Tuning the Interfacial Chemistry of Redox-Active Polymer for Bifunctional Probing

Redox-active conducting polymers are ubiquitous in sensing devices, owing to their good conductivity, ease of synthesis, and processability. The necessity of proton transfer during their redox energy modulations projected them as universal pH sensors, owing to Nernstian dependence of electromotive force (EMF) on the pH. Here, we show that the pH-dependent Nernstian EMF of redox-active conducting polymeric interfaces becomes universal only with a cation-selective ionomer cover, without which the interface transforms depending on the chemistry of the conjugate base, owing to the dependency of the polymer's redox energy on the nature of the conjugate base. For an uncovered redox-active polymer, galvanostatic intermittent titration (GITT) and electrochemical quartz crystal microbalance (EQCM) studies demonstrate a clear distinction in the kinetics of psuedocapacitive charge injection between spherical and tetrahedral anions, leading to a multifunctional ion-sensing probe. Nonetheless, for a polymer with a cation-selective ionomer cover, GITT and EQCM measurements demonstrate a unified response, irrespective of the nature of the conjugate bases, owing to rectified cationic transport, leading to a universal pH sensor. A bifunctional ion-sensing meter is constructed by coupling the redox-active polymer probe with a non-polarizable interface and it can signal the end user the cation concentration and the type of conjugate base in a single measurement.