A simple colorimetric method based on "on-off-on" mode for detection of H₂S and Hg²⁺ in water

It is of great significance to develop efficient platforms for the detection of hypertoxic Hg²⁺ and H₂S. Colorimetric have received much attention for the detection of H₂S and Hg²⁺ in the last decades. In this work, an "on-off-on" mode colorimetric method based on MnO₂/multi-wall carbon nanotubes (MnO₂/MWCNTs) composite was constructed. MnO₂/MWCNTs composite can oxidize TMB directly to form blue product (ox TMB) with a good simulated oxidase activity. In the presence of H₂S, it can decompose the MnO₂/MWCNTs composite causing the absorbance of the chromogenic system to decrease. When Hg²⁺ is introduced, the formation of Hg-S bond between Hg²⁺ and H₂S inhibited the decomposition ability of H₂S toward MnO₂ composite, thus resulting in a color change from colorless to blue. Based on this phenomenon, the proposed "on-off-on" colorimetric sensor can be used for detection of H₂S (off) and Hg²⁺ (on). Under optimized experimental conditions, this sensor showed a satisfactory linear relationship of H₂S and Hg²⁺ with pleasant repeatability, acceptable method accuracy and stability. More importantly, the proposed colorimetric sensor has been successfully applied to the detection of H₂S and Hg²⁺ in real samples, which not only provides a simple and cost-effective method to detect H₂S and Hg²⁺ but also hopefully makes a certain contribution to environmental protection.