Carbon quantum dots (CQDs), as an emerging class of carbon-based nanomaterials, have attracted widespread research interest. The advance of synthetic strategies is crucial for enabling CQDs practical application. This work presents a new green synthesis approach of ammonia oxidation combined with photocatalysis for synergistic conversion, aiming to fabricate coal-derived CQDs via the precise reconstruction of low-rank lignite molecules. Initially, ammonia oxidation achieved mild depolymerization of lignite macromolecular framework in the aqueous phase, and the nitrogen-rich functional groups were introduced by in-situ modification. Subsequently, the photocatalyst-driven homogeneous synthesis was employed to prepare nitrogen-rich coal-derived CQDs by oxidizing nitrogen-containing precursors with photo-generated active oxygen species. The synthesized CQDs was then served as fluorescent probes, exhibiting high sensitivity towards Cu²⁺ with a detection limit as low as 2.3 nM. It is a benefit from the rapid coordination of nitrogen-rich functional groups on the surface of the CQDs with Cu²⁺ ions to form nonfluorescent Cu²⁺/CQD complexes, resulting in pronounced fluorescence quenching. These superior attributes render the CQDs a promising platform for the ultrasensitive monitoring and early warning of trace low-concentration Cu²⁺. Overall, this work not only presents a mild and controllable route for synthesizing CQDs but also opens a new avenue for the high-value utilization of low-rank lignite.
 

lignite; CQDs; photocatalytic; ammonia oxidation; nitrogen-rich