A simple colorimetric sensor based on anti-aggregation of gold nanoparticles for Hg2+ detection

A new method has been proposed to realize the visual detection of Hg2+ via anti-aggregation of gold nanoparticles (Au NPs). The positively charged Au NPs were prepared using poly(diallyldimethylammonium) chloride (PDDA) as reducer and stabilizer. In the presence of cysteine, the color of Au NPs solution turned from ruby red to royal purple, indicating the aggregation of Au NPs. Owing to the high stability constant of cysteine with Hg2+, the pre-incubation of Hg2+ with cysteine would form Hg2+–cysteine complexes and significantly reduce the concentration of free cysteine molecules, thus the aggregation of Au NPs was interrupted since there was not enough inducer. With the increase of Hg2+ concentration, the color of the Au NPs solution would progress from purple to red, allowing the visual detection of Hg2+ ranging from 5.0 × 10−8 to 1.0 × 10−5 M, with a detection limit of 2.5 × 10−8M. The proposed method is convenient, low-cost and free of complex equipment, making it possible to analyze Hg2+ in drinking water, rain water or water extracted air samples.