Synthesis of Fiber-like Monetite without Organic Additives and Its Transformation to Hydroxyapatite

Abstract

Monetite (CaHPO4, DCPA) and hydroxyapatite (Ca10(PO4)6(OH)2, HAp) are promising biomaterials with wide application in the biomedical field. In this study, we propose a novel method to prepare fiber-like DCPA without organic additives for the first time and show that nanostructured HAp can be synthesized without changing the macroscopic morphology of DCPA. Our results show that the temperature, pH, calcium to phosphate ratio (Ca/P), and concentration of NH4 + are essential factors in the formation of fiber-like DCPA. The synthesized materials were characterized by powder X-ray diffraction analysis, scanning electron microscopy, and transmission electron microscopy, and the DCPA → HAp phase transformation process was examined. The growth of HAp on the DCPA crystal can be regulated by the composition of the hydrolyzing solution, forming HAp with its c-axis perpendicular [HAp (001)//DCPA (100)] or parallel [HAp (001)//DCPA (010)] to the elongation direction of the DCPA crystal. At a high concentration of Ca2+ in NH4OH solution, octacalcium phosphate was observed as an intermediate phase with its c-axis parallel to elongation of the initial DCPA crystal. The HAp crystals formed after complete transformation of the precursor phases have a preferred crystallographic orientation with c-axis coaxial to the elongation direction of DCPA. In the concentrated NH4OH solution, the direct transformation of DCPA to HAp was observed. At initial stages of the hydrolysis, the c-axis of newly formed HAp is perpendicular to the elongation direction of DCPA. This study provides a new insight into regulation of the crystal growth of DCPA without organic additives and a facile route to form HAp with hierarchical structures.

On the Crystallization of Hydroxyapatite under Hydrothermal Conditions: Role of Sebacic Acid as an Additive

“…This is simply because the synthesized DCPA crystals were oriented with a triclinic crystal system with the P 1̅ space group. 35 As shown in the results of the XRD pattern of hydrothermally synthesized CaP crystals under pH 7 and pH 12, all diffraction peaks can be indexed and assigned to a pure HAp using ICDD 09-0432 ( Figure 2 b). No peaks for any other phases were detected for these samples.…”

Development of a thermodynamic approach to assist the control of the precipitation of hydroxyapatites and associated calcium phosphates in open systems

“…These observations, formalized by the solubility isotherms, 31,32 are often reported to explain the formation of DCPA and HAp under acidic and neutral/basic conditions, respectively. 34,43 However, due to the different Ca/P ratios exhibited by CaPs, the comparison of their solubility isotherms cannot be used as a basis for the comprehensive understanding of their competitive precipitation. (a) Data on CaPs dissolution-precipitation equilibria were taken and/or extrapolated from the literature.…”

Facet-Specific Dissolution–Precipitation at Struvite–Water Interfaces

“…After 2 h of reaction, the formation of a lamellar phase that is composed of particles at a height of ∼55 nm was observed (Figure S2C). The precipitate composition was analyzed using SEM-EDS (Figure A1), showing that in area # I (Figure A2), lattice spaces for the (030), (211), and (22̅1) planes with corresponding d (030) = 2.20Å, d (211) = 2.51 Å, and d (22̅1) = 2.16 Å of monetite (CaHPO 4 ) − and the angles between the planes (∠ (030) – (211) = 65.3°, ∠ (211) – (22̅1) = 63.2°, and ∠ (030) – (22̅1) = 128.5°) − are in accordance with the experimental observations (Figure A3,A4). This suggests that the monetite crystalline phase resides along its [102̅] viewing zone axis (Figure A4).…”