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

Abstract

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.