Calcium phosphate nanoparticles with adjustable dispersability and crystallinity

Approximately spherical calcium phosphate nanoparticles with a diameter of about 50 nm can be prepared by rapid precipitation from water and subsequent colloidal stabilization by coating with polymers (polyvinylpyrrolidone, PVP, and polyethyleneimine, PEI). If they are either washed with isopropanol followed by air-drying, or freeze-dried, they can be easily redispersed in polar organic solvents in colloidal form. High-resolution transmission electron microscopy (HRTEM) and electron diffraction showed that their internal crystallinity depends on the drying method: After freeze-drying, the particles remain amorphous whereas all other drying methods lead to nanocrystalline particles. TEM observations also suggest a disordered surface layer (with a thickness of about 5 nm) of the crystalline particles, consisting of hydroxyapatite and calcium oxide. The presence of calcium oxide was ascribed to a surface layer of calcium carbonate that was formed by precipitation in a hydrated surface layer from atmospheric carbon dioxide and then decarboxylated in the TEM. In conclusion, we show how calcium phosphate nanoparticles can be prepared with defined crystallinity and dispersability while their external size and shape are retained.