Properties of Iron-Containing Nanohydroxyapatite- Based Composites

EPR spectra of the (a) N-a and (b) N-c samplesProperties of Iron-Containing Nanohydroxyapatite-Based Composites. Pankratov D.A., Dolzhenko V.D., Ovchenkov E.A., Anuchina M.M., Severin A.V. //Inorganic Materials. 2017. V.53. №1. P.89-98.Search the full text below. Ищи полный текст ниже.

The paramagnetic properties of compounds resulting from the synthesis of nanohydroxyapatite in the presence of Fe(III) ions have been studied by electron paramagnetic resonance, Mössbauer spectroscopy, and magnetochemistry. Based on the obtained results on the mechanism of the reaction between an orthophosphoric acid solution and an aqueous calcium hydroxide suspension, we have found conditions for incorporating Fe(III) impurity ions into hydroxyapatite. We have studied samples differing in the sequence in which reagents were mixed and in hydroxyapatite crystallite formation conditions. It has been shown that, in all instances, the composition and properties of the iron-containing phases in the composites depend significantly on both synthesis and heat treatment conditions.

A targeted search for biocompatible iron-containing composites suitable for practical application in medicine has recently been the subject of extensive studies. Examples include preparations that are used directly to compensate for iron deficiency in the human body, magnetic field-controlled targeted drug delivery means, and biocompatible superparamagnetic nanoparticles of iron oxide compounds for chemotherapy or hyperthermia under the action of an ac magnetic field. In addition, they find application as contrast materials for nuclear magnetic resonance imaging and Mössbauer targeted radionuclide-free nanomagnetbased brachytherapy and as a biocompatible and biodegradable material for cryonanoconservation. In the last case, biocompatible iron-containing composites can simultaneously solve a few problems, for example, to ensure optimal ice nucleation and bind free oxygen radicals.

One common feature of materials in question is the presence of iron-containing nanoparticles. Note that an important role is also played by other components of the composites, which are responsible for the stabilization, isolation, transport, bioaccessibility, and biocompatibility of iron-containing nanoparticles. It is known that one such potential component is hydroxyapatite. Hydroxyapatite, an inorganic matrix of bone tissue, is successfully used as a biomaterial in biochemical and medical practice (dentistry, bone surgery, implantology, orthopedics, etc.). The formation of this nanomaterial in the presence of Fe(III) ions was the subject of a previous study. In particular, that work was concentrated on the effect of small amounts of Fe(III) ions on the morphology, phase composition, and structure of products forming under the conditions of nanohydroxyapatite synthesis. It has been shown that the addition of Fe(III) impurity ions to a reaction medium at different steps of hydroxyapatite formation allows one to control the growth, morphology, and phase composition of crystals, but the stabilization of Fe ions in the crystal structure of hydroxyapatite was not confirmed in that work. Fe atoms are assumed to form their own nanophase and produce adsorption clusters on the hydroxyapatite surface.

The objectives of this work were to examine the effect of synthesis conditions on the state of iron atoms in iron-containing nanohydroxyapatite-based composites and to study the effect of Fe ions on the properties of these materials.

Instants when iron-containing solutions should be added to the reaction mixture were determined from preliminary data on the mechanisms of the formation of calcium phosphates. To this end, we examined the variation in the pH of the reaction mixture with the fraction of orthophosphoric acid relative to the stoichiometric amount, xH. The titration curve was obtained by adding, with constant stirring, an orthophosphoric acid solution to an aqueous calcium hydroxide suspension at an average rate of 1 mL/min. Since the pH of the reaction mixture was measured every 30 s, without waiting for equilibration in the system, and because of the finite rate of heterogeneous reactions, the instants of completion of reaction steps were observed in the titration curve at slightly smaller volumes of the acid than should be expected from the stoichiometry of the reactions. Nevertheless, the resultant titration curve can be divided, somewhat arbitrarily, into four distinct portions, differing in xH. In the first region (xH = 0.0÷0.8), the pH of the reaction mixture decreases slowly in the range 12.0 to 11.5. The processes that take place in this region are Ca(OH)2 dissolution and almost concurrent nucleation, growth, and agglomeration of hydroxyapatite nanoparticles. In the second region (xH = 0.8÷1.05), after almost all of the calcium hydroxide is consumed, we observe a sharp decrease in pH to 5.0. Even though the pH of the system falls beyond the stability region of hydroxyapatite (pH 7÷12), the composition of the forming product is essentially identical to that of stoichiometric hydroxyapatite, Ca5(PO4)3OH, owing to kinetic effects and the presence of the residual precursor. Further addition of orthophosphoric acid leads to hydroxyapatite dissolution (which may be accompanied as well by hydroxyapatite conversion to calcium orthophosphate, Ca3(PO4)2, but its formation cannot be detected visually) and concurrent formation of a suspension of a new phase, brushite (CaHPO4 • 2H2O), in the reaction mixture. Since the titration curve was obtained under nonequilibrium conditions and there was partial mutual encapsulation of the reactants, it is reasonable to assume that brushite formation in the reaction mixture stops at xH = 1.4. At the end of the process (in the fourth region, at xH ≥ 1.5), all of the components of the reaction mixture pass into the calcium dihydrogen orthophosphate (Ca(H2PO4)2 • H2O) suspension.

The present results demonstrate that the properties of nanohydroxyapatite-based composites that are due to Fe impurity ions depend significantly on both the sequence in which reagents are mixed during synthesis of the material and the processing method in the isolation step. As pointed out previously, the presence of Fe ions in different nanohydroxyapatite synthesis steps influences the size, morphology, and phase composition of the particles of the material. In addition, it has been shown in this work that synthesis conditions have a significant effect on the state of the iron in the final material.

It follows from the present results that iron predominantly forms its own compounds in a nanoparticulate state. The composition and properties of the iron-containing component are determined by the synthesis conditions of the composite. The capture of Fe ions by the hydroxyapatite matrix is insignificant and occurs most readily if Fe ions are added to the reaction mixture before the onset of hydroxyapatite formation. Even though the materials studied here contain Fe ions in a variety of states, all of them have only paramagnetic properties.

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link to DOI - 10.1134/S0020168517010125 ссылка на (С) издателя link to Google Scholar Ссылка в Интеллектуальной системе тематического исследования научно-технической информации МГУ
Associated translations: Свойства железосодержащих композитов на основе наногидроксиапатита. Панкратов Д.А., Долженко В.Д., Овченков Е.А., Анучина М.М., Северин А.В. //Неорганические материалы. 2017. Т.53. №1. C.94-104.

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Properties of Iron-Containing Nanohydroxyapatite-Based Composites. Pankratov D.A., Dolzhenko V.D., Ovchenkov E.A., Anuchina M.M., Severin A.V. //Inorganic Materials. 2017. V.53. №1. P.89-98.

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