Application of Raman spectroscopy and sequential injection analysis for pH measurements with water dispersion of polyaniline nanoparticles

A novel method for pH measurements between pH 7.5 and 10.4 is reported in this paper. The method combines Raman spectroscopy and the automated sequential injection analysis system (SIA) and makes use of the acid-base properties of a commercially available water dispersion of polyaniline (PANI) nanoparticles with a mean particle size of 46 nm. The useful pH range of the PANI nanoparticles is broader than for conventional acid-base indicators, due to the 1:2 reaction stoichiometry of the proton-ligand complex of the nanoparticles. The pH measurements were conducted with the 633-nm laser excitation wavelength by calculating the difference between the Raman intensities of the primary and reference wavenumbers. In this study, the pH-sensitive CHCH stretching band at 1422 cm(-1) and C-H in-plane bending band of the quinoid form at 1163 cm(-1) were chosen as the primary wavenumbers. The presented method is fast and allows pH to be measured with a precision of 0.2 pH unit. The advantage of the proposed method is that the hysteresis effect, which is usually observed for PANI films, can be overcome with the PANI nanoparticles, because a fresh nanoparticle solution is used in each measurement. It should be pointed out, that this work is a fundamental study showing the applicability of Raman spectroscopy in combination with the SIA technique for pH measurements in specific applications, where the conventional glass pH electrode cannot be used.