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.