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

Abstract

Calibration-free sensors are generally understood asanalytical tools with no need for calibration apart from the initialone (i.e., after its fabrication). However, an “ideal” and therefore“more restricted” definition of the concept considers that nocalibration is necessary at all, with the sensor being capable ofdirectly providing the analyte concentration in the sample. In theelectroanalysis field, investigations have been directed to charge-based readouts (i.e., coulometry) that allow for concentrationcalculation via the Faraday Law: The sample volume must beprecisely defined and the absoluteness of the electrochemicalprocess in which the analyte is involved must be ensured (i.e., theanalyte in the sample is ∼100% converted/transported). Herein, wereport on the realization of calibration-free coulometric ISEs basedon ultrathin ion-selective membranes, which is demonstrated for the detection of potassium ions (K+). In essence, the K+ transfer atthe membrane−sample interface is modulated by the oxidation state of the conducting polymer underlying the membrane. Theaccumulation/release of K+ to/from the membrane is an absolute process owing to the confinement of the sample to a thin-layerdomain (thickness of <100 μm). The capacity of the membrane expressed in charge is fixed to ca. 18 μC, and this dictates thedetection of micromolar levels of K+ present in ca. 5 μL sample volume. The system is interrogated with cyclic voltammetry to obtainpeaks related to the K+ transfer that can be treated charge-wise. The conceptual and technical innovative steps developed here madethe calibration-free detection of K+ possible in artificial and real samples with acceptable accuracy (<10% difference compared withthe results obtained from a current-based calibration and ion chromatography). The charge-based analysis does not depend ontemperature and appeared to be repetitive, reproducible, and reversible in the concentration range from 1 to 37.5 μM, with anaverage coulometry efficiency of 96%.