# Recent Progress and Challenges Regarding Magnetic-Based Nanoparticles for Targeted Drug Delivery
## 5 Monitoring of Magnetic-Based Nanocarriers Interacting with Cells in Biological Systems  
Transferring research on magnetite-based nanosystems from the academic laboratory level to practical biomedical applications requires an actual analysis of the processes that nanocarriers undergo in biological systems. In general, the mechanisms proposed for the delivery of magnetic nanostructures assume using a strong magnetic field near the target site after DDS injection or implantation of magnets near the target tissue before injection. In practice, the first method encounters difficulty in penetrating deep tissues by an external magnetic field, and the second method requires knowledge of the exact location of cancer cells. Moreover, only a few original works thoroughly examine this issue [38; 55; 76]. Typically, articles describe the magnetic properties of a drug nanocarier using a magnetometer, and the efficacy of targeted drug delivery therapy is assessed solely based on the magnetic saturation values. Then, it is often assumed that an external magnetic field generated by permanent magnets, along with a gradient, would be capable of concentrating the drug at the tumor site or other target tissues. However, the magnetic field is able to penetrate the body up to 2 cm from the skin, and its strength is reduced with distance; thus, efforts are underway to explore minimally invasive solutions [118; 119; 120]. Recently, a new method for focusing magnetic nanoparticles in three-dimensional space was proposed [121]. The focusing system was based on four neodymium magnets in a special configuration rotating around a sample with magnetic particles. Two magnets (magnetic induction 60 mT; diameter 70 mm) produced a strong homogeneous magnetic field of N-S, while two others (magnetic induction 30 mT; diameter 35 mm) produced a magnetic field of opposite orientation. The magnet system rotating around a sample containing ferromagnetic particles led to a rapid concentration of those particles in a three-dimensional space. Hence, this approach can potentially concentrate a magnetic drug carrier in a specific area, facilitating the release of an optimal drug dosage. The method was also applied to investigate the effect of different organic units attached to the magnetite core on the average focusing time in a saline-serum mixture [122]. The highest focusing time was observed for magnetic NPs with amine, carboxylic groups, and quaternary ammonium salts. It is presumably the effect of the strongest interactions of those groups with bioorganic molecules present in serum compared to other organic units. These findings could be significant in determining the focusing mechanism of nanoparticles as the functional groups at the surface of the magnetite core significantly influence the rate of their concentration in a desired area. On the other hand, extensive in vitro research is being carried out on monitoring cells interacting with magnetic nanocarriers in biological systems. However, it is also crucial to investigate nanoparticles upon their introduction into a biological system [25; 123; 124]. Initially, biomolecules interact with a magnetic NP's surface through physical adsorption (ionic, electrostatic interactions, van der Waals forces) or chemical linkages (covalent bond). However, within the discussed nanosystems, many have been specifically engineered to exhibit a higher binding affinity towards cells. The presence of targeted ligands (e.g., FA) at the nanocarier surface results in ligand-receptor interactions characterized by higher bond strength. Another strategy is an attachment on the NP's surface special functional groups (e.g., thiol), which results in chemical conjugation with cells. The adsorption at the NPs involves different functional groups, depending on the biomolecule: carboxylic groups in proteins; hydroxyl, amino, and carboxylic in carbohydrates; phosphate in DNA; carboxylic and phosphate in lipids.