Carbon dots for fluorescence bioimaging: Advances in synthesis, optical properties, and multimodal applications

Carbon dots (CDs) are an emerging class of carbon nanomaterials distinguished by exceptional optical properties, biocompatibility, and tunable surface chemistry, making them highly attractive for next-generation biomedical imaging. Unlike conventional semiconductor quantum dots, CDs are nontoxic, free of heavy metals, resistant to photobleaching, and readily synthesized via green, scalable routes. Advances in size control, surface modification, and heteroatom doping have enabled near-infrared emission, essential for deep-tissue imaging with minimal autofluorescence. This review summarizes recent progress in top-down and bottom-up synthetic strategies, emphasizing the mechanism property relationships that link structural design to photophysical performance. Surface passivation, oxidation, and doping are shown to govern fluorescence lifetime, quantum yield, and spectral tunability. Cutting-edge applications are highlighted, including high-resolution cellular imaging, targeted diagnostics, and theranostics, with integration into multimodal platforms such as MRI, CT, and photoacoustic imaging. Critical challenges such as reproducibility, spectral heterogeneity, and translational biosafety are analyzed alongside emerging solutions, including machine-learning-guided synthesis and ligand-mediated targeting. Collectively, CDs are positioned not merely as safer fluorophore alternatives but as programmable nanoplatforms unifying safety, sensitivity, and multifunctionality, poised to redefine diagnostic and therapeutic strategies across oncology, neurology, and regenerative medicine.