Manipulating time-dependent size distribution of sulfur quantum dots and their fluorescence sensing for ascorbic acid

Unlike the previous commonly used strong alkaline solvent sodium hydroxide, we employ an eco-friendly solvent, ethanol, as a solvent for the preparation of ultra-small-sized sulfur quantum dots (SQDs). Ethanol can disperse bulk sulfur and allow sufficient transfer of large-sized sulfur to smaller-sized SQDs through a one-pot synthesis approach. The SQDs obtained from ethanol as the solvent displays superior photoluminescence properties to those in water and sodium hydroxide. By delicately controlling the reaction conditions, including the amount of bulk sulfur, the reaction time, and the proportion of sulfur to oxidizing reagent, highly blue emissive SQDs with a photoluminescence quantum yield (PLQY) of 7.04% with ultra-high stability for several months can be successfully prepared. Furthermore, we found out that the SQDs display a dynamic photoluminescence properties and varied particle sizes as the reaction time increases, which is possibly realized via the etching-aggregation process. Morevoer, the fluorescence of SQDs-72 can be effectively quenched by CoOOH nanosheets and recovered upon addition of ascorbic acid (AA) by consuming CoOOH nanosheets through the redox reaction, leading to fluorescence recovery. Therefore, a fluorescence "off-on" nanosensor for the detection of AA with a limit of detection (LOD) of 0.85 μM was constructed.