Approaching to Calibration-Free Ion Detection Based on Thin Layer Coulometry with Ultrathin Ion-Selective Membranes

In pursuit of calibration-free all-solid-state ion-selective electrodes (ISEs), we propose a coulometry strategy based on thin-layer samples confined adjacent to the ion-selective membrane (ISM) surface, with the system being controlled under a cathodic potential sweep. The ion-to-electron transducer in the ISE is the conducting polymer poly(3-octylthiophene) (POT), the oxidation state of which changes upon the application of a cathodic sweep and triggers the accumulation of the preferred cation in the ISM. This accumulation is provided of absolute nature (i.e., the cation concentration is totally depleted in the sample) when the capacity of the membrane encompasses the charge of the cation of interest in the sample (K⁺ in this case). As such, the ion exchanger content of the ISM is fixed to 18 μC, being able to accumulate a K⁺ concentration from the solution in the range of 5-40 μM. The charge transfer in the POT film ultimately leads to the K⁺ transfer at the ISM-sample interface, depleting its content in the thin-layer sample with demonstrated efficiency (∼100% at 5 and 1 mV s^-1). The charge is directly proportional to the corresponding concentration via the Faraday law, constituting the core principle of the calibration-free approach. In essence, there is no need of calibrating the sensor, because the K⁺ concentration can be obtained from the charge by knowing the sample volume with certain precision (volume of 5 μL, with the sample thickness being 100 ± 5 μm). The conceptual innovation introduced in this Letter is accompanied by the validated calibration-free detection of K⁺ in five real samples, demonstrating the plausibility of the approach to contribute to the measurement science field, especially in the direction of fulfilling the gap between benchtop trials and the end users of electrochemical sensors. It is key to put efforts into calibration-free sensors to address real world applications such as point-of-care, wearable sensors for well-being, and environmental in situ monitoring, among others.