Polyaniline composite coatings interrogated by a nulling optical-transmittance bridge for sensing low concentrations of ammonia gas

Polyaniline (PANI) is a pi -conjugated polymer that shows electric and optical property changes, when it is in contact with oxidant/reductor chemicals. In particular, the interaction between the conductive form of PANI, the emeraldine salt (ES), and the NH3 gas results in a decrease of the polaron densities inside the band gap of the organic material. In this work, the optical-transmittance changes detected in PANI coatings as a function of the sensing ammonia gas concentrations are studied by using a nulling optical-transmittance bridge with a 632 nm laser (He-Ne) source, which allows a sensitive monitoring of optical property changes. It is observed that the conductive PANI-polymethyl methacrylate (PMMA) composite coatings are sensitive for very low concentrations of HN3 (from similar to 10 to 4000 ppm, depending on the PANI films' electrical conductivity), and their optical response is reversible as the reduction gas is removed by N-2. (C) 2001 Elsevier Science B.V. All rights reserved.