Rare-earth doped apatite nanocrystals for cell and implanted biomaterial tracking

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Li, Xiyu
Chen, Haifeng
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Apatite nanoparticles are the main inorganic component of human hard tissues, and have been used in repair of bone defects or scaffolds for bone tissue engineering. It has a stable hexagonal crystal structure, which could be employed as a host matrix for lanthanides doping to prepare both downconversion and upconversion fluorescent nanoparticles. In the thesis, lanthanide doped apatite nanoparticles have been prepared by hydrothermal synthesis, and their structure, luminescence properties, cell proliferation, and their fluorescent tracking in vitro and in vivo have been investigated. The novel apatite fluorescence probe is expected to overcome the shortcomings such as instability and photobleaching of organic fluorophores, and the potential toxicity from quantum dots. The terbium (Tb) doped fluorapatite (FA:Tb) and hydroxyapatite (HA:Tb) nanocrystals with a uniform slender morphology exhibit bright green fluorescence, good cytocompatibility and excellent cell imaging capacity, providing feasibility for imaging and tracking of cells with multilineage differentiation. The Yb3+/Ho3+ co-doped apatite nanoparticles are firstly prepared with both green and red upconversion emissions under 980 nm near-infrared excitation. The 543 and 654 nm upconversion emission signals could be assigned, respectively, to 5F4 (5S2) - 5I8 and 5F5 - 5I8 transitions of holmium after energy transfer from ytterbium under 980 nm excitation. The FA:Yb3+/Ho3+ results in a superior green luminescence, while HA:Yb3+/Ho3+ dominates in red emission. The difference in green and red emission behavior between FA:Yb3+/Ho3+ and HA:Yb3+/Ho3+ is dependent on their lattice structure and composition. Several reasonable lanthanide embedding lattice models along the fluorine channel or hydroxyl channel of FA or HA crystal cell are proposed, and reveal a necessity for coexistent substitution mechanism. The upconversion apatite nanoparticles are used for the first time to clearly distinguish the implanted material from bone tissue, and to track their respective distribution in vivo. The superposition of fluorescent images including the upconversion apatite and the Masson’s stained bone tissue indicates that new bone tissue could grow into the interval space between the apatite particles and fully integrate with these particles. The upconversion material and fluorescence superposition method provide a novel strategy for long-term discriminable fluorescence tracking of implanted materials and scaffolds. These fluorescent apatite nanoparticles can not only form bonding with bone tissue, but also provide stable and clear fluorescence, which can be utilized for long-term bioimaging, distinguishing scaffold from newly formed bone tissue or identifying the interface between implanted material and bone tissue, and tracking the distribution of degraded scaffold fragments in vivo. Both the lanthanide doped apatite nanoparticles and the method presented in this research will have a promising prospect for future biomedical applications.
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