Understanding the sensing mechanism of polyaniline resistive sensors. Effect of humidity on sensing of organic volatiles

Polyaniline (PANI) is the oldest and potentially one of the most useful conducting polymers. Among other technological applications, PANI films have been extensively used as resistive sensors of volatile analytes. However, at present the mechanism underlying the resistivity changes of the films upon exposure to volatile substances is still unclear. In this work, we investigate different effects of the volatile absorption which can cause the changes polymer electrical resistance when it acts as resistive sensor. This model takes into account three major components: (i) changes in the electronic structure of the polymeric chains (ΔRele), (ii) variations in the electron hopping process (ΔRhop) and (iii) changes in the ionic conductivity between chains due to changes in the dielectric medium between them (ΔRmed). Using two point probe resistivity, UV–visible spectroscopy, environmental ellipsometric porosimetry and atomic force microscopy we study the effect of volatiles on polyaniline films properties to improve the understanding on the mechanism of resistivity changes. Specifically, in the case of water absorption, the resistivity changes seem to be associated with dielectric medium changes and swelling effects (in the high humidity range). It is found that ambient humidity not only gives a resistive signal but also strongly affect the sensing of other volatiles (e.g. ethanol).