Synthesis Of Hollow And Porous Rare Earth Fluoride Upconversion Nanomaterials, Phase Transformation Mechanism And Application Of Biological Imaging

Rare-earth doped upconversion nanocrystals(UCNCs) are excited by the 980 nm near infrared laser. Owing to their unique luminescencent properties, such as sharp emissions, long fluorescent lifetime, the large stokes shift, superior physical and chemical stability, low long-term toxicity, numerous potential applications of rare-earth doped nanocrystals, such as the biological imaging, display devices, solar cells, optical communication, solid-state lasers and catalysis, have been explored, and UCNCs have sparked considerable interests among different technological and scentific subjects. The fluoride nanocrystals of NaYF4, NaGdF4, NaLuF4, CaF2, BaGdF5, BaLuF5 and BaYF5 have been regarded as the most commonly-used host materials, the synthetic strategies towards high quality UCNCs with uniform size and morphology are widely studied.This thesis can be divided into three parts:The first part is to synthesize LuF3 upconversion nanoparticles(80–100 nm) and Na1.5Y2.5F9 flower-like assemblies(450–550 nm) through a novel cationic surfactant assisted hydrothermal method. Controlling the initial shape of the building blocks, growth oriented by the cationic surfactant and Ostwald ripening are deemed as the possible formation processes in the hydrothermal system. In this part, we have compared four quaternary ammonium-type cationic surfactants: cetyltrimethyl-ammonium bromide(CTAB), dodecyl dimethyl benzyl ammonium chloride(DDBAC), dodecyl trimethyl ammonium bromide(DTAB), and didodecyl dimethyl ammonium bromide(DDAB). Compared with other quaternary ammonium-type cationic surfactants, samples with DDBAC exhibited a decreased spontaneous aggregation phenomenon and better dispersion, uniformity and stability.The second part is to synthesize these water-soluble hollow/porous upconversion nanocrystals, such as NaYF4, NaGdF4, NaLuF4, CaF2, BaGdF5, BaLuF5 and BaYF5, through the anion-induced hydrothermal oriented-explosive strategy. Owing to external ions corrosion and internal gas pressure caused by the decomposition of organic additives in hydrothermal conditions with high temperature, UCNCs can form cavities and hollow structure. Based on the synthetic procedure, the ?explosion? method was divided into three steps:(I) ?Deep zone decomposition? is suitable to produce hollow assemblies;(II) ?Blob and oriented decomposed techniques? is facilitating the formation of porous rods, porous hamburger structures;(III) ?Superficial zone decomposition? is leading to superficial porous nanoparticles just like ?the surface of moon?. We have successfully applied the structural mechanics finite element calculations to validate the reaction process.The third part introduces ?crystal cell oriented-rotation triggered phase transition?, ?energy zones hypothesis of phase transition from cubic to hexagonal structure?, and ?the trend hypothesis of crystal cells energy change?, they can reasonably explain the phase transformation process from cubic structure to hexagonal sturcture, and improve the upconversion luminescence intensity about one or two orders of magnitudes. These mechanisms are helpful to solve the key problem for low upconversion luminescence intensity. We have synthesized the sub-5nm NaGdF4, CaF2 upconversion ?nanoclusters? and water-soluble NaYF4 porous upconversion nanoparticles with 15-200 nm.The experiments for cell toxicity demonstrate that water-soluble porous upconversion nanocrystals are safe, non-toxixity and good biocompatibility. In vitro MGC-803 cells upconverison luminescence imaging and in vivo CT, MRI multi-mode imaging show that NaGdF4 and NaLuF4 nanocrystals can act as multi-mode imaging contrast agents. We prepared some monochrome imaging model with β-NaYF4:Yb3+, Er3+ upconversion nanocrystals, which show that water-soluble porous upconversion nanocrystals can be potential upconversion luminescence display device, and the fluorescent display device is a key technology for the rapid development of laser television.Our thesis of ?anion-induced hydrothermal oriented-explosive strategy?, ?crystal cell oriented-rotation triggered phase transition?, ?the energy zones hypothesis of phase transition? and ?the trend hypothesis of crystal cells energy change? can provide a new train of thought for the controllable synthsis of rare earth upconversion nanomaterials, and improve materials? performance through inducing nanocrystals? phase transformation.