pH-Sensing Properties of Poly(aniline) Ultrathin Films Self-Assembled on Indium−Tin Oxide

Abstract

The structural and functional properties of ultrathin (1, and (PANI/PAA)₂ films was examined using both potentiometry and ATR spectroscopy. Near-Nernstian potentiometric responses over pH 3−9 were observed for all three types of films, consistent with the weak acid−base behavior expected of polymers assembled in a LbL film. The ATR spectral sensitivity to pH increases as the number of layers in the film increases, with the highest sensitivity achieved by monitoring the absorbance at 800 nm (predominately due to the emeraldine salt form) of (PANI/PAA)₂ films. Codeposition of PANI and PAA appears to produce a wide distribution of strengths of acidic and basic sites in the film and thus a large linear dynamic range, up to six pH units. The water contact angle of (PANI/PAA)₂ is ∼16°, which is considerably more hydrophilic than either the PANI single layer or (PANI/PAA)₁ films (∼40°). This film is shown to be a suitable substrate for deposition of a planar supported phospholipid bilayer. The supported membrane is highly impermeable to protons, which makes this architecture useful for monitoring transmembrane charge transport.