Preparation and Characterization of Mono- and Biphasic Ca_1−xAg_xHPO₄·nH₂O Compounds for Biomedical Applications

This study aimed to explore the effects of the full-scale replacement (up to 100%) of Ca²⁺ ions with Ag¹⁺ ions in the structure of brushite (CaHPO₄·2H₂O). This substitution has potential benefits for producing monophasic and biphasic Ca_1−xAg_xHPO₄·nH₂O compounds. To prepare the starting solutions, (NH₄)₂HPO₄, Ca(NO₃)₂·4H₂O, and AgNO₃ at different concentrations were used. The results showed that when the Ag/Ca molar ratio was below 0.25, partial substitution of Ca with Ag reduced the size of the unit cell of brushite. As the Ag/Ca molar ratio increased to 4, a compound with both monoclinic CaHPO₄·2H₂O and cubic nanostructured Ag₃PO₄ phases formed. There was a nearly linear relationship between the Ag ion ratio in the starting solutions and the wt% precipitation of the Ag₃PO₄ phase in the resulting compound. Moreover, when the Ag/Ca molar ratio exceeded 4, a single-phase Ag₃PO₄ compound formed. Hence, adjusting the Ag/Ca ratio in the starting solution allows the production of biomaterials with customized properties. In summary, this study introduces a novel synthesis method for the mono- and biphasic Ca_1−xAg_xHPO₄·nH₂O compounds brushite and silver phosphate. The preparation of these phases in a one-pot synthesis with controlled phase composition resulted in the enhancement of existing bone cement formulations by allowing better mixing of the starting ingredients.