Nitrogen-Doped Porous Carbon Derived from Zeolitic Imidazolate Framework-67 Strung into Necklace with Carbon Nanotubes for the Detection of Calcium at Ultralow Level

Zeolitic imidazolate frameworks (ZIFs) are vital branch of metal-organic frameworks (MOFs) that have attracted plentiful interest due to their variable structures and easy-synthesis characteristics. Research on ZIFs and their derived nanostructures for electrochemical applications is increasing. Nitrogen-doped porous carbons (NPCs) derived from ZIFs have been widely used in electrochemical catalysis and energy storage. However, there is limited information on their use as potentiometric sensors. In the present work, ZIFs-derived NPCs strung into necklace with multi-walled carbon nanotube (MWCNT/NPC) were synthesized and used as an effective solid contact for calcium ion-selective electrode (ISE). The synthesized composites showed good uniformity, a high capacitance and improved electrical conductivity. Sensors fabricated from these composites displayed a very short response time (2–3 s) and a low detection limit (1.49 nM) for calcium. When the amount of calcium in untreated milk was analysed by the sensor, sensitive and accurate results were obtained. Additionally, studies of stability, reproducibility, and selectivity also confirmed that the prepared MWCNT/NPC composite was an efficient transducer, and ideal for a potentiometric sensor. This study advances the development of ISE-based sensors by improving the detection limit and provides a technical pathway for directly detecting calcium in untreated milk.