Assembly, Sepration And Optical Properties Of Noble Metal Nanoparticles

Noble metal nanomaterials have been widely studied due to their special physical and chemical properties, one of the most unique characteristic is the surface plasmon resonance (SPR) property. Gold, like most of the metal material, has many good properties, such as high electrical conductivity, reflectivity, ductility and corrosion resistance. When the size changes to nanometer, new properties and potential applications can be obtained. The most important one is the novel interactions between gold nanoparticles and light. When being irradiated with light, there will be collective oscillations of their conduction-band electrons of gold particles which is known as surface plasmon resonance (SPR). When the distance between the adjacent gold particles is short enough, there will be a "hot spot", which can greatly strengthen the localized field intensity, One-dimensional gold nanoparticles assembled nanostructures contain many "hot spots", and the longitudinal SPR properties can be adjusted through the change of the assembly degree, because of the properties of one-dimensional gold nanoparticle assembly, it has many applications, such as surface enhanced Raman scattering, biological detection, biosensing. There are many ways to realize the synthesis of nano-assemblies, but it is still hard to finely control the limited assembly degree, owing the poor stablity and the strong tendency of further aggregation. So how to control the degree of the assembly and stabilize them is still a problem. In this paper, we compared various assembly methods, realized controllable assembly and separation of Au nanoparticles based on density gradient centrifugation and the synthesis and separation of Au nanochain@SiO2 and Au nanochain@SiO2@SiO2. The research contents and results are as follows:1. Two synthesis methods are introduced to synthesize different size of gold nanoparticles:seed-mediated growth and sodium citrate reduction. We explored several different methods (PDDA induction method, NaCl induction method, cryogenic) to induce the assembly of gold nanoparticles. The TEM images showed that by changing the concentration of PDDA or NaCl, the size of the Au nano-assemblies can be tuned. When the environmental temperature is 5℃, the particles tended to form the chain-like assembly structures to maintain the lowest energy state. UV-Vis spectra show the SPR slowly and regularly red-shifted with the length increase of gold nano-assemblies.2. "One-tube synthesis" was developed to construct gold nanoassemblies, by introducing a thin salt (NaCl) solution layer into density gradient media, the very limited self-assmbly of Au nanoparticles was achieved. The electrostatic interactions between Au nanoparticles would be interfered and caused 1D assembly as passing through the salt layer. Negatively charged polymer such as poly(acrylic acid) was used as an encapsulation/stabilization layer to help the formation of 1D Au assemblies, which were subsequently sorted according to unit numbers at succeeding separation zone. Centrifugal field was introduced as the external field to overcome the random Brownian motion of NPs and benifit the assembly effect. Such facile "one-tube synthesis" approach coupled assembly and separation in one centrifuge tube by centrifugation at one time, and could be tuned by changing the concentration of salt layer, encapsulation layer and centrifugation rate. Furthermore, positively charged fluorescent polymers such as perylenediimide-poly(N, N-diethylaminoethyl methacrylate) could be encapsulated on the surface of the assemblies to provide tunable fluorescence properties.3. Au nanochain @ SiO2 @ SiO2 core-shell nanostructures with controllable morphologies and tunable shell-thickness were synthesized. First of all, gold nanoparticles with different sizes were prepared by the reduction of sodium citrate, MPA(mercapto propyl acid) was introduced to modify the surface of nanoparticles, NaCl was introduce to induced assembly and silica to stablize the structure, and the silica shell was used as a protective coating and an interlayer to prevent fluorescence quenching. Such nanostructures with precisely controllable size displayed unique optical properties, By changing the volume of ammonia, si lane and reaction temperature, the morphology and thickness of silica could be controlled.