## 4 Applications of Fe\({}_{3}\)O\({}_{4}\) Nanoparticles
### Environmental Applications  
Fe\({}_{3}\)O\({}_{4}\) nanoparticles and their nanoarchitectures have been utilized for applications in environmental treatments, such as heavy metal removal and adsorption/degradation of organic pollutants [18]. In these applications, Fe\({}_{3}\)O\({}_{4}\) NPs have been used as supporting platforms that can easily be recovered/recycled by using magnetic separation techniques. Inexpensiveness, easy fabrication, biocompatibility, and easy separation/recovery by magnetic fields are the advantages of using Fe\({}_{3}\)O\({}_{4}\) nanoparticles in environmental applications. However, easy agglomeration (for ferrimagnetic NPs) and oxidation are drawbacks that can be overcome by applying an appropriate surface coating layer or functionalized surface. The surface coating layer should be stable under harsh chemical conditions (e.g., acidic  
Figure 11: (a) Magnetic signal of streptavidin-conjugated Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\)@PAA NPs bound to a biotinylated surface. (b) Schematic illustration of the interaction between magnetic NPs and the surface for protein detection in the EXIRM analysis. (c) EXIRM data for Protein A arising from an exchange between IgG1 and IgG2 subclasses. Reproduced with permission from ref. [43]. Copyright 2018 American Chemical Society. Figure 12: (a) Schematic illustration of the contact area in the biosensing of nanocubic and nanospherical Fe\({}_{3}\)O\({}_{4}\). FIRMS data showing the (b) magnetization profiles versus applied force of nanocubes and nanospheres with similar volumes and (c) number of retained nanoparticles on the sensor at 1 pN. Reproduced with permission from ref. [42]. Copyright 2017 American Chemical Society. and basic conditions), have high thermal and mechanical stability, and be porous with a high loading capacity for pollutants. In general, magnetically loaded adsorbents are usually core-shell structures or composites of Fe\({}_{3}\)O\({}_{4}\) with other oxides (e.g., SiO\({}_{2}\), TiO\({}_{2}\)), carbon-based materials (carbon, carbon nanotubes, and graphene), and polymers or organic molecules with a good affinity for heavy metals or organic pollutants. [PERSON] et al. synthesized superparamagnetic Fe\({}_{3}\)O\({}_{4}\)@C core-shell nanoparticles with an average size of \(\sim\)250 nm and studied the adsorption kinetics of two dyes, methylene blue (MB) and cresol red (CR) [140]. The prepared magnetic NPs are dispersible in an aqueous solution, easily separated from the solution using an external magnet, and have adsorption capacities for MB and CR of 44.38 mg/g and 11.22 mg/g, respectively. Separately, carboxylatopillar [5]arene-modified Fe\({}_{3}\)O\({}_{4}\) nanoparticles synthesized using a one-pot solvothermal technique exhibit excellent cationic dye separation over a wide range of pH values and can be recycled by simple washing without decreasing the adsorption properties [141]. [PERSON] examined the ability of Fe\({}_{3}\)O\({}_{4}\) nanocubes coated with SiO\({}_{2}\) and TiO\({}_{2}\) to remove Cr(VI) [142]. [PERSON] and coworkers also prepared ferrimagnetic Fe\({}_{3}\)O\({}_{4}\)@carbon composites and studied their performance in both removing the heavy metal Cr(VI) and organic pollutant Congo red [143]. Furthermore, Fe\({}_{3}\)O\({}_{4}\)-functionalized nanoparticles and composites have been applied to remove various heavy metal cations, such as Pb(II), Cd(II), Cu(II), Hg(II), As(III), and Cr(III) [144; 145; 146; 147; 148; 149; 150], and organic pollutants, including rhodamine B, rhodamine 6G, methyl orange, and oil [151; 152; 153]. Additionally, Fe\({}_{3}\)O\({}_{4}\) NPs have been used as magnetic carriers for photocatalyst materials (e.g., TiO\({}_{2}\)) to promote the photocatalytic degradation of organic pollutants such as methylene blue and ofloxacin fluoroquinoline [154].