO in this studydid not dry out and remained as a liquid film indefinitely, owing to their extremely low (〈15 %) deliquescencerelative humidity (DRH) above which the salts become an aqueous solution by absorbing moisture from the air.Upon exposure to dimethyl methylphosphonate (DMMP) and oxalyl chloride, both of which react with water,the resistance of sensors showed a large change and the amount of change in resistance was much greaterthan that of dry CNT chemiresistors without the aqueous film. To determine the amount and direction of theresistance change based on the presence and the type of hygroscopic salt solution, Raman spectra were usedfor identification. We also tested the sensor response to compounds that are not hydrolyzed and confirmed thatour sensors coated with an aqueous film show high sensitivity and selectivity toward hydrolyzable compounds.The amount and direction of the resistance change varied with both the type of salts dissolved and theanalytes, suggesting that our sensors potentially apply to the discrimination of a wide range of water-dissolvable and/or hydrolyzable chemical compounds.