Ultratrace Naked-Eye Colorimetric Ratio Assay of Chromium(III) Ion in Aqueous Solution via Stimuli-Responsive Morphological Transformation of Silver Nanoflakes

In this work, we discover an effective domino-effect based detection mechanism for the rapid, high-selectivity, and ultrasensitive naked-eye colorimetric ratio assay of chromium(III) ion (Cr3+) in aqueous solutions. This naked-eye colorimetric ratio assay to Cr3+ relies on changes in absorbance properties that are due to the stimuli response of Cr3+ for the morphological transformation of silver nanoflakes. In the presence of Cr3+, large silver nanoflakes can be stimulated to transform to small nanoparticles (NPs), which promoted a color change in solution from bright yellow to wine red that was detected by the naked eye and spectroscopic absorption methods. The proposed naked-eye sensing strategy was coupled with an ultraviolet (UV)-spectroscopic chemosensor for the detection of trace Cr3+. The detection method showed an ultralow detection limit of 1.4 nM (nanomole) for Cr3+ in ultrapure water and 11.5 nM in human serum sample, respectively. On the basis of colorimetry, the lowest concentration similar to 50 of nM for Cr3+ in ultrapure water and human serum can be detected by the naked eye. This naked-eye-based colorimetric ratio assay for accurate and selective identification and high sensitivity of Cr3+ is expected to have large potential and wide applications such as pharmaceutical analysis, clinical diagnosis, and environmental monitoring.