LAUSR

The performance criteria for the design of all-solid-state ion-selective electrodes mainly include high electrode-to-electrode reproducibility and a low potential drift. Here, we introduce nickel cobalt sulfide (NiCo2S4) as a solid contact for ion-to-electron transduction based on multiple redox couples. NiCo2S4 materials with different morphologies can be prepared through a facile hydrothermal/solvothermal method. A NiCo2S4-based solid-contact Ca2+-ISE has been developed, which exhibits a Nernstian slope of 27.5 ± 0.2 mV/dec in the activity range from 1.0 × 10-6 to 2.9 × 10-2 M with a detection limit of 5.0 × 10-7 M. A variation of the standard potential E° for eight individual solid-contact electrodes can be obtained as low as 0.35 mV. Due to the synergistic effect of cobalt and nickel ions in the ternary sulfide, an excellent redox capacitance (565 μF) of the buried solid contact coated with the ion-selective membrane can be achieved and is much larger than those obtained from other redox solid-contact materials reported so far, thus yielding a high potential stability of 2.2 ± 0.4 μV/h. In addition, the NiCo2S4-based solid-contact Ca2+-ISE shows a reduced water layer at the sensing membrane/NiCo2S4 interface and provides an excellent resistance to the interferences from light, O2, and CO2. The proposed strategy utilizing NiCo2S4 as a solid contact is a promising alternative for the fabrication of calibration-free ASS-ISEs.