Nitrogen-Doped Carbon Quantum Dots from Poly(ethyleneimine) for Optical Dual-Mode Determination of Cu2+ and l-Cysteine and Their Logic Gate Operation

In this work, nitrogen-doped carbon quantum dots from poly(ethyleneimine) (PQDs) were synthesized by a low-cost and facile one-step hydrothermal method without other reagents. A quantum yield (QY) of up to 23.2% with maximum emission at 460 nm under an 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 PQD-Cu2+ complex exhibited selective fluorescence recovery and color disappearance on exposure to l-cysteine (Cys). The electron transfer from amino or oxygen groups on the PQDs to Cu2+ leads to fluorescence quenching, and a chromogenic reaction of the cuprammonium complex results in a 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+ in 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 detection limits of 28.11 μM (fluorescence determination) and 19.74 μM (peak shift determination at 272 nm) was obtained. Meanwhile, the PQD-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 the PQD-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.