Enhanced sinterability and in vitro bioactivity of diopside through fluoride doping

In this work, diopside (CaMgSi2O6) was doped with fluoride at a level of 1 mol.%, without the formation of any second phase, by a wet chemical precipitation method. The sintered structure of the synthesized nanopowders was studied by X-ray diffraction, Fourier transform infrared spectroscopy and field-emission scanning electron microscopy. Also, the samples’ in vitro apatite-forming ability in a simulated body fluid was comparatively evaluated by electron microscopy, inductively coupled plasma spectroscopy and Fourier transform infrared spectroscopy. According to the results, the material’s sinterability was improved by fluoride doping, as realized from the further development of sintering necks. It was also found that compared to the undoped bioceramic, a higher amount of apatite was deposited on the surface of the doped sample. It is concluded that fluoride can be considered as a doping agent in magnesium-containing silicates to improve biological, particularly bioactivity, behaviors.

💡 Research Summary

The present work investigates the effect of a modest (1 mol %) fluoride (F⁻) incorporation on the structural, sintering, and in‑vitro bioactivity characteristics of diopside (CaMgSi₂O₆), a magnesium‑containing silicate ceramic widely explored for orthopedic applications. Diopside powders were prepared by a wet‑chemical co‑precipitation route using calcium chloride, magnesium chloride, silicon tetrachloride, and magnesium fluoride as precursors. The fluoride‑doped synthesis differed only by the addition of MgF₂ before the ammonia‑induced precipitation, with the excess magnesium from MgF₂ compensated by reducing the MgCl₂ amount to preserve the stoichiometric Ca:Mg:Si ratio of 1:1:2. The resulting suspensions were dried at 120 °C, ground, uniaxially pressed at 100 MPa, and sintered at 1200 °C for 2 h (heating/cooling rate 10 °C min⁻¹).

X‑ray diffraction (XRD) of the as‑dried powders revealed amorphous silicate phases together with crystalline ammonium chloride and ammonium magnesium chloride by‑products. After sintering, both undoped and fluoride‑doped samples displayed only the diopside phase, confirming that the 1 mol % fluoride does not generate any secondary phase. However, the doped sample exhibited sharper, more intense diffraction peaks, indicating higher crystallinity and larger crystallite size (66 nm versus 52 nm for the undoped material, calculated by the Scherrer equation). This improvement is attributed to the known melting‑point depression effect of fluoride on silicate systems, which raises the homologous temperature during sintering and promotes grain growth.

Fourier‑transform infrared spectroscopy (FT‑IR) further corroborated fluoride incorporation. In addition to the characteristic Si‑O, O‑Ca‑O, and O‑Mg‑O vibrations of diopside, the doped material showed new bands at ~800 cm⁻¹ and ~930 cm⁻¹ assigned to Si‑F stretching. Moreover, the Si‑O bands shifted slightly to higher wavenumbers (2–5 cm⁻¹), reflecting the higher electronegativity of fluorine and a subtle redistribution of electron density in the Si‑O network.

Scanning electron microscopy (SEM) of the sintered bodies revealed a porous microstructure resulting from the sublimation of the large amount of NH₄Cl formed during precipitation. The doped sample displayed larger primary particles (≈800 nm) compared with the undoped one (≈300 nm) and, more importantly, considerably larger sintering necks (≈500 nm versus ≈150 nm). These observations confirm that fluoride accelerates neck formation and overall densification.

The biological performance was evaluated by immersing the sintered discs in simulated body fluid (SBF) for three days at 37 °C. Field‑emission SEM coupled with energy‑dispersive X‑ray spectroscopy (EDS) showed that the undoped surface acquired only sparse, nanometric precipitates, whereas the fluoride‑doped surface was covered by a uniform leaf‑like apatite layer on ~85 % of the area, with an average thickness of ~20 nm. EDS confirmed the presence of phosphorus, calcium, and silicon, indicating apatite formation. FT‑IR of the post‑immersion samples displayed intensified phosphate (≈970 cm⁻¹, 1080 cm⁻¹) and carbonate (≈1420–1550 cm⁻¹) bands, together with the disappearance of the hydroxyl band at 3570 cm⁻¹, suggesting the formation of hydroxycarbonate/fluorocarbonate apatite—a phase known for superior chemical stability and bone‑bonding ability.

Inductively coupled plasma atomic emission spectroscopy (ICP‑AES) measured ion concentrations in the SBF before and after immersion. Both samples released Mg and Si, while Ca concentrations increased due to partial dissolution of the diopside matrix, creating a supersaturated environment that favors apatite nucleation. The doped specimen showed a more pronounced rise in Si and Mg, consistent with its enhanced dissolution and simultaneous apatite precipitation.

In summary, a simple 1 mol % fluoride doping, introduced via a wet‑chemical route, yields a single‑phase diopside ceramic with improved sinterability (larger grains, more extensive neck growth), higher crystallinity, and a markedly superior ability to induce apatite formation in SBF. The findings demonstrate that fluoride is an effective dopant for magnesium‑containing silicate bioceramics, offering a practical pathway to enhance both processing and biological performance for orthopedic and dental applications.