MNPs show high field irreversibility, high saturation field, and superparamagnetism, which are highly dependent on particle size and surface coating. The relationship between MNP size and magnetism (coercivity) has been extensively reported [36,37,38,39,40]. The coercivity gradually increases for bulk nanoparticles to a maximum value at a particular size. In this region, the magnetization is stable and nonuniform (Weiss domains, magnetic multi-domain state). The critical size of the magnetite nanoparticles, above which they become multi-domain, has been theoretically calculated and is 76 nm and 128 nm, respectively, for cubic and spherical nanoparticles [38]. However, the experimental data indicate that the critical size of transition between single- and multi-domain magnetic structure highly depends on the crystal structure and coating [37,38,41]. By reducing the size of the nanoparticles, the coercivity rapidly decreases to zero, reaching a superparamagnetic state [36,37]. Superparamagnetism is especially important in applications such as drug delivery and imaging. Particle sizes below 20 nm are required to achieve superparamagnetism for magnetite MNPs. Superparamagnetic MNPs provide a stronger response to external magnetic fields than simple MNPs. Frenkel J. and Doefman J. in 1930 predicted that, below a critical size, MNPs would consist of a single magnetic domain [42]. However, superparamagnetic MNP synthesis was achieved half a century later.

Research on nano-emulsion began in 1943 (Figure 2, historical timeline) [43]. However, the nanotechnology concept was first proposed in 1959 by Richard Feynman in the lecture entitled “Plenty of Room at the Bottom”. This was a historical event in nanoscience. The first synthesis of iron nanoparticles by gas condensation was achieved in 1981. The concept of using magnetic forces for enhanced therapeutic and imaging performance has evolved over the years. Two of the milestones were the development of MNPs for imaging purposes in 1990 and silica-coated MNPs in 1995. Since 2000, numerous studies have investigated the potential applications of MNPs and nanocomposites with magnetic cores. Another essential development was the successful magnetic hyperthermia clinical trials in 2010. Magnetic hyperthermia utilizes MNPs that are exposed to an alternating magnetic field to generate heat in local regions [23,44]. Magnetic hyperthermia therapy was first proposed much earlier, in 1957. However, about fifty years were required to synthesize stable and non-toxic MNPs with optimal physical properties. Colloidal stability, biocompatibility, and toxicity studies are crucial for in vivo application. Recently, numerous MNP-based “Smart” nanocomposites with pH-stimuli-responsive drug release, theranostics, and multimodal constructions have been developed [5,8,13,24,25,45,46,47,48,49]. Numerous research papers have focused on the possible procedures for MNPs synthesis, coating, drug-loading, toxicity studies, and clinical trials [1,2,8,20,44,50,51,52,53,54].

Figure 2. Historic timeline for development of MNPs.