Achieving Room‐Temperature Phosphorescence in Solution Phase from Carbon Dots Confined in Nanocrystals

Carbon dots (CDs) have attracted growing interest in the construction of room‐temperature phosphorescent (RTP) materials. However, in the solution phase of CDs, it is still challenging to obtain efficient and stable phosphorescent emission due to the intense quenching effect by dissolved oxygen and solvent molecules. Herein, we report robust phosphorescence in the solution phase, achieved by encapsulating citrate‐derived CDs into NaYF4 nanocrystals via a one‐step method of high‐temperature coprecipitation. Combined characterizations show that the triplet emission from CDs is related to the abundance of C=O in the CDs, the formation of ionic‐bond networks around the CDs, and the spatial confinement and quenching inhibition effects of NaYF4 nanocrystals. Notably, the transition of CDs@NaYF4 from hydrophobicity to hydrophilicity can be easily achieved by simple surface modulation of NaYF4 nanocrystals, which allows the RTP of CDs to be maintained in either polar or nonpolar solvents. In addition, CDs@NaYF4 exhibits stable afterglow in different pH environments, suggesting its excellent stability. Finally, we demonstrated the application of CDs@NaYF4 in 3D printing, oily anti‐counterfeiting patterns, and cell imaging. Our work can serve the controllable preparation of solution‐phase RTP materials and their various applications.