Direct visual detection of MnO 2 nanosheets within seconds

The increasing application of nanomaterials will inevitably lead to their release into the environment, which may pose a threat to the environment and human health. As such, there is an urgent need to detect various nanomaterials. In the present work, we present a novel, rapid, and simple visual detection of MnO 2 nanosheets in buffer solution and environmental water sample. In this assay, a redox reaction between MnO 2 nanosheets and 3,3′,5,5′-tetramethylbenzidine (TMB) occurred, leading to the oxidation of TMB to TMB diimine. This redox reaction generated different colors dependent on the concentration of MnO 2 nanosheets, including colorless, blue, green, and yellow, which allowed semiquantitative detection of MnO 2 nanosheets with the naked eye. The detection range of the visual assay was 2–15 μg/mL, and the minimum concentration of MnO 2 nanosheets can be visually detected at concentrations down to 2.0 μg/mL. Moreover, the developed visual assay showed a high selectivity to MnO 2 nanosheets over Mn 2+ ions, tetramethylammonium hydroxide, hydrogen peroxide, graphene oxide, and graphitic carbon nitride nanosheets. Graphical abstract Visual detection of MnO 2 nanosheets