| Recently, metal nanoparticles have attracted great attention due to the unique optical and electric properties, and thus they can be widely exploited to detect some analytes such as DNA, amino acid, protein, metal ion and so on on one hand, and can assemble into one-, two-, or three-dimensional structures on the other, which can offer the unique collective properties of assembly structures.In this thesis, investigations on the assembly of gold nanorods in high ionic strength solutions, colorimetric assay of melamine have been made on the basis of the network structrue of gold nanoparticles, the morphological change of Ag nanoparticles induced by halogen and the formation of core-shell structure of Ag nanoparticles. The main contents are as follow:1.Assemblies of gold nanorods (Au-NRs) induced by captopril (Cap) in high ionic strength solutions. Captopril molecule has is a thiol group and a carboxylic group on the each side, and thus can bond to the surface of Au-NRs on one hand because of the easily formed Au-S covalent bond, and are preferable to combine with each other through cooperative hydrogen bond with its carboxylic groups on the other, which offer the foundation for introducing the assembly of Au-NRs. Furthermore, it was found that the assembly structures could be formed only in the presence of both Cap and NaCl.In the medium of low concentration of NaCl, Au-NRs can form side-by-side structures because NaCl can neutralize the electrostatic repulsion among CTAB-capped gold nanoparticles.In high concentration of NaCl, on the other hand, abundant Cl- is present around the surface of Au-NRs forming an electronic double layer, which keeps Au-NRs well dispersed. In such case, Au-NRs can realize the end-to-end assembly. At the same time, several Au-NRs can form the side-by-side assembly because Cl" can neutralize the electrostatic repulsion among Au-NRs to some extent, so the assembly of Au-NRs containing both end-to-end and side-by-side structures is formed. This assembly method will be of importance in chemical and biological sensing.2. Network assembly of gold nanoparticles coused by melamine. Thioglycolic-acid-modified gold nanoparticles (Au-NPs) were synthesized under alkaline condition, so most carboxyl groups are negatively charged.Before the interaction, HCl was used to modulate the acidity of melamine, making sure that amino groups are mainly positively charged.When mixed, electrostatic interaction occured between modified Au-NPs and melamine,resulting in the network assembly of Au-NPs. At the same time, the color of mixed solution changed from red to blue.Upon this,a colorimetric method of detecting melamine is developed.This method is simple,fast, intuitionistic and low-cost.Employing it to detect melamine in milk powder, the analytical result is satisfying, so this colorimetric assay of melamine will be promising and worth being developed.3.We investigated the morphological change of citrate-capped Ag nanoparticles (Ag-NPs) induced by halogen. From the results of UV-vis absorption spectra and SEM images, it is concluded that at room temperature, iodine could fuse Ag nanoparticles, and produce AgI.At the same condition, bromine can not fuse Ag nanoparticles, but only induce the aggregation of Ag nanoparticles.Besides, we also found that the diameter of citrate-capped Ag nanoparticles (Ag-NPs) enlarges obviously after adding Cu2+ and Br", and the whole Ag-NPs still keep well dispersed. Increasing the concentration of Cu2+ and Br-, the absorption intensity gradually decreases, and the absorption peak red shifts firstly, then blue shifts.Finally a new characteristic absorption appears at longer wavelengths(～620 nm).What is more interesting is that all the absorption spectra intersect at an isosbestic point (λ.≈360 nm, A≈0.270) perfectively. UV-vis absorption spectra, SEM images and energy dispersive spectrdmeter were employed in this study, and we speculate that Cu2+ and Br- may form some complex compound, covering the surface of Ag-NPs to form a new core-shell structure nanoparticles.
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