OPTICAL SENSING OF PH BASED ON POLYPYRROLE FILMS

An optical sensor for pH is presented which is based on the use of a t hin polypyrrole (PPy) film obtained by chemical oxidation of pyrrole a nd deposited as a <1 mu m 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 sha pe and intensity of the spectrum are pH-dependent between pH 6 and 12, the minimum being shifted to shorter wavelengths at higher pH, and th e absorbance of the longwave band increasing in the range 600-900 nm r ange. The apparent pK(a) is around 8.6, but once exposed to a pH 6, th e films need to be reconditioned with 0.1 M HCl in order to give the s ame response to pH. The films are an interesting alternative to indica tor-based pH sensor films in that they do not require a dye to be immo bilized, are fully compatible with LED and diode laser light sources, and can easily be prepared.