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.