Optical pH Measurements Based on UV-VIS and Raman Spectroscopy with Water Dispersion of Polyaniline Nanoparticles

A new method for optical pH measurements with a commercially available water dispersion of polyaniline (PANI) nanoparticles (mean particle size: 46 nm) is presented [1,2]. The pH measurements are based on the acid-base equilibrium of PANI and were carried out either by combining the automated sequential injection analysis system (SIA) and UV-vis or Raman spectroscopy (excitation wavelength : 633 nm). The detection was done in continuous mode at 800 nm by using the SIA technique for transporting the sample to light path of the UV-vis or Raman instrument. With a fibre-optic lightguide, the UV-vis detection was done in batch mode at 400 nm and 580 nm. In both methods, fresh pH reagent (PANI) solution was used in each measurement, thus overcoming the problem with hysteresis (memory effect), which is usually observed with PANI films. Fast pH measurements can be done with the PANI nanoparticles between pH 6-10.5 depending on the measuring technique.

The nanoparticles were characterized with UV-vis spectroscopy in pH buffer solutions between pH 2-12 and a protonation constant of log K =4.4 was calculated from these data. The protonation constant provide important information about the pH sensitivity of different types of polyaniline and can therefore be used for choosing right PANI materials for specific applications where either pH sensitivity or insensitivity is required. The acid-base equilibrium is established entirely in the solution phase. The protonation equilibrium is therefore established faster in small nanoparticles than in films, which usually have a thickness of at least several hundreads of nanometers.

Potential application areas of the presented optical pH measurement methods are e.g. automated flow analysis systems where the application of the glass pH electrode is difficult, determination of pH under elevated pressures and pH measurements of sea water where the sodium error may limit the use of the glass pH electrode. The limitations of the absorbance measurements of real samples should, however, be considered when developing new optical methods. It should be stressed, that this is a fundamental study showing the applicability of PANI nanoparticles in optical pH measurements.

[1] www.zipperling.de (D1012W-1)
[2] T. Lindfors, L. Harju, A. Ivaska, Anal. Chem., submitted 12/2005