Enhanced single-frequency effective capacitance C sensing for solid-contact ion-selective electrodes

In this study, a single-frequency effective capacitance ( C ec ) readout was further explored as a simplified sensing strategy for poly(3,4–ethylenedioxythiophene) PEDOT-based glassy carbon electrodes (PEDOT/GC) and K + -selective solid-contact ion-selective electrodes (K + -SCISEs). The C ec of both systems increased as frequency decreased from 1 MHz to 10 mHz. A linear logarithmic effective capacitance with respect to logarithmic of applied single frequency ( logC ec – logf ) relationship was observed at high frequencies (1 MHz–10 Hz), indicating dominant double layer and bulk capacitive behavior. Ionic strength strongly influenced C ec at high frequencies, while the bulk capacitance of PEDOT prevailed below 1 Hz. Reduced low-frequency capacitance occurred at low electrolyte concentrations. PEDOT/GC in 0.1 M KCl exhibited near-ideal capacitive behavior at low frequencies (1 Hz–10 mHz), with a logC ec – logf slope of −0.016. For K + -SCISEs, the slope in the same range increased from −0.4 to −1 as the valinomycin-doped PVC membrane thickened, reflecting increased diffusion impedance. The single-frequency C ec method provides direct correlation with AC potential, offering a simplified and precise readout for SCISEs. Low-frequency C ec values at 10 mHz of PEDOT/GC were approximately 55 μF, 220 μF, and 420 μF for 1 mC, 5 mC, and 10 mC PEDOT films, respectively, with high stability (e.g., 0.27–1.36% standard deviation). Although efficient, the method still requires for a potentiostat capable of accurate impedance measurements within frequency ranging from 100 Hz to 10 mHz.