## 3 Results and Discussion  
The material studied, Fe\({}_{3}\)O\({}_{4}\)@TEMPOL, was initially characterized using physico-chemical methods (Figure 3). The XRD pattern of magnetite was confirmed by reflections at 2 theta: 29.7, 35.1, 42.4, 53.8, 57.6, and 65.1 (Joint Committee for Powder Diffraction Studies, JCPDS No. 74-0748). The diffraction reflections associated with Fe\({}_{3}\)O\({}_{4}\) were also observed in Fe\({}_{3}\)O\({}_{4}\)@GLY and Fe\({}_{3}\)O\({}_{4}\)@TEMPOL samples. This confirmed the stability of the magnetite structure during the organic functionalization of its surface. TEM images indicated irregular shapes of nanoparticles with average sizes ranging from 10 to 20 nm. The presence of an organic layer was confirmed by thermogravimetric measurement. Pure magnetite and the nanoparticles with organic units already demonstrated initial weight losses (1%) below 120 \({}^{\circ}\)C due to adsorbed water molecules. However, the decomposition of organic units resulted in an increased weight loss at 200-400 \({}^{\circ}\)C compared to the initial Fe\({}_{3}\)O\({}_{4}\). Notably, the total mass loss for Fe\({}_{3}\)O\({}_{4}\)@TEMPOL was slightly greater than that for Fe\({}_{3}\)O\({}_{4}\)@GLY, suggesting an increase in the organic layer on the magnetite surface. In the next step, the presence of TEMPOL on the magnetite surface was confirmed by electron spin resonance (ESR) spectroscopy. In previous papers, we showed the application of the ESR method to monitor the uptake of functionalized magnetite nanoparticles by yeast cells and cancer cells [16; 17]. These pilot studies concentrated on the effect of a magnetic field on the course of this process. Figure 4a,b show a typical ESR spectrum of magnetite nanoparticles recorded at a temperature of 260 K. The broad line (Figure 4a) corresponds to the magnetic core and the narrow one to the TEMPOL spin label attached to the surface of the nanoparticles. This line is in fact a triplet, resulting from the hyperfine interaction of the spin of the unpaired electron with the spin of the nitrogen nucleus (Figure 4b). Figure 4c,d demonstrate the spectra of nanoparticles and TEMPOL attached to them, recorded at 295 K. The ESR signal from the TEMPOL spin label recorded at low temperature (Figure 4b) was taken to observe the interactions between nanoparticles and cells and the uptake of nanoparticles. According to our previous results, such a spectrum consisting of three lines obtained in the frozen sample indicated the penetration of nanoparticles into cells [16; 17; 32]. The next research step concerned the influence of the magnetic field generated by a neodymium magnet on the magnetic nanoparticle penetration rate into cells (Figure 1). The results are shown in Figure 5. The concentration of free radicals in the sample during the incubation decreased due to reactions with radicals and reactive oxygen species, which probably occur in cellular mitochondria. As a result of such redox reactions, the attached  
Figure 3: (**a**) XRD pattern and TEM image (inset) of Fe\({}_{3}\)O\({}_{4}\) nanoparticles; (**b**) thermogravimetric curves of Fe\({}_{3}\)O\({}_{4}\), Fe\({}_{3}\)O\({}_{4}\)@GLY, and Fe\({}_{3}\)O\({}_{4}\)@TEMPOL. spin label was reduced to stable hydroxylamines [17; 32]. Importantly, the control and the sample incubated in a magnetic field behaved differently (Figure 5). In the case of the control sample, the concentration of free radicals in subsequent measurements was lower than that in the sample incubated in the presence of the magnetic field. Moreover, the signal decayed earlier in the control sample. It can, therefore, be concluded that incubation of the sample in a constant magnetic field slowed down the interactions between nanoparticles and cells. The results were statistically significantly different for the defined significance level of 0.05. A similar relationship has been described in the literature [51]. The authors studied the effect of a pulsed magnetic field and compared the results for a constant magnetic field.