Materials challenges in solid-state sensors for continuous monitoring of ions in water.

Accurate detection of ions in aqueous environments, ranging from trace to high concentrations, is essential for monitoring natural water resources, treatment facilities, and wastewater systems. Solid-state sensors have emerged as versatile platforms for this purpose due to their adaptability in geometry, compatibility with electronic integration, portability, and low energy requirements. A wide variety of active materials have been investigated, including metal oxides, graphene, carbon nanotubes, silicon nanowires, AlGaN/GaN, MXenes, transition metal dichalcogenides, and organic polymers all of which can be tailored for sensor development. Sensor selectivity and functionality can be enhanced through strategies such as defect engineering, nanoparticle doping, surface functionalization with organic molecules, or incorporation of advanced recognition elements like metal-organic frameworks, covalent organic frameworks, ion-imprinted polymers, and biomaterials. Hybrid nanocomposites and ion-selective membranes further expand the design space, enabling customized performance for specific applications. This review systematically examines the materials employed in solid-state ion sensors, their transduction mechanisms, and analyte interactions with sensing media, while critically evaluating advantages, limitations, fabrication approaches, and performance metrics. In addition, recent advances in sensor arrays are highlighted to demonstrate progress toward multiplexed detection. By consolidating these developments, this work provides a comprehensive framework to guide the rational selection of active and sensing materials for designing electrochemical and electrical devices capable of reliable, real-time ion monitoring in water.