A specific nanoprobe for cysteine based on nitrogen-rich fluorescent quantum dots combined with Cu2+

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 Cu2+ 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 Cu2+ for the coordination of amino-containing groups with Cu2+. The strong affinity between Cu 2+ and Cys leads to the formation of Cu2+-Cys complexes and cause the detachment of Cu2+ 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 5.3 nM. As expected, the proposed NRQDs-Cu2+complex-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.