Multi-frequency measurements of organic conducting polymers for sensing of gases and vapours

Electrically conducting organic polymers display electrical conductivities that are dependent on the concentration of dopant ions incorporated in the material. The conductivity, usually measured using direct current (dc) techniques, may be modulated reversibly and rapidly at ambient temperature by adsorption and desorption of volatile chemicals. This phenomenon has immense practical use for gas and odour sensing. By using arrays of conducting polymer sensors having broadly overlapping specificity to a range of volatiles, we are able to measure and assign descriptors to the volatiles. In this paper we show that similar descriptors can be generated by using alternating current (ac) at suitable frequencies to follow the changes in sensor capacitance, conductance and dissipation factor. We also show that using a single sensor we can obtain discrimination between chemical species. The ac response characteristics of these sensors have been modelled using a simple electrical circuit equivalent and we show that the model is a good predictor of sensor performance.