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.