Low-cost colorimetric mercury sensor based on immobilisation of rhodamine B thiolactone in a sustainable agar-agar gel substrate

The global impact of mercury (Hg) pollution requires the development of improved low-cost analytical sensors for Hg determination. Rhodamine B thiolactone (RBT) has been proposed as a colorimetric sensor for Hg2+ as it undergoes ring-opening in the presence of Hg to give a fast, reliable, and easily observed, colour change. In this study a naked-eye biopolymer-based sensor for Hg2+ detection was developed based on RBT-doped agar-agar membranes supported on filter papers. The chromogenic reagent was stable at −18 °C for over two years, whilst the RBT-doped sol–gel membranes prepared from 1% (w/v) agar colloid had a shelf-life of at least 12 weeks at room temperature when stored in the dark. The limit of detection (LOD) for naked-eye sensing was 0.4 μg L−1. For Hg2+ quantification, images of the membranes were recorded using a flatbed scanner and analysed with public-domain ImageJ software. The linear range based on greyscale intensity in the green channel was 0.2–6.0 μg L−1 and the LOD was 0.2 µg L−1. Precision was 10% (n = 3) and spike recoveries were in the range 97–103%. The sensors were regenerated using 10% w/v KI and successfully reused up to 4 times. Compared with other RBT-doped sol–gel sensors, the agar-agar membranes were simpler to prepare, more environmentally friendly, and gave a superior detection limit.