Synthesis and surface structural characterization of the nano-bio interface for amorphous and nanocrystalline hydroxyapatite morphologies

Biological hard tissue, such as bone and teeth, are comprised of nanocrystalline hydroxyapatite (HAP) and protein constituents with a hierarchical structural organization. The HAP biocompatibility and superior mechanical features strongly depend on the atoms present on HAP surfaces and the molecular organization of the hard-soft interface. The assembly of HAP and protein are found to be highly surface-selective however, the atomic and molecular details of the surface structure are still lacking. In the present study, HAP nanocrystals grown along different crystallographic lattice planes with distinct morphologies have been synthesized under controlled conditions including nanoparticles in the forms of amorphous, isotropic and nanocrystalline material with rod and platelet geometry.¹ Their morphology, nanocrystallinity, molecular structure and surface chemistry have been characterized by transmission electron microscopy (TEM), scanning electron microscope (SEM), X-ray Diffraction (XRD) and solid-state nuclear magnetic resonance (NMR), see Figure.
The HAP surfaces are charged and hydroscopic, which will be immediately protonated and hydrated upon formation. These surface species including calcium atoms, hydroxide and phosphate groups will be ionized or protonated in response to the pH or chemicals in the aqueous synthetic environment. Due to their different atomistic surface termination, each lattice plane is expected to be protonated in distinct patterns. These differences were clearly observed by solid-state ¹H magic angle spinning (MAS) NMR as displayed in the Figure. It was found that the c-plane carries more surface protons than the a(b)-plane and shows a higher capacity for surface proton sites. Future work will focus on elucidating the surface specific interactions between HAP and biomolecules to determine the mechanism of protein directed biomineralization and the proteins and site-specific intermolecular interactions. A series of collagen-like peptides are being probed at the bio-nano interface of HAP nanomaterials and their assembly at molecular level are being characterized with proton-based MAS solid-state NMR methods.