Responsive Janus Droplets for Metal Ion Detection in Water Quality-Monitoring Platforms: Enhanced Specificity via Selective Ion Complexation

Abstract Water quality monitoring is essential across various industries, including environmental management, industrial processes, food safety, and agriculture. Among the many contaminants, heavy metal ions are particularly concerning due to their toxicity and persistence in water systems. Here, we present a liquid–liquid sensing platform for metal ion detection based on surfactant-stabilized biphasic oil-in-water emulsions. The detection mechanism relies on changes in surfactant behavior upon interaction with metal ions, including variations in critical micelle concentration (CMC), surface excess, and interfacial stabilization. The size and charge of metal ions significantly influence surfactant effectiveness, enabling both total ion content analysis and identification of specific aqueous signatures. To enhance specificity, we introduce selective ion complexation using chelating agents, such as EDTA, which reduces the impact of chelated ions on surfactant behavior and improves sensitivity toward unbound target ions. This approach also enables the indirect detection of chelating agents in water samples, including glyphosate, a widely used herbicide. Our findings expand the potential of Janus droplet-based sensing assays and support the development of modular, adaptive liquid-monitoring platforms for real-time water quality monitoring.