**2.2. Magnetic nanomaterials**

The magnetic nanomaterials used in biology and medicine generally fall into three categories: zero dimensional nanomaterials such as nanospheres; one-dimensional nanomaterials such as nanowires and nanotubes; and two-dimensional nanomaterials such as thin films. Usually, all the nanospheres, nanorods, nanowires and nanotubes are called nanoparticles, among which, nanorods, nanowires and nanotubes are high aspect-ratio nanoparticles. In most of the biomedical applications, magnetic nanoparticles are suspended in appropriate carrier liquids, forming magnetic fluids, also called ferrofluids. Among the three types of magnetic nanoparticles, magnetic nanospheres are most widely used in biomedicine. To realize their biomedical applications, the magnetic nanospheres should be stably suspended in the carrier liquid, and they should also carry out certain biomedical functions. The magnetic material most often used is iron oxides, and the carrier liquids are

usually water, kerosene or various oils. Due to their small size, the magnetic nanoparticles in carrier liquids neither form sediment in the gravitational field or in moderate magnetic field gradients, nor do they agglomerate due to magnetic dipole interaction. However, a stable suspension can only be achieved if the particles are protected against agglomeration due to the van der Waals interaction. Usually this protection can be achieved by one approach is the electric charge stabilization. In this approach, a thin layer of gold is coated on the surface of the nanospheres. Meanwhile, the thin gold layer can also serve as an ideal base on which chemical or biological agents can be functionalized. These molecules generate a repulsive force, preventing the particles from coming into contact and thus suppressing the destabilizing effect of the van der Waals interaction. In practical applications, this approach is often used in combination for the majority of ferrofluids, since this allows the synthesis of suspensions which are stable over years (*Could 2004*).

Laser and Radiofrequency Induced Hyperthermia Treatment via Gold-Coated Magnetic Nanocomposites 105

Hydrothermal synthesis techniques are an alternative method for the preparation of highly crystalline iron oxide nanoparticles *(Wang et al, 2005)*. In this case a mixture of iron salts dissolved in aqueous media is introduced in a sealed Teflon container and heated above the boiling temperature of water, and consequently the reaction pressure is increased much above atmospheric pressure. The synergistic effect of high temperatures and pressures strongly improves the quality of the nanocrystals and hence their magnetic features. However, and in contrast to the biological technique, there is no straightforward way to control the size and the shape of the final particles and usually polydisperse samples are

Biological methods, since nanomaterials have comparable dimensions to biological aggregates, bio-related synthesis methods have been explored for novel nanoparticle synthesis. In biological methods, synthesis and assembly of crystalline inorganic materials can be regulated by biological organisms under environmentally benign conditions and desired chemical compositions and phases can be achieved. For example, the nucleation of semiconducting nanoparticles can be initiated in the presence of viruses expressing material-specific peptides. Other examples are the use of porous protein crystals, manipulation of bacteria to produce oxide nanoparticles and selection of metal-specific polypeptides from combinatorial libraries *(Reiss et al. 2004)*. In biological methods, biological entities usually serve as templates for nanoparticles formation. In all cases, the biological entities were used not only to encapsulate the nanoparticles, but to strictly regulate the dimension of the crystals. To prepare magnetic nanoparticles, ferritin can be used which consists of 24 nearly identical subunits. Self-assembly of ferritin will form a spherical cage with a 7.5–8.0 nm-diameter cavity, which can be used for the biological storage of iron in the form of ferrihydrite, an iron (III) oxy-hydroxide? The protein cage is able to withstand relatively high temperatures for biological systems (up to 65 ◦C) and various pH values (~ 4.0–9.0) for certain periods of time. Therefore this protein template is quite strong and will

There are many categories of iron oxide nanoparticles based on their overall diameter (including iron oxide core and hydrated coating). Iron oxide nanoparticles can be distinctly classified into super-paramagnetic iron oxide nanoparticles (SPIO NPs) between 300 nm and 3.5 *μ*m; standard SPIO (SSPIO) of approximately 60–150 nm; ultra small SPIO (USPIO) of approximately 10–40 nm (*Weissleder et al., 1990)*; monocrystalline iron oxide nanoparticles (MION—a subset of USPIO) of approximately 10–30 nm and cross-linked iron oxides

On the other hand, the magnetic materials are characterized by the presence of magnetic dipoles generated by the spinning of some of their electrons. Each of these polarized electrons can be aligned in a parallel or antiparallel fashion with respect to the neighboring ones in the crystal lattice, and this type of interaction is what gives rise to the macroscopic magnetic effect that we can measure. Based on the magnetic response, the magnetic

(CLIO) which is a form of MION with cross-linked dextran coating (*Shen et al., 1993)*.

not cause any significant disruption of the quaternary structure.

*2.2.3. Classification of iron oxide nanoparticles* 

obtained.
