Nanocrystalline Ni0.35Zn0.65Fe2O4 mixed ferrite was obtained from the Fe2(Ni0.35,Zn0.65)(OH)4(C2H2O4)2⋅H2O complex combination that corresponds to the atomic ratio Ni(II) : Zn(II) : Fe(III) = 0.35 : 0.65 : 2; the complex combination was decomposed at 325°C and the resulting oxides mixture was annealed in the temperature range of 400–1000°C for 2 h. The thermal analysis of the synthesized complex combination was done by TG–DTA techniques. It has been shown by means of x-ray diffraction that even at 400°C Ni–Zn mixed spinel ferrite is formed with an face-centred cubic structure and a lattice parameter that is in agreement with the reported value. Thus, the formation temperature of ferrite was drastically reduced (by cca. 900°C) compared to that of the conventional ceramic method. The magnetic measurements showed the increase of the saturation magnetization σs and a maximum of the coercivity Hc of the nanocrystalline system with the increase of the annealing temperature. These changes can be attributed to the increase of the average diameter of the nano-sized crystallites from 14.6 to 46.3 nm when the temperature increases from 400°C to 1000°C. The nanocrystallites are single-domain up to ∼28 nm; above this value they have an incipient structure of Weiss domains, a result that is in agreement with the critical diameter of the single-domain deduced from theoretical calculation.
The cancer therapy with the lowest possible toxicity is today an issue that raises major difficulties in treating malignant tumors because chemo- and radiotherapy currently used in this field have a high degree of toxicity and in many cases are ineffective. Therefore, alternative solutions are rapidly being sought in cancer therapy, in order to increase efficacy and a reduce or even eliminate toxicity to the body. One of the alternative methods that researchers believe may be the method of the future in cancer therapy is superparamagnetic hyperthermia (SPMHT), because it can be effective in completely destroying tumors while maintaining low toxicity or even without toxicity on the healthy tissues. Superparamagnetic hyperthermia uses the natural thermal effect in the destruction of cancer cells, obtained as a result of the phenomenon of superparamagnetic relaxation of the magnetic nanoparticles (SPMNPs) introduced into the tumor; SPMNPs can heat the cancer cells to 42–43 °C under the action of an external alternating magnetic field with frequency in the range of hundreds of kHz. However, the effectiveness of this alternative method depends very much on finding the optimal conditions in which this method must be applied during the treatment of cancer. In addition to the type of magnetic nanoparticles and the biocompatibility with the biological tissue or nanoparticles biofunctionalization that must be appropriate for the intended purpose a key parameter is the size of the nanoparticles. Also, establishing the appropriate parameters for the external alternating magnetic field (AMF), respectively the amplitude and frequency of the magnetic field are very important in the efficiency and effectiveness of the magnetic hyperthermia method. This paper presents a 3D computational study on specific loss power (Ps) and heating temperature (ΔT) which allows establishing the optimal conditions that lead to efficient heating of Fe3O4 nanoparticles, which were found to be the most suitable for use in superparamagnetic hyperthermia (SPMHT), as a non-invasive and alternative technique to chemo- and radiotherapy. The size (diameter) of the nanoparticles (D), the amplitude of the magnetic field (H) and the frequency (f) of AMF were established in order to obtain maximum efficiency in SPMHT and rapid heating of magnetic nanoparticles at the required temperature of 42–43 °C for irreversible destruction of tumors, without affecting healthy tissues. Also, an analysis on the amplitude of the AMF is presented, and how its amplitude influences the power loss and, implicitly, the heating temperature, observables necessary in SPMHT for the efficient destruction of tumor cells. Following our 3D study, we found for Fe3O4 nanoparticles the optimal diameter of ~16 nm, the optimal range for the amplitude of the magnetic field of 10–25 kA/m and the optimal frequency within the biologically permissible limit in the range of 200–500 kHz. Under the optimal conditions determined for the nanoparticle diameter of 16.3 nm, the magnetic field of 15 kA/m and the frequency of 334 kHz, the magnetite nanoparticles can be quickly heated to obtain the maximum hyperthermic effect on the tumor cells: in only 4.1–4.3 s the temperature reaches 42–43 °C, required in magnetic hyperthermia, with major benefits in practical application in vitro and in vivo, and later in clinical trials.
This article studies the dynamic magnetic behavior of a system of colloidal Fe3O4 nanoparticles dispersed in kerosene. By increasing the frequency of the magnetizing field from 50 to 640 Hz, and by reducing the temperature from 300 to 77 K, the measuring time is getting closer to the magnetic relaxation time. Under these conditions, it was possible to observe how the dynamic behavior is modified by the parallel arrangement of the easy magnetization axes of the particles from the frozen ferrofluid, as opposed to the situation when the axes are randomly oriented. Unlike the superparamagnetic behavior at room temperature, at low temperatures the magnetization has a hysteresis loop. This behavior is due to Néel relaxation processes. It has been shown that the relaxation time resulting from the Néel theory is determined by an effective anisotropy constant that takes into account the magnetocrystalline anisotropy, as well as the shape and surface anisotropy. The relaxation time becomes greater when the easy magnetization axes of the nanoparticles are aligned in the direction of the magnetization field, as opposed to the case when the axes are oriented in all directions. The results show that the remanence increases both with the decrease of the temperature and with the increase of the frequency of the magnetization field. At the temperature of 77 K, the saturation magnetization Msat of the colloidal suspension increases by 57.1% compared to the value at the temperature of 300 K, whereas the saturation (spontaneous) magnetization Ms of bulk Fe3O4 increases by only 6.6% in the same temperature range. Using the core-shell model, we assumed that the surfactant decreases the superexchange interaction in the shell, as opposed to the core of the particle; this leads to an increase of the magnetic diameter when the temperature is decreasing.
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