Highly Stable Solid Contact Calcium Ion-Selective Electrodes: Rapid Ion–Electron Transduction Triggered by Lipophilic Anions Participating in Redox Reactions of Cu_nS Nanoflowers

Abstract

Solid contact (SC) calcium ion-selective electrodes (Ca2+-ISEs) have been widely applied in the analysis of water quality and body fluids by virtue of the unique advantages of easy operation and rapid response. However, the potential drift during the long-term stability test hinders their further practical applications. Designing novel redox SC layers with large capacitance and high hydrophobicity is a promising approach to stabilize the potential stability, meanwhile, exploring the transduction mechanism is also of great guiding significance for the precise design of SC layer materials. Herein, flower-like copper sulfide (Cu n S-50) composed of nanosheets is meticulously designed as the redox SC layer by modification with the surfactant (CTAB). The Cu n S-50-based Ca2+-ISE (Cu n S-50/Ca2+-ISE) demonstrates a near-Nernstian slope of 28.23 mV/dec for Ca2+ in a wide activity linear range of 10–7 to 10–1 M, with a low detection limit of 3.16 × 10–8 M. Cu n S-50/Ca2+-ISE possesses an extremely low potential drift of only 1.23 ± 0.13 μV/h in the long-term potential stability test. Notably, X-ray absorption fine-structure (XAFS) spectra and electrochemical experiments are adopted to elucidate the transduction mechanism that the lipophilic anion (TFPB–) participates in the redox reaction of Cu n S-50 at the solid–solid interface of ion-selective membrane (ISM) and redox inorganic SC layer (Cu n S-50), thereby promoting the generation of free electrons to accelerate ion-electron transduction. This work provides an in-depth comprehension of the transduction mechanism of the potentiometric response and an effective strategy for designing redox materials of ion–electron transduction triggered by lipophilic anions.