In situ grafting polyethylene glycol chains onto amorphous calcium phosphate nanoparticles to improve the storage stability and organic solvent redispersibility

Amorphous calcium phosphates (ACPs) usually exhibit superior bioactivity and bioresorbability over crystalline calcium phosphates. However, due to the tendency of transforming into more stable hydroxyapatites (HAs) during storage, the commercialization of ACPs is still restricted. In this work, a monophosphate-terminated methoxy-poly(ethylene glycol) (mPEG-OPO3H2) was used as an in-situ chemical stabilizer and surface modifier to ACPs' nanoparticles which were synthesized in methanol/water mixtures. The results indicate that the high content of methanol (50% and 75%, v/v) favors the formation of ACPs and the low content (0% and 25%) of methanol results in HA crystals. All the calcium phosphates (CaPs) displayed a colloidal appearance in the corresponding mother solution. In addition, the centrifuged precipitates can be redispersed in some organic solvents. For example, the methanol colloids still showed narrow particle size distributions (20 and 120 nm) without any sedimentation after 3 months. The ACP powders can maintain their amorphous structure up to 1 year. Such long-term storage stability and unique organic redispersibility are attributed to the formation of polyethylene glycol (PEG) brushes onto the individual ACP nanoparticles.