2.1. Synthesis of Hydroxyapatite Nanoparticles (HAp)

The hydroxyapatite, nanoparticles, Ca₁₀(PO₄)₆(OH)₂, were obtained using an adapted coprecipitation method which was previously detailed in [25,26].

The reagents for synthesis, calcium nitrate [Ca(NO₃)₂·4H₂O] and ammonium dihydrogen phosphate [(NH₄)₂HPO₄] were purchased from Alpha Aesare, Karlsruhe, Germany. The Ca₁₀(PO₄)₆(OH)₂, with x = 0 (HAp), nano powder was synthesized by an adapted co-precipitation method (Ca/P molar ratio: 1.67) using [(NH₄)₂HPO₄] and Ca(NO₃)₂·4H₂O [27]. The hydroxyapatite nanoparticles was effectuated by adjustment the atomic ratio Ca/P as 1.67. A designed amount of Ca(NO₃)₂·4H₂O were dissolved in deionised water to obtain 400 mL Ca-containing solution. In the same time, a designed amount of ammonium dihydrogen phosphate (NH₄)₂HPO₄ was dissolved in deionised water to make 400 mL P-containing solution. The two solutions were agitated at 100 °C during 40 min. The P-containing solution was added drop by drop into the Ca-containing solution. The pH of resulting mixture was kept constant during the reaction by adding NH₃ and stirred continuously for 4 h. The resulting final mixture was filtered and washed several times with deionized water. Finally, the recovered powder was dried in an oven at 100 °C for 72 h. The resulting powder after drying was used in the experiments in this study.