Ultrasensitive detection of Cu2+ with the naked eye and application in immunoassays

In this study, a novel method for the fast, sensitive and selective detection of Cu2+ using gold nanoparticles (AuNPs) was developed and used in immunoassays. In the presence of L-cysteine, L-cysteine can bind to the surface of citrate-stabilized AuNPs through Au-S bonds. As a result, aggregation of AuNPs occurs through electrostatic interactions between the cysteine-bound AuNPs. In contrast, in the presence of Cu2+, Cu2+ can catalyze O-2 oxidation of cysteine, leading to the quick formation of disulfide cystine. An increase in the concentration of Cu2+ decreased L-cysteine-induced AuNPs aggregation by decreasing the number of free cysteine thiol groups, and the solution color changed from purple to red. Therefore, the concentration of Cu2+ can be detected with the naked eye or with ultraviolet-visible spectroscopy, and the detection limits of Cu2+ were 20 nM and 10 nM, respectively. This sensitivity was approximately three orders of magnitude higher than that of traditional AuNPs-based colorimetric Cu2+ detection methods. Because of the high sensitivity of the proposed method, we further used it with a labeled antibody in colorimetric immunoassays. The detection limit of the cancer biomarker alpha-fetoprotein was 2 ng ml(-1), which is comparable to the detection limit of the enzyme-linked immunosorbent assay method. NPG Asia Materials (2012) 4, e10; doi: 10.1038/am.2012.18; published online 16 March 2012