Original Paper Counter-Ion Influence on Polypyrrole Potentiometric pH Sensitivity

TLDR: In this paper, the effect of doping and supporting electrolyte anions' effect on the potentiometric sensitivity and response time of polypyrrole (PPy) electrodes towards changes of solution pH were studied.

Abstract: The pH sensitivity of conducting polymer films is an important issue from the sensor design point of view. The doping and supporting electrolyte anions' effect on the potentiometric sensitivity and response time of polypyrrole (PPy) electrodes towards changes of solution pH were studied. It was found that (i) the response of PPy doped by easily exchangeable com- mon anions (Cl � ,N O 3 � , ClO4 � ) in their solutions (KCl, KNO3, NaClO4) is slow. In contrast, (ii) poly- pyrrole films deposited in the presence of weak acid anions (phthalates, oxalates, salicylates) were charac- terised by instantaneous responses in the above men- tioned solutions. On the basis of electrochemical experiments (open circuit potential vs. time dependen- cies, cyclic voltammetry, EQCM), the observed differ- ences were attributed to different mechanisms of pH sensitivity of tested films. The long response times are related to the incorporation of the solution ions into the film in order to compensate charges created due to protonation. On the other hand, if the ion-exchange is hindered as in the case of (ii), instantaneous open circuit responses are observed due to polarisation of the oxidised polymer layer, analogously to the metal elec- trode. Moreover, for these films the internal pH buffer- ing within the polymer membrane will weaken the pH change effect. The mechanisms were confirmed in the course of studying the pH effect in solutions containing anions easily (KCl, NaClO4, KNO3) or hardly exchangeable with polypyrrole (K2SO4, sodium poly(4-styrenesul- phonate) solutions) acidified with H2SO4.