Capacitive Coulometric Readout of Polyaniline Membrane-Based pH Sensors in Combination with Cyclic Voltammetry and Electrochemical Impedance Spectroscopy

In this study, a polyaniline (PANI)-based solid-contact pH sensor was fabricated, and its amperometric and coulometric response was investigated both without and in series with capacitors (10 and 47 µF). The conducting polymer PANI membrane was electropolymerized on the electrode surface to serve as an ion-to-electron transducer. The amperometric and coulometric performance of the PANI-based sensor in series with a capacitor (10 µF) was reduced to the order of seconds, and the cumulated charge <i>Q</i> was standardized, significantly minimizing the influence of applied potential. Electrochemical impedance spectroscopy, constant potential coulometry, and cyclic voltammetry demonstrated that a larger low-frequency capacitance corresponds to a greater cumulated charge, reflecting the doping level of the electropolymerized PANI membrane. The growth of the PANI membrane, represented by charge <i>Q</i>, increased exponentially with the number of polymerization cycles, following a power-law relationship with exponents (α) of 2.14 (1-25 cycles) and 2.97 (30-100 cycles), consistent with a transition from a layered (10 cycles) to a porous morphology (50 cycles). Furthermore, a linear dependence of cumulated charge <i>Q</i> on pH was observed, demonstrating that capacitive coulometric readout offers a promising and practical approach for wearable ion sensors.