Self-Calibrated Ion-Selective Electrodes.

Abstract

Ion-selective electrode (ISE) potentiometry is reliable only if on-site calibration using a standard solution is performed before ion measurements. The complex device and operation required for calibration hinder the implementation of ISEs in decentralized sensing. Reported herein is a new type of ISE that is calibrated by a built-in component of the sensor without requiring any fluid handling processes. The indicator and reference electrodes are connected by a thin ionic conductor such as an aqueous phase containing the measuring ions in a capillary tube. This connection establishes a baseline electromotive force (EMF) that incorporates phase boundary potentials across multiple interfaces of the electrochemical cell and serves as a one-point calibration. Unlike conventional ISEs that rely on one EMF reading for each measurement, the proposed sensor utilizes a sample-induced EMF change relative to the baseline for each ion measurement. The variability in relative EMF is found to be <2.0 mV for multiple full potentiometric sensors consisting of plasticizer-based K+ ISEs and hydrogel-based Ag/AgCl reference electrodes. This value is significantly smaller than the variability of absolute EMF readouts in similar sensors without the self-calibration design. Moreover, when the ion-conducting calibration bridge has a low concentration of primary ions, low ion mobility, and/or a small contact area with the indicator and reference phases, it does not compromise the Nernstian response slope toward the analyte ions in the sample and therefore does not need to be removed for sample testing. The accuracy of the single-use self-calibrated K+ sensor in testing undiluted human blood samples is validated using a commercial blood gas analyzer as the reference method. Although this study focuses on disposable sensors consisting of tubes, the fluidics-free self-calibration strategy may be adapted to other sensor configurations such as planar sensors.