Experimental and computational studies on carbonated apatite

Abstract

The inorganic component of mineralized tissues (bone and teeth) is an analogue of carbonated apatite (CAp), it is therefore important to elucidate the crystal structure of CAp for the fundamental comprehension of bone regeneration in bodies and further for developing new artificial bones with desirable bioactivity and biocompatibility. However, the details of the crystal-chemical relationship between carbonate ions and apatite structure are not well known. Lack of single crystals of CAp with large enough size for direct structure analyses has kept the problem unresolved. This doctoral study aims to: 1) develop synthesis of biomimetic CAp nanocrystals; 2) Systematically study the carbonate effect on the structure, chemical composition, crystal size, morphology, and crystallinity of the apatite; 3) investigate the exact location and configuration of carbonate substitution in the apatite structure by ab initio simulation; 4) verify and refine the criterion to distinguish two types of CAp by infrared (IR) spectroscopy. CAp nanocrystals were successfully synthesized by controlled precipitation from solutions containing calcium, phosphate, and carbonate ions at pH = 11. In view of experimental evidence from XRD studies, Ca/P molar ratio changes and IR spectra difference, carbonate indeed substituted for partial phosphate in the precipitated CAp. Hydrothermal method was also developed to prepare CAp nanocrystals. The optimum parameters for synthesis of well crystalline CAp nanocrystals are pH = 10, C/P = 2/3, 200°C for 24 h. The structure of CAp has been investigated by ab initio simulations. The results show that type-A CAp are energetically preferred to type-B CAp. The most energetically favored configuration of type-A CAp had its carbonate triangular plane almost parallel to c-axis, making an angle of about 2° at z = 0.46. In the lowest energy structure of type-B CAp, the nearest Ca(2) ion was replaced by a sodium ion and the carbonate group was lying almost flat in b/c-plane of the apatite lattice, the normal to carbonate plane making an angle 88° to c-axis. Of all the models considered, mixed substitution type-AB where two carbonate ions replacing one phosphate group and one hydroxyl group was the most stable structure. The carbonate ions in the apatite lattice tend to be parallel to c-axis in both type-A and type-B sites. The lattice parameters a and b in type-A CAp unit cell expanded but the c parameter contracted. However, the most stable type-B CAp structure showed the opposite trend. The lattice parameter changes due to different substitution types may also be used as evidence to distinguish CAp. The bands at ~880 cm-1, ~1413 cm-1, and ~1450 cm- should not be used to identify CAp individually since they may result from carbonate absorption on apatite crystals. The IR signatures of carbonate environment in apatites are: v3 band at ~1465 cm-1 due to CO32- substituting for PO43- ; and v3 band at ~1546 cm-1 due to CO32- substituting for OH-. Based on these IR signatures, we can identify that the precipitated CAp is type-B substitution, the CAp prepared by hydroxyapatite reaction with CO2 at high temperature and hydrothermal method, and biological apatites are mixed type-AB substitution. This study is expected to contribute to synthesize CAp biomaterials with excellent biological properties and to help understanding the chemical and structural properties of mineralized tissues