Stability-limited ion-exchange of calcium with zinc in biomimetic hydroxyapatite

The exchange of Ca²⁺ ions in hydroxyapatite (HAp) with Zn²⁺ ions into Zn-HAp is of interest for applications ranging from bone tissue engineering to the use as a precursor in subsequent ion-exchange reactions. Previous studies, using direct synthesis, showed that ~ 20 mol% Zn²⁺ ions can be incorporated into HAp, before byproducts are observed. However, this is realized at the cost of a loss in crystallinity and control over crystal size and shape with increasing amounts of Zn²⁺ ion incorporation. In this work a simple post-synthetic ion-exchange strategy for the formation of Zn-HAp has been investigated. By merely exposing HAp to high concentrations of zinc nitrate in water, up to 22 mol% of the Ca²⁺ ions can displaced by Zn²⁺ ions without any measured loss in crystallinity and preservation of crystallite size and shape. It was found that the incorporation of Zn²⁺ ions destabilizes the HAp crystals resulting in their gradual dissolution and reprecipitation. Consequently, promoting the exchange of Ca²⁺ with Zn²⁺ions using increased reaction times, sonication and increased temperature results in an increased dissolution of HAp and precipitation of hopeite crystals, thereby preventing the formation of more zinc rich Zn-HAp.