Preparation Of Znse Quantum Dots (QDs) And Interaction Between QDs And The Protein

Quantum dots(QDs) is a kind of semiconductor nanoparticles consisting of Ⅱ～Ⅵ or Ⅲ～Ⅴ family elements. Quantum Dots, with excellent optical properties, are widely used in the life sciences and medicine. At present, QDs containing heavy metals Cd, were studied by most of the researches. However, the QDs in the presence of Cd will have a certain of negative impact on organisms and the environment. So its biological toxic effects become a new researching foucus. The surface of QDs entering into the bio-organic body can bind protein, which can affect protein structure and function. We can understand the biological toxic effects of QDs through the interaction study between QDs and protein. In addition, the lighting conditions are the important factors that affect the structures stability of quantum dot and their cytotoxic effect. But the light toxicity caused by QDs have been little studied. The interaction of quantum dots under light conditions and protein and their microscopic mechanism of interaction have not been reported. Therefore, the preparation of a kind quantum dots with low toxicity, high quantum yield, good stability and biocompatibility advantages, and the study of the role of light impacting on the quantum dots, and the role of light through quantum dots before and after the interaction with proteins to understand the its toxic effects, have all important significance.This paper starts from the toxicity, the fluorescence efficiency and stability of QDs in the current practical problem of its application. Different stabilizers, surface modifiers and different synthesis methods are adopted to study the preparation of ZnSe QDs. QDs are irradiated by different types of light to study the effects of role of light on the structure and properties of QDs, and to explore its mechanism of action of light. Bovine serum albumin was selected as a model protein to study the interaction between QDs and protein at the molecular level. The contents and conclusions of the research are as follows:(1) Thioglycolic acid (TGA) and glutathione (GSH) were used as stabilizers, and ZnSe QDs were prepared by hydrothermal and microwave methods, respectively. The synthesis condition were optimized. The structure of QDs were characterized. ZnSe/GSH QDs that prepared by microwave method were modified by BSA cross-linking. Results showed that, the fluorescence and stability properties of ZnSe/GSH QDs is much better than ZnSe-TGA QDs’s in the hydrothermal synthesis. The microwave method is superior to hydrothermal method for ZnSe/GSH QDs’s synthesis. The optimal microwave synthesis of ZnSe/GSH QDs conditions were that the moral ratio of GSH:Zn:Se was1.6:1:0.1, pH10.5, the heating time60min, and the heating temperature95℃. In optimal condition, we obtained the ZnSe/GSH QDs, with a half peak width of emission peak25nm, diameter about2-3nm, and hydroxyl and carboxyl two kinds of functional groups in their surface. The fluorescence intensity of ZnSe/GSH QDs modified by BSA cross-linking increased. QDs-BSA complex fluorescent probe system was formed.(2) The interaction of QDs and BSA was investigated by UV-visible absorption spectrum, fluorescence spectrum, circular dichroism and isothermal titration calorimetry, and the thermodynamic constant, binding sites, and the distance between molecules of QDs and BSA interaction were calculated. The results showed that QDs cause distinct quenching on the intrinsic fluorescence of BSA, the quenching mechanism is static quenching mode with the formation of non-fluorescent complexes. The magnitude order of binding constants for three QDs is ZnSe/TGA QDs, CdSe/L-cysteine QDs, ZnSe/GSH QDs. The main reason is that the QDs have different surface groups. ZnSe/GSH QDs and BSA interaction is a common enthalpy-entropy-driven process. The main interaction forces are electrostatic and hydrogen bonding.The ionic strength and pH also affected the interaction between them.(3) ZnSe QDs were irradiated by dark, ultraviolet light and incandescent light, respectively. The affect of light on the structure and property of QDs were investigated by UV-visible absorption spectrum, fluorescence spectrum, ICP-AES, HRTEM, XRD and FTIR. Results showed that the physical size and element contents of ZnSe QDs under different light were changed, but the crystal structure of ZnSe QDs was not changed obviously. Compared with dark ZnSe/GSH QDs, UV irradiation ZnSe/GSH QDs possessed good sphericity, size distribution and dispersion, but there were also a small amount of agglomeration. The content of S elements in UV irradiation ZnSe/GSH QDs was higher relatively. The role of light impact on the O-H stretching vibration peaks greatly, however, little impact on other bonds. This is because that the light makes the ZnSe/GSH QDs’s surface-SH group and-OH groups interact by chemical bond. Comprehensive comparison, UV light impact on physical and chemical properties of ZnSe/GSH QDs is greater than incandescent and dark conditions. (4) The interaction of ZnSe/GSH QDs illuminated and BSA was investigated by fluorescence spectroscopy, circular dichroism and FTIR, and the thermodynamic constant and binding sites were calculated. The results showed that three kinds of ZnSe/GSH QDs illuminated quenched on the intrinsic fluorescence of BSA, the quenching mechanism is static quenching. The magnitude order is UV light QDs, incandescent light QDs, Dark QDs. The values of△H and△S are negative (△H<0,△S<0). These results indicate that hydrogen bonds and van der Waals interactions play a major role in the binding reaction between UV light QDs and BSA. The△H<0and△S>0suggested that electrostatic interactions play a major role in the binding reaction between UV light QDs and BSA.but the hydrophobic and coordination interactions are also important. Light changed the QDs surface ligand structure, which effected indirectly on the conformation of BSA. UV QDs conformational change of BSA greater impact. The amino acids microenvironment of BSA was sensitive to the binding of QDs to BSA, while the secondary structure of BSA was still predominant a-helix.(5) The spatial binding mode of QDs to BSA was modeled by Autodock and Sybyl8.1software. Result indicated that QDs capped by TGA, QDs capped by L-cysteine and QDs capped by GSH all can bind with surface of BSA. There were many binding sites, but their interaction force type and strength were different. The type of interaction force of QDs capped by TGA and was mainly polar interaction that include van der Waals force and hydrogen bond. The bonding strength was weak. QDs capped by L-cysteine combind with hydrophobic bond and hydrogen bond, and bonding strength was strong. QDs capped by GSH are combind strongly by hydrogen bond, and bonding strength was very strong. Among the surface modifiers, the interaction force of BSA and QDs modified by GSH is the most strongest, the interaction force of BSA and QDs modified by L-cysteine was the second, interaction force of BSA and QDs modified by TGA was the smallest. Molecular modeling results were in accordance with the spectral analysis’s partly.