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he effect of preparation conditions on the composition and structure of three-dimensional composite nanomaterials based on opal matrixes (packing of spherical particles of amorphous SiO2) has been studied. The experimental part of the work was performed with the samples of opal matrixes with a diameter of spherical SiO2 particles equal ~260 nm. Composite nanomaterials were formed by repeatedly filling of opal matrixes with solutions of metal salts (oxides) and holding the samples at 623–723 K, and after that, they were annealed at 973–1473 K. Chemical reactions and phase transformations of substances in nanopores of opal matrixes depended on the annealing parameters, and chemical properties of intermediate compounds. There was confirmed the formation of SiO2 crystallites in nanopores of composite nanomaterials, as well as the products of their interaction with SiO2. Composite nanomaterials with filling of opal matrix nanopores with metals, ferroelectrics and piezoelectrics, multiferroic and other substances have been obtained. The resulting composite nanomaterials had ordered components (substances) with a given composition and crystallite size in the range of 10–90 nm. The influence of the composition and structure of composite nanomaterials containing ferromagnetically ordered nanostructured magnetic orthovanadates of rare-earth metals, Ni-Zn-Fe-, Co-Zn-Fe-spinel, and also Co and Ni together, on their properties was demonstrated. Present research was executed under financial support by RFBR (Grant N 18-29-02076).

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Out of all the cardiac manifestations of post-COVID syndrome, tachycardia has been studied to a lesser extent, while almost all patients complain of palpitations.The purpose of the study is to reveal the incidence of post-COVID tachycardia syndrome in apparently healthy patients and to evaluate the effectiveness of ivabradine in this category of patients.Materials and methods. 48 patients revealing no comorbid pathology were examined 12 or more weeks after COVID-19. Materials of instrumental research methods, functional tests, quality of life before and during treatment with ivabradine were studied.Results. In all patients included in the study, against the background of an autonomic imbalance with a predominance of the activity of the sympathetic part and deterioration in quality of life, the syndrome of post-COVID tachycardia was diagnosed, and a positive effect of ivabradine on heart rate control was revealed.Conclusion. Tachycardia can be considered a marker of post-COVID syndrome in patients who were apparently healthy before coronavirus infection; it can manifest itself in POTS, NST, or their combination, as well as be accompanied by other cardiac arrhythmias including PVC, SVE, non-sustained SVT, and persistent form of AF. Ivabradine is able to control heart rate and level the manifestations of post-COVID tachycardia syndrome.