Preparation And Application Of Fluorescent Sulfur Quantum Dots

Following carbon quantum dots,silicon quantum dots and black phosphorus quantum dots,sulfur quantum dots（SQDs）,as a new non-metallic fluorescent quantum dots,have gradually become the focus of human research due to their excellent fluorescence performance,good biocompatibility and low toxicity.SQDs have shown great applications in the fields of fluorescence sensing detection,biological imaging and optoelectronic devices.This paper focuses on the use of simple synthesis methods,revolves around the preparation and performance of SQDs,and mainly explores the application of SQDs in fluorescence sensing and detection.The details are as follows:1.We used ethanol as a solvent to prepare small-size SQDs by solvothermal method.Ethanol can disperse the bulk sulfur and enables sufficient transfer of large sized sulfur to smaller sized one under one-pot synthesis approach.The SQDs obtained with ethanol as the solvent showed better photoluminescence than water and Na OH.By delicately controlling the reaction conditions including the amount of bulk sulfur,the reaction time,the proportional of sulfur to oxidizing reagent,highly blue emissive SQDs with a photoluminescence quantum yield（QY）of 7.04%with extra-long stability for several months can be successfully prepared.Furthermore,we found out that the SQDs displays a different photoluminescence property and varied particle size as the reaction time increases which is proposed by the etching-aggregation process.Interestingly,the fluorescence of the SQDs-72 can be effectively quenched by CoOOH nanosheets and recovered upon addition of ascorbic acid（AA）by consuming CoOOH nanosheets through redox reaction,leading to the fluorescence recovery.Therefore,a fluorescence“off-on”nano sensor for detection of AA with a limit of detection（LOD）of 0.85μM was constructed.2.Using hydrogen peroxide（H₂O₂）as both solvent and etching agent,evenly distributed SQDs of sublimation sulfur and passivation polyglycol-400（PEG-400）were prepared via a one-pot solvothermal method at elevated temperature and used for the detection of Fe³⁺and cell imaging.The H₂O₂ acts as a dual-functional reagent which could suspend the sulfur powder and etching bulk sulfur to construct uniform SQDs.Under elevated temperature conditions,the sulfur powder is easily converted into the liquid phase sulfur in the molten state,and can form a uniform reaction solution with the solvent.The resulting SQDs have a uniform size distribution,excellent PL properties,superior dispersion,excellent PL stability,and QY up to 10.3%.Furthermore,we show that the mechanism of SQDs formation at different reaction times follows the"etching-assemble-etching"process.Finally,the resultant SQDs was utilized as a sensitive iron ions(Fe³⁺)sensor with limit of detection（LOD）up to 10.80 n M as well as their investigation for fluorescent cellular imaging.3.For the first time,when ethylenediamine is used as a solvent,the sublimated sulfur is directly prepared into green nitrogen-doped sulfur quantum dots（N-SQDs）through one-step solvothermal method.Through their characterization,N-SQDs are characterized by uniform size distribution,surface modifiable,and excellent fluorescence.The quantum yield of N-SQDs prepared by this method can reach 25.3%.Moreover,the resulting N-SQDs have a clear concentration effect that enables blue to green light emission.As the N-SQDs slowly move from low to high concentrations,the color slowly changes from blue to cyan and eventually to green.As the concentration of N-SQDs changes from low to high,the color gradually changes from dark blue-light blue-cyan-green.