Sensitive photoluminescent detection of Cu2+ in real samples using CdS quantum dots in combination with a Cu(2+)-reducing reaction.

By reducing free and/or weakly complexed Cu(2+) with a Cu(2+)-reducing agent (ascorbic acid in the present study) and detecting the photoluminescence peak of Cu(2)S-covered CdS quantum dots (QDs) at 650 nm, Cu(2+) concentrations ranging from 1 nM to 1 μM can be readily determined. Unlike other related reports, the present method takes advantage of the more efficient chemical reduction of Cu(2+) to Cu(+) (with respect to the photochemical reduction inherent in CdS QDs) and the facile deposition of Cu(2)S. As a result, a detection limit of 0.5 nM was achieved, which is at least 2-3 orders of magnitude lower than QD-based detection methods. In contrast with other methods requiring sample pretreatment or Cu(2+)-specific ligands capping QDs, the selectivity of the method towards Cu(2+) is excellent. Among a number of metal ions examined, only Cu(2+) causes the red shift of the CdS photoluminescence. A process causing the shift of the CdS photoluminescence was investigated and described. The matrix effect on the photoluminescent behavior of CdS QDs and the amenability of this method for real samples were also studied. Analyses of Cu(2+) in a river water sample and Cu(2+) complexed by amino acids and proteins in cerebrospinal fluids were performed. The latter analysis reveals that our method can differentiate weakly complexed Cu(2+) ions from the more strongly bound ones. This simple method was also demonstrated to be highly sensitive, accurate and reproducible.