Low frequency a.c. response of polypyrrole gas sensors

Electrically conducting organic polymers change their conductivity and relative permittivity when exposed to volatile chemicals [1.2]. The changes depend on the frequency, the concentration and the type of the chemical. In particular the frequency dependence could be used for making more selective sensors. A polypyrrole (PPY) film gas sensor was designed and constructed in order to probe it. The low frequency (20 Hz−10 KHz) a.c. response was studied when it was exposed to 200 ppm of four different vapours: methanol, acetone, ethyl acetate and ethanol. Response patterns to each vapour were obtained by varying the measurement frequencies. The pattern for each vapour investigated was very different from the others, and so it has been possible to discriminate between them. The results demonstrate that it is possible to increase the sensor selectivity using the low frequency a.c. response rather than d.c. resistance change. Ageing and temperature dependence of resistance and capacitance are also reported.