Step-by-step from amorphous phosphate to nano-structured calcium hydroxyapatite: monitoring by solid-state H and P NMR and spin dynamics

• Published in: Physical chemistry chemical physics : PCCP 2022-08, Vol.24 (31), p.18952-18965
• Main Authors: Klimavicius, Vytautas, Maršalka, Ar nas, Kizalaite, Agne, Zarkov, Aleksej, Kareiva, Aivaras, Aidas, K stutis, Hirschinger, Jérôme, Balevicius, Vytautas
• Format: Article
• Language: English
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/d2cp02108k

The solid-state 1 H, 31 P NMR spectra and cross-polarization (CP MAS) kinetics in the series of samples containing amorphous phosphate phase (AMP), composite of AMP + nano-structured calcium hydroxyapatite (nano-CaHA) and high-crystalline nano-CaHA were studied under moderate spinning rates (5-30 kHz). The combined analysis of the solid-state 1 H and 31 P NMR spectra provides the possibility to determine the hydration numbers of the components and the phase composition index. A broad set of spin dynamics models (isotropic/anisotropic, relaxing/non-relaxing, secular/semi-non-secular) was applied and fitted to the experimental CP MAS data. The anisotropic model with the angular averaging of dipolar coupling was applied for AMP and nano-CaHA for the first time. It was deduced that the spin diffusion in AMP is close to isotropic, whereas it is highly anisotropic in nano-CaHA being close to the Ising-type. This can be caused by the different number of internuclear interactions that must be explicitly considered in the spin system for AMP (I-S spin pair) and nano-CaHA (I N -S spin system with N ≥ 2). The P-H distance in nano-CaHA was found to be significantly shorter than its crystallographic value. An underestimation can be caused by several factors, among those - proton conductivity via a large-amplitude motion of protons (O-H tumbling and the short-range diffusion) that occurs along OH − chains. The P-H distance deduced for AMP, i.e. the compound with HPO 4 2− as the dominant structure, is fairly well matched to the crystallographic data. This means that the CP MAS kinetics is a capable technique to obtain complementary information on the proton localization in H-bonds and the proton transfer in the cases where traditional structure determination methods fail. Structural and dynamical evolution in a series of samples including amorphous phosphate, composite, and nano-structured calcium hydroxyapatite is probed by NMR cross-polarization kinetics.