Comprehensive structural studies of ultra-fine nanocrystalline calcium hydroxyapatite using MAS NMR and FT-IR spectroscopic methods

Three nanoapatites produced using a microwave solvothermal synthesis (MSS), including one with a mean crystal size of only 6 nm, were characterized by high-resolution solid-state NMR (1H and 31P) and FT-IR spectroscopies. It was found that nanoapatite particles had an inner crystal core covered by a non-apatitic surface hydrated layer with an outermost liquid-like zone. Various physicochemical properties of those apatite compartments were determined. Important structural details, such as deficiency and disorder of structural OH− ions were discovered and discussed. The materials were also compared with respect to the reaction time and thermal post-synthesis treatment in order to optimize their preparation conditions to the required performance. The study emphasizes the influence of the surface hydrated layer on the physicochemical properties and biological activity of nanoapatites.