Tunable Ion-Sensing Using Coulometric-Based Protocols with Permselective Nanomembranes

Herein, we investigate all-solid-state ion-selective electrodes (ISEs) based on permselective nanomembranes (thickness ∼230 nm) in a coulometric mode. The detection of the potassium ion (K⁺) has been selected as proof of concept, implementing two electrochemical protocols based on the anodic and cathodic readouts of the same ISE. The electrode consists of an ITO glass substrate with the conducting polymer poly(3-octylthiophene) (POT) electrodeposited on it and a potassium-selective nanomembrane spin-coated over the POT layer. The K⁺ transfer at the membrane-sample interface is mediated by the redox activity of POT, which is in excess with respect to the dopant in the membrane (i.e., the anion part of the cation exchanger, R^-). In the cathodic protocol, the entry of the K⁺ into the membrane is promoted by the POT⁺ reduction to POT⁰; while in the anodic interrogation, first, K⁺ enters the membrane with a previous accumulation step, and then it is expelled during the oxidation of the POT⁰ to POT⁺. Both protocols were studied under linear sweep voltammetry and chronoamperometry, followed by signal integration to obtain the charge corresponding to K⁺. It is demonstrated that this charge is directly proportional to the K⁺ concentration in the bulk solution. We found two distinct response ranges: 3-20 μM in the cathodic protocol and 200-1000 nM in the anodic one. In addition, the cathodic coulometry strategy revealed excellent repeatability and reversibility within the linear range of response. The developed analytical approach demonstrates suitability in the quantification of real samples, i.e., human urine, horse serum, canal water, and standard KCl solution, while providing a linear and tunable coulometric response over a broad concentration range from the nanomolar to the micromolar level. Moreover, the sensor can be readily integrated into microfluidic devices, additionally offering the advantage of small sample volume requirements. The demonstrated reversibility, along with the ability to customize the ionophore in the membrane for an analysis of different ions, renders the proposed concept adaptable and exceptionally suitable for clinical analysis and environmental monitoring.