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 O2 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 α-fetoprotein was 2 ng ml−1, which is comparable to the detection limit of the enzyme-linked immunosorbent assay method. Huang-Hao Yang and co-workers at Fuzhou University have devised an ultra-sensitive method to detect copper ions (Cu2+), whose accumulation in the body has been linked to several diseases. The researchers' method relies on gold nanoparticles, popular in colourimetric sensing because they turn a solution from red to purple on aggregation. Here, however, instead of triggering the aggregation directly, the copper ions catalyse a ‘supporting reaction’ – the oxidation of a monomeric amino acid (cysteine) into a dimeric one (cystine). Only cysteine triggers the aggregation of the nanoparticles, which means the presence or absence of copper results in a red or purple solution, respectively. The method's sensitivity is very high because only catalytic quantities of copper are needed. The researchers further used this rapid, convenient, sensitive method in an immunoassay for a human protein. A novel method based on the AuNPs for fast, sensitive and selective detection of Cu2+ was developed and applied in immunoassays.