CdTe Quantum Dots (QDs) Based Kinetic Discrimination of Fe2+ and Fe3+, and CdTe QDs-Fenton Hybrid System for Sensitive Photoluminescent Detection of Fe2+

A method based on the quenching kinetics for the fluorescence of glutathione capped CdTe quantum dots (GSH-CdTe QDs) was developed for discriminating Fe2+ and Fe3+, and a GSH-CdTe QDs-Fenton hybrid system was constructed for sensitive and selective determination of trace Fe2+. Although both Fe2+ and Fe3+ could quench the fluorescence of GSH-CdTe QDs, the quenching kinetics was quite different for Fe2+ and Fe3+. The fluorescence of the GSH-CdTe QDs (30 nM) was quenched by about 18% in 1 min after the addition of Fe3+ (10 mu M), and remained unchanged with further increase of reaction time. In contrast, the fluorescence intensity of the GSH-CdTe QDs decreased by about 65% in the first 5 min after the addition of Fe2+ (10 mu M), then slowly decreased by 15% in the next 25 minutes. Other transition metal ions like Cu2+, Ni2+ and Co2+, Zn2+, and Mn2+ also gave very different quenching kinetics of the GSH-CdTe QDs from Fe2+. No significant effect of the capping agents (GSH, thioglycolic acid, and mercaptopropionic acid) for the QDs on the pattern of the time course of the fluorescence of the QDs for Fe2+ or Fe3+ was observed. To achieve selective determination of Fe2+ in the presence of Fe3+, trace H2O2 was introduced to establish a QDs-Fenton hybrid system. The Fenton reaction between Fe2+ and H2O2 resulted in hydroxyl radicals which can effectively quench the fluorescence of the QDs through electron transfer from the conduction band of the QDs to the single occupied molecular orbit of hydroxyl radicals. The high redox potential of hydroxyl radicals (2.8 V) permits more effective quenching of the fluorescence of the QDs than Fe2+. The detection limit of the developed method was 5 nM for Fe2+. The recovery of Fe2+ spiked in water samples ranged from 96% to 105%.