Ultrasensitive Colorimetric Chromium Chemosensor Based on Dye Color Switching under the Cr(VI)-Stimulated Au NPs Catalytic Activity

In view of the high toxicity of Cr­(VI), simple and rapid on-site analytical approaches are in high demand for environment monitoring. Herein, an innovative chemosensor is developed for on-site sensitive detection of Cr­(VI) in minutes by the naked eye. The chemosensor consists of gallic acid-capped gold nanoparticles (GA-Au NPs), methylene blue (MB), and NaBH4. In the presence of Cr­(VI) ions, the reduction of MB by NaBH4 is able to greatly accelerate due to the Cr­(VI)-stimulated catalytic activity of GA-Au NPs, resulting in a color switching of MB from blue to colorless for the quantitative detection of Cr­(VI). The chemosensor in solution exhibits excellent selectivity and ultrahigh sensitivity to Cr­(VI), with the detection limits of 0.05 nM by UV–vis spectroscopy and 0.1 nM by the naked eye. Similarly, a paper-based chemosensor is obtained by immobilization of GA-Au NPs and MB onto a piece of filter paper, offering a more convenient approach for rapid on-site detection of Cr­(VI). In addition, H2O2 as an oxidizing agent is employed to convert Cr­(III) into Cr­(VI), thus achieving speciation analysis of Cr. The applicability of a chemosensor is also validated by the detection of Cr speciation in water samples with satisfactory recoveries.