A synergistic coordination strategy for colorimetric sensing of chromium(III) ions using gold nanoparticles

We report a simple, facile, and reliable colorimetric system for detection of chromium(III) ions (Cr³⁺) using citrate- and thiourea-modified gold nanoparticles (AuNPs). The colorimetric sensing strategy is based on the synergistic coordination interaction of citrate and thiourea toward Cr³⁺ on the surface of AuNPs, leading to the aggregation of AuNPs which produces a color change from red to purple. Under the optimal conditions, this colorimetric sensing system shows an excellent selectivity and sensitivity for Cr³⁺, and the limit of detection (LOD) is estimated to be 0.05 μM at a signal-to-noise ratio of 3, which is far below the current standard stipulated by U.S. Environmental Protection Agency (1.9 μM). Moreover, this LOD is one and a half orders of magnitude lower than those of previously reported modified AuNPs-based colorimetric methods. Visual color change can be observed when 50 μM of Cr³⁺ was introduced to the sensing system. Furthermore, this colorimetric sensing system can be employed for detection of Cr³⁺ in diluted natural water samples.