Highly efficient red-emissive carbon dots in aqueous solution with prolonged circulation for enhanced tumor Theranostics.

The development of water-dispersible red-emissive carbon dots (CDs) for biological applications is urgently needed but has been constrained by fluorescence quenching in aqueous media and inadequate tumor accumulation resulting from ultrafine dimensions. Here, we report a straightforward one-step solvothermal synthesis of CDs demonstrating pure red emission (r-CDs). Surface engineering with polyethylene glycol (PEG) markedly augments their (r-CDs-PEG) red fluorescence in water, elevating the photoluminescence quantum yield from 4 % to 64 % while preserving a high photothermal conversion efficiency. This PEGylation strategy not only mitigates aqueous quenching effects and promotes higher cellular uptake but also prolongs blood circulation duration without altering their size, consequently enhancing tumor accumulation in mouse models and significantly improving the efficacy of photothermal therapy. Theoretical calculations revealed that the intense red-light emission of r-CDs originates from the conjugated domains constructed by graphitic nitrogen and the modulation of electron transitions by external CO functional groups. The r-CDs-PEG also exhibited excellent water solubility, photostability, and biocompatibility. Capitalizing on these attributes, we successfully applied the r-CDs-PEG for in vivo high-contrast tumor imaging and photothermal therapy, achieving remarkable efficacy. This study addresses the longstanding challenge of water-induced quenching in red-emitting CDs, providing a robust platform for advanced bioimaging and tumor theranostics.