Application of Raman Spectroscopy and Sequential Injection Analysis for pH Measurements with Water Dispersion of Polyaniline Nanoparticles

Abstract

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 CH=CH 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.

Tuning the Interfacial Chemistry of Redox‐Active Polymer for Bifunctional Probing

“…Since these anions have dissimilar sizes, charges and mobility, such polymeric films are expected to exhibit different conductivity with respect to the nature of the anion. This further demonstrates that surrounding media can modulate PANI's redox energy . The vibrations corresponding to the wavenumber regions 1066 cm −1 , 1130 cm −1 and 990 cm −1 correspond to perchlorate, sulphate and phosphate respectively .…”

“…A minor peak pair observed in the mid potential region is attributed to defects in the polymeric backbone by many researchers . It is well known that ES is the conducting and EB is the non‐conducting forms of PANI and the surrounding environment can tune their redox energy . For example, an acidic pH favor the formation of ES state whereas alkaline pH favor the formation of non‐conducting EB state owing to the protonation/deprotonation of nitrogen backbone in PANI with concomitant anion insertion/deinsertion reactions to maintain charge neutrality .…”

“…[21][22][23][24] It is well known that ES is the conducting and EB is the non-conducting forms of PANI and the surrounding environment can tune their redox energy. [25,26] For example, an acidic pH favor the formation of ES state whereas alkaline pH favor the formation of non-conducting EB state owing to the protonation/deprotonation of nitrogen backbone in PANI with concomitant anion insertion/ deinsertion reactions to maintain charge neutrality. [27][28][29] This suggests that redox transition in PANI is pH dependent and the Nernst equation for such a reaction predicts an electrode potential shift of 59 mV per change in pH.…”

State Key Laboratory of Nonlinear Mechanics (LNM), Institute of Mechanics, Chinese Academy of Sciences, Beijing 100080, China

“…A novel method for pH measurements between pH 7.5 and 10.4 with a precision of 0.2 pH unit was reported in [22]. The method combines Raman spectroscopy and the automated sequential injection analysis system (SIA) and makes use of the acidbase properties of a commercially available water dispersion of PANI nanoparticles with a mean particle size of 46 nm.…”

“…The method combines Raman spectroscopy and the automated sequential injection analysis system (SIA) and makes use of the acidbase properties of a commercially available water dispersion of PANI nanoparticles with a mean particle size of 46 nm. 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, where the pH-sensitive CH=CH 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 [22]. Lindfors et al [23, 24, 25, 26] conducted in-depth fundamental studies of optical pH sensitivity of PANI conductive polymers and the research is ongoing.…”