Magnetic nanoparticles with specific features can be obtained using ferromagnetic elements such as Fe, Ni, Co or metal oxides (Fe₂O₃, Fe₃O₄), alloys (CoPt, FePt), and ferrites (MnFe₂O₄, CoFe₂O₄).

The electronic and magnetic properties of nanoparticles depend on their size [12]. In particular, finite-size effects derived from the electron quantum confinement, and surface effects related to the symmetry of the crystal structure, dominate the nanoparticles’ magnetic properties. Large particles have a multi-magnetic domain structure, with a remanent magnetization in the absence of an external magnetic field, and exhibit a ferromagnetic behavior. Decreasing the particle size to the nanoscale (typically around 10–25 nm) results in a single magnetic domain structure with all the spins lined in the same direction, but with a superparamagnetic behavior [13]. When an external magnetic field is absent, superparamagnetic nanoparticles exhibit zero magnetization, no coercivity, and less tendency to agglomerate at room temperature, which makes them good candidates for biomedical and adsorptive applications. However, for magnetic separation, particles with ferromagnetic properties are mainly used [14].

The strong connection between the size, the shape, and the magnetic properties of the nanoparticles leads to the development of a wide number of synthetic procedures to achieve high crystallinity, a narrow size distribution, uniform morphology, and tuneable properties [15]. Magnetic nanoparticles can be obtained by physical or chemical methods. Physical strategies include top-down approaches, leading to nano-sized materials from bulk materials (molecular beam epitaxy [16], chemical vapour deposition [17], and spray pyrolysis [18]). The main disadvantage of these techniques is the formation of powders with a wide side distribution. Chemical methods involve, on the contrary, bottom-up approaches, since they use molecular precursors to synthetize nanocrystals. Chemical methods for the synthesis of high-quality magnetic nanoparticles include co-precipitation [19], microemulsion [20], hydrothermal treatment [21], and thermal decomposition in the presence of molecular precursors [22]. The co-precipitation method is extensively used for the synthesis of MNPs, with a good control on size and magnetic properties for biomedical applications. However, MNPs obtained by this technique tend to agglomerate because of their small particle size [19]. The microemulsion technique allows for the obtainment of MNPs, with a good control over size and composition and high saturation magnetization. However, the type of surfactant used affects nanoparticles’ properties, and represents a great disadvantage [20]. The hydrothermal or solvothermal method is mainly used for the synthesis of ultrafine powder and crystals of different materials. For example, Zheng et al. [23] have reported the hydrothermal synthesis of Fe₃O₄ NPs in the presence of sodium bis (2-ethylhexyl) sulfosuccinate as surfactant. The main disadvantage associated with this synthetic route is that nanoparticles smaller than 10 nm in size cannot be obtained [21]. Among the various chemical methods used for the fabrication of MNPs, the thermal decomposition of organometallic precursors in the presence of stabilizing agents such as surfactants best allows the synthesis of inorganic nanoparticles in a wide range of composition, including oxides, metals, and semiconductors, with a good control of their size, shape, size dispersion, crystallinity, and, accordingly, the resulting physicochemical properties. MNPs obtained by means of such synthetic approaches are dispersible in organic solvent, and require post-synthetic treatments for their application in biological fields [22]. Alivisatos et al. [24] reported the synthesis of maghemite nanocrystals with size of 4–10 nm by the thermal decomposition of iron cupferron complexes (FeCup₃). Very recently, the γ-irradiation method, commonly named as the radiolytic method, has emerged as a new green synthetic route for magnetic oxides, exploiting the interaction between high energy γ-photons and an aqueous phase [25]. Recently, Jurkin et al. [26] have reported for the first time the synthesis of δ-FeOOH nanodiscs by γ-irradiation of a deoxygenate iron(III) chloride alkaline solution in the presence of diethylaminoethyl-dextran (DEAE-dextran) polymer, which is able to disperse the NPs and form colloidal solutions rather than suspensions.

Magnetic nanoparticles derived from iron, in the form of magnetite (Fe₃O₄) or maghemite (γ-Fe₂O₃), are the most extensively studied in the last decades, and have become promising candidates for various applications because of their magnetic properties, chemical stability, low toxicity, biological compatibility, tuneable size, and particle shapes that can be controlled by varying the synthesis conditions, as well as the fact that they can be easily coated by surface functionalization [27].

For most applications, the chemical stability of magnetic nanoparticles is crucial in order to prevent agglomeration, precipitation, or oxidation. Moreover, their surface functionalization is essential not only to make them stable against degradation, but also to convey additional properties that enable their specific activity towards target cells, such as tumor cells in order to address hyperthermia, towards biological ligands for the development of electrochemical sensors, and also towards pollutants for the uptake of contaminants from water, thus leading to the fabrication of various nanocomposites with applications in many technological fields [28,29,30,31,32]. The strategies developed for the protection of magnetic nanoparticles, their surface engineering, and their integration in functional structures and materials, can be divided into two major groups: surface coating with inorganic materials (silica shell [33,34], carbon [35,36], metals [37]) and coating with organic materials (surfactants, polymers [38,39,40,41]). In recent years, biopolymers including cellulose, alginate, chitosan, polyethylene glycol (PEG), and synthetic eumelanin-type biopolymers such as mussel-inspired polydopamine (PDA) have received much more attention for MNPs coating owing to their physicochemical properties, which are useful for different applications in various research areas. Surface modification of MNPs can be carried out by two main strategies, i.e., in situ and ex situ processes [42]. In the case of in situ surface functionalization, the coating is carried out during the synthesis of nanoparticles and it starts at the same time of nucleation, avoiding further growth of MNPs [42]. Generally, nanocomposites obtained this way have core-shell or mosaic structures [43] with a variable polymer shell in terms of morphology and thickness. The ex situ surface functionalization procedure, on the contrary, relies on two different stages: the synthesis of nanoparticles and their successive coating with biopolymers, allowing for better control of the nanocomposites’ morphology. In both cases, the interactions attending the adsorption mechanism of biopolymers on the surface of MNPs are mainly electrostatic and hydrophobic interactions, and hydrogen bonding [42]. All strategies available for the surface functionalization of MNPs lead to several magnetic bio-nanocomposites characterized by different structures, including core-shell, shell-core-shell, multicores or matrix-dispersed structures, and Janus-type hetero-structures [44,45,46,47,48,49].

3. Polydopamine Functionalized Iron Oxide Nanoparticles: Synthesis and Structures

4. Technological Applications of Polydopamine Functionalized Iron Oxide Nanoparticles