Efficient Conversion of Elemental Sulfur to Robust Ultrabright Fluorescent Sulfur Quantum Dots Using Sulfur-Ethylenediamine Precursor

The direct utilization of abundant and cheap elemental sulfur (S) to synthesize sulfur quantum dots (SQDs) with satisfactory yield and photoluminescence quantum yield (PLQY) remains a big challenge. Here, we report a simple and effective method for mass-producible synthesis of ultrabright fluorescent SQDs based on one-pot solvothermal treatment of S-ethylenediamine (S-EDA) solution. The PLQY of synthesized SQDs and the corresponding S-to-SQDs conversion efficiency (CE) can reach as high as 87.8% and 15.9%, respectively, both of which are the highest values to date. In addition, the SQDs simultaneously show good water solubility, superior storage stability, stable fluorescence against ionic strength variation and pH change, and low toxicity, which enable them promising for numerous applications such as fabrication of fluorescent polymer composite and ink. Furthermore, SQDs also present strong two-photon emission, and their use for two-photon fluorescence imaging is reported here for the first time. This work thus highlights enormous opportunities not only for bringing SQDs from laboratory research to real applications but also for the value-added utilization of S resource.