Comparing Selectivity of Functionalized Graphenes Used for Chemiresistive Hydrocarbon Vapor Detection

Portable or wearable sensors for volatile organic compounds (VOCs) such as benzene and naphthalene are important for occupational health monitoring of workers near refueling operations. Six commercially available, plasma-processed, functionalized-graphene nanoplatelet (fGNP) materials were dispersed between electrodes as disordered films to form chemiresistors. Putative functional groups included amino, carboxyl, fluoroalkyl, and hydroxyl. Sensor response trends were determined upon exposure to both nonpolar (fuel related alkanes and arenes) and polar compounds (alcohols, acetone, trichloroethylene, and water). The relative sensitivity to the fuel-related hydrocarbon compounds did not correlate with the functional group; but any functionalization significantly increased the sensor response compared to unfunctionalized graphene. Most notably, there is increased sensitivity to polar protic vapors. The results suggest that the dominant mechanism for sensor response is swelling caused by intercalation of vapor between particles, with sorption enhanced by defects introduced during processing. N2-fGNP was the most sensitive to alcohol vapors, demonstrating limits of detection below 10 ppm for ethanol and methanol vapors in dry air. Among the target fuel-related compounds, some selectivity was demonstrated. Sensors prepared from the O2-fGNP showed limited (∼20%) selectivity for isooctane over benzene; in contradistinction, the carboxyl-fGNP showed over 2x sensitivity for benzene over isooctane for identical exposure levels. The fluoroalkyl fGNP had an attenuated response to all analytes. The carboxyl-fGNP was particularly sensitive to the target refueling vapors with room temperature limits of detection below 50 ppbV for naphthalene and below 1 ppmV for benzene, in dry air.