One-step synthesis of ultrabright amphiphilic carbon dots for rapid and precise tracking lipid droplets dynamics in biosystems.

Fluorescent probes enabling precisely labeling lipid droplets (LDs) in complex systems are highly desirable in life science for studying LDs-related physiological processes and metabolic diseases. However, most of the current LDs fluorophores fail to achieve rapid wash-free LDs labeling, especially in vivo labeling due to their strong hydrophobicity and poor water solubility. We report here one-step synthesis of highly efficient carbon dots (CDs) that feature robust solvatochromic emission, high quantum yield (QY) up to 76.35% in oil, good water solubility and lipophilicity, thus allowing to stain LDs in a bright and selective manner. Detailed characterizations reveal the presence of a well-defined molecule, 2-dimethylamino-5-fluorobenzimidazole in a large amount in CDs. Its D-π-A structure and dimethylamino-induced spatial torsion configuration and extended π-electron conjugation account for solvatochromic emission with high QY. Notably, the CDs can image LDs with many advanced merits (high brightness, ultrafast staining within 10 s, wash-free, excellent LDs specificity, good biocompatibility) and have been successfully applied to monitor cellular LDs dynamics. Moreover, the CDs for the first time allow in situ labeling of LDs and epidermal cell membranes simultaneously in live zebrafish. This work expands the diversity for optical properties and applications of CDs, facilitating the design of new LDs-targeting CDs.