## 2 Structures, Synthesis, and Magnetic Properties of Fe\({}_{3}\)O\({}_{4}\) Nanoparticles
### Fe\({}_{3}\)O\({}_{4}\) Spherical Nanoparticles (SNPs)
Consequently, the magnetic properties of these nanospheres depend on the size of the nanosized subunits, the overall size of the nanospheres, interactions of the primary crystals, surface properties, and morphology. These NPs are either ferrimagnetic or superparamagnetic, depending on both their overall size and the size of the subunits. These types of nanoparticles are also called \"colloidal nanocrystal clusters\" (CNCs) [53] or colloidal superparticles (SFs) [67; 68]. For example, [PERSON] and coworkers prepared iron oxide NPs with a size of 5.8 nm through the thermal decomposition of iron(III) oleate [56] and then used nanoparticle micelle formation to form larger supercrystalline colloidal SPs with sizes of 120, 190, and 560 nm via solvophobic interactions [67; 68]. These colloidal SPs have superparamagnetic properties at room temperature along with excellent stability in polar solvents. [PERSON] et al. reported single-crystalline, monodisperse ferrite NPs with tunable sizes ranging from 200 to 800 nm using a solvothermal reduction method [48]. A TEM image of the obtained mean 200 nm spherical Fe\({}_{3}\)O\({}_{4}\) NPs is shown in Figure 4d. Later, this method was widely applied to synthesize SNPs for various applications, including protein detection [43] and drug delivery [69]. In the presence of a polyethylene glycol (PEG) surfactant, FeCl\({}_{3}\)-6H\({}_{2}\)O was reduced in ethylene glycol at 200 \({}^{\circ}\)C assisted by sodium acetate, an electrostatic stabilizer and a reductive-mediated agent. The size of NPs is simply controlled by the reaction time in the range of 8 to 72 h. [PERSON] and colleagues modified this recipe by adding different surfactants, polyvinylpyrrolidone, refluxing at 180 \({}^{\circ}\)C, and manipulating the reaction time from 4 to 24 h to generate NPs with diameters ranging from 100 to 275 nm [42]. Separately, [PERSON] and coworkers successfully synthesized highly water-dispersible Fe\({}_{3}\)O\({}_{4}\) SNPs with sizes from 30 to 180 nm by controlling the hydrolysis of FeCl\({}_{3}\) during high-temperature reduction in diethylene glycol [53]. In this recipe, poly(acrylic acid) (PAA) was used as the surfactant and dictated the release of water for hydrolysis upon the addition of NaOH. Thus, the size of NPs can be tuned by adding different amounts of NaOH stock solution in DEG. The packing of small primary crystals with a size of approximately 10 nm to form colloidal nanocrystal clusters (mean size 30-180 nm) has many advantages, such as maintaining superparamagnetic behavior and water-dispersible properties, as well as enhancing the overall magnetization of the nanoparticles. In addition to synthesizing different sizes of secondary structure SNPs with similar crystalline subunits, the preparation of NPs of similar sizes with a tunable degree of crystallinity was reported by [PERSON] and coworkers. They developed a method to prepare secondary structures of CNCs with a diameter of 280 nm that have tunable crystallite sizes ranging from 5.9 to 21.5 nm [46]. By employing different ratios of sodium acetate and sodium acrylate, grain sizes were controlled without changing the overall size of NPs. Consequently, similar sizes of NPs with different magnetization values were obtained. Moreover, a novel strategy to tune the size of secondary structural Fe\({}_{3}\)O\({}_{4}\) SNPs from 6 to 170 nm was introduced that used different solvent mixture compositions of EG and DEG. Due to its bulky molecules, DEG slowed the aggregation of primary crystals and created more seeds for growing NPs, causing smaller NPs to form [43; 46; 47]. Utilizing this binary solvent system, the size of spherical NPs was even manipulated in a wider range from 20 to 300 nm [47]. These NPs have either superparamagnetic or ferrimagnetic properties that are dictated by varying the water concentration in the synthesis reaction. [PERSON] et al. studied the effect of the water volume fraction on the crystallite sizes and sizes of NPs in solvothermal synthesis using ethylene glycol as the solvent [70].