Preparation And Biological Imaging Application Of Ⅱ-Ⅵ Quantum Dots And Their Composite Nanostructures With Silica

Quantum dots (QDs) as a novel class of biological fluorescent labeling material, effectively overcome the drawbacks of current fluorescent dyes and fluorescent proteins in terms of photobleaching, narrow excitation and wide emission spectrum. QDs showed great potential in biomedical research including multicolor imaging, real time and dynamic fluorescent tracking, high sensitive biomolecule detection and immuno assay in recent years. Along with the progress of QD application research, there is a rising demand on enhanced emission properties, stability and biocompatibility of QD probes. Exploring efficient synthetic methods of aqueous QDs as well as surface modification strategies of organic QDs towards water solubilization and bio-functionalization serves as fundamental prerequisite for QDs analytical applications. Moreover, QDs based fluorescent microspheres and nano composite structures with multi-functionalities have shown broad prospects in high sensitive biological sensing, multi-target analysis, multi-modal imaging and simultaneous diagnosis and therapy in medical research. In this thesis, we developed practical and efficient synthetic approach for aqueous ZnSe QDs and silanization method for oil soluble CdSe/ZnS QDs. Demonstrated new strategy for preparing and silanization of oil soluble QDs-nanocarriers hydrophobic assemblies, which was further expanded to the building of bi-functional nano-material. Performed biological imaging application of QDs based probes. The main works are as follows:1. Novel synthetic approach for high quality aqueous ZnSe QDs by microwave assisted single-step manner was developed. Commercially available and air stable Na2SeO3was adopted as Se precursor, to avoid commonly used unstable NaHSe or H2Se, allowing the reaction taken place under ambient atmosphere condition. The unique properties of microwave irradiation such as rapid and homogeneous heating, favored the strong bandgap emitting and narrowing in size distribution of ZnSe QDs. XRD result indicates the formation of ZnS shell around ZnSe QDs that probably originated from decomposition of GSH under high temperature releasing active S2-. This shell effectively passivated the surface state of original ZnSe QDs, realizing brilliant bandgap emission (PLQY-18%), low trap emission and narrow FWHM (26-30nm). The effects of GSH amount, Zn/Se molar ratio and solution pH on QDs growth rate, trap emission intensity and quantum yield were discussed in detail. 2. Direct and high efficient silanization strategy for oil soluble QDs via organosilane micellization was demonstrated. Multi-cored CdSe/ZnS@SiO2fluorescent nanobeads were prepared from n-octyltriethoxysilane (OTES)-QDs precursor by ultrasonication assisted self-emulsification, base catalyzed hydrolyzation and silanol condensation/silicate deposition process. The micelle size and its loading amount could be scaled by adjusting the initial QDs concentration in OTES which subsequently changed the mutual solubility of QDs-silane precursor and water phase. Replacing OTES with n-octyltrimethoxysilane (OTMS) resulted in single QD encapsulated silane micelle. The silica bead formation process was characterized by dynamic light scattering (DLS), gel electrophoresis and transmission electron microscope (TEM), revealing that QDs@OTES micelles formed during ultrasonication stage possessed poor colloidal stability, which was significantly enhanced in subsequent active silica deposition stage together with efficiently fixed micelle morphology. We confirmed the encapsulation of oil soluble QDs by OTES mainly relied on hydrophobic interaction instead of ligand exchange, based on the observation that1-dodecanethiol (DDT) capped CuInS2/ZnS QDs after OTES coating formed similar QDs@SiO2structure. Adopting the controlled SiO2growth on silanized nanoparticles in Stober system, the SiO2spheres embeding hydrophobic domain in the core were obtained, which allows further modification and assembly.3. The fluorescent properties of CdSe/ZnS QDs in micellization and SiO2deposition stages were investigated, indicating a partial fluorescent quench during coalesce of silane and water phase caused by impermanent environmental changes around QD surface. Nevertheless, the quantum yield gradually recovered approaching90%of the initial value accompanied by active silica deposition. Fluorescent decay curves and their bi-exponential fitting results further confirmed such a reversible change. Accordingly, we prepared highly luminescent (QY～45%) and narrow emitting (FWHM～30nm) CdSe/ZnS@SiO2nanobeads, which also fitted for CdSe/ZnS QDs with various emission wavelength. The fluorescent nanobeads exhibited excellent colloidal and PL stability under cellular culture conditions, and no apparent cytotoxicity was observed within1～1000μg/ml concentration of CdSe/ZnS@SiO2. Epoxy groups were introduced onto nanobead surface by GPTMS silanization, followed by covalently conjugation with transferring (Tf) to form functionalized probes, which were applied to HeLa cells targeted fluorescent imaging.4. High efficient assembly of oil soluble QDs with SiO2carriers and their silanization strategy has been developed. Weak ligands capped QDs were immobilized onto strong ligand (mercaptopropyl group) grafted SiO2carriers driven by metal-ligand affinity in organic phase, forming densely packed (near monolayer) SiO2@QDs hydrophobic assemblies. Expanding organosilane micellization approach to silanize these assemblies, the single-phase hydrolyzation system of OTMS/methnol/ammonia and using stronger ligands for immobilization and weaker ligands for silanization were required to ensure a high QDs loading during ultrasonication and hydrolyzation (micellization) stage. High luminescent (quantum yield up to84%of initial value) and colloidally stable QDs@SiO2@QDs fluorescent microspheres were obtained after a period of active silica deposition. On this basis, a size series (56nm,93nm and200nm) of fluorescent microspheres and AuNR@SiO2@QDs@SiO2composite material combining near infrared absorption/light scattering and fluorescent emitting were produced, by altering the size and composition of silica carriers, which brings new perspectives for high sensitive fluorescent sensing, multi-modal imaging and simultaneous diagnosis and therapy researches.