# Recent Progress and Challenges Regarding Magnetic-Based Nanoparticles for Targeted Drug Delivery
## 5 Monitoring of Magnetic-Based Nanocarriers Interacting with Cells in Biological Systems
Conversely, the benefits encompass the capability to detect nanoparticles in cells and tissues and the acquisition of qualitative and quantitative data on their chemical composition. TEM can be integrated with other techniques to enhance its utility. Correlative light and electron microscopy (CLEM) aligns the overall cellular view with detailed information from TEM. Addressing the issue of thin samples, scanning transmission electron microscopy (STEM) offers a spatial resolution of 5-10 nm [127]. Various microscopic techniques have been employed to quantify iron oxide nanoparticles and assess their 3D distribution within cells and tissues. In investigations involving cells and superparamagnetic iron oxide nanoparticles, TEM and 3D electron tomography can be utilized. Another approach to gaining insights into the 3D interaction between iron oxide NPs and cancer cells involves combining TEM with cryo-soft X-ray tomography. Additionally, the 3D distribution of magnetite nanoparticles, coated with a fluorescent unit, within the intracellular environment could be determined using fluorescence microscopy in conjunction with scanning transmission X-ray tomography and ptychography. A study of iron oxide nanoparticles with cells employed a combination of scanning transmission electron microscopy (STEM), electron tomography, and EDX chemical analysis. Moreover, effective detection of magnetic nanoparticles in cancer cells can be achieved using electron spectroscopic imaging (EELS) combined with STEM [125]. Among other methods, atomic force microscopy (AFM) is worth mentioning for the visualization of single molecules and cells. The samples in liquid environments can be observed with sub-nanometer resolution. To enhance its capabilities, AFM can be synergistically integrated with fluorescence microscopy and total internal reflection fluorescence (TIRF). It facilitates a more comprehensive characterization of single-molecule structures and dynamic interactions [127]. Moreover, magnetic relaxometry (MRX) is another method capable of detecting and locating magnetic nanoparticles. For medical usage, it is crucial to specify the size of the nanoparticles (typically in the order of tens of nanometers) and ensure they possess high magnetic moments [129]. Finally, Raman microscopy can also be used to observe nanoparticles inside cells and give information on biochemical interactions within the cells. Surface-enhanced Raman scattering (SERS) is a non-destructive and water-free interference technique appropriate for biological samples [131]. Endocytosis can also be examined at the levels of protein and gene expression. Inhibitors that block endocytosis are employed. However, this approach may introduce potential side effects, such as cytotoxicity and the upregulation of the entire process. Genetic approaches offer greater specificity than chemical inhibitors, yet they have certain limitations. Hence, it is advisable to combine various approaches and controls. The examination of protein levels (proteomics) is carried out using mass spectrometry (MS). Studies at this level yield extensive information about endocytosis processes and other cellular biological processes linked to cell proliferation, inflammation, and apoptosis [127]. The structure and dynamics of proteins and nucleic acids for endocytosis can be studied by electron spin resonance (ESR) spectroscopy and confocal laser scanning microscopy [132]. Nevertheless, investigations utilizing ESR and associated techniques involve using stable free radicals known as spin labels--stable free radicals that can also serve in functionalizing magnetic nanoparticles [133; 134]. The discussion on the interactions between nanoparticles and cells is closely related to reactive oxygen species (ROS), which are intracellular free radicals characterized by one or more unpaired electrons in their valence shell [135]. ROS can originate from both exogenous and endogenous sources, with mitochondria as one of the primary endogenous contributors within a cell [136].