**4. Sterically functionalized magnetite core-shell particles**

#### **Magnetite surface chemistry**

Magnetite particles act as Lewis acids in aqueous systems and coordinate water or hydroxyl groups. This is why the particles surface chemistry is highly dependent of pH value; at low pH the surface of the particles is protonated and at high pH the surface is negatively charged (Figure 2). The hydroxyl groups from magnetite surface are the reactive parts, which can react with acid or base. The HO- groups are reacting with other organic or inorganic anions and may adsorb protons or cations, also.

Fig. 2. Magnetite particles behavior at low/high pH.

Magnetic ferrofluids based on magnetite particles have a high capacity of agglomeration due to the fact that two types of forces co-exist: attraction and repulsive forces. The stabilization of the magnetic particles requires the control of these forces. To obtain stable magnetic dispersions, the attraction forces between magnetic particles must be overcomed by electrostatic or steric stabilization (Figure 3).

Fig. 3. Electrostatic (a) and steric (b) stabilization of the particles.

In order to stabilize magnetite particles, monomers/polymers with functional groups that can bind to the surface of the particles and act as surfactants can be used. The rest of the chain is solvated in dispersion medium or in a fluid. The process, known to be entropic or

The influence of ionic species concentrations on the properties of magnetite particles was also followed. It was noted that the Fe2+/Fe3+ molar ratio was a determining factor in obtaining sub-micron sizes, while by increasing the ratio, the mean diameter of the magnetic

Magnetite particles act as Lewis acids in aqueous systems and coordinate water or hydroxyl groups. This is why the particles surface chemistry is highly dependent of pH value; at low pH the surface of the particles is protonated and at high pH the surface is negatively charged (Figure 2). The hydroxyl groups from magnetite surface are the reactive parts, which can react with acid or base. The HO- groups are reacting with other organic or

Magnetic ferrofluids based on magnetite particles have a high capacity of agglomeration due to the fact that two types of forces co-exist: attraction and repulsive forces. The stabilization of the magnetic particles requires the control of these forces. To obtain stable magnetic dispersions, the attraction forces between magnetic particles must be overcomed

In order to stabilize magnetite particles, monomers/polymers with functional groups that can bind to the surface of the particles and act as surfactants can be used. The rest of the chain is solvated in dispersion medium or in a fluid. The process, known to be entropic or

particles increased, but unfortunately the yield decreased (Babes et al., 1999).

**4. Sterically functionalized magnetite core-shell particles** 

inorganic anions and may adsorb protons or cations, also.

Fig. 2. Magnetite particles behavior at low/high pH.

by electrostatic or steric stabilization (Figure 3).

Fig. 3. Electrostatic (a) and steric (b) stabilization of the particles.

**Magnetite surface chemistry** 

steric, refers to the inhibition of particles aggregation by an entropic force, which appears when the particles are closed to each other. As surfactants monomers, homopolymers, block copolymers and polymers with terminal functional groups can be successfully used. After the coating process, these particles are re-suspended in proper solvents and form homogenous suspensions named ferrofluids (Shen et al., 2004).


Table 1. Core-shell magnetite particles.

Tailored and Functionalized Magnetite Particles for Biomedical and Industrial Applications 157

nickel and cobalt) (O'Brien, 2003). Magnetite has been intensively studied in the past decades due to its application in biotechnology and biomedicine: cancer treatment (Jordan et al, 2001, Jordan et al., 1999, Herrera et al., 2008), sensors, catalysis, storage media, clinical

Especially for biomedical application, the particles must present some specific properties

In order to respond to an external magnetic field magnetic particles must have a large saturation magnetization, an important characteristic for targeted drug delivery systems and contrast agents for MRI (magnetic resonance imaging) (Bulte, 2006, Modo et al., 2005, Burtea et al., 2005, Boutry et al., 2006, Babes et al., 1999, Sonvico et al., 2005, Corot et al., 2006, Corot et al., 2007)). In the presence of an external magnetic field the magnetic moments are oriented with the field (Figure 5). Magnetic nanoparticles modified with organic molecules and used for biomedical applications can be magnetically controlled by applying

The stability of the particles can be enhanced by coating with biocompatible surfactants (Ma et al., 2003) capable of interactions with hydroxyl groups on the magnetite surface and ensuring the ferrofluid stability. The coating agent should have specific functional groups

The magnetic particles with biomedical applications must have controllable sizes ranging from a few nanometres up to tens of nanometres, smaller than those of a cell (10–100 μm) (Pankhurst et al., 2003), a virus (20–450 nm), a protein (5–50 nm) or a gene (2 nm wide and 10–100 nm long) because they can 'get close' to a biological entity of interest. Magnetic particles encapsulated into a polymer matrix with dimensions more than 1 µm or submicron is more effective in the bloodstream through the veins or arteries (Lazaro et al.,

Metals such as Fe, Co and Ni, which oxidize easily, are not recommended in biomedical uses due to the higher probability of oxidizing inside human bodies. This problem can be solved using magnetite particles that not only present a greater stability to oxidation, but can be coated with different specific surfactants, increasing their resistance to

The polymers used as surfactants can be synthetic or natural molecules able to yield stable colloidal suspensions designed for drug delivery. Nevertheless, synthetic polymers have the

The colloidal magnetic particles obtained for biomedical application (Asmatulu et al., 2005) as targeted drug delivery will have to pass through a few mechanisms of releasing drug molecules: diffusion, degradation of polymeric shell, swelling of the shell followed by diffusion of active principle outside the magnetic particle. The diffusion involves drug

advantage of high purity and reproducibility over natural polymers.

diagnosis and treatment of some diseases (Torchilin et al., 2001, Jordan et al., 2001).

like:

2005).

oxidation.

1. Magnetic properties

an external magnetic field.

3. Nano- sized dimensions

4. Oxidation stability

2. Stability in colloidal dispersions

for further functionalization for specific applications.

The synthesis methods for magnetic particles with magnetite core and monomer/polymer shell, named core-shell magnetic particles, are presented in the literature (Table 1). The surfactant used must have functional groups to interact by hydrogen or covalent bonds (Figure 4) with the hydroxilic groups from pre-formed magnetite surface (trialkoxysilanes or compounds with carboxylic groups) (Kazufumi et al., 2008; Wormuth, 2001; Mondini et al., 2008; Shukla et al., 2007; Wilson et al., 2005) and other functional groups to permit their dispersing in a transport fluid (aminic, epoxy, vinyl groups, etc.).

Fig. 4. Schematic representation of hydrogen bond (a) and covalent bond (b) between core and shell.
