Crystallographic Aspects Regarding the Insertion of Ag⁺ Ions into a Hydroxyapatite Structure

The objective of this study was to evaluate how silver can be inserted into hydroxyapatite (HA) via two distinct processes: co-doping with CO₃^2- via precipitation in an aqueous medium and immersion of preformed HA crystals into Ag⁺ solutions. It was concluded that although Ag⁺ and Ca²⁺ have different radii, the accommodation of Ag⁺ ions in the Ca²⁺ sites of the hydroxyapatite lattice can be explained by the models proposed for inserting monovalent ions such as Na⁺. In this case, because Ag⁺ ions are larger than Ca²⁺ ions and have a different charge, the Ag⁺ ions are stabilized in the HA structure by co-substitution with CO₃^2- ions in both the A- and B-type sites. This simultaneous insertion of Ag⁺ and CO₃^2- appears to thermally stabilize the HA phase because no phase transformation is observed after calcination. In addition, the doping of HA with Ag⁺ ions can clearly occur via two routes: co-precipitation in the presence of these ions or diffusion in preformed hydroxyapatite crystals. This result appears to indicate the possibility of doping HA with Ag⁺ using less complex routes at ambient temperature and with prefabricated implants or biomaterials, which reduces the costs of producing devices with antibacterial effects.

Keywords:
hydroxyapatite; silver; doping; carbonate; diffusion