A simple and portable optical sensor for Hg²⁺ detection with naked-eye response, smartphone-based quantification and environmental monitoring

A colorimetric and “turn-on” fluorescent sensor (PRB) for the selective detection of Hg2+ ions was developed from readily available chemical namely 2 hydroxynaphthaldehyde and 1,3-propanedithiol though a single step reaction. The sensor operates via an Hg2+-triggered thioacetal deprotection mechanism. In the presence of Hg2+, the 1,3-dithiane unit is rapidly converted into its formyl analogues (PRB-CHO), resulting in a distinct colour change from colourless to yellow along with a pronounced fluorescence enhancement within seconds, enabling naked-eye detection. The sensing mechanism was confirmed by 1H NMR titration studies. PRB exhibits excellent selectivity and sensitivity towards Hg2+ over a wide range of competing metal ions. The limit of detection (LOD) was determined to be 0.01788 μM (0.0035 ppm), which is significantly lower than the permissible limit (0.03 ppm), while the limit of quantification (LOQ) was calculated to be 0.06 μM (0.012 ppm). For practical applications, PRB was immobilized in a starch–PVA hydrogel matrix and on paper substrates, enabling simple and portable on-site detection. Furthermore, a 3D-printed portable device integrated with a custom Android application (Mercury Sensor) was developed for automated data acquisition and real-time smartphone-based quantification of Hg2+. The device demonstrated excellent reliability under field-testing conditions. The sensor was successfully applied to Hg2+ detection in real samples, including water, cosmetic products, and herbal samples, with high accuracy and strong anti-interference performance. This work provides a rapid, sensitive, and user-friendly sensing platform with significant potential for environmental and on-site mercury monitoring.