Synthesis And Application Of Aqueous Quantum Dots And Quantum Dots/SiO₂ Nanoparticles

Quantum dots (QDs), also named semiconductor nanocrystals or semiconductor nanoparticles, are a kind of spherical or quasi-spherical nanoparticle, which are usually composed of atoms from groups II–VI, III–V, or IV–VI of the periodic table. QDs are nanocrystals with the diameter within 10 nm, composed of nanometer-sized crystalline clusters of a few hundred to a few thousand atoms, which have different properties compared with the bulk crystals. As a result of their small size, QDs possess high surface-to-volume ratios, such that many of their chemical and physical properties are dominated by their surface, not by their bulk volume. One extraordinary property of QDs is that the particle size determines many of the QDs'properties, most importantly the wavelength of fluorescence emission. Typical QDs have been exploited in inorganic ion sensors, organic small molecule sensors, biological macromolecule sensors, biological labeling, cell labeling, cellular effector and reporters, animal imaging and therapy. Thus far, one of the fastest developing and most exciting interfaces of nanotechnology is the use of quantum dots (QDs) in biology.In chapter 1, we described the extraordinary optical properties of QDs and developments in methods for their synthesis. Then we focused on the biomedical application, and the toxicity of QDs and potential barriers to their use in practical biomedical applications. Finally, we provided insights into improvements aimed at decreasing the toxicity of QDs, and the significance and contents of this dissertation.In chapter 2, we studied the synthesis and application of aqueous CdTe QDs. We choose a new ligand mercaptosuccinic acid (MSA) as stabilizer for CdTe QDs synthesis in both refluxing and hydrothermal routes. The stabilizer MSA composed of both thioglycolic acid (TGA)-like and 3-mercaptopropionic acid (MPA)-like moieties, which can accelerate the growth of CdTe QDs in the whole synthesis process. We optimized the condition of two routes by varying the ratio of precursors and reaction temperature. Ultimately, the CdTe QDs obtained by two routes all have high PL QY and narrowing size distribution. These MSA modified CdTe QDs have carboxyl groups on their surface and can be easily combined with antibodies or antigens for biomedical applications. Then using the synthesized CdTe QDs, we fabricated multilayer CdTe QDs films on quartz slides by LbL assembling of poly(dimethyldiallyl ammonium chloride) (PDDA) and CdTe QDs capped with mercaptosuccinic acid (MSA). Through the electrostatic interactions, PDDA and CdTe QDs were alternately deposited on the quartz slides, thus photostable QDs multilayer films were obtained. The fluorescence of PDDA/QDs multilayer films was sensitive to the existence of Hg (II) ions. The fluorescence of multilayer films was quenched effectively by Hg (II) ions. The concentration of Hg(II) in aqueous solution can be determined through the fluorescence quenching of multilayer films.In chapter 3, we embedded aqueous CdTe QDs in silica spheres by reverse microemulsion method and functionalized the silica spheres as photostable fluorescent probes were applied to biological labels. As the intermediate silica species can carry negative charges at pH 11, the same pH as CdTe QDs stabilized by MSA, only one QD can be coated by silica sphere, which may weaken the fluorescence intensity of CdTe@SiO₂ core-shell nanoparticles and limit their applications. With the aim of embedding more CdTe QDs in silica spheres, we used poly(dimethyldiallyl ammonium chloride) (PDDA) to balance the electrostatic repulsion between CdTe QDs and silica intermediates, which enhanced the fluorescence intensity of CdTe/SiO₂ composite nanoparticles effectively. The modified surface of silica nanoparticles has amino groups as functional groups which combine with biomolecules and methylphosphonate groups as stabilizing groups which reduce aggregation of silica nanoparticles. The CdTe/SiO₂ composite nanoparticles were linked with biotin-labeled mouse IgG via covalent binding. Furthermore, they can recognize FITC-labeled avidin and avidin on the surface of polystyrene microspheres successfully through the high affinity between avidin and biotin. Finally, the CdTe/SiO₂ composite nanoparticles were used to label the MG63 osteosarcoma cell, which demonstrates the application of CdTe/SiO₂ composite nanoparticles as fluorescent probes in bioassay and fluorescence imaging is feasible.In chapter 4, we attempted to prepare Mn:ZnSe d-dots by a nucleation–doping strategy with mercaptopropionic acid as the capping reagent in aqueous solution. Mn:ZnSe d-dots are new generation fluorescence emitter, which have great application potentials in biomedical fields for no heavy metal element addition during the synthesis process. Compared with organometallic synthesis, aqueous synthesis is cheaper, simpler, less toxic and Mn:ZnSe d-dots synthesized in aqueous solution are biologically compatible, making them much more suitable for biomedical applications. The optimal precursor ratio and the kind of stabilizer for obtaining Mn:ZnSe d-dots with good PL emission properties were studied in detail. As compared with CdTe QDs synthesized in aqueous solution, Mn:ZnSe d-dots have much better photostability, indicating that they can be applied as outstanding fluorescent labels for biological assays, imaging of cells and tissues, and even in vivo investigations.In chapter 5, we explored another fluorescent method for systematically investigated the DNA damage induced by a series of previously synthesized water-soluble nanoparticles, including CdTe QDs, CdTe/SiO₂ composite nanoparticles (CdTe/SiO₂ NPs), and Mn-doped ZnSe QDs (Mn:ZnSe d-dots). Ethidium bromide (EB) is a fluorescent compound, which is normally used to probe DNA structure in drug-DNA and protein-DNA interactions by its intercalating in the DNA double helix to enhance its fluorescence. Once the double helix structure of DNA molecules is destroyed, the fluorescence of EB will be quenched. It was found that ionic strength, pH value and UV irradiation influenced the PL emission properties of CdTe QDs, CdTe/SiO₂ NPs and Mn:ZnSe d-dots, and also influenced the interaction of DNA molecules with them. Among the three kinds of nanoparticles, DNA molecules were most easily damaged by CdTe QDs whether in the dark or under UV irradiation. The CdTe/SiO₂ NPs led to much less DNA damage when compared with CdTe QDs, as a silica overcoating layer could isolate the QDs from the external environment. Mn:ZnSe d-dots as a new class of non-cadmium doped QDs demonstrated almost no damage for DNA molecules, which have great potentials as fluorescent labels in the applications of biomedical assays, imaging of cells and tissues, even in vivo investigations.