Disposable Screen-Printed Ca2+ Sensor Using NH2-MIL-125@rGO as Solid-Contact Layer for Raw Milk Analysis

Solid-contact ion-selective electrodes have been extensively utilized for calcium ion detection, particularly in applications involving milk samples. In this study, a solid-contact was developed using a composite material of Ti8O8(OH)4(H2bdc-NH2)6 (NH2-MIL-125) and reduced graphene oxide (rGO) to prepare calcium SC-ISE (NH2-MIL-125@rGO/Ca2+-ISE). The NH2-MIL-125@rGO/Ca2+-ISE demonstrated high double-layer capacitance (222 μF) and rapid electron transfer capabilities. Within the calcium ion concentration range of 10−7~10−1 M, the electrode exhibited an excellent Nernstian response, with a rapid response time (2~3 s) and a low detection limit of 10−7.2 M. Additionally, the electrode performed exceptionally in water layer and anti-interference tests, showing enhanced potential stability and response speed while being unaffected by environmental factors such as light, CO2, and O2. These results confirm the superior performance of the NH2-MIL-125@rGO composite as a solid-contact layer, therefore constructed disposable screen-printed calcium potentiometric sensor. In particular, it demonstrated the capability for rapid and accurate analysis of calcium content in untreated milk samples, highlighting the significant practical potential for development of calibration-free SC-ISEs.