Metal Charge Transfer Doped Carbon Dots with Reversibly Switchable, Ultra-High Quantum Yield Photoluminescence

As a class of the heteroatom-doped carbon materials, metal charge-transfer doped carbon dots (CDs) exhibited an excellent optical performance and were widely used as fluorescent probes. To improve fluorescence quantum yield (QY) remains one of the fundamental and challenging issues in the carbon dots field. Herein, we prepared a novel manganese doped CDs (Mn-CDs), which exhibited an ultrahigh quantum yield of 54%, the highest quantum yield for metal-doped CDs. Various spectroscopic measurements revealed an in situ change of dopant oxidation state during the synthesis. Our further study indicated the presence of metal–carbonate, which served as an important component for high quantum yield. We have also studied the reversibly switchable fluorescence property of Mn-CDs by adding Hg2+/S2–, as well as elucidating the underlying mechanism of this switching fluorescence phenomenon. By using the Mn-CDs as fluorescent probes, we developed an extremely sensitive detection method for heavy metal Hg2+ detection at a nM detection limit level.