Different-sized CdTe QDs on the detection of Cu2+ ions: Combining experimental investigation with first-principles verification

We report the interaction between different sizes of CdTe quantum dots (CdTe QDs) and copper (II) ion (Cu2+) via experimental and theoretical aspects. Different sizes of CdTe QDs are synthesized and size effects on the selectivity in the detection of Cu2+ ions are studied. Experimental results indicate that with the size of CdTe QDs from 1.8 nm to 2.7 nm and to 3.2 nm increases, the linear range gets wider. For the smallest size of QD (QD524, 1.8 nm), the linear range is 1.0–5.0 × 10−5 M with a limited detection of 2.8 × 10−5 M. The Stern–Volmer plots at different temperatures demonstrate that the fluorescence of CdTe QDs is quenched by Cu2+ ion through a dynamic quenching mechanism, and the physiological mechanism is analyzed. Electrons are transferred from CdTe QDs to Cu2+ ion, and Cu2+ ion acquire electrons and are reduced to Cu+ ion, leading to fluorescence quenching. Meanwhile, with the increase in the number of transferred electrons, the phenomenon of fluorescence quenching is more obvious. In order to validate the process of electron transfer, the amount of transferred charge between Cu2+ and CdTe QDs are calculated by using first principles density functional theory (DFT). Calculation results show that the smaller the QDs are, the more electrons they transfer. The linear range of the smaller size QDs (QD524) is the narrowest, indicating fluorescence quenching is indeed the most obvious of all three sizes.