Approaching\nto Calibration-Free Ion Detection Based\non Thin Layer Coulometry with Ultrathin Ion-Selective Membranes

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