| In the past ten years,the properties of quantum dots(QDs)have been studied intensively.The relationship between surface ligands and QDs has been discussed in a great deal of work.In this thesis,organic CdSe quantum dots(QDs)with uniform size distribution and good optical properties were synthesized.Aqueous QDs were prepared by ligand exchange.The effects of auxiliary ligands were discussed by representing the conversion efficiency,conversion rate and properties of aqueous quantum dots during ligand exchange.In addition,we used the behavior of adsorption and desorption in ligand exchange process to intuitively discuss the relationship between ligand density and QD properties.Based on this,we found the process of the QD-ligand is deviated from Langmuir model,and then we systematically studied the degree of deviation and the treatment method.Then,a large number of aqueous QDs with good biological functionalization were prepared by simple means of one-step synthesis according to the theoretical rules obtained by us.We proposed a direct functionalization strategy for QDs.The research results are of great significance to further understand the changes of surface and optical properties of QDs in the application process,and enrich the qualitative and quantitative depiction of physical images on the surface of QDs.This thesis consists of four chapters.The first Chapter briefly introduces the basic background of QDs,including synthesis,optical properties and applications.The surface,surface characterization methods,surface physical and chemical processes of QDs are introduced in detail.In Chapter 2,we discuss the preparation of CdSe and CdSe/Zn S QDs and optimize the preparation process.The effects of different feeding and shell thickness on fluorescence properties of QDs were discussed,and the core-shell CdSe/Zn S QDs with high quantum yield were optimized.We use mercaptopropionic acid as the main body of aqueous quantum dot ligand exchange,to discuss the effects of three auxiliary ligands on the quantum dot ligand exchange process: tetramethylenediamine(TMEDA),1,2-dimethylenediamine(1,2-DMEDA),ethylenediamine(EDA).Therefore,we prepared aqueous CdSe/Zn S QDs with different optical properties.In Chapter 3,we find that TMEDA and EDA have different characteristics on the surface of QDs.TMEDA can inhibit the adsorption of ligands on the surface of QDs,so as to control the density of ligands on the surface of QDs.EDA can be re-adsorbed on the surface of QDs with exposed sites.These two behaviors can be part of ligand exchange together,so it makes sense to study them separately.Based on this,we designed the dynamics and thermodynamics models of the surface ligands of quantum dots.We use TMEDA's model to construct quantum dots with different ligand densities.Due to the nature of MPA,the difference between the bonded state and the unbonded state can be directly displayed on NMR,thus making it possible to quantify the ligands on the surface of QDs.We find that the binding of surface ligands of quantum dots conforms to Hill equation and the exponential term is up to 3.8.This means that the surface adsorption thermodynamic process of QDs is a positive promoted form,widely off the Langmuir model.EDA as the model of adsorption process,we used isothermal titration microcalorimetry(ITC)to test the thermodynamics of our adsorption process.We found that the degree of purification has a significant effect on the ligands of QDs,which is clearly shown in our ITC results.This result is consistent with the qualitative description of our Hill equation.From the result,the higher the purification degree,the smaller the value of the binding constant Ka.In order to make up for the defects on the surface of QDs,we proposed to use ethylenediamine,a small molecule capable of spontaneous adsorption,to reduce or avoid the decline in fluorescence quantum yield of QDs under the condition of completion of purification,and we conducted 5 cycles of fluorescence quantum yield to verify the rationality of our idea.Experiments concerning kinetics of adsorption and desorption are also discussed.Consistent with our previous conclusion,under the ideal Langmuir model,there is a large deviation in data fitting.On the basis of the original formula,we added the factors of ligand influence factors,and all the data could be well fitted.In Chapter 4,after we proposed the physicochemical model of QD surface and ligand,we tried to use this conclusion to propose preparation strategy to obtain biologically functionalized quantum dots with practical significance.Lysosomal targeting,mitochondrial targeting and cancer cell targeting three different quantum dots can be obtained by ligand exchange in one step,while the functional ligand could not obtain stable QDs originally.We hope that this preparation strategy can simplify the preparation process of functionalized QDs and provide a new idea for the way of functionalized modification.Chapter 5:Summary and outlook. |
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