Fine Structure Of Synthesized Zn-Doped Nano-Hydroxyapatite

As the main organic component of the mineralized tissues （e.g. bone and dental e-namel） of mammalian, hydroxyapatite （HAp） and apatite-like materials （e.g. fluorine、magnesium and zinc doping） have been extensively studied in recent years. SynthesizedHAp has been considered for medical application such as bioceramics and bone graftsurgery. Modifications of HAp-like materials have also been extensively studied to enhancemechanical and physiochemical stabilities in order to improve their biocompatibility andsuitability for restoration of hard tissues.Zn-doped hydroxyapatite can enhance biomineralization and restrain bone resorption.It has been recently demonstrated that Zn incorporation into implants may promote boneformation around materials. In this dissertation, we synthesized low concentration of zincsubstitution for calcium site in hydroxyapatite （ZnHAp）. Depending on various instrumentsand methods, synthesized Zn-doped hydroxyapatite were characterized and analyzed. Den-sity functional theory （DFT） are performed for cation-substituted hydroxyapatite in orderto investigate the relative cation exchange ability, such as Mg²⁺, Sr²⁺, Zn²⁺and Cd²⁺. ByDFT calculation and experimental results, the mechanism of Zn incorporated into hydrox-yapatite is novel when the concentration of Zn is lower than0.1mol.%. The researches aresummarized as following:Based on the constant drop method, a series of ZnHAp samples have successfully beensynthesized. We determined the total zinc concentration in the product by Inductively Cou-pled Plasma Atomic Emission Spectrometer （ICP-AES）, but the results show that the zincconcentration in ZnHAp are74～81%of the nominal values. The typical morphology ofthe ZnHAp nanoparticles have a rod-like shape, a diameter of about30nm and a lengthof about100nm. The zinc concentration in such low concentration （0.1mol.%～1mol.%）has no obvious influence on the final particle morphology. The synthetic ZnHAp powderswere analyzed by XRD （X-ray diffraction） to determine the crystal phase. XRD patterns are compared with the HAp crystal standard profile （PDF09-0432）, meanwhile the estimatedunit volume shrinks0.38%when zinc concentration increase from0.1mol.%to1mol.%.The plane wave DFT calculation（PSeudopotentials Plane-Wave，PSPW） was per-formed with NWChem program, using a periodic supercell model. The substitutional en-ergies and atomic relaxations of divalent cations such as Mg²⁺, Sr²⁺, Zn²⁺and Cd²⁺arecalculated from total energies of supercells. The calculations indicated that the Ca²⁺to M²⁺substitutions induce different local distortions on the hydroxyapatite lattice.The X-ray absorption near edge structure （XANES） results show that Zn in the Ca sitesof HAp is a single dominant coordination environment, especially for the0.5mol.%and1mol.%samples. Extended X-ray absorption fine structure （EXAFS） results demonstratedthat Zn located in the center of Zn-O₄tetrahedral coordination. When the zinc substitutionconcentration is as low as0.1mol.%, the first shell fitting shows that Zn-O₄structures donot match the tetrahedral coordination. By PSPW calculations, the substitution energyis negative for the inserting mechanism which suggests that the zinc atoms prefer to beinserted along the hydroxyl column energetically.