Visual Chronometric Assay for Chromium(III) Ions Based on the Cu 2 O Nanocube-Mediated Clock Reaction

We report a visual chronometric assay for Cr based on the Cu O nanocube-mediated clock reaction of methylene blue (MB) and N H . MB with a blue color can be easily reduced by excess N H to form colorless leuco-MB in the presence of Cu O nanocubes as a catalyst in a short time. However, the addition of Cr decreases the catalytic activity of Cu O nanocubes owing to the coordination interactions between them, leading to an increase in the reaction time. The reaction time change was employed for selective detection of Cr . This assay has a wide dynamic range from 0.03 to 600 μM with a limit of detection of 2.7 nM. Remarkably, different concentrations of Cr can be directly detected with the naked eye by observing the color-fading speed. Additionally, this assay was successfully used to determine Cr in real water samples.