Wet-chemical synthesis of Mg-doped hydroxyapatite nanoparticles by step reaction and ion exchange processes

Magnesium-doped hydroxyapatites (Mg-HAs) with different feeding molar ratios of Ca : Mg were synthesized by a wet-chemical method at 90 °C based on the step reaction and ion exchange processes. Firstly, magnesium nitrate (Mg(NO₃)₂·6H₂O) and diammonium hydrogen phosphate ((NH₄)₂HPO₄) with a Mg : P molar ratio of 1.67 were used as starting materials and ammonia water was used as the agent for pH adjustment. Perfect long hexagon shape plates 4-10 μm in size with 200-300 nm thickness were obtained. These particles were then used as precursors, and calcium nitrate (Ca(NO₃)₂·4H₂O) solutions with feeding Ca : Mg molar ratios of 2.5 : 1, 5 : 1, 7.5 : 1, 10 : 1, 12.5 : 1, and 15 : 1 were added, followed by the addition of (NH₄)₂HPO₄. The (Ca + Mg) : P molar ratio was kept at 1.67 during the reaction process. Magnesium ions in the precursor particles were substituted by calcium ions during the process of ion exchange in solution. As a result, the particle size (ranging from nano- to micro-scale), morphology, and magnesium content (1.3-4.2 wt%) in the final Mg-HAs were well controlled by the precursor particles and the original addition of different Ca : Mg molar ratios. The transmission electron microscopy (TEM) images showed the morphology changes with different Ca : Mg feeding ratios. The X-ray powder diffraction (XRD) analysis showed that the lattice disorder increased with Mg substitution in the hydroxyapatite. The (Ca + Mg) : P molar ratio, chemical properties, and thermal stability properties were investigated by inductively coupled plasma emission spectrometry (ICP), Fourier transform infrared spectroscopy (FTIR), and thermal gravimetric analysis (TGA), respectively. In addition, methyl-thiotetrazole (MTT) assay demonstrated that the Mg-HA materials exhibited quite low cytotoxicity. The formation mechanism of the Mg-HA particles could be explained by a precursor particle template and ion exchange process. The present work provides a novel approach to prepare well-controlled Mg-doped HA nanoparticles.