Influence of sulfur doping on the molecular fluorophore and synergistic effect for citric acid carbon dots* *Project supported by the National Natural Science Foundation of China (Grant No. 51571085) and the Key Scientific Research Projects of Colleges and Universities in Henan Province, China (Grant No. 20A430015).

Abstract

In citric acid-based carbon dots, molecular fluorophore contributes greatly to the fluorescence emission. In this paper, the nitrogen and sulfur co-doped carbon dots (N,S-CDs) were prepared, and an independent sulfur source is selected to achieve the doping controllability. The influence of sulfur doping on the molecular fluorophore was systematically studied. The introduction of sulfur atoms may promote the formation of molecular fluorophore due to the increased nitrogen content in CDs. The addition surface states containing sulfur were produced, and S element exists as –SO3, and –SO4 groups. Appreciate ratio of nitrogen and sulfur sources can improve the fluorescence emission. The photoluminescence quantum yields (PLQY) is increased from 56.4% of the single N-doping CDs to 63.4% of double-doping CDs, which ascribes to the synergistic effect of molecular fluorophores and surface states. The sensitivity of fluorescence to pH response and various metal ions was also explored.