title: Probing the surface structure of hydroxyapatite using NMR spectroscopy and first principles calculations.
abstract: The surface characteristics of hydroxyapatite (HA) are probed using a combination of NMR spectroscopy and first principles calculations. The NMR spectrum is taken from a bone sample and the first principles calculations are performed using a plane-wave density functional approach within the pseudopotential approximation. The computational work focuses on the (100) and (200) surfaces, which exhibit a representative range of phosphate, hydroxyl and cation bonding geometries. The shielding tensors for the 31P, 1H and 17O nuclei are calculated from the relaxed surface structures using an extension of the projector augmented-wave method. The calculated 31P chemical shifts for the surface slab are found to be significantly different from the bulk crystal and are consistent with the NMR data from bone and also synthetically prepared nanocrystalline samples of HA. Rotational relaxations of the surface phosphate ions and the sub-surface displacement of other nearby ions are identified as causing the main differences. The investigation points to further calculations of other crystallographic surfaces and highlights the potential of using NMR with ab initio modelling to fully describe the surface structure and chemistry of HA, which is essential for understanding its reactivity with the surrounding organic matrix.
authors.affiliation: Department of Earth Sciences, University of Cambridge, Cambridge, UKCB2 3EQ.
authors.forename: Helen
authors.initials: H
authors.lastname: Chappell
authors.forename: Melinda
authors.initials: M
authors.lastname: Duer
authors.forename: Nicholas
authors.initials: N
authors.lastname: Groom
authors.forename: Chris
authors.initials: C
authors.lastname: Pickard
authors.forename: Paul
authors.initials: P
authors.lastname: Bristowe
mesh_terms: D003198:Computer Simulation; D017886:Durapatite / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D008968:Molecular Conformation; D010710:Phosphates / Q000737:chemistry; D013499:Surface Properties
chemical_list: D010710:Phosphates; D017886:Durapatite