Calcium hydroxyapatite (HAp) is the main inorganic component of natural bones. In this review, the crystal structure and fabrication methods of HAp were first introduced; then the synthesis and applications of metal-ion-doped HAp nanoparticles were mainly reviewed. HAp was usually synthesized through solid state reaction and wet chemical precipitation, of which the properties depended on the fabrication method. Due to its crystal structure, HAp can tolerate ion-exchange with a high amount of

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... er ions without significantly affecting its lattice structure. Hence ion-exchange reaction of HAp has been used to remove heavy metal ions such as Pb 2+ or Cd 2+ in waste water or soils, and immobilize radioactive waste. The adsorption capacity depended on the crystallinity and specific surface area of HAp. Meanwhile other metal ions besides Ca can be introduced into HAp through ion exchange or co precipitation method to form metal ion doped HAp. Due to the presence of these metal ions, the properties of HAp were either enhanced or new properties were introduced. The electrical conductivity and catalysis effect of HAp were increased due to ion substitution; the metal ion doped HAp demonstrated potential for better performance in tissue engineering, can be paramagnetic or used as fluorescence probes. The applications of HAp were thus broadened.