Spectroscopic properties of thin layers of sulfamic acid-doped polyaniline and their application to reagentless determination of nitrite

Thin layers of sulfamic acid-doped polyaniline (abbreviated PANI-SFA) were prepared on quartz substrates by chemical polymerization using ammonium peroxydisulfate as an oxidizing agent. A specific feature of PANI-SFA is its reactivity towards nitrite anions, which results in significant changes in the UV–vis-NIR spectra of the PANI-SFA layer accompanied with a decrease of its conductivity. The chemical mechanism of these changes can be described as follows: for low nitrite concentration it involves the removal of the dopant via oxidation of amine nitrogen to N2 whereas for higher nitrite concentration an additional phenomenon such as reduction of polyaniline from its semi-oxidized (emeraldine) state to the fully reduced (leucoemeraldine) one takes place. The observed phenomena enabled us to use PANI-SFA layers as optodes for reagentless determination of nitrite. In buffer (pH 2) solutions of nitrites the relative absorbance of the PANI-SFA layer, at 816 nm, changes linearly with log Cnitrite. For nitrite concentrations exceeding 10−4 M determination based on the dc conductivity measurements is also possible. The optodes can easily be regenerated by treatment with 1 M sulfamic acid.