Composite and free standing PANI-PVA membranes as flexible and stable optical pH sensors

Composite free standing membranes made by solvent casting of polyaniline (PANI) synthesized in the presence of water-soluble polyvinyl alcohol (PVA) as a steric stabilizer, were deposited on solid substrates to yield homogeneous and porous membranes. The acid used to dope PANI, either hydrochloric acid or formic acid, has a marked influence on the size distribution of PANI and the morphology as well as the conductivity of the composite PANI-PVA membranes. The absorption spectra of the membranes obtained using formic acid during the synthesis were sensitive to pH changes which allowed to design a dual wavelength (603 nm and 845 nm) optical pH sensor. The membranes displayed a marked hysteresis during the pH 12 to pH 2 titration with respect to the titration performed from acid to basic solutions. The effect of this hysteresis could be suppressed by prolonged incubation in a solution at pH 2. The second acid to basic titration curve was then identical to the first one. In addition to the optical response of the composite, the PANI-PVA membranes displayed excellent time stability, allowing to use such membranes as stable and reusable pH meters not only in solution but also in moist gas phases.