Carbon Dots@Upconversion Nanoparticles: Conjugate Manipulation and Luminophor Confinement Enabling Aqueous‐Phase Orthogonal Multicolor Phosphorescence‐Upconversion Luminescence

Abstract Developing single‐particle nanocomposite with aqueous‐phase orthogonal multicolor phosphorescence or multimodal luminescence holds great significance for optical coding, anti‐counterfeiting encryption, bioimaging, and biosensing. However, it faces challenges such as a limited range of emission wavelengths and difficulties in controlling the synthesis process. In this work, a conjugate structure manipulation integrated luminophor confinement strategy is proposed to prepare carbon dots@upconversion nanoparticles (CDs@UCNPs) featuring aqueous‐phase orthogonal multicolor room‐temperature phosphorescence‐upconversion luminescence (RTP‐UCL) through wet‐chemical synthetic methods. Four types of CDs are synthesized by introducing molecules with varying degrees of conjugation, while the intersystem crossing process is enhanced by constructing charge‐transfer states to narrow the energy gap between the excited singlet and triplet states. Aqueous‐phase orthogonal multicolor RTP (green, yellow, and orange) and UCL (blue, green, yellow, and red) are achieved by confining CDs with different conjugation degrees within a NaBiF 4 matrix doped with various lanthanide ions. Notably, NaBiF 4 UCNPs can crystallize at low temperatures, serving as a matrix to immobilize CDs, thereby preventing their vibration and rotation, and minimizing interference from water and oxygen. Additionally, the versatility of this strategy is demonstrated by constructing multicolor CDs@Cs 2 NaGdCl 6 : 5%Yb 3+ ,10%Er 3+ perovskite nanocomposites. This strategy offers valuable guidance for the preparation of advanced aqueous‐phase orthogonal multicolor materials.