Solid-state MAS NMR, TEM, and TGA studies of structural hydroxyl groups and water in nanocrystalline apatites prepared by dry milling

Abstract

A series of nanocrystalline calcium hydroxyapatites was prepared by dry milling and characterized using proton and 31P MAS NMR, TEM, and TGA methods. The samples contained stubby rod-shaped crystals, which length and width varied in the 130–30 and 95–20 nm ranges, respectively. It was confirmed that concentration of structural hydroxyl groups in nanocrystalline apatites decreases with the decreasing crystal size. In the series of the studied apatites, the decrease was from 86 to ca. 50 % in reference to stoichiometric apatite. Water was found in the surface hydrated layer and in the c-axis channels, in which compartments existed as adsorbed and structural, respectively. Molecules of the adsorbed water were capable of moving from the crystal surface into the lattice c-axis channels of apatite. This process introduced considerable structural disorder within and around those channels and reduced the content of the structural hydroxyl groups, particularly in the region underneath the apatite crystal surface.Electronic supplementary materialThe online version of this article (doi:10.1007/s11051-013-1868-y) contains supplementary material, which is available to authorized users.

Morphological and functional changes in RAW264 macrophage‐like cells in response to a hydrated layer of carbonate‐substituted hydroxyapatite

“…The reduction in OH − was accompanied by increasing atomic disorder within and around c ‐axis channels, as the existence of Ca vacancies and/or sodium substitutions were assumed to widen the channels in CAp. The disorder is caused by uptake of adsorbed water into the channels, which is thought to be associated with the resorption capacity of CAp as well as a small grain size and microstrain . The OH − vacancies are presumed to interact with water and OH − of hydroxyproline in collagen fibrils .…”

“…The disorder is caused by uptake of adsorbed water into the channels, which is thought to be associated with the resorption capacity of CAp as well as a small grain size and microstrain. 17,[29][30][31] The OH 2 vacancies are presumed to interact with water and OH 2 of hydroxyproline in collagen fibrils. 17 The DRIFT analysis also revealed that more H 2 O was adsorbed on the surface of CAp as compared to HAp.…”

Selenium-Doped Hydroxyapatite Nanocrystals–Synthesis, Physicochemical Properties and Biological Significance

“…The content of structural hydroxyl groups was calculated, and it was observed that there was a significant loss of HA crystals with incorporated selenite ions (49% OH groups relative to stoichiometric HA) and selenate (63%) when compared to pure HA (77%). It was also proven that this reduction in the content of structural hydroxyl groups was related to the decreased size of the crystals [43,57] due to the absorption of water from the crystals' surface into the columns of the hydroxyl groups. Thus, the authors postulate that the reason for the loss of structural hydroxyl groups is two-fold: not only are OH groups removed during substitution, but also water is present in the OH columns [41].…”

“…The signal in the 31 P CP NMR spectra was deconvoluted into two components: narrow, derived from phosphate groups inside the crystal located near the protons of structural hydroxyl groups, and broad, derived from phosphate groups located near to the water-rich environment (especially from phosphors from the hydrated surface layer) [41,57]. The wide component was relatively the most intense in the spectrum of the sample containing SeO 3 2− , indicating a more extensive hydrated surface layer and, thus, a more developed surface.…”

The solid-state proton NMR study of bone using a dipolar filter: apatite hydroxyl contentversusanimal age

“…This latter aspect is still studied and debated, because the ionic and water lling of the crystal channels containing apatite hydroxyl ions is crucial for the bone mineral structure and its properties. [15][16][17] It is known that the OH À concentration in the apatite crystals decreases with increase of the carbonate content caused by A-type (CO 3 2À for OH À ) and/or B-type (CO 3 2À for PO 4 3À ) substitution, [18][19][20][21] and with the decreasing size of apatite nanocrystals 4,15,22,23 and with increasing disorder in their crystal lattice. 4,15 Maturation of bone apatite, which is a chemical, thermodynamically driven process, 24 increases the apatite OH À content.…”