Controllable Synthesis And Structure And Properties Of Gold Nanoparticles And Their Nanocomposites

Gold nanoparticles are one of the most promising nanomaterials for numerous applications in biomedicine, sensor, optoelectronic devices, and catalysis, due to their superior biocompatibility, special optical, electronic, and catalytic properties. The mechanism of nucleation and growth of gold nuclei in solution-based synthesis, the principle and routes of controllable synthesis of gold nanoparticles with well-defined size and morphology, as well as multifunctional modification and assembly are the focus and hot topics of gold nanoparticles. In this dissertation, stable monodisperse Au₁₁ nanoclusters and water-soluble gold nanocrystals have been successfully prepared via the modified Brust synthesis. The influence of the reaction parameters on the formation and growth of gold nuclei as well as the ordered assembly of nanoparticles has been investigated. Furthermore, using the prepared gold nanoparticles as building blocks, multifunctional NaYF₄:Yb³⁺/Ho³⁺@SiO₂/Au and Fe₃O₄/Au nanocomposites have been fabricated. The relationship between properties and microstructures of gold nanoparticles and their nanocomposites have been clarified.By employing 3-mercaptopropionic acid (MPA) as ligand and adjusting the mole ratio of Au3+ to MPA, the adding rate of NaBH4 and the reaction temperature, one-sized stable Au₁₁(SCH₂CH₂COO-)₇(TOA⁺)₇ nanoclusters are prepared by a modified Brust biphasic synthesis. The blue luminescence with excitation and emission maxima at 355 and 417 nm, respectively, is observed from the Au₁₁ nanoclusters. The luminescence quantum yield is determined to be as high as 8.6 %, which is attributed to the protection of nanoclusters from quenchers in the solution by the high-density (SCH₂CH₂COO-)(TOA⁺) ligands and one-sized nanoclusters prepared to eliminate the influence of other sized nanoclusters. By using the uniform Au₁₁ nanoclusters as building blocks, well-defined ordered Au₁₁ nanocluster superlattices have been fabricated. Compared to the emission maximum of diluted solutions of Au₁₁ nanoclusters, the superlattices exhibit a red-shift of 56 nm.The highly stable monodisperse water-soluble gold nanocrytals are prepared by a modified Brust single-phase method. It is found that the formation of homogeneous Au0 nuclei and uniform growth rate for every nucleus are key to obtain monodisperse gold nanocrystals. The homogeneous Au0 nuclei can be prepared by controlling the size distribution of Au1+ intermediates. The fast stirring speed can make every gold nuclei have a uniform growth rate. By tuning the adding rate of NaBH4, the different-sized gold nanocrystals can be obtained. The MPA-stabilized gold nanocrytals prepared exhibit a high X-ray attenuation and superior stability. When the concentration being 0.2 mol/L, 6.6nm-sized gold nanocrystals have an X-ray attenuation value of 1663 HU, while the commercial computed tomography iodinated contrast agent, Ousu, only has a value of 826 HU. The gold nanocrystals can be stored for as long as 14 months as a powder state and can keep stable in a solution containing NaCl, even with the concentration of NaCl being 240 mmol/L.A new type of bifunctional NaYF₄:Yb³⁺/Ho³⁺@SiO₂/Au nanocomposites is successfully fabricated by a layer-by-layer technology. The prepared nanocomposites possess upconversion (UC) luminescence, surface plasmonic resonance and easy modification. The gold nanoparticles loaded on the nanocomposites only decrease the intensity of UC luminescence, but have no influence on the UC mechanism. The main reason for the decrease of the intensity of UC luminescence is that NaYF₄:Yb³⁺/Ho³⁺ nanocrystals and gold nanoparticles constitute a fluorescence resonant energy transfer system.A novel and simple layer-by-layer assembly method is proposed to fabricate bifunctional Fe₃O₄/Au nanocomposites which are formed by a strong chemical Au-S bond linkage. With the increase of the gold content in the nanocomposite, the absorption peak is broadened and exhibits a red shift, and the magnetization decreases. The nanocomposites possess superparamagnetic property and exhibit high surface enhanced Raman scattering activity. When employing the nanocomposite as a probe to detect the nitrobenzene in water, the nanocomposites exhibit high selectivity of nitrobenzene and the process is facile with high sensitivity. The detection limit for nitrobenzene is 10-7 mol/L, which is lower than the maximum level of nitrobenzene in water permitted by the Chinese government.