Study On Synthesis And Assembly Of Optical Function Nanoparticles Through Density Gradient Ultracentrifuge Rate Separation

Due to the special size effect of nano materials, a substantial change of material’s optical properties have been demonstrated, which make the optical function nanoparticles potential application in bio-labeling and display etc. Lanthanide-doped upconversion crystal and noble metal were widely noted. In this thesis, we demonstrate detail growth mechanism of NaYF4:Yb/Er and Au assembly through density gradient rate separation method. Details are illustrated as follows:1. Separation and synthesis of NaYF4:Yb/ErLanthanide ions exhibit unique luminescent properties, including the ability to convert near infrared long-wavelength excitation radiation into shorter visible wavelengths through a process known as photon upconversion. Lanthanide-doped upconversion nanocrystals have been developed as a new class of luminescent optical labels that have become promising alternatives to organic fluorophores and quantum dots for applications in infrared display, biological assays, medical imaging and so on. It was found that the NaYF4was potential candidates for host materials, because of low phonon energies and close lattice matching to dopant ions.We know the properties and applicatons of NaYF4:Yb/Er closely related to their composition, morphology and size. It is usually assumed that the atomic doping in final products are uniform and the same as feeding ratio, even the yield is not100%, and in a mixed condition. During the growth procedure of NaYF4:Yb/Er, there commonly exists phase transition from cubic kinetically stable α-phase to thermally stable β-phase, however, whether there exist segregations during the nucleation or phase transition is unexplored, which lags far behind their monodisperse counterparts. To find out the relationship between size, shape, composition, and properties (e.g. photoluminescence) in a mixture, a sorting procedure is required.We prepared NaYF4:Yb/Er nanocrystal with different morphologies and different phases by solvothermal method. Then, for the first time, NaYF4:Yb/Er nanocrystal were sorted according to size using a density gradient ultracentrifuge rate separation (DGURS) method. We investigated the dependence of compositions, phase and optical properties difference on sizes and morphologies of each fraction. Compared with the feeding ratio Y:Yb:Er=78:20:2, the cubic a-phased NaYF4was rich in Y, majored in red luminescence, while hexagonal β-phased NaYF4had relatively higher Yb and Er atomic ratio, majored in green emissions, which suggested segregation during nucleation step in such system. To verify the evolution of composition by monitoring the whole formation process involved nucleation and growth, we performed the experiments with different reaction time and detailed control experiments with single rare-earth ion:NaYF4, NaYbF4, NaErF4. We proposed a plausible mechanism of composition evolution accompany with phase transition. Inspired by the separation results, we discovered that the pure β-NaYF4:Yb/Er could be controlled by varying the amount of NH3H2O in the synthesis process, also formed different morphologies.2. Au assembly in a centrifugal fieldFor Au nanostructure, many special properties such as surface plasmon resonance, surface-enhanced Raman scattering, catalytic properties make gold nanostructures widely used in many areas such as sensors, drug delivery, biological imaging, catalysis and photothermal heat. Fabrication of elaborate nanodevices requires precise control over size, morphology, stoichiometry, as well as symmetry. More and more people pay attention to heterogeneous assembly. Compared with these homogeneous assemblies, heterogeneous assembly could link two or more kinds of distinct particles together. It can not only combine properties associated with different kinds of particles, but also possibly show unique and superior properties, such as chemical and electronic anisotropy. Centrosymmetric hetero-assembly could be achieved through spontaneously assembly of two kinds of NPs with different surface ligands. However, for asymmetric hetero-assembly, only templated assembly of NPs could achieve such symmetry control. Which greatly hindered the applications of the assembled structures in solution. Herein, we introduced a centrifugal field to the colloidal assembly system to overcome Brown motion effect and control the directional movements of NPs. Details are as follows:Take60nm and20nm Au NPs functionalized with different ligands for example, when centrifugal field was applied, since big NPs had larger radius and apparent density, they would fall much faster than small ones according to the sedimentation equation, thus "crash" would happen when two kinds of NPs met. The sedimentation velocity of spherical colloidal particles is much faster than Brown motion, so centrifugal force was dominant to control the directional movements of NPs and overcome Brown motion effect, guaranteeing asymmetric hetero-assembly. We increased the concentration of small NPs to increase the "crash" probability. We randomly selected assembled particles from "crash reaction" and normal "random" crosslinking reaction to evaluate how much the small nanoparticles bind off-centered by analyzing their gravity center. Such quantified data demonstrated asymmetric hetero assembly is taking place in "crash reaction" system. The collision unit could be tuned by increasing the amount of ligands used for functionalization of20nm Au NPs, which can regulate the assembly morphologies from Janus to comet-like or even tadpole-shaped hetero-assembly. Next, we studied the closed packed Au assembly at the water/oil interface in centrifugal field. Then further enhancement of the effective collision probability was carried out by introducing a water/oil interface to the density gradient.