Modulated Monoclinic Hydroxyapatite: The Effect of pH in the Microwave Assisted Method

• Main Authors: Daniel Sánchez-Campos, Maria Isabel Reyes Valderrama, Susana López-Ortíz, Daniela Salado-Leza, María Eufemia Fernández-García, Demetrio Mendoza-Anaya, Eleazar Salinas-Rodríguez, Ventura Rodríguez-Lugo
• Format: Article
• Language: English
• Published: MDPI AG 2021-03-01
• Series: Minerals
• Subjects: hydroxyapatite; microwave hydrothermal synthesis; pH effect; hydroxyapatite nanorods; monoclinic phase
• Online Access: https://www.mdpi.com/2075-163X/11/3/314

Hydroxyapatite (HAp) is a natural hard tissue constituent widely used for bone and tooth replacement engineering. In the present work, synthetic HAp was obtained from calcium nitrate tetrahydrate (Ca(NO₃)₂·4H₂O) and ammonium phosphate dibasic (NH₄)₂HPO₄ following an optimized microwave assisted hydrothermal method. The effect of pH was evaluated by the addition of ammonium hydroxide (NH₄OH). Hence, different characterization techniques were used to determine its influence on the resulted HAp powders’ size, shape, and crystallinity. By Transmission Electron Microscopy (TEM), it was observed that the reaction pH environment modifies the morphology of HAp, and a shape evolution, from sub-hedral particles at pH = 7 to rod-like nanosized HAp at pH = 10, was confirmed. Using the X-ray Diffraction (XRD) technique, the characteristic diffraction peaks of the monoclinic phase were identified. Even if the performed Rietveld analysis indicated the presence of both phases (hexagonal and monoclinic), monoclinic HAp prevails in 95% with an average crystallite size of about 23 nm. The infrared spectra (FTIR) showed absorption bands at 3468 cm⁻¹ and 630 cm⁻¹ associated with OH⁻ of hydroxyapatite, and bands at 584 cm⁻¹, 960 cm⁻¹, and 1090 cm⁻¹ that correspond to the PO₄³⁻ and CO₃²⁻ characteristic groups. In summary, this work contributes to obtaining nanosized rod-like monoclinic HAp by a simple and soft method that has not been previously reported.