Calcium environments, proton defects, and facet-dependent fluoride interactions in hydroxyapatite nanostructures probed by multinuclear solid-state NMR

Abstract

Hydroxyapatite (HAP) nanostructures expose morphology-dependent surfaces that govern surface chemistry, yet the calcium environments and proton defect structures underlying these differences remain unclear. We combined multinuclear solid-state NMR methods, including ⁴³Ca MAS and 3Q-MAS, ¹H–⁴³Ca TRAPDOR, and ¹⁹F MAS, together with solution ¹⁹F NMR quantitation, to probe calcium coordination, hydroxyl defects, and fluoride partitioning in nanorods (rHAP), nanowires (aHAP), and nanosheets (cHAP). 3Q-MAS resolved the two crystallographic calcium sites (Ca1 and Ca2) and showed conserved isotropic shifts and quadrupolar parameters across morphologies, indicating consistent average Ca–O coordination. Morphological effects were reflected mainly in line broadening linked to crystallinity. Importantly, ¹H–⁴³Ca TRAPDOR revealed strong early-time dephasing for both channel OH⁻ groups (0 ppm) and surface protons at 0.8–1.3 ppm, identifying the latter as Ca-associated hydroxyl defects enriched at nanostructured surfaces. These proton defects are a defining feature of nanoscale apatite, yet fluoride uptake is determined not only by defect density but also by lattice plane termination. rHAP, with mixed facets exposing (001) OH⁻ channel ends, incorporated fluoride as fluorapatite; aHAP, dominated by (100) surfaces, instead yielded CaF₂; and cHAP, with extensive (001) exposure and higher defect density, supported both processes. Thus, fluoride partitioning arises from the interplay of proton defects and lattice plane exposure. This integrated NMR workflow provides a general strategy to disentangle structure, defects, and ion incorporation at apatite interfaces.