Voltammetric Ion-Selective Electrodes in Thin-Layer Samples: Absolute Detection of Ions Using Ultrathin Membranes

Calibration-free sensors are generally understood as analytical tools with no need for calibration apart from the initial one (i.e., after its fabrication). However, an "ideal" and therefore "more restricted" definition of the concept considers that no calibration is necessary at all, with the sensor being capable of directly providing the analyte concentration in the sample. In the electroanalysis field, investigations have been directed to charge-based readouts (i.e., coulometry) that allow for concentration calculation via the Faraday Law: The sample volume must be precisely defined and the absoluteness of the electrochemical process in which the analyte is involved must be ensured (i.e., the analyte in the sample is ∼100% converted/transported). Herein, we report on the realization of calibration-free coulometric ISEs based on ultrathin ion-selective membranes, which is demonstrated for the detection of potassium ions (K⁺). In essence, the K⁺ transfer at the membrane-sample interface is modulated by the oxidation state of the conducting polymer underlying the membrane. The accumulation/release of K⁺ to/from the membrane is an absolute process owing to the confinement of the sample to a thin-layer domain (thickness of <100 μm). The capacity of the membrane expressed in charge is fixed to ca. 18 μC, and this dictates the detection of micromolar levels of K⁺ present in ca. 5 μL sample volume. The system is interrogated with cyclic voltammetry to obtain peaks related to the K⁺ transfer that can be treated charge-wise. The conceptual and technical innovative steps developed here made the calibration-free detection of K⁺ possible in artificial and real samples with acceptable accuracy (<10% difference compared with the results obtained from a current-based calibration and ion chromatography). The charge-based analysis does not depend on temperature and appeared to be repetitive, reproducible, and reversible in the concentration range from 1 to 37.5 μM, with an average coulometry efficiency of 96%.