Intercalation Compounds as Inner Reference Electrodes for Reproducible and Robust Solid-Contact Ion-Selective Electrodes

With billions of assays performed every year, ion-selective electrodes (ISEs) provide a simple and fast technique for clinical analysis of blood electrolytes. The development of cheap, miniaturized solid-contact (SC-)ISEs for integrated systems, however, remains a difficult balancing act between size, robustness, and reproducibility, because the defined interface potentials between the ion-selective membrane and the inner reference electrode (iRE) are often compromised. We demonstrate that target cation-sensitive intercalation compounds, such as partially charged lithium iron phosphate (LFP), can be applied as iREs of the quasi-first kind for ISEs. The symmetrical response of the interface potentials towards target cations ultimately results in ISEs with high robustness towards the inner filling (ca. 5mVdec(-1) conc.) as well as robust and miniaturized SC-ISEs. They have a predictable and stable potential derived from the LiFePO4/FePO4 redox couple (97.0 +/- 1.5mV after 42days).