Optical sensing of pH based on polypyrrole films

An optical sensor for pH is presented which is based on the use of a thin polypyrrole (PPy) film obtained by chemical oxidation of pyrrole and deposited as a < 1 μm film on the walls of a polystyrene cuvette. The spectrum of PPy displays strong absorption in the near infrared at around 900 nm, and a characteristic minimum at around 550 nm. The shape and intensity of the spectrum are pH-dependent between pH 6 and 12, the minimum being shifted to shorter wavelengths at higher pH, and the absorbance of the longwave band increasing in the range 600–900 nm range. The apparent pKa is around 8.6, but once exposed to a pH 6, the films need to be reconditioned with 0.1 M HCl in order to give the same response to pH. The films are an interesting alternative to indicator-based pH sensor films in that they do not require a dye to be immobilized, are fully compatible with LED and diode laser light sources, and can easily be prepared.