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

Abstract

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 (N-CDs) and nitrogen/sulfur co-doped (N,S-CDs) carbon dots synthesized hydrothermally (with purification by 24 h dialysis, with a cutoff of 3.5 kDa), with ethylenediamine and sodium thiosulfate as 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 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 S—precursor. Future works 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.