Blue-to-Red Colorimetric Sensing Strategy for Hg²⁺ and Ag⁺ via Redox-Regulated Surface Chemistry of Gold Nanoparticles

Abstract

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.

Colorimetric Detection of Mercury Species Based on Functionalized Gold Nanoparticles

“…As is well-known, the citrate ions, which keep Au NPs from aggregation on the surfaces, can be easily displaced by thiol groups. 20,27,28 Thus, it is supposed that the Hg−DDTC complex could be attached to the surface of Au NPs through the Au−S linkage, as shown in Scheme 1B. Consequently, the system became so destabilized and thereby resulted in the aggregation of Au NPs and a color change from red to blue, which is different from the reported aggregation induced by the deposition of Hg 2+ onto the Au NP surface.…”

A highly selective and sensitive colorimetric sensor for iodide detection based on anti-aggregation of gold nanoparticles

“…Our group has investigated a blue-to-red colorimetric sensor that was based on the Hg 2+ and Ag + deposition on the gold surface which could inhibit the cysteine-induced aggregation of Au NPs [22].…”

Novel Hg2+-imprinted polymers based on thymine–Hg2+–thymine interaction for highly selective preconcentration of Hg2+ in water samples

“…IIPs are often puzzled by the problem of low selectivity because many metal ions have the same charge, similar ionic radius and properties. Mercury ion (Hg 2+ ), an environmentally and toxicologically important element, being a research hotspot [17][18][19][20], also has been used as template to prepare Hg 2+ -IIPs for preconcentration of trace Hg 2+ [21][22][23]. However, the most commonly used functional monomers for HgIIPs, such as 4-VP and 3-mercaptopropyltrimethoxysilane (MPTAS), can provide coordination for many ions and thereby results in the low selectivity [21][22][23].…”