Highly stable Li⁺ selective electrode with metal-organic framework as ion-to-electron transducer

Detection of Li⁺ ions is vital due to their applications as therapeutic drugs in medicine. In addition, finding Li⁺ in geothermal brines is gaining interest because of its application in energy storage systems. Monitoring Li⁺ levels in an aqueous environment can be achieved using chemical sensors. Solid-contact sensors (SCS) have attracted significant attention because of their portability, high sensitivity and lack of requirement of calibration. However, most solid-contact sensors suffer from low stability, especially in the long term. As a result, ion-to-electron transducers have been utilized to mitigate this problem. Recently Metal-Organic Frameworks (MOFs) have proven to be excellent candidates for use as ion-to-electron transducers in SCSs. In this study, we have used Ni-HAB MOF to produce a highly stable Li⁺ selective electrode. Increasing the thickness of the MOF to 3.28 µm enhanced the sensor's capacitance 100-fold leading to the lowest drift in Li⁺ SCSs reported in the literature, 1.15×10⁻⁶ mV/h with a low limit of detection (LOD) of 9.94×10⁻⁷M and a 57.6 mV/dec sensitivity. The sensor exhibited a linear output and a fast response time of less than 1 s. In addition, the sensor developed was used in a real brine to detect the concentration of Li⁺ ions, where the obtained results were in good agreement with the actual concentration of Li⁺ ions. This paper offers a solution for the persistent issues of solid-contact sensors such as drift, response time, and limit of detection and paves the way for the miniaturization of sensors to be used in real-life applications.