Coulometric ion sensing with Li⁺-selective LiMn₂O₄ electrodes

A coulometric signal readout method, which was originally developed for solid-contact ion-selective electrodes, was investigated in this work using LiMn₂O₄ (LMO) as a combined ion-recognition and signal-transduction layer. The redox process of LMO, which is associated with reversible intercalation/expulsion of Li⁺ ions, allowed coulometric sensing of Li⁺ ions in aqueous solutions. On increasing the active area (mass loading) of LMO, the coulometric signal increased for a given change in Li⁺ ion activity. The excellent redox reversibility of LMO and its relatively low resistance were instrumental in achieving a high signal amplification together with a relatively fast response. Coating the LMO layer with a conventional Li⁺-selective plasticized PVC membrane was found to dramatically lower the coulometric response. Hence, the application of LMO as a combined Li⁺-selective electrode material and ion-to-electron transducer was found to be highly compatible with the coulometric signal readout method, especially for detecting small Li⁺ activity changes at high Li⁺ concentrations.