| Gold nanoflowers(GNFs) has shown great potential applications in SERS,catalysis and biomedicine due to their unique optical and electromagnetic properties.The use of pre-synthesized seeds is a very popular choice for the preparation and morphology control of monodisperse gold nanoflowers. Ascorbic acid was regarded as an ideal reducing agent during the synthesis of GNFs based on its nontoxicity and biocompatibility. But it always comes with the assistance of surfactants in AA reduction while it is known to bring higher surface protection to the GNPs and lead to a decreased surface activity. Surfactantless synthesis of GNFs by AA reduction had been reported, but the mechanisms were still unclear while the morphology and properties of GNFs were not optimized, either.In this dissertation, the growth kinetics of the GNFs by AA reduction with seeding approach were investigated systematically. The various morphology of GNFs was obtained. SERS performance of GNFs with different branch length and morphology was evaluated.1. Mechanism in the preparation of gold nanoflowers by using the mixture of HAu Cl4 and ascorbic acid(AA) as growth solution was investigated. It was identified that the AA concentration affected the attachment of the small gold particles on the seeds and the intraparticle ripening of the gold nanoflowers, thus resulting in the formation of the flowers with different morphologies and optical properties. It is effective to tune morphology and optical properties of the gold nanoflowers by optimizing the AA concentration and p H of the growth solution simultaneously.Surface enhanced Raman scattering(SERS) results indicated the as-prepared goldnanoflowers showed good SERS activity to Rhodamine 6G, suggesting the clean surface character of the flowers prepared by AA reduction.2. As a weak ligand, AA couldn’t suppress the intraparticle ripening of the GNPs which is a disadvantage for the GNFs’ formation. Thus, Na3 Ct was introduced into the system, which is a stronger ligand than AA. Correlation between random attachment and intraparticle ripening was further investigated. It is revealed that Na3 Ct could suppress the intraparticle ripening, while it also provided stabilization to the newly reduced Au0 and accelerated the self-seeding growth. With larger concentration of AA,the stabilization caused by Na3 Ct could be weakened to avoid the self-seeding growth and enough to suppress the intraparticle ripening. Morphology and optical properties of the gold nanoflowers were further tuned by optimizing the Na3 Ct concentration and p H of the growth solution. The negative effects brought by Na3 Ct to the SERS and catalytic properties of GNFs could be removed by ligand-exchange.3. To further improve the morphology and optical properties of the GNFs, we exchanged the seeds of 25 nm gold nanoshperes with 25-28 nm gold dimers which could bring in the hot spots effect with the special architecture of it. The hot spots effect located on the gaps between two spheres provides an enormous electromagnetic enhancement of 103~104times. With the previous understanding of the AA reduction system, gold dimers with rough surface(gold dimer-flowers) was synthesized, of which the branch length was successfully tuned in a certain range.4. SERS properties of GNFs is basically determined by its morphology and optical properties. SERS spectra of the GNPs as synthesized in Chapter 2, 3, and 4were collected by using p-MBA as probe. The enhancement factors were calculated for GNPs with various morphologies. The experimental results indicated that the enhancement factors(EF) of the single-cored GNFs increased with the increased branch length while the GNFs are of the same size. The enhancement factors of gold dimer-flowers were much larger than those of the single-cored GNFs, which attributed to the local electromagnetic enhancement on the branch tips and the gaps ofthe dimers. The increasing of the branch length of the gold dimer-flowers could also result in the increasing of the EF. |
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