Portable and selective chloride ion detection in high-salinity water via a microwave sensor and synergistic...

Accurate detection of chloride ions (Cl^-) in water is of great significance for ensuring human health and environmental safety. This study reports a cost-efficient, portable, and flexible microwave sensing system for detecting Cl^- in high-salinity water. This system employs a split ring resonator fabricated on a printed circuit board (PCB) substrate as the microwave sensor. The passivation layer covers the sensor to improve the sensitivity, prevent cross-contamination, and increase robustness. The integration of the sensor with a palm-sized LiteVNA for microwave signal acquisition facilitates portable Cl^- detection. To avoid the influence of other impurity ions and achieve high specificity in Cl^- detection, silver nitrate (AgNO₃) is added into the sample during testing. The reaction between Cl^- and AgNO₃ induces changes in the microwave signal caused by variations in dielectric properties, which corresponds to the Cl^- concentration. The detection is carried out over a Cl^- concentration range of 0-40 g L^-1 within the microwave frequency range of 0.5-3.5 GHz, whereas a characteristic peak appears between 0.9 and 1.2 GHz. The frequency shift before and after AgNO₃ addition is proportional to the Cl^- concentration and remains unaltered from various impurity ions. The optimized sensor design and AgNO₃ amount facilitate the system to obtain high sensitivity, excellent specificity, and rapid detection with a response time of 10 s.