Solid-contact polymeric membrane ion-selective electrodes based on electrodeposited NiCo₂S₄ nanosheet arrays

In situ growth of quasi-superhydrophobic porous NiCo₂S₄ nanosheet arrays with a one-step electrodeposition method was provided. A calcium ion-selective electrodes (Ca²⁺-ISE) was subsequently constructed by using the prepared NiCo₂S₄ as a solid contact layer. The proposed Ca²⁺-ISE exhibits a good Nernstian slope of 30.7 ± 0.3 mV/dec and a detection limit of 1.6 × 10^-7 M. Due to the large redox capacitance of 1.8 mF, the Ca²⁺-ISE based on NiCo₂S₄ nanosheet arrays shows a high potential stability of 1.9 ± 0.5 μV/h over 90 h. Excellent reproducibilities for the NiCo₂S₄-based Ca²⁺-ISEs can be obtained with the single batch and batch-to-batch E° standard deviations of 0.3 (n = 6) and 0.7 mV (n = 5), respectively. The NiCo₂S₄ nanosheet arrays have a large contact angle of 148 ± 1.4^°, which effectively suppresses the formation of a water layer at the sensing membrane/NiCo₂S₄ interface.