A simple colorimetric method based on "on-off-on" mode for detection of HS and Hg in water.

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