Magnetic iron oxide nanoparticles

Magnetic iron oxide nanoparticles (MINPs) are also attractive nanoparticles with an excellent magnetic property that is suitable for application in biomedical approaches. Although MINPs are easily oxidized with air, similar to GNPs, their surface can be modified to provide better stability than the naked form. Various types of inorganic coatings, such as metal substance, silica, sulfide, metal oxide, and nonmetal material, can be used for coating the surface of MINPs. Besides these inorganic materials, MINPs can also be indirectly or directly coated with organic materials. The common organic materials that have been used to coat the surface of MINPs are dextran, starch, poly(ethylene glycol) (PEG), poly(D, L-lactide), and polyethylenimine. In general, after coating MINPs with these materials, they can be further modified with designed molecules (Wu et al., 2008, 2015). It is commonly known that there are three popular forms of MINPs: hematite (a-Fe2O3), magnetite (Fe2O4), and maghemite (y-Fe2O3) (Fig. 7.2) (Wu et al., 2015). Their morphology relies on an iron oxide precursor with other processes such as nucleation, growth, and adsorption, which cause more difficulty in controlling the shape and size of MINPs than gold nanoparticle synthesis. Nevertheless, different shapes and sizes of MINPs can be prepared through various techniques, such as hydrothermal synthesis, thermal decomposition, coprecipitation, and sonochemical synthesis (Roca et al., 2009; Wu et al., 2008). The aggregation is an important factor on synthesis of MINPs (Mohapatra and Anand, 2010). Different techniques for MINP synthesis were reviewed by Laurent et al. (2008). As mentioned previously, it is important to make MINPS stable against aggregation stimulated by the magnetic field and biological fluid. To overcome this aggregation problem, controlling electrostatic and steric repulsions is a key factor. Therefore many approaches of surface coatings have been applied to increase the stability of MINPs. However, the coating effect on magnetic property and biocompatibility of MINPs needs to be taken into consideration.

Different crystal structure of different MINPs

Figure 7.2 Different crystal structure of different MINPs. The black, green, and red balls represent Fe2+, Fe3+, and O2- respectively. MINPs, magnetic iron oxide nanoparticles. Waiting for permission from Wu, W., Wu, Z., Yu, T., Jiang, C., Kim, W.S., 2015. Recent progress on magnetic iron oxide nanoparticles: synthesis, surface functional strategies and biomedical applications. Science and Technology of Advanced Materials 16, 023501.

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