**3. M-IONPs synthesis methods**

in the degree of internalization in endothelial cells of human umbilical bladder. Thus, the superparamagnetic nanoparticles absorption depends not only on their surface properties

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,

but also on cell type.

234 Iron Ores and Iron Oxide Materials

**Figure 3.** The behavior of Fe3

O4

nanoparticles depending on pH.

**2.4. Surface functionality and colloidal stability**

In the past decade, numerous synthesis methods have been developed to obtain M-IONPs. On the basis that the method of preparation plays an essential role in obtaining nanoparticles with tailored properties, the research work regarding the development of new synthesis methods to control the size, shape, morphology and magnetic properties of these nanoparticles is a permanent challenge. In the same time, the synthesis method has to be environmentally friendly, simple, inexpensive and reproducible. Many scientific publications have described efficient synthesis methods, which allow the obtaining of monodisperse magnetic nanoparticles, stable for a long time with controlled shape.

The synthesis method has to ensure the obtaining of magnetic nanoparticles with specific properties to their application domain by changing the experimental reaction conditions. For biomedical applications, superparamagnetic iron oxide nanoparticles with a specific surface chemistry (for *in vivo* applications), high magnetization values and a narrow size distribution of the particles with size below 100 nm are needed. Magnetite (Fe3 O4 ) and maghemite (γ-Fe<sup>2</sup> O3 ) have attracted particular attention because, under certain synthesis conditions, they are superparamagnetic, being also biocompatible, thus becoming the magnetic nanomaterials most commonly used in medical applications.

Hereinafter, the most popular synthesis methods used for obtaining M-IONPs will be described. After the chemical surface modification of magnetic nanoparticles by binding drugs, proteins, enzymes, antibodies, etc., they can be directed to an organ, tissue or tumor with the help of an external magnetic field. The methods described below allow to obtain magnetic nanoparticles with narrow size dimensions, desired shape and morphology, by changing the conditions and/or parameters of the synthesis. The most used and popular method for the synthesis of magnetite, being in the same time simple and efficient, is the chemical precipitation of iron salts [35–39].

#### **3.1. Precipitation method**

The first synthesis of superparamagnetic iron oxide nanoparticles was reported by Massart, and the method consists in mixing two salts of Fe3+ and Fe2+ in a molar ratio of 2:1 in aqueous medium followed by precipitation of these salts using a precipitating agent (a base – NH3 ) under inert atmosphere or at elevated temperature, resulting a black magnetic precipitate [40]. The equation of the chemical reaction which underlies the formation of magnetite may be written as Eq. (1):

$$\text{Fe}^{2+} + 2\text{Fe}^{3+} + 8\text{OH}^- \rightarrow \text{Fe(OH)}\_2 + 2\text{Fe(OH)}\_3 \rightarrow \text{Fe}\_3\text{O}\_4 \downarrow + 4\text{H}\_2\text{O} \tag{1}$$

Magnetite is not stable at room temperature, being sensitive to oxidation in contact with air, easily transforming into maghemite, according to Eq. (2):

$$\text{2Fe}\_3\text{O}\_4 + \text{ '} \&\text{O}\_2 \rightarrow \text{ 3}\text{\textgreater Fe}\_2\text{O}\_3\tag{2}$$

Other parameters that can influence the size, shape and composition of the magnetic iron oxide nanoparticles are the nature of iron salts (chlorides, perchlorates, nitrates, sulfates, etc.) and the molar ratio Fe3+/Fe2+. Roth and co-workers published a good analysis regarding the influence of the reaction conditions on the formation of superparamagnetic iron oxide nanoparticles. The authors demonstrated that for obtaining particles with a size between 3 and 17 nm with high saturation magnetization a higher reaction temperature, higher iron salt concentration, Fe3+/Fe2+ molar ratio below 2 and a molar ratio of hydroxide ions/iron ions of

Preclinical Aspects on Magnetic Iron Oxide Nanoparticles and Their Interventions as Anticancer…

Wu et al. investigated the effect of the vacuum drying method on the change of the morphology and magnetic properties of magnetic iron oxide nanoparticles (M-IONPs). They revealed that the obtained nanoparticles tend to agglomerate more easily when their average diameter decreased, but the structure and morphology are maintained better by ambient air drying. They also obtained magnetic nanoparticles with high saturation magnetization after drying

ing ultrasonic-assisted chemical co-precipitation. They used high purity iron separated from iron ore tailings by an acidic leaching method and obtained superparamagnetic iron oxide nanoparticles without a protecting gas [46]. Pereira and co-workers have synthesized super-

properties by one-step aqueous precipitation route based on the use of a new type of alkaline agents [47]. The alkaline agents that they have used include alkanolamines, isopropanol amine and diisopropanolamine. The base that they have used, instead of the most used—NaOH, leads to smaller particle sizes (up to 6 times) and enhanced saturation magnetization (up to 1.3 times). Generally, the size of the particles is proportionally with the magnetization saturation, but the above results showed improved magnetic properties while keeping their small size.

Besides the many advantages of the precipitation method (high saturation magnetization, rapid synthesis with high yield, versatility, nanoparticles with the desired morphology and characteristics), it shows several disadvantages, like oxidation, magnetic nanoparticles with particle size distribution that cannot be controlled, polydispersion and weak crystallization

Thermal decomposition of organometallic compounds in high boiling organic solutions in the presence of stabilizers is also a popular method for the synthesis of the spinel structured

The method is based on the decomposition of an iron precursor at high temperature in the presence of solvents which contain stabilizing surfactants (such as oleic acid or oleylamine) [49–51]. By varying the reaction mixtures and modifying the synthesis condition, it can be

 and a very promising technique for obtaining high-quality superparamagnetic iron oxide nanoparticles. The magnetic nanoparticles obtained by this method proved to be superior to those obtained by precipitation, because the nucleation process can be separated by the

nanoparticles with small particle size (4.9–6.3 nm) and improved magnetic

O4

nanoparticles by utiliz-

http://dx.doi.org/10.5772/intechopen.74176

237

1.4:1 are needed [45].

paramagnetic Fe3

O4

**3.2. Thermal decomposition**

Fe3 O4

the obtained nanoparticles in a vacuum at 70°C [36].

The same group of researchers in another study has synthesized Fe3

which leads to nanoparticles with low saturation magnetization.

growth process and the hydrolysis reaction is avoided [48].

The precipitation process is based on two steps regarding the formation of the solids [41, 42]: (i) nucleation—a very short period, occurs only when the concentration of the constituent species reaches suprasaturation and (ii) slow controlled growth of the preformed nuclei, by diffusion from the solutions to the surfaces of the crystal. To avoid the formation of polydispersed nanoparticles, it is necessary that the two stages to be separated, i.e., nucleation does not take place simultaneously with crystal growth. By controlling the two processes, monodispersed magnetic particles can be obtained. If the nuclei start to form in the same time, the growth of these nuclei leads to particles with very narrow size distribution. Therefore, the size of the obtained particles can be controlled but only in the nucleation step because the size of the particles does not change during the growth process.

It has been shown that by controlling both the pH of the reaction medium and the ionic strength, it is possible to control the mean size of the particles. Jiang et al. have demonstrated that the size of the particles has an inverse proportionality with the pH and the ionic strength of the precipitation medium [43]. These two parameters (pH and the ionic strength) also affect the chemical surface of the crystals and the electrostatic surface charge [44].

Other parameters that can influence the size, shape and composition of the magnetic iron oxide nanoparticles are the nature of iron salts (chlorides, perchlorates, nitrates, sulfates, etc.) and the molar ratio Fe3+/Fe2+. Roth and co-workers published a good analysis regarding the influence of the reaction conditions on the formation of superparamagnetic iron oxide nanoparticles. The authors demonstrated that for obtaining particles with a size between 3 and 17 nm with high saturation magnetization a higher reaction temperature, higher iron salt concentration, Fe3+/Fe2+ molar ratio below 2 and a molar ratio of hydroxide ions/iron ions of 1.4:1 are needed [45].

Wu et al. investigated the effect of the vacuum drying method on the change of the morphology and magnetic properties of magnetic iron oxide nanoparticles (M-IONPs). They revealed that the obtained nanoparticles tend to agglomerate more easily when their average diameter decreased, but the structure and morphology are maintained better by ambient air drying. They also obtained magnetic nanoparticles with high saturation magnetization after drying the obtained nanoparticles in a vacuum at 70°C [36].

The same group of researchers in another study has synthesized Fe3 O4 nanoparticles by utilizing ultrasonic-assisted chemical co-precipitation. They used high purity iron separated from iron ore tailings by an acidic leaching method and obtained superparamagnetic iron oxide nanoparticles without a protecting gas [46]. Pereira and co-workers have synthesized superparamagnetic Fe3 O4 nanoparticles with small particle size (4.9–6.3 nm) and improved magnetic properties by one-step aqueous precipitation route based on the use of a new type of alkaline agents [47]. The alkaline agents that they have used include alkanolamines, isopropanol amine and diisopropanolamine. The base that they have used, instead of the most used—NaOH, leads to smaller particle sizes (up to 6 times) and enhanced saturation magnetization (up to 1.3 times). Generally, the size of the particles is proportionally with the magnetization saturation, but the above results showed improved magnetic properties while keeping their small size.

Besides the many advantages of the precipitation method (high saturation magnetization, rapid synthesis with high yield, versatility, nanoparticles with the desired morphology and characteristics), it shows several disadvantages, like oxidation, magnetic nanoparticles with particle size distribution that cannot be controlled, polydispersion and weak crystallization which leads to nanoparticles with low saturation magnetization.
