## 3 Results and Discussion
It was found that, in a constant magnetic field, the transport of magnetic nanoparticles across cells was slower. Furthermore, their microscopic observations revealed that, under a constant magnetic field, this transportation was hindered by the formation of large aggregates of nanoparticles. These aggregates, owing to their size, were unable to undergo endocytosis. Also, [PERSON] et al. [52] discovered that using very strong magnetic fields to deliver nanoparticle-based drugs caused their aggregation within minutes to sizes exceeding the size of intercellular pores in cancer tissues, thus preventing them from reaching their target sites. Since, in our study, the samples were always stable on the magnet (there was no movement), the nanoparticles could accumulate in the form of larger aggregates on the magnet. Figure 4: Representative ESR spectra of TEMPOL-functionalized magnetite nanoparticles with yeast cells: (**a**) Magnetic core with TEMPOL recorded at 260 K, magnetic sweep width of 650 mT; (**b**) signal from attached TEMPOL recorded at 260 K, magnetic sweep width of 13 mT; (**c**) magnetite core with TEMPOL recorded at 295 K, magnetic sweep width of 650 mT; (**d**) signal from attached TEMPOL recorded at 295 K, magnetic sweep width of 8 mT. In the second experiment, we applied a rotating magnetic field to check its impact on magnetic nanoparticle uptake by cells using the previously designed apparatus for nanoparticle focusing (Figure 2) [64; 65; 66]. The results are shown in Figure 6. In the first part of the experiment (Figure 6a), samples were kept in a rotating magnetic field for the appropriate time and then incubated. For the significance level of 0.05, the results were statistically significantly different for the measurements performed after 1.5 and 2.25 h (3 and 4 points in the graph). However, the general tendency was similar to the results observed in the experiment with the neodymium magnet (Figure 5). In this experiment, the samples were incubated after exposure to a rotating magnetic field and measured at subsequent time intervals. Moreover, the induction of the magnetic field used in this case was only 22 mT in the center of the Petri dish with the sample and 11 mT at its edge. An opposite tendency was observed in the second stage of this experiment (Figure 6b). The concentration of free radicals decreased faster in the sample exposed to a rotating magnetic field just before the measurement compared to the control sample. Such changes can probably be connected with the prolonged production of reactive oxygen species (ROS) in cells caused by a magnetic field [50]. For the significance level of 0.05, the results were statistically significantly different for the measurements performed after 0.75 h and 1.5 h (2 and 3 points in the graph). The effect of a rotating magnetic field on cancer cells was studied by [PERSON] et al. [67]. An electromagnetic field increases the reactive oxygen species (ROS) level in cells. Additionally, a weak magnetic field can impact spin pairing with redox-active radicals. Thus, it may disrupt the flow of electrons in the mitochondrial electron transport chain. This could explain the faster decay of the ESR signal in the samples exposed to the rotating field in our experiment. The differences between the obtained results were not as apparent as in the case of the experiment with the neodymium magnet because the magnetic field induction was also much lower. The number of yeast cells was controlled with an optical microscope during the experiments. The results are shown in Tables 1 and 2. Magnetic nanoparticles and a magnetic field had a negligible effect on cell number, with two exceptions marked in Table 2 without a statistical significance at the defined significance level of 0.05. The biological effects caused by low-frequency electromagnetic fields on yeast cells have been studied by [PERSON] et al. [68]. The authors exposed yeast cells to the magnetic field for 8 h and observed an inhibitory effect on cell growth and multiplication.