## 4 Biomedical Applications  
This section of the review will display recent studies regarding magnetic nanoparticles which have garnered great interest regarding the most important techniques used in biomedical applications. One of them, the MRI, is a diagnostic technique used to visualize the internal structure of the body in detail. This technique has the advantage of obtaining a high display of soft tissues and is non-invasive, compared with computed tomography [131]. Also, unlike other diagnostic methods such as computed tomography (CT), sonography, nuclear scintigraphy, and X-ray imaging, MRI does not cause radiation damage and offers a high resolution of soft tissues which allows this method to be successfully applied to diagnosing a variety of diseases [7]. Along with the drug delivery capability of these systems, they can generate hyperthermia which can be used either to enhance delivery or to kill tumoral cells. Hyperthermia that treats cancer is also called thermal therapy, thermal ablation, or thermotherapy [53; 54; 62; 88]. Figure 3: Graphical representation of a surface functionalization model. [PERSON] et al. [107] developed core@shell nanostructures from modified silica magnetite nanoparticles loaded with a photosensitizer (PS) and a model drug \"methylene blue\" (MB) for biomedical applications such as drug delivery. The main concern of modern medicine is to treat cancer with few side effects. Based on this concept, photodynamic therapy (PDT) has been developed [132], in which certain photosensizers (PS) were loaded into drug delivery vehicles (DDVs) because of their ability to induce photothermry or to assist the drug delivery in a personalized manner. First, the targeted place for the implementation of PS is reached and, then, through the irradiation, it is directed to a specific, targeted area. Even if PS touches normal tissues, it is toxic only when exposed to light, and thus no, or limited, side effects occur in healthy tissues. [PERSON] was the first to initiate the PDT concept by using a dye as a PS in the photodynamic therapy. Studies concluded that acridine dye activated by light could kill paramecia, which was a finding that led to the basis of PDT [133; 134]. More recent research has shown the significant progress that photosensitizers have brought to cancer treatments. Studies in clinical photodynamic therapy have received increased attention since the hospital administrations have permitted using photosensitizing drugs in curative and palliative treatments of diverse diseases [135; 136]. Studies conducted by [PERSON] [5] and [PERSON] et al. [137] confirmed that the magnetite core of the SPIONs coated with silica shells present promising applications in the biomedicine field, such as contrast agents for magnetic resonance imaging and drug delivery systems [25; 138]. Since ancient times, the early diagnosis of various diseases, such as cancer and sepsis, has always been a major problem of vital importance in medicine. Thus, special attention has been paid to the study of efficient methods of auxiliary diagnosis [1]. Magnetic resonance imaging (MRI) is one of the most powerful medical diagnostic methods that provides MRI imaging with excellent anatomical details based on soft-tissue contrast and functional information in a non-invasive method of real-time monitoring [6]. MRI results are strongly influenced by both the different imaging methods used in detecting and establishing medical problems, and the contrast agent (CA), which can significantly improve the contrast of biological targets around the tissue [9]. However, because the conventional contrast agent RM (CA) is expensive, and confers a low spatial resolution, early tiny tumours cannot be detected and, at the same time, no distinction can be made between benign and malignant tumours [8]. Therefore, the development of new CAs can effectively enhance the imaging effect.