Solid‐State NMR Study of Mn²⁺ for Ca²⁺ Substitution in Thermally Processed Hydroxyapatites

Abstract

In the present study calcium hydroxyapatites enriched at 0.08 wt% in Mn 2+ ions (Mn-HA) and their unsubstituted forms (HA) were synthesized using the same standard wet chemical route. Mn-HA and HA were both calcined at 800°C to give Mn-HAc and HAc, respectively or sintered at 1250°C, to give Mn-HAs and HAs, respectively. The influence of the heat treatment on physicochemical properties of Mn-HA was investigated using powder X-ray diffraction (PXRD), scanning, and transmission electron microscopy (SEM and TEM), and solid-state nuclear magnetic resonance (ssNMR). Mn-HAc and Mn-HAs were compared to each other and to HAc and HAs, respectively. Assignment of the proton ssNMR peaks from high-temperature-treated apatites has been revised. It was found that Mn-HAc and HAc were nanocrystalline, while Mn-HAs and HAs comprised micrometer sized, partially fused particles (SEM and TEM). PXRD and ssNMR demonstrated that the incorporation of Mn 2+ into the crystal lattice of hydroxyapatite significantly facilitates its dehydroxylation and decomposition to oxyhydroxyapatite during calcination at 800°C, and induces its transformation to tetracalcium phosphate (TTCP) and alpha-tricalcium phosphate (a-TCP) at 1250°C. Contamination by CaO has also been detected. The 1 H? 31 P NMR cross-polarization experiments have indicated that the Mn 2+ ions preferentially occupied the Ca(I) position in the crystallographic unit cell of Mn-HAc. In Mn-HAs, the Mn 2+ ions were evenly distributed between the Ca(I) and Ca (II) positions.

A Solid-State NMR Study of Selenium Substitution into Nanocrystalline Hydroxyapatite

“…, CO 3 2− ) and tetravalent (SiO 4 4− ) anions [14,15]. It is important to note that all substitutions may be the source of lattice distortion, vacancies, crystal defects or hydrogen bonds in OH groups’s columns, which in turn play a significant role in HA crystal size and morphology [6,8]. …”

Paramagnetic Manganese in the Atherosclerotic Plaque of Carotid Arteries

“…As we have already mentioned, the gathered set of the experimental facts allows for ascribing the obtained spectra to Mn 2+ ions in hydroxyapatite. The incorporation of small amount of Mn 2+ ions into the hydroxyapatite structure does not lead to any substantial alterations in the parameters of the crystallographic unit cell and does not affect hydroxyapatite crystallinity [32, 33]. Surprisingly, the linewidths of the EPR spectra components in APs are quite narrow to distinguish the different contributions (cf.…”

“…A location (position) of the introduced manganese in either biogenic material or synthesized samples is still debated [17, 32, 33]. As for the natural apatites, two nonequivalent calcium positions Ca(1) and Ca(2) distinguishable in the HAp structure are proposed for the Mn 2+ substitutions.…”

“…It is conventionally assumed that Mn 2+ ions preferentially occupy the Ca(1) position in the crystallographic unit cell but can be also distributed between the Ca(1) and Ca(2) positions [17, 32, 33]. In principle, EPR techniques should feel the difference between these locations.…”

Dual Doping of Silicon and Manganese in Hydroxyapatites: Physicochemical Properties and Preliminary Biological Studies

“…Other mechanisms of manganese’s beneficial effects on bone tissue metabolism include osteocalcin stimulation, increased alkaline phosphatase activity and collagen type I production [13,14,19]. Syntheses of hydroxyapatites substituted with various amounts of manganese were carried out, from trace amounts (a few ppm) to 11.9 wt% [14,16,19,20,21,22,23,24]. However, it is important to note that in vitro studies on osteoblast cells showed significantly better results for lower manganese contents [16,19], while a Mn content of 1 wt% was regarded as high and caused drastically lower cell viability [24].…”