Sensitive photoluminescent detection of Cu2+ in real samples using CdS quantum dots in combination with a Cu2+-reducing reaction

By reducing free and/or weakly complexed Cu2+ with a Cu2+-reducing agent (ascorbic acid in the present study) and detecting the photoluminescence peak of Cu2S-covered CdS quantum dots (QDs) at 650 nm, Cu2+ concentrations ranging from 1 nM to 1 mu M can be readily determined. Unlike other related reports, the present method takes advantage of the more efficient chemical reduction of Cu2+ to Cu+ (with respect to the photochemical reduction inherent in CdS QDs) and the facile deposition of Cu2S. 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 Cu2+-specific ligands capping QDs, the selectivity of the method towards Cu2+ is excellent. Among a number of metal ions examined, only Cu2+ 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 Cu2+ in a river water sample and Cu2+ complexed by amino acids and proteins in cerebrospinal fluids were performed. The latter analysis reveals that our method can differentiate weakly complexed Cu2+ ions from the more strongly bound ones. This simple method was also demonstrated to be highly sensitive, accurate and reproducible. (C) 2012 Elsevier B.V. All rights reserved.