Water-soluble CuInS2 ternary quantum dots (QDs) capped by mercaptopropionic acid (MPA) were directly synthesized in aqueous solution, and were then applied to the detection of Cu2+ and Cd2+ ions. The ternary QDs exhibited obvious photoluminescence (PL) quenching in the presence of Cu2+ and marked enhancement in the presence of Cd2+ respectively. Other physiological cations such as K+, Ca2+ Mg2+ and Zn2+ had no effect on the PL intensity under similar conditions. The detection limits were 0.1 µmol L−1 for Cu2+ and 0.19 µmol L−1 for Cd2+. The fluorescence quenching of the ternary CuInS2 QDs by Cu2+ could be explained by the reduction of Cu2+ to Cu+ on the surface of the QDs, whereas the fluorescence enhancement by Cd2+ was attributed to activation of surface states. Ternary QDs modified by sufficient Cd2+ showed enhanced sensitivity for Cu2+ determination compared with unmodified QDs, and a detection limit of 0.037 µmol L−1 was obtained for Cu2+. Utilizing the property that Cu2+ can be easily removed by addition of iodide, we could eliminate the interference of Cu2+ on the Cd2+ detection. To the best of our knowledge, this is the first report on the selective detection of Cu2+ and Cd2+ by quenching and enhancing the fluorescence of the near-infrared CuInS2 QDs in aqueous solution. This novel method has been successfully applied to the detection of trace Cu2+ and Cd2+ in real water samples with satisfactory results.
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