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

Abstract

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. 5 mV dec(-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±1.5 mV after 42 days).

Prussian Blue Analogues: A Versatile Framework for Solid‐Contact Ion‐Selective Electrodes with Tunable Potentials

“…The phase boundary potential , which forms upon distribution between the lipophilic membrane and the aqueous sample solution, follows the Nernst equation. It can therefore be used to determine the activity of . For single‐use sensors which can be used in point‐of‐care diagnostics or field analysis, however, the incorporation of an iRE of second kind with an IF is not viable.…”

“…For the redox control of K‐NiHCF, a part of the PBA was chemically reduced by ascorbic acid and subsequently mixed with pristine K‐NiHCF. Electrodes were prepared by lab‐scale techniques similar to those used in the research and production of lithium ion batteries . PBA and graphite powder as conducting additive were dispersed in a solution of PVdF in NMP.…”

“…For SC‐ISEs, PBA‐electrodes were glued onto ISM discs using a small amount of THF. The PBA‐ISM assembly was subsequently glued to a PVC tube in the same manner and contacted from inside the tube . For details of electrochemical experiments see SI.…”

“…It can therefore be used to determine the activity a I s of I. [4,5] For singleuse sensors which can be used in point-of-care diagnostics or field analysis, however, the incorporation of an iRE of second kind with an IF is not viable.…”

“…We therefore recently revisited intercalation compounds (ICs) as potential candidates to meet these challenges. [5] Similar to CPs, ICs provide a large redox capacity and exhibit a Nernstian potential response when interfacing a solution containing the intercalated cations and can thus be considered as electrodes of pseudo-first kind. Moreover, they exhibit welldefined intercalation potentials owing to the formation of discrete crystal phases.…”