Interactions between gold, thiol and As(iii) for colorimetric sensing.

As(iii) or arsenite is extremely toxic, and various colorimetric sensors were reported for its on-site detection. A highly cited example was based on gold nanoparticles (AuNPs) modified with a few thiol-containing compounds including dithiothreitol (DTT), reduced glutathione (GSH), and cysteine (Cys). As(iii) was believed to crosslink these surface ligands to aggregate AuNPs and produce a red-to-blue color change. Since As(iii) can also adsorb on AuNPs, we herein carefully studied this effect on these ligand-capped AuNPs. For citrate-capped AuNPs, 10 mM free citrate resulted in a strong blue color in the presence of As(iii) attributable to the elevated ionic strength, while common divalent cations resulted in no color change due to the chelation effect of free citrate. For AuNPs capped with the three thiol compounds, more than 5 mM As(iii) was needed to produce a color change, which was very different from the previously reported color change with nanomolar concentration of As(iii). Our color change was attributed to the displacement of the surface ligands by As(iii) instead of crosslinking by it. This conclusion was made based on the irreversibility of the color change, kinetics of the reaction, and high As(iii) concentration needed. This work has revealed that any two species from AuNPs, thiol and As(iii) can react. It also calls for care in the interpretation of related colorimetric sensing mechanisms, and the need to consider the previously overlooked As(iii) adsorption onto AuNPs.