Study On The Preparation And Analytical Application Of Environment Friendly Sulfur Quantum Dots(SQDs) Fluorescent Probes

As one kind of semiconductor nanocrystals,quantum dots（QDs）possess the properties of photoluminescence,quantum confinement and consecutive tunable spectra.Quantum dots,especially pure element quantum dots,not only have unique photoluminescence properties,but also avoid the potential ecological and environmental risks brought by quantum dots containing heavy metals.Therefore,the construction of analytical chemical sensing system based on their photoluminescence properties has increasingly become the research hot spot in the field of analysis and detection.As a new kind of pure element quantum dots,sulfur quantum dots（SQDs）are non-toxic,eco-friendly and have excellent photoluminescent properties.Herein,β-cyclodextrin was employed as the stabilizer to prepare a fluorescent sulfur quantum dots which combined the molecular recognition and assembly functions.Then the high-performance sensing systems for L-cysteine and alkaline phosphatase were constructed based on the molecule assemblies ofβ-cyclodextrin-SQDs on the surface of MnO₂ nanosheets.The present research not only enriched the study of photoluminescence mechanism of sulfur quantum dots,but also further broadened the application research field of sulfur quantum dots.The main research contents are as follows.Chapter 1,the conception,structure,properties,synthesis,and corresponding applications of QDs were comprehensively reviewed here.Carbon,Silicon,Phosphorus quantum dots and other pure element quantum dots have attracted a lot of attention.Sulfur quantum dots,as a kind of pure element quantum dots,is becoming a new star-up in quantum dots due to its unique photoelectric properties,biological non-toxic and relatively high resource of sulfur in natural environment.On this basis,the research topic of this study was also briefly introduced.Charter 2,in this work,we prepared a new kind of SQDs using sublimed sulfur as precursor andβ-cyclodextrin as the surface passivation ligand by hydrothermal method.The results showed that the as-prepared SQDs have good dispersibility in aqueous solution with uniform spherical shape.The diameter of the SQDs lies in the range of 2.4-6.6 nm with an average size of 4.02 nm.The SQDs emits strong fluorescence at 440 nm and the relative quantum yield was determined to be 2.06%.FT-IR spectra showed that the surface of SQDs was fully capped withβ-cyclodextrin and XPS analysis demonstrated that SQDs was composed of S[0]and a series of sulfonic groups,which was also supported by UV-Vis spectra of SQDs with three absorption at 213 nm,337 nm and 358 nm,respectively.Finally,it is demonstrated that the photoluminescence of SQDs can be interpreted through quantum confinement and surface states.Chapter 3,in this chapter,usingβ-cyclodextrin-SQDs as the fluorescent probe,a sensitive sensing fluorescent nano sensor for L-cysteine（L-Cys）was constructed based on the efficient assembly ofβ-cyclodextrin-SQDs on the surface of MnO₂ nanosheets（MnO₂ NS）,in which the inner filtration effect（IFE）and fluorescence resonance energy transfer（FRET）happened between MnO₂ NS and SQDs.MnO2 NS played difunctional role of quencher for SQDs and target recognition unit for L-Cys in the sensing process.The fluorescence intensity of SQDs was quenched in an exponential mode as a result of IFE and FRET between SQDs and MnO₂ NS.It is tested to be a dynamic quenching of MnO₂ NS on the fluorescence SQDs,whereas,the fluorescence of the sensing system can be recovered based on the redox reaction of L-Cys with MnO₂ NS after the introduction of L-Cys.This sensing assay could achieve sensitive detection of L-Cys in the linear range of 25-1000μM with LOD of 4.11μM.Apart from this,the sensing system showed an excellent selectivity towards L-Cys among the natural amino acids and could be used for the detection of L-Cys in real sample like human serum.Chapter 4,an indirect sensing fluorescent strategy was developed in this work for the determination of alkane phosphatase（ALP）by utilizing the as-prepared SQDs as the sensitive fluorescent probe.MnO₂ NS exhibited strong quenching effect towards the fluorescence of SQDs via IFE and FRET,and sensitive detection of ALP was achieved through redox reaction between MnO₂ NS and AA which was generated from the hydrolysis of AAP by ALP.It was found that the fluorescence intensity of SQDs at 440 nm was linearly restored with the increasing of ALP in the range of 10 ng/m L-1.2μg/m L,of which the LOD was detected to be 4.4ng/m L.Simultaneously,this fluorescent sensor possessed a good selectivity for ALP and could be applied to the detection in real samples such as tap water and human serum.Chapter 5,in this part,the synthesis of SQDs and its application in the analysis and detection of L-Cys and ALP was summarized and the prospect of the study was also briefly illustrated.