Synthesis And Properties Of Upconversion NaYF₄/Metal Composite Nanomaterials

Recently, rare-earth doped upconversion nanocrystals (UCNs) have received enormous attention due to their unique UC property which enables them wide applications ranging from biological medicine, disease diagnosis, solid-state laser to optical fiber. Among these materials, sodium yttrium fluoride (NaYF4) is much popular and has been proven to be the best host matrix for lanthanide doping to give the highest upconverting efficiency. Uniform NaYF4NCs have been developed in nonpolar solvents in recent years. However, they could not be directly applied to biological systems due to their poor biocompatibility resulted from their hydrophobic surface. As such, it is desirable to synthesize monodisperse and high-quality NaYF4NCs with hydrophilic surface using convenient solution-based method. On the other hand, it is still challenging to construct a multifunctional NC which combines UCN with other functional materials. Such kind of multifunctional nanomaterials may be applied in a variety of fields.Metal NCs possess unique SPR properties, excellent stabilities and high biocompatibilities, so that they have widely used in the fields of medicine, energy, catalysis, electronics, and biology and so on. It is a hotspot in present research that combining noble metals with rare earth doped UCNs. Modification of noble metals on UCNs can not only enhance their fluorescence intensities, but also improve the biocompatibility, thus satisfying the demands of biological medicine, pharmacy and so on. However, it is a grand challenge to synthesize monodisperse and hydrophilic composite materials consisting of UCNs and nobel metals..In this thesis, we demonstrate a simple strategy to surface modify NaYF4NCs through producing a carbonaceous layer on them. The carbonaceous layer is formed by carbonization of glucose molecules in hydrothermal condition. Such a surface is much hydrophilic and has lots of function groups such as-OH and C=O around, favoring grafting to various biomolecules for biomedical applications. The influence of surfactant and concentration of glucose on the morphology of NaYF4@Carbon has been studied in detail. Controllable thickness of the layer can be achieved by adjusting the experimental parameters.Furthermore, nobel NCs can be attached to the NaYF4NCs, taking the advantage of the carbonaceous layer. We proposed a simple method to prepare NaYF4@C@Au and NaYF4@C@Ag hybrids, in which the carbonaceous layer on the surface serves as a "bridge" between two types of NCs. Two different structures have been produced in the NaYF4:Yb,Er@C@Au hybrids, namely ’inside’ and ’outside’, using Au precursors with positive or negative charges, respectively. The green-to-red emissions of the UCNs can be conveniently tuned by altering the amount of gold NCs. It is anticipated that the controllability in UC fluorescence in this hybrid nanostructure may provide a platform for widely exploring applications in biological imaging, detection, and sensing. In the NaYF4:Yb,Er@C@Ag composite materials, continuous red-shift of SPR band from Ag NCs can be realized by controlling the thickness of the carbon shell or the amount of Ag NCs. The SPR band could be tuned from400nm to NIR region, which may have promising application in biomedical engineering such as photothermal therapy.In addition, we developed a facile route to prepare hydrophilic NaYF4nanorods using hydrophobic NaYF4nanospheres as precursor. It is interesting that hydrothermal treatment of oleic acid-capped NaYF4nanocrystals can not only induce anisotropic growth of these nanocrystals but also change their surface properties. Hydrothermal treatment of nanospheres not only changes their surface properties but also induces anisotropic growth of these nanocrystals through Ostwald ripening. The hydrophilic NaYF4nanorods were well characterized and formation mechanism was also proposed.