Visual Chronometric Assay for Chromium(III) Ions Based on the Cu2O Nanocube-Mediated Clock Reaction.

We report a visual chronometric assay for Cr3+ based on the Cu2O nanocube-mediated clock reaction of methylene blue (MB) and N2H4. MB with a blue color can be easily reduced by excess N2H4 to form colorless leuco-MB in the presence of Cu2O nanocubes as a catalyst in a short time. However, the addition of Cr3+ decreases the catalytic activity of Cu2O 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 Cr3+. 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 Cr3+ can be directly detected with the naked eye by observing the color-fading speed. Additionally, this assay was successfully used to determine Cr3+ in real water samples.