Comparison of Hydrothermal and Sol-Gel Synthesis of Nano-Particulate Hydroxyapatite by Characterisation at the Bulk and Particle Level

Abstract

Hydrothermal and sol-gel synthesis methods have been used to prepare nano-particulate hydroxyapatite (HA) powders for detailed characterisation. Bulk elemental analysis data are compared from X-ray diffraction, X-ray fluorescence and Fourier transform infrared spectroscopy. These show the presence of secondary phases in the sol-gel powders which can be attributed to evaporative loss of precursor phosphite phases during specimen preparation and breakdown of the primary HA phase during calcination. Only the primary HA phase is detected in the hydrothermally prepared powder. In addition, Ca/P ratios of each powder are determined at the particle level using transmission electron microscopy with energy dispersive X-ray spectroscopy (TEM-EDX), having first established a threshold electron fluence below which significant electron-beam-induced alteration of the composition of HA does not occur. The TEM-EDX results show a greater compositional variability of particles from the sol-gel preparation route compared to the hydrothermal route. Overall it is the combination of the analysis techniques that shows the hydrothermal synthesis route produces nearstoichiometric, single phase, hydroxyapatite.

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“…Selected area electron diffraction from these particles showed them to be polycrystalline having lattice spacings consistent with an apatite structure when the lattice spacings were matched to the strong reflections of the hydroxyapatite x-ray standard (Supplementary Figure 1A [inset] and Supplementary Table 1). Infrared analysis was consistent with hydroxyapatite (HA) cultured in TCM ( Figure 1H & I) since amine adsorption bands from the serum proteins could be identified at 1600-1670 cm -1 [20], carbonate adsorption bands were present at 1465-1410 cm -1 and potential OH broadening from residual water with the main OH band were present at 3400 cm -1 [21]. The remaining bands at lower wavenumber (<1100 cm -1 ) are due to lattice absorption and have previously been assigned to HA [22] since there are no other absorption bands for any other calcium phosphate, such as dicalcium phosphate dihydrate and octacalcium phosphate, present in the current spectra [23].…”

“…Rigorous identification of calciumdeficient hydroxyapatites (i.e., Ca:P ratios in the range 1.5-1.67) requires the use of several complementary characterization techniques and thermal treatment of powders [24]. We do not present powder XRD or thermal treatment results here; however, the morphology, composition and electron diffraction pattern of what we assume to be representative particles, plus the infrared fingerprint of the bulk material, do indeed invoke the formation of a nonstoichiometric apatite phase [20][21][22]. Since cells are exposed to the freshly formed hydrated species we use the terms 'apatite' and 'apatitic' throughout.…”

Influence of Growth Parameters on the Formation of Hydroxyapatite (HAp) Nanostructures and Their Cell Viability Studies

“…Nanostructured HAp has been prepared using several methods, e.g., hydrothermal [ 9 , 10 ], high gravity [ 11 ], sol-gel [ 10 ], solid state reaction [ 12 ], microwave [ 13 , 14 ], reflux condensation [ 15 ], ultra sonication [ 16 , 17 ], green synthesis [ 18 , 19 ], mechano-chemical [ 20 , 21 ], micro emulsion [ 22 , 23 ] and complexing agent-assisted precipitation [ 24 – 26 ] techniques. In all of these methods, the tweaking of parameters for achieving the desired morphology and good crystalline nanostructures is not easy.…”

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“…Other approaches for synthesis of nano/microparticle of biocompatible materials such as CaP include template-directed growth methods, using surfactants, ligands or solid membrane templates. Solvothermal technique is considered to be a conventional technique offering an effective route for the production of controlled morphology of calcium phosphate materials in the presence of templates or surfactants [26]. However, there is a concern in using template materials during conventional solvothermal synthesis which may lead to formation of unwanted organic residues being incorporated into the final products [27].…”