Solid-state ¹H and ³¹P NMR and FTIR spectroscopy study of static and dynamic structures in sol-gel derived calcium hydroxyapatites

Abstract

Calcium hydroxyapatite containing amorphous phosphate phase (ACP-CaHA) and nano-structured hydroxyapatite (CaHA) have been prepared by two sol-gel synthesis routes. The structural organization of hydroxyl groups in both materials has been determined by means of 1 H MAS NMR and FTIR spectroscopy. It has been shown that the amount of structural -OH groups in nano-structured CaHA is significantly higher than that from adsorbed water and vice versa in ACP-CaHA. A precise signal shape analysis has been carried out for both studied samples. The 31 P NMR signals have been found being Voigt-shaped in the wide-line as well as in MAS spectra. The 1 H and 31 P spin-lattice and spin-spin relaxation time measurements have revealed that the fast spin motion takes place in ACP-CaHA. The corresponding correlation time τ ~ 7 · 10 -7 s at ~300 K has been determined. The effect of MAS rate on the 31 P signal shape also confirms that this motion runs in the time scale of microseconds or even nanoseconds. The magnitude of the anisotropic broadening 1220 ± 20 Hz determined for nano-structured CaHA is very close to the maximum of the dipolar 1 H-31 P coupling distribution profile estimated using CP MAS kinetics. The dynamics of 1 H-31 P spin interactions in nano-structured CaHA (τ ~ 3.3 · 10 -5 s) is much slower than in ACP-CaHA.

Subnanoscale Order and Spin Diffusion in Complex Solids through the Processing of Cross-Polarization Kinetics

“…This choice can be supported by the fact that the 31 P NMR signals were found being in many cases Gauss-or Voigt-shaped with dominant Gauss contribution. 11,22 The width parameter (w) reflects the extent of the overall spin coupling distribution and the relaxation rates. The results of simulation are shown in Figure 6.…”

“…The 1 H and 31 P spin−lattice and spin−spin relaxation time (T 1 and T 2 ) measurements push toward the presence of significant amount of the adsorbed water in amorph-CaHA. 11 These experiments have revealed the fast spin motion with the correlation time τ = 6.9 × 10 −6 s at ∼300 K takes place in amorph-CaHA. The effect of MAS rate on the 31 P signal shape supports that the correlation time of this motion gets into the time scale of microseconds or even nanoseconds.…”

“…38 The line width upon MAS becomes dependent on the spin interaction strength, the time scale of the motion, and the spinning rate. 11,38 Hence we can speak about a certain decimation of dipolar interactions that is reflected in the changes of NMR signal widths and in the spatial distribution of interacting spins (Figure 8).…”

“…Among a broad variety of physical characterization techniques, solid-state NMR spectroscopy can be considered as the method that provides probably the most insightful view of the structure and dynamics on the molecular and nanoscopic length scales. Therefore, NMR magic-angle spinning (MAS), cross-polarization (CP) kinetics, heteronuclear correlation (HETCOR), and so forth experiments have been widely exploited in the studies of calcium phosphates and related systems. − In addition, some recent works on 1 H and 31 P spin–lattice and spin–spin relaxation times measurements and the effect of variable MAS rate on the 31 P signal shape, phosphate-ion distribution by employing 31 P multiple-quantum coherence, ultrahigh speed MAS , and NMR crystallography have to be noted as very advanced and promising. The growing interest in the area called as “NMR crystallography” was noted in ref .…”

Influence of Thermal Treatment on the Antimicrobial Activity of Silver-Doped Biological Apatite

“…The bands found in the spectral range 3550–3600 cm −1 attributed to the water lattice are presented in the top right corner of each spectrum. The vibrational bands from 3571, 3587, and around 3594 cm −1 are characteristic to the O-H vibrations [ 9 , 44 , 45 ]. Previous studies [ 9 ] have already proved that the band from 3571 cm −1 is characteristic to the hydroxyapatite phase.…”