Carbon quantum dots as fluorescent probes: recent developments and applications in environmental and biomedical fields

Carbon quantum dots (CQDs) have emerged as a versatile class of fluorescent nanomaterials owing to their unique optical properties, low cytotoxicity, excellent biocompatibility, and tunable surface chemistry. These attributes have driven extensive interest in their application across biosensing, bioimaging, photodynamic therapy, and drug delivery. This review provides a comprehensive overview of recent advances in the synthesis, physicochemical properties, and functional applications of CQDs. Both top-down approaches, including laser ablation and arc discharge, and bottom-up strategies, such as hydrothermal, microwave-assisted, pyrolysis, and green biomass-derived methods, are critically discussed. The structural and optical characteristics of CQDs, including size-dependent photoluminescence and surface-state-mediated emission, are summarized in relation to their functional performance. Key fluorescence mechanisms relevant to sensing applications—such as static and dynamic quenching, inner filter effect, aggregation-induced quenching, and aggregation-induced emission—are systematically reviewed. CQD-based fluorescent probes have demonstrated high sensitivity and selectivity toward a wide range of analytes, including heavy metal ions, pesticides, and antibiotics. In addition, the bioimaging capabilities of CQDs are highlighted through in vitro and in vivo studies, including cancer cell lines, zebrafish, and small animal models, benefiting from their tunable emission, photostability, and low toxicity. Furthermore, emerging applications of CQDs in photodynamic therapy and drug delivery are discussed, emphasizing their multifunctional theranostic potential. Finally, current challenges related to reproducibility, biosafety, scalability, and clinical translation are outlined, providing perspectives for future research and practical implementation of CQD-based technologies.