Synthesis of nanohydroxyapatite in the presence of iron (III) ions
Synthesis of nanohydroxyapatite in the presence of iron (III) ions. Severin A.V., Pankratov D.A. //Russian Journal of Inorganic Chemistry. 2016. V.61. №3. P.265-272.
The effect of small amounts of iron (III) ions on the morphology, phase composition, and structure of the products of the hydroxyapatite (HAP) synthesis has been studied by electron microscopy, X-ray powder diffraction, and Mossbauer spectroscopy methods. It has been demonstrated that the introduction of dopant iron (III) ions into the reaction mixture at different stages of HAP formation makes it possible to control crystal growth, morphology, and phase composition. The iron ions are not incorporated into the HAP crystal structure; rather, they form their proper nanophase, as well as adsorption clusters on the HAP surface.
Hydroxyapatite (HAP)—the inorganic matrix of bone tissue—is among the most popular biomaterials in medical and biochemical practice (bone surgery, stomatology, creation of implants or coatings for metal implants in orthopedics, etc.). The presence of transition-metal impurities in HAP (when they are substituted for Сa2+ ions or when they form stable adsorption complexes on the HAP crystal surface) can noticeably change its physicochemical properties, such as solubility, thermal stability, biochemical activity, magnetic properties, and mechanical durability. Some biologically active metals (for example, zinc, magnesium, iron, or manganese) present as impurities in HAP can stimulate bone tissue formation and differentiation since they are natural components of bone tissue. Nevertheless, a specific role of many metals in rigid tissue and their potential medical application is still unclear. At the same time, composite materials containing HAP nanoparticles with transition-metal (Cu, Co, Fe, etc.) compounds as nanocatalysts (for example, for glycerol conversion or formaldehyde oxidation) have found increasing application in recent years. Substitution of transition metal for calcium ions in HAP can lead to the formation of colored products, which can be used as safe pigment dyes.
Among HAP composites with metal compounds, iron holds a special position. First, apatite-like structures in which iron ions can be substituted for calcium ions can function as precursors of de novo bone tissue and enhance the proliferative activity of osteoblasts (cells that make bone). Second, numerous recent efforts have focused on composites of HAP crystals with magnetic iron-containing particles of different nature. Heightened interest in such compounds is due to the development and improvement of methods of magnetic therapy for cancer, in particular, for different types of bone cancer. In this case, HAP particles serve as both the carriers of the therapeutic magnetic iron and the building material for healthy bone tissue. Third, in addition to widespread medical applications, HAP–Fe composites as specific carriers of different metal-containing nanocatalysts enhancing their activity have been reported.
Available literature data demonstrate that HAP composites with iron (III) ions are mainly of three types: composites where iron (III) ions are incorporated in the HAP crystal structure through substitution for one of the calcium ions in it; composites in which these ions are adsorbed (as ions or nanoclusters) on the surface of HAP nanocrystals; composites in which iron (III) compounds form their proper nanophase. Each of these forms is synthesized by a special procedure with a wide variation of synthesis and post-synthesis treatment conditions. In the present work, control of the moment of introduction of iron (III) compounds into the system of reagents allowed us to produce all three forms of composites in a single experiment. As distinct from the earlier experiments, we introduced a maximal iron amount possible under given conditions (but not more than 7.5% of the HAP weight).
The formation of aqueous suspensions of nano-HAP in the presence of iron (III) sulfate has been studied. It has been demonstrated that parameters of the reaction products, such as particle size, morphology, and phase composition, can be modified by varying the moment of introduction of the modifying compound into the reactor. The strongest effect is achieved when the iron-containing solution is introduced before the onset of the basic synthesis reaction. In this case, the impurity particle of calcium sulfate and iron (III) hydroxide can act as HAP aggregation and growth centers. In addition, the ability of iron (III) ions to suppress the HAP crystallization is most pronounced (formation of the amorphized HAP phase). The introduction of an iron-containing solution at later stages has an effect (insignificant) mainly on the HAP particle size. The data obtained in this study demonstrate that iron itself can form both its own nanophase (oxo hydroxo species in samples 1a and phosphate species in samples 2a and 3a) and ion clusters adsorbed on the HAP nanoparticle surface. Being annealed at 500°С, the iron-containing particles undergo the incomplete thermal transformation. The relative content of different iron species in samples can be tailored by varying both the moment of introduction of the modifying agent and its concentration. The strategy of introduction of bioactive metal ions at early stages of HAP synthesis can be used for producing bone tissue precursors with different morphology and microcomponent content. If it is necessary to use HAP as the carrier of magnetic particles, iron ions should be introduced at final stages of HAP synthesis in relatively large amounts. However, under our experimental conditions, it is inadvisable to use iron sulfate for increasing the iron content in samples because of the formation of large amounts of calcium sulfate as a byproduct.