Elucidating Manganese Single‐Atom Doping: Strategies for Fluorescence Enhancement in Water‐Soluble Red‐Emitting Carbon Dots and Applications for FL/MR Dual Mode Imaging

Abstract The absence of the enhancement of fluorescence in carbon dots (CDs) through doping with transition metal atoms (TMAs) hinders the advancement of multi‐modal bio‐imaging CDs with high photoluminescence quantum yield (PLQY). Herein, Mn‐atomically‐doped R‐CDs (R‐Mn‐CDs) with a high PLQY of 41.3% in water is presented, enabling efficient in vivo dual‐mode fluorescence/magnetic resonance (MR) imaging. The comprehensive characterizations reveal that the incorporation of Mn atoms leads to a Mn‐N 2 O 2 coordinating structure, resulting in five significant effects: an increase in sp 2 conjugation domains, a reduction in band gap, a decreased oxidation level, an increase in photo‐excited electron numbers, and the suppression of non‐radiative electron relaxation pathways. Collectively, these factors contribute to the remarkable PLQY of R‐Mn‐CDs. Additionally, the doping of Mn atoms also endows R‐Mn‐CDs with superior MR imaging capabilities due to, which highlights their promising prospect as a dual‐modal bio‐imaging platform for fluorescence/MR imaging. Furthermore, the findings indicate that the introduction of various TMAs, such as Mn, Zn, Ni, and Cu, can universally improve the PLQY of water‐soluble CDs through the construction of TMAs─O bonds. This research provides valuable theoretical insights into the mechanisms underlying the fluorescence enhancement induced by TMAs doping and offers guidance for the future design of high PLQY CDs.