All-solid-state sulfide-ion sensor incorporating a supercritically water-treated Zr-doped LLTO ceramic reference electrode and Ag/Ag₂S working electrode

This study successfully fabricated high-performance zirconium-doped lithium lanthanum titanate (Zr-LLTO) ceramics exhibiting high relative density (>99 %) and high ionic conductivity (5.89 × 10⁻⁵ S/cm), using a powder synthesized via an elevated-temperature solid-state method as the raw material. To simulate the deep-sea environment featuring high temperature, high pressure, and corrosivity, the fabricated Zr-LLTO ceramics were treated in supercritical water (400 °C, 35 MPa) for structural and performance stability evaluation. Based on this stable ceramic, a novel all-solid-state reference electrode suitable for electrochemical measurements in high-temperature, high-pressure marine environments was constructed. Meanwhile, a dense Ag₂S film (Ag/Ag₂S) was grown in situ on silver wires by hydrothermal sulfidation, significantly enhancing the film uniformity and substrate adhesion. This Ag/Ag₂S electrode maintained structural stability after supercritical water treatment. A fully solid-state S²⁻ sensor was fabricated by integrating the Zr-LLTO reference electrode with the Ag/Ag₂S working electrode. The sensor exhibits rapid response, high sensitivity, and a wide linear detection range (10⁻⁶ to 1 mol/L, R² ≥ 0.96) in simulated seawater, effectively overcoming interference issues of traditional Ag/AgCl reference electrodes in high‑sulfur environments. This system offers a novel and robust solution for in situ sulfide monitoring in deep-sea extreme environments, offering high environmental adaptability, simple fabrication, and reliable performance.