Accurate characterization of pure silicon-substituted hydroxyapatite powders synthesized by a new precipitation route

TL;DR: The results, particularly those from infrared spectroscopy, raise serious reservations about the phase purity of previously prepared and biologically evaluated SiHA powders, pellets and scaffolds in the literature.

About: This article is published in Acta Biomaterialia.The article was published on 2013-06-01 and is currently open access. It has received 88 citations till now. The article focuses on the topics: Infrared spectroscopy & Fourier transform infrared spectroscopy.

Summary

1. Introduction

• In order to correctly describe the physical, chemical and biological properties of SiHAs and to compare them to routinely implanted HA and β-TCP, well-characterized pure SiHAs powders first need to be prepared.
• Therefore, this work was devoted to the development of a new route to synthesize monophasic SiHA powders with controlled stoichiometry.
• To this purpose, a solution of soluble silicate was first prepared from TEOS via a sol-gel route, and then accurate powder analysis was carried out by means of ICP/AES, Xray powder diffraction, Rietveld refinement, high resolution electron transmission microscopy (HR-TEM) with energy dispersive spectroscopy (EDS) as well as infrared (FT-IR/ATR) and solid-state NMR spectroscopy.
• Two pH levels of precipitation were studied, as well as six Si/P molar ratios.

2.1 Powder synthesis

• The as-synthesized powders were heated under air using an alumina crucible.
• The heating and cooling rate was fixed at 4°C min -1 .

2.2.1 X-ray powder diffraction and Rietveld refinement

• Crystalline phases were identified by means of a Siemens D5000 θ/2θ X-ray diffractometer.
• The evolution of the crystallinity of the samples after calcination at 1000°C for 15 h was evaluated by means of the full width at half maximum (FWHM) of the (211) peak at 2θ=31.8°, as it had the highest intensity and minimal overlap with neighboring peaks.

2.2.4 Electron microscopy (HR-TEM, SAED and EDX)

• Gold was then distributed as crystallized nano-domains which were used as a reference in the selected area electron diffraction (SAED) patterns to calculate as precisely as possible the lattice parameters.
• The SAED patterns obtained from regions with or without gold on the HA part were the same.

3.2.4 Electron microscopy

• The results are the average of about ten intervals per pattern.
• Moreover, other experimental patterns for different zone axes (not shown here) were obtained and compared to theoretical electron diffraction patterns calculated by means of the Java Electron Microscopy Simulation (JEMS) software [87] .
• The results indicate that the experimental and simulated patterns are perfectly superimposed for 0.734 ≥ c/a ≥ 0.729.

Figures

Fig. 8. 1H-31P HETCOR CP MAS experiment recorded on (a) Si1.00HA powder calcined at 400°C for 2h, and (b) Si0.50HA powder calcined at 1000°C for 15h.

Fig. 10. FTIR spectra of calcined HA and SiHA powders (1000°C / 15h) collected at room temperature, symbol ● shows additional vibrations for Si1.0HA at 500, 515, 535 cm-1.

Fig. 2. X-ray diffraction patterns of raw SiHA powders synthesized at pH = 10.8 with different silicon concentration.

Table 2 NMR parameters.

Fig. 12. FTIR spectra of calcined Si1.0HA powder (1000°C / 15h) under different partial pressure of steam (pH2O = 0, 80, 200 mbar and air).

Fig. 5: (a) selected area electron diffraction (SAED) pattern of heat treated Si1.0HA powder (1000°C/15h), (b) SAED of heat treated Si1.0HA powder (1000°C/15h) for the [0 1 0] zone axis, c/a= 0.730

Table 4 IR bands and shoulders assignment proposed by our wrk

Table 1 Chemical conditions of preparation of HA and SiyHA samples and final composition of the calcined powders (1000°C/15h).

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