Elucidating Manganese Single-Atom Doping: Strategies for Fluorescence Enhancement in Water-Soluble Red-Emitting...

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-N₂O₂ coordinating structure, resulting in five significant effects: an increase in sp² 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.