Self-Calibrated Ion-Selective Electrodes

Ion-selective electrode (ISE) potentiometry is reliableonly ifon-site calibration using a standard solution is performed beforeion measurements. The complex device and operation required for calibrationhinder the implementation of ISEs in decentralized sensing. Reportedherein is a new type of ISE that is calibrated by a built-in componentof the sensor without requiring any fluid handling processes. Theindicator and reference electrodes are connected by a thin ionic conductorsuch as an aqueous phase containing the measuring ions in a capillarytube. This connection establishes a baseline electromotive force (EMF)that incorporates phase boundary potentials across multiple interfacesof 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 relativeto the baseline for each ion measurement. The variability in relativeEMF is found to be <2.0 mV for multiple full potentiometric sensorsconsisting of plasticizer-based K+ ISEs and hydrogel-basedAg/AgCl reference electrodes. This value is significantly smallerthan the variability of absolute EMF readouts in similar sensors withoutthe self-calibration design. Moreover, when the ion-conducting calibrationbridge 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 analyteions in the sample and therefore does not need to be removed for sampletesting. The accuracy of the single-use self-calibrated K+ sensor in testing undiluted human blood samples is validated usinga commercial blood gas analyzer as the reference method. Althoughthis study focuses on disposable sensors consisting of tubes, thefluidics-free self-calibration strategy may be adapted to other sensorconfigurations such as planar sensors.