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Magnetic NP's detection and diagnosis strategies have recently received considerable attention. Magnetic nanoparticles combined with other diagnostic systems offer exclusive advantages over conventional methods. Specifically, deep tissue imaging magnetic nanomaterials are used because a biological sample does not exhibit magnetic background so it has become a point‐of‐care diagnostics as well. The magnetic NPs are widely used in many applications (Figure 1.21). Magnetic Resonance Imaging (MRI) is a noninvasive imaging modality that has been widely used in clinical diagnosis (Sun et al. 2008; Gao et al. 2014). The novel magnetic NPs in vivo enable strong image contrast of targeted anatomical sites, also sense the local molecular environments to catalyze contrast switching. In ex vivo diagnostics, magnetic nanomaterials are seamlessly incorporated into reduced biosensing platforms, thereby enabling the detection of rare and diverse molecular targets without requiring for extensive sample preparation. From these synergistic developments, it is likely that magnetic detection will have broad applications in biomedical research as well as clinical transformation. Nowadays, tumor morbidity is increasing, and early tumor diagnosis is of vital importance. MRI is one of the common methods for tumor diagnosis (Shokrollahi et al. 2014).

Figure 1.21 Different applications of magnetic NPs.

Anbarasu et al. (2015) labeled the PEG‐coated Fe₃O₄ NPs with a monoclonal antibody and then implanted it into the colon cancer mouse model. They successfully conducted targeted localization by MRI. Stem cell has attracted widespread attention in research on biomedicine owing to its excellent proliferative capacity and differentiation potential. Presently, there are two methods to label stem cells using superparamagnetic NPs. Additionally, cells injected into the tumor tissue were recognized histologically to be the superparamagnetic NP‐labeled stem cells. The function and activity of these cells are not affected, suggesting that superparamagnetic NPs can be used for labeling stem cells (Kim et al. 2016; Guo et al. 2018). Magnetic nanoparticles based on a novel nano biosensor have been developed by Grimm et al. (2004) for rapid screening of telomerase activity in biological samples (Kaittanis et al. 2009). Hassen et al. (2008) defined a method based on DNA hybridization to detect the hepatitis B virus using the nonfaradic electrochemical impedance spectroscopy method. They modified DNA probes with biotin on streptavidin‐based magnetic nanoparticles and then immobilized nanoparticles onto the bare gold electrode using a magnet. Sayhi et al. (2018) developed a technique with the aim of isolation and detection of influenza A virus H9N2 subtype. They first attached an anti‐matrix protein 2 antibody to iron magnetic nanoparticles (Saylan et al. 2019).

Superparamagnetic nanoparticles are beneficial for cell‐tracking and for calcium sensing. Ferrofluids consist of a magnetic core surrounded by a polymeric layer coated with antibodies for capturing cells (Atanasijevic et al. 2006). Superparamagnetic nanoparticles with 2–3 nm sizes have been used in conjunction with MRI to reveal small and otherwise undetectable lymph‐node metastases. A dextran‐coated iron oxide nanoparticle increases MRI visualization of intracranial tumors for more than 24 hours (Gao et al. 2014). Ma et al. (2011) presented a novel method of cocaine detection. They used a fluorescence biosensor based on aptamer and rolling circle amplification of short DNA strand separated by magnetic beads. The cocaine aptamers were immobilized onto gold nanoparticles functionalized magnetic beads hybridized with short DNA strand. The rolling circle magnification and the separation by magnetic beads decrease the background signal. Furthermore, compared with other reported cocaine sensors, their method exhibited excellent sensitivity. In addition, new strategy may provide a platform for the detection of numerous proteins and low molecular weight analytes.