Gold And Silver Nanomaterials: Synthesis, Characterization And Research In SERS

The preparation and the application of nanomaterials has been known for a long time. In recent years, nanomaterials have been exploited for the extensive potential applications in optics, electronics, magnetics, sensing, biomedicine, etc,due to their peculiar surface effect, volume effect, and quantum effect that are different from those of bulk materials and molecular compounds. Because of these properties, nanomaterials have attracted more and more people to devote themselves into this field. Among all kinds of nanomaterials, gold and silver nanomaterials which have fascinating optical and electrical properties, have been the subjects of a numbers of studies for fundamental interests and practical applications. The unique properties of nanomaterials are determined by their size, shape, composition, crystalline and assemble structures,thereby size-controlled and morphology-controlled preparation of nanomaterials is very important. At present, various synthesis methods for gold and silver nanoparticles(NPs) have been reported, however, most of them are complicated. So how to synthesize gold and silver NPs with controllable size and shape in a convenient way is still a barrier and challenge to further study its unique properties and broaden their application. Besides, under irradiation of lights with a proper wavelength, the surface plasmon in metal particles can be excited, as a result, the local electric field close to the particles can be greatly enhanced. Based on this phenomenon, the research of surface enhanced Raman spectroscopy(SERS) in gold and silver NPs is a hotspot. This dissertation is focused on the synthesis, characterization and related SERS enhancement of silver and gold/silver bimetallic nanomaterials, in particular the research of SERS in interaction between molecule and metal NPs. The details are summarized as follows:1. Synthesis, structure and growth mechanism of size and shape tunable Au/Ag bimetallic NPs (Chapter 2). By simply adding ascorbic acid in advance of AgNO3, the size and shape controllable Au/Ag bimetallic NPs were prepared in the traditional Au growth solution free of seed at room temperature. The size distribution of NPs is well uniform with ca. 10～15% standard deviation in diameter. The size and shape of NPs are tunable by changing the Cetyltrimethylammonium bromide(CTAB) concentration. An enhancement factor varied from 4.3×104 to 1.1×105 was obtained for the surface-enhanced Raman spectroscopy (SERS) behavior of the prepared NPs centered at ca. 64±8 nm. The electrochemical cyclic voltammetric results revealed that the so formed NPs is Au enriched Au/Ag bimetallic NPs. The formation of Au/Ag bimetallic NPs might be due to the disproportionation reaction of Au+ prompted by Ag+ and the under potential deposition process of Ag+ on Au.2. Synthesis, characterization and growth mechanism of Ag nanowires(NWs), NPs and Ag/Au bimetallic NPs(Chapter 3). By simply adding ascorbic acid in AgNO3 solution, with CTAB as protective agent, the Ag NWs and NPs were prepared at room temperature. We separated and concentrated the NWs and NPs through centrifugation. Furthermore, when the concentration of CTAB is too high that the reaction can't proceed successfully, we continued adding HAuCl4 in the former solution before heating in 40℃, then the Ag enriched Ag/Au bimetallic NPs were prepared. Characterized by UV-visible spectroscopy, scanning electron microscopy and transmission electron microscopy, the Ag NWs and NPs have large outputs after centrifugation; Ag NWs are about 4μm long and 23 nm wide, diameters of Ag NPs are about 47 nm. About the Ag riched Ag/Au bimetallic NPs, UV-Vis spectroscopy show the Ag/Au bimetallic NPs have the single plasmon absorptions, and these absorptions locate in the traditional silver plasmon absorption area. With decrease of Au3+ or increase of CTAB, the absorptions shift towards long wave length region, meanwhile, the size of Ag/Au bimetallic NPs increase slightly.3. The adsorption orientation of bifunctional molecules at the nanojunction region(Chapter 4): SERS technique was applied to investigate the adsorption behavior of PATP (4-aminothiophenol) on the SERS active substrates formed with Au (Ag) nanoparticle self-assembled monolayers. The formation and mechanism of laser induced molecular junctions were investigated with respect to the laser power, laser excitation wavelength and temperature dependent SERS spectra.