Aleksey E. Sokolov

Carbon-encapsulated cobalt nanoparticles (NPs) were synthesized by thermal decomposition of a solution of organic components and cobalt acetate followed by heat treatment in an argon atmosphere at different temperatures Theat from 400 to 1000 °C. The crystal structure, morphology, magnetic properties, and magnetic circular dichroism (MCD) of NPs formed at each Theat were studied. The decomposition resulted in the formation of Co3O4 NPs in a carbon shell with an average diameter of ∼10 nm. Heat treatment ensured the reduction of Co3O4 first to CoO monoxide and then to metallic cobalt with an increasingly perfect crystalline structure and a higher degree of carbon shell graphitization as Theat increased. The as-synthesized (initial) Co3O4 NPs were paramagnetic in the temperature range used in magnetization measurements. Starting with NPs treated at Theat = 500 °C, a hysteresis loop was observed. The shape of the hysteresis loop was identical for all the samples heat-treated at Theat ≥ 500 °C. The saturation magnetization (Ms) increased significantly with increasing Theat and reached 110 emu/g at Theat = 1000 °C, with a low coercivity Hc (75 Oe). The value of the MCD signal and its spectrum varied significantly with Theat.

The effect of the Mn3+ ion on the local distortions of the FeO6 octahedra in orthoferrite samples was investigated. Mössbauer spectroscopy measurements for a series of the HoFe1−xMnxO3 (x=0.0−0.7) orthoferrite samples with space group Pnma was carried out at temperatures above the Néel point (700 K). The electric field gradient (EFG) tensor on Fe ions for these compounds was found using the first-principle calculations. The concentration dependence of the quadrupole splitting was resulted from the experimental and theoretical data. The absence of an electron contribution to the quadrupole splitting at all the manganese concentrations was demonstrated. It was shown by the theoretical calculations that the values of all the electric field gradient components increase significantly with the manganese concentration in the system, and the eigenvectors Vxx and Vyy of the electric field gradient tensor sharply change their direction at concentrations of x > 0.1.

One of the promising novel methods for radical tumor resection at a single-cell level is magneto-mechanical microsurgery (MMM) with magnetic nano- or microdisks modified with cancer-recognizing molecules. A low-frequency alternating magnetic field (AMF) remotely drives and controls the procedure. Here, we present characterization and application of magnetic nanodisks (MNDs) as a surgical instrument (“smart nanoscalpel”) at a single-cell level. MNDs with a quasi-dipole three-layer structure (Au/Ni/Au) and DNA aptamer AS42 (AS42-MNDs) on the surface converted magnetic moment into mechanical and destroyed tumor cells. The effectiveness of MMM was analyzed on Ehrlich ascites carcinoma (EAC) cells in vitro and in vivo using sine and square-shaped AMF with frequencies from 1 to 50 Hz with 0.1 to 1 duty-cycle parameters. MMM with the “Nanoscalpel” in a sine-shaped 20 Hz AMF, a rectangular-shaped 10 Hz AMF, and a 0.5 duty cycle was the most effective. A sine-shaped field caused apoptosis, whereas a rectangular-shaped field caused necrosis. Four sessions of MMM with AS42-MNDs significantly reduced the number of cells in the tumor. In contrast, ascites tumors continued to grow in groups of mice and mice treated with MNDs with nonspecific oligonucleotide NO-MND. Thus, applying a “smart nanoscalpel” is practical for the microsurgery of malignant neoplasms.

The morphology, structure, and magnetic properties of Fe₃O₄ and Fe₃O₄@C nanoparticles, as well their effectiveness for organic dye adsorption and targeted destruction of carcinoma cells, were studied. The nanoparticles exhibited a high magnetic saturation value (79.4 and 63.8 emu/g, correspondingly) to facilitate magnetic separation. It has been shown that surface properties play a key role in the adsorption process. Both types of organic dyes—cationic (Rhodomine C) and anionic (Congo Red and Eosine)—were well adsorbed by the Fe₃O₄ nanoparticles’ surface, and the adsorption process was described by the polymolecular adsorption model with a maximum adsorption capacity of 58, 22, and 14 mg/g for Congo Red, Eosine, and Rhodomine C, correspondingly. In this case, the kinetic data were described well by the pseudo-first-order model. Carbon-coated particles selectively adsorbed only cationic dyes, and the adsorption process for Methylene Blue was described by the Freundlich model, with a maximum adsorption capacity of 14 mg/g. For the case of Rhodomine C, the adsorption isotherm has a polymolecular character with a maximum adsorption capacity of 34 mg/g. To realize the targeted destruction of the carcinoma cells, the Fe₃O₄@C nanoparticles were functionalized with aptamers, and an experiment on the Ehrlich ascetic carcinoma cells’ destruction was carried out successively using a low-frequency alternating magnetic field. The number of cells destroyed as a result of their interaction with Fe₃O₄@C nanoparticles in an alternating magnetic field was 27%, compared with the number of naturally dead control cells of 6%.

This work is devoted to the study of magnetic Fe₃O₄ nanoparticles doubly coated with carbon. First, Fe₃O₄@C nanoparticles were synthesized by thermal decomposition. Then these synthesized nanoparticles, 20–30 nm in size were processed in a solution of glucose at 200 °C during 12 h, which led to an unexpected phenomenon—the nanoparticles self-assembled into large conglomerates of a regular shape of about 300 nm in size. The morphology and features of the magnetic properties of the obtained hybrid nanoparticles were characterized by transmission electron microscopy, differential thermo-gravimetric analysis, vibrating sample magnetometer, magnetic circular dichroism and Mössbauer spectroscopy. It was shown that the magnetic core of Fe₃O₄@C nanoparticles was nano-crystalline, corresponding to the Fe₃O₄ phase. The Fe₃O₄@C@C nanoparticles presumably contain Fe₃O₄ phase (80%) with admixture of maghemite (20%), the thickness of the carbon shell in the first case was of about 2–4 nm. The formation of very large nanoparticle conglomerates with a linear size up to 300 nm and of the same regular shape is a remarkable peculiarity of the Fe₃O₄@C@C nanoparticles. Adsorption of organic dyes from water by the studied nanoparticles was also studied. The best candidates for the removal of dyes were Fe₃O₄@C@C nanoparticles. The kinetic data showed that the adsorption processes were associated with the pseudo-second order mechanism for cationic dye methylene blue (MB) and anionic dye Congo red (CR). The equilibrium data were more consistent with the Langmuir isotherm and were perfectly described by the Langmuir–Freundlich model.

Fe₃O₄@SiO₂ core-shell nanoparticles (NPs) were synthesized with the co-precipitation method and functionalized with NH₂ amino-groups. The nanoparticles were characterized by X-ray, FT-IR spectroscopy, transmission electron microscopy, selected area electron diffraction, and vibrating sample magnetometry. The magnetic core of all the nanoparticles was shown to be nanocrystalline with the crystal parameters corresponding only to the Fe₃O₄ phase covered with a homogeneous amorphous silica (SiO₂) shell of about 6 nm in thickness. The FT-IR spectra confirmed the appearance of chemical bonds at amino functionalization. The magnetic measurements revealed unusually high saturation magnetization of the initial Fe₃O₄ nanoparticles, which was presumably associated with the deviations in the Fe ion distribution between the tetrahedral and octahedral positions in the nanocrystals as compared to the bulk stoichiometric magnetite. The fluorescent spectrum of eosin Y-doped NPs dispersed in water solution was obtained and a red shift and line broadening (in comparison with the dye molecules being free in water) were revealed and explained. Most attention was paid to the adsorption properties of the nanoparticles with respect to three dyes: methylene blue, Congo red, and eosin Y. The kinetic data showed that the adsorption processes were associated with the pseudo-second order mechanism for all three dyes. The equilibrium data were more compatible with the Langmuir isotherm and the maximum adsorption capacity was reached for Congo red.