One‐Step In Situ Synthesis of Carbon Dot‐Based Nanospheres With Underwater Color‐Tunable, Dual‐Mode Long‐Afterglow Emission

ABSTRACT Carbon dot (CD)‐based afterglow materials have recently proliferated in various areas, but color‐tunable, carbon dot‐based underwater afterglow nanomaterials remain largely unexplored. Moreover, the existing synthesis methods are generally complex, and there has been a notable lack of efficient, one‐step preparation techniques. We present a simple one‐step hydrothermal method to synthesize CD‐based nanomaterials that exhibit stable dual‐mode underwater emission comprising room‐temperature phosphorescence (RTP) and thermally activated delayed fluorescence (TADF). Using 3‐aminopropyltriethoxysilane, a densely cross‐linked SiO 2 matrix forms, embedding in situ generated CDs. This confined structure stabilizes triplet excitons, enabling an ultra‐long underwater afterglow lifetime of 1218 ms and a quantum yield of 46.7%. By doping with halide ions, the afterglow wavelength was tuned from 402 to 560 nm, achieving multicolor emission. The nanocomposites maintain stable afterglow performance in harsh conditions, including strong acids, alkalis, oxidants, and polar solvents. Given its exceptional stability and multicolor afterglow, the material was used to construct a sophisticated triple anti‐counterfeiting system, demonstrating great potential for advanced information encryption.