Synthesis And Biological Imaging Applications Of Water Soluble Rare-earth Upconversion Nanoparticles

Rare-earth upconversion nanoparticles （UCNPs） have been considered as excellentfluorescent probes for in vivo fluorescence imaging due to their many unique merits, such assuperior photostability, absence of photodamage to live organisms, low autofluorescencebackground, and high light penetration depth in biological tissues. In vivo fluorescenceimaging probe can be used to monitor gene expression and cellular activity directly, that hascaused wide public concern in recent years. Consequently, the development of biocompatiblerare-earth UCNPs holds great significance for bioluminescence imaging.In this thesis, rare-earth UCNPs NaYF₄:Yb³⁺/Er³⁺(Tm³⁺) are synthesized byhydrothermal or thermal decomposition methods. On this basis, the composite nanoparticlesNaYF₄:Yb³⁺/Er³⁺@Au and NaYF₄:Yb³⁺/Er³⁺@Ag are synthesized at140oC for2-4h in1-octadecene. Then chemical composition, crystal morphology and optical properties of therare-earth UCNPs are characterized by XPS, XRD, TEM, ultraviolet spectroscopy andfluorescence spectroscopy. The results show that the prepared rare-earth UCNPsNaYF₄:Yb³⁺/Er³⁺(Tm³⁺) are cubic phase or hexagonal phase. The rare-earth UCNPs areuniform with partical size in20nm in diameter. With excitation of a980nm laser, theupconversion fluorescence spectrum of the rare-earth UCNPs exhibites two distinct emissionbands at540nm and660nm. The fluorescence intensity ratio of emission peak at660nm tothat at540nm can be increased3.1times by modified the NaYF₄:Yb³⁺/Er³⁺nanoparticalswith Au or Ag nanostructures. Not only tunable spectrum is realized, but also tissuepenetration is improved. The rare-earth UCNPs may have a potential for in vivo fluorescentimaging probes.Ligand oxidation and ligand exchange methods are used to modify the surface ofrare-earth UCNPs. The ligand-modified rare-earth UCNPs were quite stable and water solublein water. The characterization of UCNPs with TEM, fluorescence spectroscopy andfourier-transform infrared spectroscopy shows that the particle size, crystalline phase, crystalmorphology and upconversion fluorescence properties of modified rare-earth UCNPs are notobvious changed. However, the ligand oxidation method can quench UCNPs in polar solventslike water. Therefore, the ligand exchange method is a simple procedure for UCNPs surfacemodification. Ligand with a plurality of hydrophilic and same charge groups is conducive tomake rare-earth UCNPs more stable and well-distributed in water. In vivo fluorescence imaging with UCNPs suggests that the water-soluble rare-earthUCNPs NaYF₄:Yb³⁺/Er³⁺modified with citric acid injected into mouse muscle show clearfluorescence imaging signal. The tissue penetration capability of the fluorescence at660nm issignificantly higher than that at540nm. Therefore, the water-soluble rare-earth UCNPs mayhave great potential as fluorescent probes for in vivo fluorescene imaging.