Visual Method for Selective Detection of Hg²⁺ Based on the Competitive Interactions of 2-Thiobarbituric Acid with Au Nanoparticles and Hg²⁺

A facile visual strategy is presented for specific identification of mercury ions (Hg2+) in an aqueous environment based on the competitive interactions of 2-thiobarbituric acid (TBA) with Au nanoparticles (AuNPs) and Hg2+. This approach depends on the unique optical performance of AuNPs and the sulfophile affinity of AuNPs and Hg2+. With the addition of TBA, AuNPs aggregate because of the ligand exchange between surface-loaded citrate ions of AuNPs and the thiol group of TBA, which induces a red shift in the absorption spectrum, and the color changes to blue from wine red. However, the added Hg2+ in advance can restrain the agglomeration of AuNPs induced by TBA since the mercaptan group is more likely to react with Hg2+ than AuNPs through the formation of Hg2+ sulfur. Based on the competitive interactions of TBA with AuNPs and Hg2+, the method can specifically detect Hg2+ in the absence of masking reagents without reacting with the major interfering metal ions including Cu2+, Pb2+, and Cd2+ compared to previous studies. Moreover, the detection limit of this original method can be as low as 2.4 nM, and it can be effectively used for Hg2+ detection in environmental aqueous samples.