Synthesis Of Monodisperse,Quasi-spherical Gold Nanocrystals In Water And Their Application

Gold nanocrystals (Au NCs), known since the times of ancient Romans, are the most stable metal nanoparticles. Au NCs have aroused great concern and have been extensively studied because of their special physical and chemical properties and attractive prospects in science and technology applications, especially in the emerging field of nanocrystals synthesis and self-assembly. Soluble Au NCs have stimulated extensive interest due to their good biocompatibility, wide academic study and fascinating practical application in this field. The Turkevich method was the most commonly used one to synthesize Au NCs due to its flexibility to tune the size of Au NCs from16to147nm by varying the molar ratio of citrate to HAuCCl4. The polydispersity increases with the increase of size of Au NCs and the shape of Au NCs shows elliptical or other irregular shapes; the stability of Au NCs decreases, and the repeatability can hardly control between batches. Particularly, the reproducibility can hardly control between batches. Xia and Wang et al reported a simple way to produce quasi-spherical Au NCs with a narrow size distribution in water by rapidly adding a mixture solution of HAuCL4, sodium citrate, and a trace amount of silver nitrate into boiling water. The sizes of quasi-spherical Au NCs obtained increases from12±1nm to36±3nm. This approach efficiently makes up the shortages of the classical Turkevich method with respect to the reproducibility and uniformity of the size and shape. Currently, Au NCs with size bigger than40nm were usually prepared by seed-growth method, which needed mane steps and was complicated to manipulate. In the past more than5decades, countless studies of the Turkevich method were conducted in multifarious applications. Many influence factors have been investigated, such as the amount of reductant, the sequence of adding reagents, reaction time, and pH value, etc. However, the sizes of Au NCs prepared by Turkevich method are different in the range of12to16nm under the same recipe by different researchers. This phenomenon is not still understood very well until now. So far, there was no consistent explanation for the mechanism governing the nucleation and crystal growth of Au NCs in the Turkevich method. How to synthesize monodisperse gold nanocrystals with controlled size and uniform morphology is evidently quite a challenge.Ultra-small noble metal nanocrystals (<2nm) were commonly known as noble metal nanoclusters, such as Au nanoclusters and Ag nanoclusters. Highly fluorescent, water-soluble noble metal nanoclusters have stimulated extensive study due to its discrete, size-tunable electronic transitions throughout the visible and near IR. Au nanoclusters have attracted considerable interest because of their attractive superiority: ultimately small sizes, excellent biocompatibility, favorable stability, and water solubility. Recently, fluorescent Au NCs are recognized as promising candidates for cell labeling, biosensing, and selective detection of trace metal ions. However, there are still some problems to be solved in the process of application in biological systems. For example, the preparation of nanoclusters modified by biocompatible molecular with uniform size is still difficult; The improvement of the stability of the gold nanoclusters is a big challenge; Moreover, the fluorescence quantum yield of most gold clusters is not very high, which cannot meet the needs of the actual application.On consideration of the above content, this thesis focuses on the monodisperse quasi-spherical gold nanocrystals. The effect of temperature and gas bubbles on the formation of Au NCs by citrate reduction of HAuCl4by classic Turkevich method and our premixing approach was further researched. The nucleation-growth mechanism was explored at the optimal conditions. Under the guidance of the resulting mechanism, one-pot synthesis of monodisperse, quasi-spherical Au NCs with size range from2nm to330nm via citrate reduction was achieved, which was further verified our comprehension and knowledge about the mechanism. In addition, fluorescent GS/C-Au NCs with Au(0)@Au(I)-GS/C core-shell structure was prepared by selective reduction via co-reduction of glutathione (GSH) and citrate.This thesis is totally divided into four parts:Chapter2, the effect of latent heat on the formation of Au NCs by citrate reduction method was investigated in detail. Transmission electron microscopy (TEM), dynamic light scattering (DLS) and UV-Vis spectroscopy were used to characterize the samples. The results indicate that the effect of latent heat is the key factors on size variation of Au NC synthesis by Turkevich method under the same recipe. Moreover, our results suggest that a prerequisite for effectively suppressing secondary nucleation in a colloidal synthesis is that the primary nucleation must produce a critical amount of nuclei and the rate of atoms for growth (Vgrowth) should be close to the rate of formed ones (Vformed) in general. Secondary nucleation or polydispersity would happen due to excessful Au0atoms for growth (Vgrowth<Vformed) or insufficient Au0atoms for growth (Vgrowth> Vformed), resepctively. The bubbles may provide one way to control nucleation rate in the synthesis of monodisperse colloidal nanocrystals.Chapter3, start with the abnormal color change in the process of preparation of Au NCs, the mechanism of nucleation-growth was investigated by measured the size and Zeta potential of intermediate products and determined the consumption of AuCl4-ions. The change of surface charge density during growth has a great influence on the shape and size of the intermediate. The SADC-Au+cluster complex would aggregate or decompose induced by the variation of charge density. The final size of Au NCs is irrelevant to the aggregation behavior, but it is in contact with the number of initial nuclei. It was found that the appearance of blue color during formation process of Au NCs was due to agglomeration of particles of cluster complex of SADC-Au+(primary nuclei). The disappearance of abnormal color change can be achieved by increasing the surface charge density or introducing assistant ligands; the final size and shape of Au NCs can be controlled by the regulation of growth process. It was found that the amount of "new nuclei" can be controlled by introducing other ligands to adjust the size of primary nuclei. The results also proved that high concentration of sodium citrate inhibits the reduction AuCl4-, while SADC can speed up the rate of formation of nuclei at low concentration of sodium citrate. The above analysis shows that the nucleation process of Au NCs can be effectively controlled by tuned the number of nuclei and the aggregation and dispersion of SADC-Au+clusters.Chapter4, Au NCs with huge size range of2nm～330nm was successfully prepared by modified Turkevich method and premixing method. UV-vis, DLS and TEM were used to characterize the size, shape, structure and optical properties of the Au NCs.(i) Au NCs of6nm to39nm with size interval of3nm were prepared by premixing method. The amount of particles of complex cluster of SADC-Au+as nuclei can be finely tuned by the molar ratio of citrate to HAuCl4(R) and premixing time. With the decrease of R, the premixing time is reduced and the size increased,(ⅱ) Au NCs of2nm and4nm were prepared by premixing method via adding GSH as assistant stabilizer due to the strong affinity between Au and S.(ⅲ) Au NCs of40nm to95nm were prepared by premixing method via adding Tris-base as assistant stabilizer. The agglomerations of particles of complex cluster of SADC-Au+as new primary nuclei (Scheme lc) would be mainly stabilized by Tris-base; citrate is used to guarantee subsequent growth.(ⅳ) Au NCs of105nm to330nm were prepared by Turkevich method via adding Tris-base as assistant stabilizer. The amount of particles of complex cluster of SADC-Au+as nuclei would be mainly formed by citrate reduction and would be mainly stabilized by Tris-base.Chapter5, synthesis of water-soluble, highly fluorescent Au NCs by co-reduction of GSH and citrate at low temperature (50℃) within24h. The high content of Au(Ⅰ)-thiolate complexes (about75%) on the surfaces of core-shell structured GS/C-Au NCs obtained under optimal concentration of GSH and citrate is responsible for their strong fluorescence generated by the AIE. The citrate is used to (ⅰ) lower the reaction temperature;(ⅱ) control formation rate of Au(0) cores by selective reduction of Au(Ⅲ) ions under optimal conditions; and (ⅲ)enhance the colloidal stability of GS/C-Au NCs in the wide pH range from4.1to8.6due to differently stable states of glutathione-citrate complex. In addition, fluorescence intensity of GS/C-Au NCs obtained is pH-dependent and can be reversibly adjusted in the pH range from4.1to8.6due to changes in their surface charge density stemming from the transitions among differently stable states of glutathione-citrate complex. Our preliminary study also demonstrates that GS/C-Au NCs can be used as fluorescent nanoprobes in bio-imaging.In this paper, the nucleation-growth mechanism of sodium citrate reduction was investigated detailedly. New synthesis strategy has been designed according to the resulting mechanism to synthesize quasi-spherical Au NCs with size range from2nm to330nm via citrate reduction, which makes up the shortages of the uncontrollability of Turkevich method, size limitation of premixing method and cumbersome operation of seed-growth method. The monodisperse quasi-spherical Au NCs greatly broadened their practical applications in biological detection and biosensing techniques. The fluorescent GS/C-Au NCs has great potential application value in cell imaging and pH detection due to their good stability in the pH range from4to9, and the linear relationship between fluorescence intensity and pH value.