Blue-to-red colorimetric sensing strategy for Hg²⁺ and Ag⁺ via redox-regulated surface chemistry of gold nanoparticles.

Here we report a "blue-to-red" colorimetric method for determination of mercury ions (Hg²⁺) and silver ions (Ag⁺) based on stabilization of gold nanoparticles (AuNPs) by redox formed metal coating in the presence of ascorbic acid (AA). AuNPs were first stabilized by Tween 20 in phosphate buffer solution with high ionic strength. In a target ion-free system, the addition of N-acetyl-L-cysteine resulted in the aggregation of Tween 20 stabilized AuNPs for mercapto ligand self-assembled on the surface of AuNPs, which induced the AuNPs to be unstable. This would lead to a color change from red to blue. By contrast, in an aqueous solution with Hg²⁺ or Ag⁺, the ions could be reduced with the aid of AA to form Hg-Au alloy or Ag coating on the surface of AuNPs. This metal coating blocked mercapto ligand assembly and AuNPs kept monodispersed after addition of N-acetyl-L-cysteine, exhibiting a red color. Therefore, taking advantage of this mechanism, a "blue-to-red" colorimetric sensing strategy could be established for Hg²⁺ and Ag⁺ detection. Compare with the commonly reported aggregation-based method ('red-to-blue'), the color change from blue to red seems more eye-sensitive, especial in low concentration of target. Moreover, selective analysis of Hg²⁺ and Ag⁺ was simply achieved by the redox nature of target ions and the application of classic ion masking agents, avoiding the design and selection of ion chelating moieties and complicated gold surface modification procedure. This method could selectively detect Hg²⁺ and Ag⁺ as low as 5 nM and 10 nM in pure water with a linear range of 5 × 10⁻⁷ to 1 × 10⁻⁵ M for Hg²⁺ and 1 × 10⁻⁶ to 8 × 10⁻⁶ M for Ag⁺, respectively. It was successfully applied to determination of Hg²⁺ and Ag⁺ in tap water and drinking water.