## 2 Iron Oxide Nanoparticles  
Iron oxide nanoparticles were first introduced as liver-specific MRI contrast agents [10, 11, 12]. Since then, however, their application has been successfully expanded in the rapidly developing field of cardiac and vascular imaging [6, 7, 9, 13]. Iron oxide nanoparticles are naturally found in the environment and as matter in air pollution and volcanic eruptions [14]. Superparamagnetic iron oxide nanoparticles may be generated as emissions from industry and power stations, but they can also be chemically synthesised through various methods [15]. These include classical synthesis by co-precipitation, reactions in constrained environments, the polyol method, flow injection synthesis, and sonolysis [14, 15]. In addition, novel methods of nanoparticle synthesis have been described, such as the generation of magnetic \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\)/Fe\({}_{3}\)O\({}_{4}\) heterogeneous nanoparticles via a facile solution combustion process and the synthesis of iron oxide nanoparticles by the rapid inductive heating method [16, 17]. Their potential applications, safety, and bioavailability are mainly determined by two of their most essential characteristics: their coating and their size [7, 18]. The core of these nanoparticles comprises iron oxide, more commonly magnetite (Fe\({}_{3}\)O\({}_{4}\)) or maghemite (\(\gamma\)-Fe\({}_{2}\)O\({}_{3}\)), and its radius ranges between 5 and 15 nm [19, 20] (Figure 1). Iron oxide itself is hydrophobic; hence, the nanoparticles are usually modified with an external coating formed of a biocompatible hydrophilic polymer such as dextran or polyethylene glycol [7, 21]. This coating is essential in order to prevent the opsonisation (i.e., binding with plasma proteins) of the nanoparticles, which would unavoidably occur in the bloodstream without it [19, 21]. Additionally, through electrostatic interactions, the coating also inhibits the aggregation of nanoparticles that would otherwise take place due to their hydrophobic properties [19]. Another important consideration is an excess of iron ions may lead to the overproduction of reactive oxygen species or free radicals and subsequently cause protein, lipid, and DNA damage and, finally, cellular apoptosis [22]. Thus, the other critical 'task' of the external coating is to provide a stable cover that prevents the oxidation of the iron and its subsequent harmful effects, allowing the safe use of the nanoparticles [22]. Apart from the above, the coating that surrounds the core of the nanoparticle also acts like a 'harbour' to which ligands, drug molecules, and other molecular targets can bind [6]. Taking all of these into consideration, it is obvious why molecular imaging and targeted drug delivery are some of the areas where iron oxide nanoparticles have been successfully applied [23, 24, 25]. According to their size, iron oxide particles can be grouped into four different categories: very small superparamagnetic particles of iron oxide (VSPIO, diameter \(<\) 20 nm), ultrasmall superparamagnetic particles of iron oxide (USPIO, diameter 20-50 nm), small superparamagnetic particles of iron oxide (SPIO, diameter 20-250 nm), and micro-sized particles of iron oxide (MPIO, diameter 1-8 \(\upmu\)m) [6, 7]. However, the vast majority of the proposed applications for iron oxide nanoparticles use small and ultrasmall particles with diameters of 50 nm or less [6]. The concept of the superparamagnetism of nanoparticles was expressed for the first time almost a century ago [26]. The magnetic nanoparticles induce local magnetic field inhomogeneities through which water molecules will diffuse, leading to the magnetic relaxation of the water protons, from which image contrast is generated [27]. Iron oxide particles form crystalline structures that contain multiple iron ions [28]. Sufficiently large crystal-containing regions constitute a magnetic domain that comprises thermodynamically independent particles that have a net magnetic dipole that is larger than the total of the individual unpaired electrons [28].