Red-to-blue colorimetric detection of chromium via Cr (III)-citrate chelating based on Tween 20-stabilized gold nanoparticles

In this work, a simple “red-to-blue” colorimetric method for highly sensitive and selective determination of Cr (III) and Cr (VI) was presented, which was based on the aggregation of Tween 20-stabilized gold nanoparticles (Tween 20-Au NPs). This strategy relied upon the distance-dependent optical properties of gold nanoparticles (Au NPs), strong chelating between Cr (III) and citrate, and the reduction of Cr (VI) to Cr (III) by ascorbic acid (AA). Cr (III) could chelate with citrate to form a stable Cr (III)-citrate complex and thereby induce the aggregation of Tween 20-Au NPs. Thus, the sensing of Cr (III) and Cr (VI) based on aggregation of Au NPs was developed with the color of the Au NPs changing from red to blue, which was readily seen by the naked eye. The color change and surface plasmon resonance (SPR) absorption were used to monitor Cr (III) and Cr (VI) levels. Under the optimal assay conditions, good linear relationship was obtained, and the detection limit of Cr (III) and Cr (VI) were 0.016   μM and 0.009   μM in standard aqueous solution, respectively. Furthermore, the colorimetric method exhibited a good selectivity over other metal ions and was successfully applied to detect Cr (III) and Cr (VI) in real water samples.