Water-based Synthesis, Characterization And Biomedical Application Of Rare-rarth Doped NaYF₄ Nanocrystals

In the recent years, with the development of nanoscience and nanotechnology, fluorescent nanocrystals have attracted much attention due to their unique optical properties. There has been considerable research on upconverting phosphors since initial interest in the late 1950s. Rare-earth-doped upconverting materials have wide potential applications in many fields, including phosphors, display monitor, lasers and amplify for fiber-optic communications. Compared with other fluorescent materials, such as organic dye, the upconverting materials have several advantages in optical properties of narrow band emissions, high photostability, low background light, nonfading, and no significant influence of environment under near infrared radiation, so they can be used as biological labels materials. Rare-earth-doped upconverting nanocrystals have been reported to be used for the cell imaging, detection of nucleic acid and immunoassay. The nanocrystals should be highly efficient emission, size-controlled and monodisperse for biological applications. So developing synthetic technologies and researching the quenching mechanisms of luminescence are very important to both fundamental research and practical application. Surrounding the rare-earth doped fluoride nanocrystals, this dissertation presents a systematic research about preparation, characterization and biomedical application of nanoparticles. Now, some original results are obtained from our experiments, the main results are outlined as followings:1. Develop a simple hydrothermal method for synthesis of different sizes and morphology of cubic and hexagonal structure of NaYF₄: Yb³⁺, Er³⁺ nanocrystals using sodium citrate as a chelating agent. The effects of the amount of Fˉions and citrate as well as the hydrothermal temperature and hydrothermal time on the nano-crystalline morphology, size, structure, and up-conversion luminescence properties were analyzed in detail. The upconversion luminescence properties of the NaYF₄: Yb³⁺, Er³⁺ nanocrystals were also studied. It is found that excessive levels of Fˉion can effectively reduce the crystallization temperature of the sample, at a relatively low hydrothermal temperature, you can get nanocrystals with better crystallization; the size, morphology and crystal structure of the samples can be effectively controlled by adjusting the amount of sodium citrate; the strong chelating ability between rare earth cation and citric acid molecules can be able to effectively control the growth rate, phase-change time of NaYF₄: Yb³⁺, Er³⁺ nanocrystals; due to the selective coordination role of citrate with the {0001} crystal plane of the hexagonal NaYF₄ nuclei, during the crystal growth process, the growth rate of crystal relatively fast in the six symmetric directions:±[101₀],±[011₀] and±[11₀0] and the hexagonal NaYF₄ sub-micron plates eventually formed. In the 980 nm laser excitation, the upconversion luminescence of cubic phase nanoparticles and hexagonal sub-micron plates is observed and its green and red upconversion luminescence process belongs to the two-photon process. Relative to green emission, the red up-conversion emission of samples is relatively strong. Experiments show that the multi-phonon relaxation induced by the high energy vibration groups of the citrate adsorbed on the sample surface and the cross-relaxation interaction between Er³⁺ ions in the samples are the main reasons for a strong red up-conversion emission.2. The hexagonal-phase NaYF₄:Yb³⁺, Er³⁺ hollow nanospheres have been successfully prepared for the first time via a hydrothermal route with the aid of polyethylenimine (PEI) as a surfactant. The nitrogen adsorption/desorption isotherms demonstrated the porous nature of the NaYF₄ hollow nanospheres. Through discussing the effects of the PEI concentration, hydrothermal temperature and hydrothermal time on the morphology, size and structure of samples, the growth mechanism of hollow nanospheres was proposed. It was found that the appropriate PEI concentration and hydrothermal reaction conditions are the key factors to form hexagonal NaYF₄: Yb³⁺, Er³⁺ hollow nanospheres. By analysing the character of the samples prepared in different hydrothermal time, it was revealed that the samples suffered morphology evolution and the phase transition from the cubic phase NaYF₄: Yb³⁺, Er³⁺ nanocrystals to the hexagonal phase NaYF₄: Yb³⁺, Er³⁺ self-assembled nanowires, to the hexagonal phase NaYF₄: Yb³⁺, Er³⁺ hollow nanospheres; PEI-induced Ostwald ripening process is the main mechanism of the formation of the sample. In the 980 nm light excitation, the particles showed strong up-conversion luminescence emission. As the strong cross-relaxation process exist among neighboring Er³⁺ ions due to the relatively higher doping concentration and the multi-phonon relaxation induced by the organic high-energy vibration of the sample surface, the samples showed relatively strong red upconversion emission.3. The effect of branched polyethylenimine with different chain lengths, used as the surfactants in the preparation of water-soluble NaYF₄:Yb³⁺, Er³⁺ nanocrystals following solvothermal approach, on the infrared (IR) to visible photon upconversion has been studied. It was found that under the same hydrothermal conditions, the size of the sample synthesized with high molecular weight polymer (HPEI) is smaller than those with low molecular weight polymer (LPEI). Interestingly, in the same power 980 nm laser excitation, the upconversion luminescence intensity of the sample with small size is stronger than that of the samples with large size. Combining the crystal growth theory, we believe that the difference in the sample crystallinity is the main reason for the different up-conversion luminescence intensity. High molecular weight polymer can reduce the particle growth rate and favor the formation of nanoparticles with small size and good crystallinity. Cytotoxicity test and fluorescent immunoassay results show the obtained nanoparticles have good biocompatibility.