Solid-Contact Ion Sensing Without Using an Ion-Selective Membrane through Classic Li-Ion Battery Materials

The solid-contact ion-selective electrodes (SC-ISEs) are a type of potentiometric analytical device with features of rapid response, online analysis, and miniaturization. The state-of-the-art SC-ISEs are composed of a solid-contact (SC) layer and an ion-selective membrane (ISM) layer with respective functions of ion-to-electron transduction and ion recognition. Two challenges for the SC-ISEs are the water-layer formation at the SC/ISM phase boundary and the leaking of ISM components, which are both originated from the ISM. Herein, we report a type of SC-ISE based on classic Li-ion battery materials as the SC layer without using the ISM for potentiometric lithium-ion sensing. Both LiFePO4- and LiMn2O4-based SC-ISEs display good Li+ sensing properties (sensitivity, selectivity, and stability). The proposed LiFePO4 electrode exhibits comparable sensitivity and a linear range to conventional SC-ISEs with ISM. Owing to the nonexistence of ISM, the LiFePO4 electrode displays high potential stability. Besides, the LiMn2O4 electrode shows a Nernstian response toward Li+ sensing in a human blood serum solution. This work emphasizes the concept of non-ISM-based SC-ISEs for potentiometric ion sensing.