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.