High-Reproducibility and -Stability All-Solid-Contact Nitrate Ion-Selective Electrode with CoWSe₂ as Solid Contact for Nitrate Monitoring in Wetland Soil

The monitoring of nitrate ions is of great significance for human health, agricultural development, and environmental protection. All-solid-state nitrate ion-selective electrodes (ASS-NO₃^--ISEs), as an important NO₃^- analysis method, still have two challenges of poor stability and reproducibility due to the ill-defined phase boundary between the solid-contact (SC) layer and the ion-selective membrane (ISM). In this work, a novel strategy for constructing the ASS-NO₃^--ISEs based on CoWO₄, CoWS₄, CoWSe₂, or CoSe₂ as SC layers was reported for improving the stability and reproducibility. The result shows that the developed CoWSe₂-based NO₃^--ISE exhibits a good Nernstian response slope of -61.9 ± 0.4 mV dec^-1 in the activity range from 1.0 × 10^-6 to 7.5 × 10^-2 M and a detection limit of 1.0 × 10^-6 M. A good long-term stability (as low as 2.3 ± 0.4 μV h^-1) of the CoWSe₂-based NO₃^--ISE is the primary reason for the high redox capacitance of the ternary selenide. Experimental results show a surprisingly good reproducibility of approximately 0.5 mV for five individual ASS-NO₃^--ISEs. Notably, electrochemical experiments and scanning electron microscopy mapping tests are used to predict the ion-electron transduction mechanism in which the lipophilic anion (tetrakis(4-chlorophenyl)borate) participates in the transduction process at the SC/ISM interface to stabilize the electrode potential and provide high reproducibility. It was further proved that the introduction of CoWSe₂ as the SC layer maintains an excellent anti-interference to water layers, light, and gas. Hence, the CoWSe₂-based ASS-NO₃^--ISEs achieve accurate detection for free NO₃^- in wetland soil and the estuary of the Yellow River delta.