Insights into the role of common sulfur precursors in hydrothermally synthesized N,S-doped carbon dots: fluorescence modulation via surface oxidation rather than sulfur doping

While heteroatom doping is widely used to enhance the fluorescence of carbon dots, the actual mechanism underlying this claimed enhancement remains unclear. Herein, we report a systematic comparison between nitrogen-doped carbon dots (N-CDs) and nitrogen/sulfur co-doped carbon dots (N,S-CDs) synthesized hydrothermally, with ethylenediamine and sodium thiosulfate as the N- and S-dopants, respectively. Contrary to expectations, the inclusion of the S-precursor significantly reduced the quantum yield by 63%, from 35.9% to 13.2%, while leaving the emission profile and response largely unchanged. Characterization assays revealed that S-doping in N,S-CDs was residual, whereas the surface oxygen content increased markedly, indicating surface oxidation. The data suggest that sodium thiosulfate acted not as a dopant but overall as an oxidant, generating oxygen-based surface defects that introduced nonradiative recombination pathways. Thus, our findings show that co-doping with this S-precursor can decrease the quantum yield by promoting surface oxidation and the effective formation of more oxidized N-CDs, rather than heteroatom incorporation and production of co-doped CDs. This decreasing effect on the quantum yield was also observed when using thiourea as the S-precursor. Future work should include the study of other common S-precursors. In conclusion, our research shows that specific heteroatom-based precursors can alter the optical characteristics of CDs through redox/surface-modification effects instead of actual compositional doping, and thus, claims based on heteroatom doping may need closer examination.