Plant-Derived Nitrogen-Doped Carbon Quantum Dots for Selective Cu²⁺ Detection and In Vivo Fluorescence Imaging

This study employed Cibotium barometz (L.) J. Sm. as a carbon source and carbamide as a nitrogen source to conduct a straightforward one-step hydrothermal synthesis of nitrogen-doped carbon quantum dots (N-CQDs). The N-CQDs were fully characterized in terms of their morphological features, chemical constituents, and optical behaviors. The synthesized N-CQDs exhibited the highest fluorescence intensity with excitation and emission wavelengths set at 305 nm and 382 nm, respectively. Notably, copper ions (Cu²⁺) could induce obvious fluorescence quenching of the N-CQDs, endowing the system with high sensitivity and selectivity. The limit of detection (LOD) for Cu²⁺ ions was determined to be 3.52 µM within the linear range of 1-10 µM. The fluorescence lifetime of N-CQDs was shortened from 3.38 ns to 3.27 ns after adding Cu²⁺. The analysis shows that the N-CQDs were selective and sensitive to Cu²⁺ due to the static quenching mechanism. Furthermore, this method was applied to analyze the content of Cu²⁺ in actual samples with satisfying results. More importantly, studies in zebrafish embryos and Arabidopsis thaliana verified the biocompatibility of N-CQDs at low concentrations, allowing efficient in vivo bioimaging, while Cu²⁺ co-exposure caused fluorescence reduction. A facile strategy for synthesizing N-CQDs from natural plant-derived precursors is proposed in this work, revealing their great prospects as both fluorescent probes and bioimaging agents.