**2.4. Surface functionality and colloidal stability**

Both the surface chemistry of magnetite particles and its properties are particularly important in various applications. Iron atoms at the surface of the magnetite particle that are not bound to oxygen atoms act as Lewis acids and coordinate the molecules that can give a pair of electrons. In aqueous systems, these atoms coordinate water molecules that rapidly dissociate resulting magnetite with functionalized surface with Fe-OH hydroxyl groups. So, the chemistry of the surface of magnetite particles is strongly dependent on the pH value; at low pH values, the surface of the magnetite particles is protonated (positively charged), and at high pH values, it is negatively charged (**Figure 3**). The preformed hydroxyl groups on the surface of magnetite have amphoteric character; therefore, they can react either as acids or bases [30].

Another problem that arises after obtaining the magnetic iron oxide nanoparticles (M-IONPs) is their agglomeration that is installed due to the van der Waals forces and the magnetic forces. Nanoparticles without coatings (naked nanoparticles) are not stable in aqueous environments, easily aggregating and precipitating. After application *in vivo*, nanoparticles often form aggregates in the bloodstream and are retained by the macrophages. Therefore, they must be covered with a variety of fragments which have the property to eliminate or minimize their aggregation in physiological conditions [31]. The magnetic nanoparticles are coated with an impervious wrapper so that oxygen does not reach at the surface of the magnetic nanoparticles in order to ensure an effective stabilization of iron oxide nanoparticles. Some stabilizers, such as a surfactant or a polymer, usually are added during preparation to prevent the aggregation of nanosized particles. Most of these polymers stick to the nanoparticles surface in a specific substrate manner. Nanoparticle surfaces can be composed of several organic and inorganic materials, including polymer. Also, polymer coating materials can be classified in turn into synthetic and natural. Polymers such as poly-ethylene-co-vinyl acetate, poly-vinylpyrrolidone,

**Figure 3.** The behavior of Fe3 O4 nanoparticles depending on pH.

poly-acid-lactic-co-glycolic, polyethylene glycol, etc. are typical examples of synthetic polymeric systems. Natural polymer coatings include gelatin, dextran, chitosan, etc. The molecules used for stabilization of magnetic nanoparticles must be biocompatible and biodegradable. The most common surfactant molecules are oleic acid, lauric acid, acids, sulfonic acids, alkanes and alkane phosphonates. The surfactants are amphiphilic compounds and they manifest their role at the interface between nanoparticles and solvent. However, magnetic nanoparticles covered with organic compounds, in suspension cannot be used for biological purposes, especially in the delivery of medicines. Changing the surface of nanoparticles post-synthesis is known as core-shell nanoparticles, also used widely. The most commonly used materials are polymers, silica or metals (e.g. gold, cadmium, selenium, silver). Coating materials protect the core against oxidation and therefore keep the magnetic property of nanoparticles. It is known that the iron oxide nanoparticles are non-toxic, but some coating materials may be toxic. For example, silicon dioxide is biocompatible, but is not biodegradable [28].

Many researchers have prepared magnetic nanoparticles covered with various surfactants or biomolecules that have been introduced directly in the synthesis process. For example, Salavati-Niasari et al. [32] have synthesized Fe3 O4 nanoparticles covered with octanoic acid using a facile chemical precipitation method. The surfactant was present in the reaction system to improve dispersity. The authors have obtained magnetic nanoparticles with a size range of 25 nm. Liu et al. prepared magnetic nanoparticles coated with chitosan, for the immobilized lipase, using the co-precipitation method. They replaced water with 2% chitosan in acetic acid solution during the reaction process [33].

Atomic transfer radical polymerization (ATRP) is another common way to cover magnetic nanoparticles, developed by Wang et al. [34]. Due to the magnetic interaction of the iron oxide nanoparticles with biological fluids, the process of formation of free radicals of oxygen reactive species may be increased. To protect the environment *in vivo* from these toxic by-products, some materials have been used for biocompatible and rigid coatings, such as gold [28].
