Nitrogen-Doped Carbon Quantum Dots from Polyethyleneimine for Optical Dual Mode Determination of Cu2+ and L-Cysteine and Its Logic Gate Operation.

In this work, nitrogen doped carbon quantum dots from polyethyleneimine (PQDs) was synthesized by a low-cost and facile one-step hydrothermal method without other reagent. A quantum yield (QY) up to 23.2% with maximum emission at 460 nm under the excitation wavelength of 340 nm was ascribed to the high nitrogen doping (20.59 %). The PQDs selectively form a blue complex with Cu2+ accompanied by strong quenching of the fluorescence emission. Meanwhile, the PQDs-Cu2+ complex exhibited selective fluorescence recovery and color disappearing on exposure to L-cysteine (Cys). The electron transfer from amino or oxygen groups on the PQDs to Cu2+ leads to the fluorescence quenching and chromogenic reaction of cuprammonium complex results in the color change. The strong affinity between Cys and Cu2+ causes the detachment of Cu2+ from the surface of PQDs, so the color of the solution disappears and the fluorescence of PQDs recovers. Under the optimized condition, the proposed sensor was applied to detect Cu2+ with the linear range of 0-280 μM. A detection limit of 4.75 μM is achieved using fluorescence spectroscopy and 4.74 μM by monitoring the absorbance variation at 272 nm. For Cys detection, the linear range of 0-800 μM with the detection limit of 28.11 μM (fluorescence determination) and 19.74 μM (peak shift determination at 272 nm) were obtained. Meanwhile, the PQDs-Cu2+ system exhibits distinguishable responses to other biothiols such as L-glutathione (GSH) and DL-homocysteine (Hcy). Based on the multimode signals, an "AND" logic gate was constructed successfully. Interestingly, besides Cu2+, Fe3+ can also quench the fluorescence of PQDs and PQDs/Fe3+ system exhibits superior selectivity for Cys detection. Most importantly, the proposed assay is not only simple, cheap and stable, but also suitable for detecting Cu2+ and Cys in some real samples.