Issue 31, 2022

Step-by-step from amorphous phosphate to nano-structured calcium hydroxyapatite: monitoring by solid-state 1H and 31P NMR and spin dynamics

Abstract

The solid-state 1H, 31P 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 1H and 31P 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 (IN–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 HPO42− 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.

Graphical abstract: Step-by-step from amorphous phosphate to nano-structured calcium hydroxyapatite: monitoring by solid-state 1H and 31P NMR and spin dynamics

Supplementary files

Article information

Article type
Paper
Submitted
09 May 2022
Accepted
19 Jul 2022
First published
19 Jul 2022

Phys. Chem. Chem. Phys., 2022,24, 18952-18965

Step-by-step from amorphous phosphate to nano-structured calcium hydroxyapatite: monitoring by solid-state 1H and 31P NMR and spin dynamics

V. Klimavicius, A. Maršalka, A. Kizalaite, A. Zarkov, A. Kareiva, K. Aidas, J. Hirschinger and V. Balevicius, Phys. Chem. Chem. Phys., 2022, 24, 18952 DOI: 10.1039/D2CP02108K

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