Fluorosurfactant-capped gold nanoparticles-based label-free colorimetric assay for Au3+ with tunable dynamic range via a redox strategy

Gold nanoparticles-based colorimetric assay possesses several unique advantages, and has been applied for a wide range of targets, varying from nucleic acids to different metal ions. However, due to the lack of proper coordinating ligand, gold nanoparticles-based colorimetric sensing system for Au3+ has not been developed so far. It is well-known that Au3+ could induce the oxidation transition of thiol compounds to disulfide compounds. In this article, for the first time we converted such thiol masking reaction into colorimetric sensing system for label-free detection of Au3+ via a target-controlled aggregation of nanoparticles strategy. In the new proposed sensing system, fluorosurfactant-capped gold nanoparticles were chosen as signal reporter units, while an Au3+-triggered oxidation of cysteine (Cys), which inhibited the aggregation of gold nanoparticles, acted as the recognition unit. By varying the amount of Cys, a tunable response range accompanied with different windows of color change could be obtained for Au3+, illustrating the universality of the sensing system for Au3+ samples with different sensitivity requirements. Under optimized condition, the proposed sensing system exhibits a high sensitivity towards Au3+ with a detection limit of 50 nM, which is lower than previously reported spectroscopic methods. It has also been applied for detection of Au3+ in practical water samples with satisfactory result.