Detection of ammonia, 2,4,6-trinitrotoluene, and common organic vapors using thin-film carbon black-metalloporphyrin composite chemiresistors

Abstract

Thin-film chemiresistive composites of octaethylporphine-based transition-metal complexes (Ph(M), M = Co, Cu and Zn) and carbon black (CB) have been fabricated and tested as chemical vapor sensors. The sensing performance of such sensor composites was compared to the sensing performance of composites of metallophthalocyanines (Phtc(M)) and CB. The relative differential resistance response of Ph(M)/CB sensor films upon exposure to organic vapors, such as n-hexane, n-heptane, n-octane, iso-octane, cyclohexane, toluene, ethyl acetate and ethanol, was dependent on the nature of the metal center. An array of chemiresistive Ph(M)/CB vapor sensors therefore provided discrimination between the organic vapor analytes that had different polarities, specifically classifying non-polar vapors, aprotic polar vapors and protic polar vapors. However, discrimination was not observed for analytes that had mutually similar polarities. The Ph(M)/CB sensors showed reversible responses toward ammonia, NH_3(g), at concentrations below the 8 h permissible exposure level (50 ppm). Ph(M)/CB composites exhibited a slightly larger resistance response than Phtc(M)/CB composites, consistent with the Ph(M) species having less π-stacked molecular aggregates, resulting in an increase in the number of adsorption sites relative to the Phtc(M)/CB composites. Resistance responses with a signal-to-noise ratio value of ∼900 were obtained upon exposure to vapor pulses saturated with 2,4,6-trinitrotoluene.