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. (C) 2013 Elsevier B.V. All rights reserved.