Frequency-resolved characterization of thin-layer solid-contact ion-selective electrodes: Deconvoluting electron and ion transfer with AC voltammetry

This work investigates the coupled electron-ion transfer (ET-IT) phenomena occurring in solid-contact ion-selective electrodes based on a poly(3-octylthiophene) (POT) film backside contacted with a plasticized nanomembrane through cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and alternating current voltammetry (ACV). The CVs presented the typical thin-layer behavior, whereas EIS revealed distinct regions corresponding to membrane resistance, charge transfer, and capacitive responses, allowing for the separation and quantification of the ET-IT contributions. The analysis by electrical equivalent circuit demonstrated that, as the potential approaches the redox peak, the membrane resistance markedly increases from 2.6 kΩ to 40 kΩ due to ion expulsion. Charge transfer resistance followed the "v"-shaped dependence expected for thin-layer systems, with a minimum at 0.86 kΩ. Time constant analysis confirmed that ionic dynamics were significantly faster than electron transfer processes. Building on this insight, ACV was used to bridge the spectral deconvolution capability of EIS with the simplicity of CV. The ACV responses were found to strongly depend on frequency. At low frequencies (<10 Hz), the signals were dominated by faradaic ET-IT peaks; while at higher frequencies (>250 Hz), the response transitioned to a sigmoidal profile governed by ion transport and membrane resistance. This frequency selectivity enabled distinguishing between ions of different lipophilicity (e.g., Na⁺ vs. TBA⁺) in membranes exclusively containing a cation exchanger, demonstrating ion-specific peak shifts when using ionophore-based membranes (e.g., valinomycin for K⁺). Finally, the possibility of determining signal changes with variations of the K⁺ concentration in the nanomolar range by employing CV and ACV, together with a comparison of the sensing performance with both techniques, was demonstrated.