## 3 Results and Discussion
Due to the potential use of iron oxide nanoparticles, among others, in medicine, it is essential to investigate their toxic effects, both functionalized and unfunctionalized, on cells. Yeast cells were also used to study the toxic effect of the TEMPO spin label [42]. The authors examined both the effect of TEMPO concentration (0.1 mM, 0.5 mM, 1 mM, 3 mM, and 5 mM) and incubation time of cells with the radical. It was observed that TEMPO at high concentrations (3 mM and 5 mM) inhibited the growth rate of yeast cells. In the second stage, for a concentration of 3 mM, the incubation time of cells with the radical was examined. The studies showed that a longer incubation time and higher concentration of the radical had an inhibitory effect on the growth of the tested cells. On the other hand, the described studies also confirmed the antioxidant properties of TEMPO [32; 41; 43]. [PERSON] et al. [60] measured the impact of bare iron oxide nanoparticles on yeast cell viability. For two concentrations of nanoparticles (0.1 gL\({}^{-1}\) and 1 gL\({}^{-1}\)) and two strains of yeast cells, no significant impact within 6 h was observed. After 24 h, in one of the strains for the concentration of 1 gL\({}^{-1}\), a slightly negative effect (5%) on their viability was noticed. Our study used a low concentration of TEMPO-functionalized magnetite nanoparticles (0.5 gL\({}^{-1}\)) to eliminate their potential toxic effect on cells. However, the present study aimed to show the impact of a magnetic field on iron oxide nanoparticle uptake by yeast cells and the application of ESR to investigate this process. The functionalized Fe\({}_{3}\)O\({}_{4}\) nanoparticles possessed three distinct regions that facilitated accomplishing the research goals. The magnetite core, Fe\({}_{3}\)O\({}_{4}\), rendered the molecule magnetically responsive, while the silica layer shielded Fe\({}_{3}\)O\({}_{4}\) from external factors. Furthermore, the organic functional groups of silane facilitated the creation of robust hydrogen bonds with -OH, -NH, and COOH groups present on the cell surface, promoting the interaction of the magnetic NP with the cell surface. Finally, a spin label with oxidizing and radical-scavenging properties enabled rapid monitoring of the binding process of the magnetic nanoparticles in solution by recording ESR spectra. Detailed examinations of the molecular-level interactions between Fe\({}_{3}\)O\({}_{4}\) particles without silica coating and yeast cells were conducted by [PERSON] [69][70] and [PERSON] [60]. Conversely, investigations of the interactions between spin labels and cells were carried out by [PERSON] [70] and [PERSON] [42]. Additionally, SEM images were captured and EDS analysis was performed for yeast cells incubated in the presence of Fe\({}_{3}\)O\({}_{4}\)@TEMPOL. In the SEM image obtained at the beginning of the incubation (Figure 7), the predominant findings involved yeast cells featuring functionalized magnetite nanoparticles on their surfaces, along with a minority of yeast cells existing independently without Fe\({}_{3}\)O\({}_{4}\). Furthermore, there were a few observed aggregates of Fe\({}_{3}\)O\({}_{4}\)@TEMPOL that were not linked to yeast cells within the field of view. Yeast cells displayed attachment of the functionalized Fe\({}_{3}\)O\({}_{4}\) nanoparticles on their surfaces at various locations. Nevertheless, the individual binding of single nanoparticles by yeast cells was not observed. In order to determine the elemental composition of the selected yeast cells, EDS measurements were performed (Figure 8). Yeast cells without and those containing Fe\({}_{3}\)O\({}_{4}\)@TEMPOL were chosen for investigation. The elemental distribution obtained in both instances affirmed the binding of magnetite to the yeast cell surface, marked by a substantial increase in signals for Fe and O and the emergence of a signal for Si.