A specific nanoprobe for cysteine based on nitrogen-rich fluorescent quantum dots combined with Cu.

As a new member of the carbon quantum-dot family, fluorescent nitrogen-rich quantum dots (NRQDs) were prepared by a mixed solvothermal method using 2-azidoimidazole and aqueous ammonia as reactants. These NRQDs are rich in nitrogen up to 40.2%, which are endowed with high fluorescence quantum yield, good photostability, water-solubility and favourable biocompatibility. We further explored the use of NRQDs combined with Cu as a nanoprobe for sensing fluorescently of cysteine (Cys) in complex biological samples. In this sensing system, the fluorescence is significantly quenched via energy transfer from NRQDs to Cu for the coordination of amino-containing groups with Cu. The strong affinity between Cu and Cys leads to the formation of Cu-Cys complexes and cause the detachment of Cu from the surface of NRQDs, thus the fluorescence of NRQDs recover. This nanoprobe allows analysis of Cys by modulating the switch of the fluorescence of NRQDs with a detection limit of 5.3nM. As expected, the proposed NRQDs-Cucomplex-based nanoprobes were successfully applied for the determination of Cys in human serum and plasma samples with recoveries ranging from 97.2% to 105.7%. The probe ensemble was also successfully applied to imaging of Cys in living cells with satisfactory results, which shows strong potential for clinical diagnosis.