## 1 Introduction
In other studies, TEMPO was applied as a redox-sensitive MRI contrast agent, and the effects of TEMPO-type nitroxides on ascorbate reduction were tested [44; 45]. The cell membrane acts as a barrier separating the interior of the cell from its external environment. It regulates ion and particle flow to a cell [46; 47]. The particles can interact with the membrane and then pass through by endocytosis and direct permeation, but the former dominates in the interactions between nanoparticles and cells [48]. The manner of nanoparticle internalization depends on its size and shape, surface charge, and topography. An important feature is surface functionalization [12; 49]. Depending on the size, endocytosis is divided into two types: phagocytosis, during which the uptake of large particles (>500 nm) occurs, and pincytosis, which includes different mechanisms such as macropinocytosis, clathrin/caveolin-mediated endocytosis, and clathrin/caveolin independent endocytosis. The magnetic field is considered a factor that influences the endocytosis process. Its role has been studied and described in the literature for its effects on cell functions and the transport of magnetic nanoparticles to targeted cells and tissues [50; 51; 52]. [PERSON] et al. described the effect of a high-gradient magnetic field on cell life [53]. They observed that a magnetic field of about 1 T with a gradient of up to 1 GT/m can significantly affect cells. Similarly, in studies concerning prostate cancer cells and human macrophages, the presence of a high gradient magnetic field increased the internalization of superparamagnetic iron oxide nanoparticles [54; 55]. Additionally, [PERSON] et al. [56] observed that the application of a high intensity (7 T) pulsed magnetic field increases the uptake of superparamagnetic iron oxide nanoparticles through the cell membrane. The literature also analyzes the influence of magnetic fields on reactive oxygen species (ROS) [57]. These are highly reactive free radicals, capable of disrupting cell function and initiating various diseases, including cancer. The impact of various types of magnetic field on human and animal cells is described in the literature [57]. Generally, a magnetic field increases ROS concentrations in cells and tissues, but there are exceptions to this rule. The effect depends on the type of magnetic field applied, its intensity, exposure time, frequency, and the kind of biological samples studied. The research conducted with magnetic fields, biological materials, and drug-functionalized magnetic nanoparticles can contribute to clinical applications, especially for ROS-related diseases. Therefore, we propose applying the ESR method as a suitable technique for the combined study of the influence of a magnetic field on the uptake of magnetic nanoparticles by cells and the concentration of free radicals within them. Yeast cells are used as a model in several studies to analyze processes similar to these in human cells [58; 59]. [PERSON] et al. [58] characterized the oxidative stress in yeast cells after internalizing fluorescent nanodiamonds proposed as free radical biosensors. [PERSON] et al. [60] described the impact of nanoparticles on yeast cell viability and [PERSON] [42] reported the antioxidant properties and toxic effect of the TEMPO spin label. Yeast cells were also applied under various cellular stress impacts, including oxidative, osmotic, and ethanol accumulation [59]. In turn, [PERSON] and [PERSON] applied yeast cells in the study of magnetic nanoparticle uptake and the antioxidant properties of a TEMPO spin probe using ESR [16; 17; 32]. This study aimed to investigate the effect of a magnetic field on the transport of TEMPOL-functionalized magnetite nanoparticles into yeast cells by ESR spectroscopy.