Aqueous Synthesis And Biological Effect Foundation Of Water-soluble Quantum Dots

Quantum dots (QDs) have special fluorescence properties and a wide application prospect in biological and medicinal fields. Direct aqueous synthesis of QDs is simple, inexpensive. The as-prepared QDs have better biocompatibility and can be directly applied to biological system, therefore aqueous synthesis has gradully become a hot topic of QDs synthesis. At the same time, the biological effects of QDs also attracted increasing attention. However, the existing conclusions of the biological effects of QDs are numerous, jumbled, and even mutual contradiction, which attribute to various QDs, different synthetic methods, and diverse physicochemical properties of surface. In addition, all studies are limited to a certain point of view of QDs toxicity. No one has systematically illuminated the biological effects mechanism of QDs, resulting in unsatisfactory findings.In this thesis, CdTe QDs of different sizes and surface ligands were prepared using direct aqueous synthesis method. Moreover, we have developed a hydrothermal method to synthesize novel alloyed CdxZn1-xSe QDs, and investigated the alloyed mechanism. Finally, the effects of CdTe QDs modified with different ligands on cell proliferation, bacterial metabolism, and activity of serum protein were examined, then a possible mechanism of toxicity was proposed. The research results will provide a lot of important theoretic information on the synthesis and biosafty evaluation of QDs, and contribute to the development and application of nanomatierals as well, thus improving and perfecting the methodology of nano-biological activity.This thesis consists of six chapters.Chapter1:This chapter introduced a more comprehensive overview of optical properties, biological applications, progress in synthesis, and biological effects of QDs.Chapter2:CdTe QDs of different sizes and surface ligands were prepared using aqueous synthesis method. The structures and optical properties of the obtained CdTe QDs were characterised by UV-vis absorption spectroscopy, photoluminescence spectroscopy, infrared spectroscopy. X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscope, etc. The effects of pH value on optical properties of QDs were also investigated.Chapter3:We have developed a hydrothermal method to synthesize ZnSe and CdxZn1-xSe QDs, using glutathione as a stabilizer. The structures and optical properties of the as-prepared QDs were characterised by UV-vis absorption spectroscopy, photoluminescence spectroscopy, infrared spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscope, etc. The growth mechanism and optimum reaction temperature were also investigated. The results show that the maximum quantum yield of as-prepared ZnSe QDs is17.3%. The obtained CdxZn1-xSe QDs exhibit a tunable photoluminescence emission from433.0nm to486.5nm. When the Zn/Cd molar ratio in precursor solution was1:1, the quantum yield of the QDs solution reached a maximum value. Furthermore, the growth mechanism of alloyed CdxZn1-xSe QDs was explored. The ZnSe/CdSe core/shell nanocrystals were obtained when zinc precursor was added to the ZnSe QDs solution. With increasing reaction temperature, the formation of alloyed CdxZn1-xSe QDs can result from the diffusion of cadmium ions into the lattice structure of ZnSe QDs. When increasing reaction temperature, the quantum yield of the alloyed quantum dots increased gradually, the quantum yield would reach its maximum at160?, then decay with a further increasing in the reaction temperature.Chapter4:The inhibition of CdTe QDs modified with different ligands on the HEK293and HeLa cell proliferation were measured by MTT method. The effects of QDs on the mitochondrial transmembrane potential (??m), cell cycle, and ultrastructure were also detected using flow cytometry, laser confocal microscopy, transmission electron microscopy, and spectroscopic methods. Finally, we investigated the behavior in the culture medium, the cytotoxicity mechanism, and the uptake mechanism of QDs. The results show that CdTe QDs exhibited anti-proliferative activity in the dosage and time-dependent manner. QDs can induce necrosis and the decrease of ??m in HEK293cells, but have no effect on cell cycle. Furthermore, QDs can cause ultrastructure change of cells, including the nuclear enlargement, mitochondrial swelling, reticulum expansion, and ribosomal aggregation, etc. The cytotoxicity of QDs is related to the uptake of quantum dots, the release of cadmium ions, and the generation of reactive oxygen species, whereas the generation of reactive oxygen species is not the major reason of cytotoxicity. The clathrin-dependent endocytosis is one of the endocytosis mechanisms by which quantum dots enter cells, which may depend on serum in the culture medium. Finally, QDs will be transferred to lysosomes.Chapter5:The toxicity of CdTe QDs modified with three different ligands, namely, mercaptopropionic acid (MPA), N-acetyl-L-cysteine (NAC), and glutathione (GSH) were investigated via microcalorimetric, spectroscopic, and microscopic methods. The results of the calorimetric experiments and optical density measurements indicate that the QDs inhibited the growth of Gram-negative E. coli. The toxicity order of three QDs is MPA-CdTe QDs> GSH-CdTe QDs> NAC-CdTe QDs. The toxicity mechanism of QDs may be related to their bacterial adhesion, which confirmed by transmission electron microscopy and inductively coupled plasma atomic emission spectroscopy results. In addition, the effects of QDs coated with different ligands on the membrane fluidity and permeability were investigated. GSH-CdTe QDs have greater effects on the membrane function of E. coli than those of MPA-CdTe and NAC-CdTe QDs. This result may be attributed to the stronger lipophilicity of GSH compared with those of MPA and NAC.Chapter6:We investigated the interactions between human serum albumin (HSA) and CdTe quantum dots (QDs) capped with four different ligands (MPA, NAC, and GSH are negatively charged; CA is positively charged) under physiological conditions. The investigation was carried out using fluorescence spectroscopy, circular dichroism (CD) spectra, UV-vis spectroscopy, and dynamic light scattering (DLS). The results of fluorescence quenching and UV-vis absorption spectra experiments indicated that the formation of the complex of HSA and negatively charged QDs (MPA-CdTe, NAC-CdTe, and GSH-CdTe), which was also reconfirmed by the increasing of the hydrodynamic radius of QDs. The Ka values of the three negatively charged QDs are of the same order of magnitude, indicating that the interactions are related to the nanoparticle itself rather than the ligands. The electrostatic interactions play predominant roles in the adsorption process. Furthermore, it was also proven that QDs can induce the conformational changes of HSA. However, our results demonstrate that the interaction mechanism between the positively charged QDs (CA-CdTe) and HSA is significantly different from negatively charged QDs. According to the DLS results, some large-size agglomeration also emerged.