Tuning the optical sensing of pH by poly(diphenylamine)

Thin films of poly(diphenylamine) (PDPA) and copolymers of diphenylamine (DPA) with aniline were deposited on indium tin oxide (ITO) coated glass employing constant-potential electropolymerization. Polymer films were characterized for optical transitions by UV-Visible (UV-Vis) spectroscopy. Though PDPA is one of the substituted poly(aniline)s (PANIs), distinct differences could be noticed in the optical characteristics of PDPA and copolymers in contrast to PANI. The presence of diphenoquinone imine structures in PDPA-based polymers cause clear variations in the pH ranges in which protonation/deprotonation of imine units of polymer occur. Optical characteristics of PDPA and the copolymers were followed in a wide pH range (2–13) using UV-Vis spectroscopy, and titration curves were developed. PDPA and the copolymers show pH-dependent optical variations (titration curves) which are quite different from PANI. The pH sensitivity, reproducibility, and response time of PDPA and the copolymers were evaluated and compared. PDPA showed a lower response time (7 s) than PANI.