Zn2+ and SeO32- co-substituted hydroxyapatite: Physicochemical properties and biological usefulness

In this study, a series of hydroxyapatites (HAs) containing different quantities of Zn2+ and SeO32- ions, was synthesized using the aqueous precipitation method. To evaluate and compare the physico-chemical properties of the samples, various instrumental methods were applied. Transmission Electron Microscopy (TEM) indicated that the obtained materials were nanocrystalline and performed a strong tendency to agglomerate. Introduction of Zn2+ and SeO32- ions influenced the shape and size of the crystals. The chemical structure and composition of the powders was determined using Fourier-Transform Infrared Spectroscopy (FTIR), Powder X-ray Diffraction (PXRD), solid state Nuclear Magnetic Resonance (ssNMR) and Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). The results of the employed analyses proved that all the samples were HAs with no impurities in phase. The elemental analysis demonstrated that both Zn and Se were successfully incorporated into their structure. The experimental (Ca+Zn)/(P+Se) molar ratio differed significantly from the nominal one and proved the complex character of Zn2+ introduction. The Zn2+ ions were both inserted between the structural columns of hydroxyl groups and replaced the Ca2+ ions within the crystal lattice of HA. Consistent with the results of FTIR studies, ssNMR experiments indicated that the Zn2+ ions stimulate formation of the amorphous, hydrated surface layer surrounding the core of HA crystals. As the outer layer contain phosphorous atoms, it contributed to a significant decrease of the experimental (Ca+Zn)/(P+Se) molar ratio. The size of the layer increased with the increasing content of the Zn2+. Additionally, release kinetics tests on the ionic dopants were conducted using the ICP-OES method. The results of Se release confirmed that selenite ions were both substituted into the crystal core and located in the hydrated surface layer. To evaluate the biological usefulness of the synthesized materials, preliminary cytotoxicity tests were conducted.