Semi-quantitative colorimetric and supersensitive electrochemical sensors for mercury using rhodamine b hydrazide thio derivative

Mercury is a toxic heavy metal which pollutes air, land, and water posing a serious threat to the environment and human health. For a first-hand estimate, a qualitative method (colorimetric) for detection of mercury could suffice. However for precise measurement, below the threshold value, a quantitative method (electrochemical) is needed. The colorimetric strips were produced using mercapto-propionic acid derivatized rhodamine hydrazide (RS). RS developed a pink color when in contact with mercury (Hg2+) and the shade became darker with an increase in concentration. A new polymeric hydrogel was used to entrap RS to fabricate sensing strips for quasi-quantitative detection of Hg2+ through the naked eye. An electrochemical sensor was also developed for accurate quantification of Hg2+ at ppt level by modified glassy carbon electrode with the same RS and green reduced graphene oxide (RS-gRGO/GCE) via differential pulse anodic stripping voltammetry (DPASV). The dipstick exhibited high sensitivity, good selectivity and low detection limit (0.29 ppb) whereas metal free, label-free modified electrode performed very well with a high sensitivity (70.22 μA·ppb·cm−2), low detection limit (57 ppt), good selectivity, and reproducibility. These sensors will serve the purpose of common people for onsite estimation as well as scientific laboratories requiring precision measurements.