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

We report a visual chronometric assay for Cr3+ based on the Cu2 O nanocube-mediated clock reaction of methylene blue (MB) and N2 H4 . MB with a blue color can be easily reduced by excess N2 H4 to form colorless leuco-MB in the presence of Cu2 O nanocubes as a catalyst in a short time. However, the addition of Cr3+ decreases the catalytic activity of Cu2 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 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.