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Magnetic nanoparticles (MNPs) are widely used in biomedical applications such as biosensing, bioimaging, hyperthermia, and drug delivery. 1 MNPs exhibit properties that are noticeably different from their bulk materials due to the particle size closely approaching the domain size as well as an increased surface-to-volume ratio afforded by the smaller dimensions. 2 Given that biosensors are devices that provide both qualitative and quantitative information regarding the composition of the molecules in which the sensor is situated, 3 biosensing applications require specific properties that strongly depend on size, shape, functionality, stability, and magnetization of the MNPs, as summarized in Figure 1.

Typically, MNPs are classified as diamagnetic, paramagnetic, ferromagnetic, ferrimagnetic, antiferromagnetic, and superparamagnetic based on intrinsic magnetic dipoles and the net magnetization in the presence and absence of an external magnetic field. 2 In the presence of an external magnetic field, the maximum magnetization observed when the magnetic moment of the MNPs is aligned along the direction of the external magnetic field is defined as saturation magnetization (Ms). 4 Upon removal of an external magnetic field, the magnetic moment of MNPs retains the previous direction and magnetization, which is defined as remnant magnetization (Mr). In general, MNPs with high Ms values are preferred for biosensing applications due to their high sensitivity, which provides enhanced sensing efficiency; 5 furthermore, Ms values generally increase with the size of the MNPs. Even though there is no clear consensus with regard to shape, some …