Amorphous 1-D nanowires of calcium phosphate/pyrophosphate: a demonstration of orientated growth of amorphous minerals

Amorphous inorganic solids are traditionally isotropic, thus, it is believed that they only grow into morphologies of nanospheres or irregular aggregates of nanoparticles. However, in the presence of (ortho)phosphate (Pi) and pyrophosphate ions (PPi) which have synergistic roles in biomineralization, amorphous calcium phosphate-pyrophosphate nanowires (ACPPNs) form in an aqueous additive-free solution. Cryo-TEM shows that the single nanowire has an average diameter of 2-3 nm, and lengths of up to hundreds of nanometers. In ACPPNs, amorphous calcium orthophosphate and amorphous calcium pyrophosphate are distributed at separated but close sites. The ACPPNs grow via either the preferential attachment of ~2 nm nanoclusters in 1-dimension way, or the transformation of bigger nanoparticles. We propose that the anisotropy of ACPPNs microstructure, which is corroborated experimentally, causes their oriented growth. This study proves that, unlike the conventional view, amorphous minerals can form via oriented growth without external regulation, demonstrating a novel insights into the structures and growth behaviors of amorphous minerals.