The influence of non-specific molecular partitioning of analytes on the electrical responses of conducting organic polymer gas sensors

A systematic study is presented of the steady-state electrical resistance responses of a variety of conducting polymer gas sensors to 34 volatile organic chemicals comprising alcohols, esters, aromatics and alkanes. The sensors were prepared from alkyl substituted polypyrroles and a polyalkylterthiophene: poly(3′,3″-didecyl-2′,2″:5′,2″- terthiophene), doped with a variety of counteranions. Relationships between the vapour concentration producing a fixed amplitude of sensor response (Cg) and analyte saturated vapour pressure (SVP) have been investigated both theoretically and experimentally. Homologous analytes within the investigated classes displayed linear correlations between Cg and SVP indicating that volatile organic compound (VOC) partition and signal transduction in these cases are non-specific processes. In the case of alcohols it was found that responses to branched primary, secondary and tertiary species generally correlate closely with the straight chain primary series, indicating that the sensors studied show little shape or size selectivity for non-homologous members of this analyte class. In the case of the polyalkylterthiophene material, the responses to heteroatom-containing aromatic compounds did not correlate closely with hydrocarbon-only aromatics, suggesting the occurrence of selective π-π interactions. The apparent sensitivity order followed alcohols > esters > aromatics ≈ alkanes, which is consistent with broad trends in analyte polarity.