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Magnetic Nanoparticles (MNPs)

Magnetic nanoparticles (MNPs) are found to exhibit interesting and considerably different magnetic properties due to their finite size effects, such as the high aspect ratio and different crystal structures, than those found in their corresponding bulk materials.

Magnetic materials are those materials that show a response to an applied magnetic field. They are classified into five main types; ferromagnetic, paramagnetic, diamagnetic, anti-ferromagnetic, and ferrimagnetic.

Magnetic nanoparticles clusters that are composed of a number of individual magnetic nanoparticles are known as magnetic nanobeads with a diameter of 50–200 nanometers. Magnetic nanoparticles clusters are a basis for their further magnetic assembly into magnetic nanochains. The magnetic nanoparticles have been the focus of much research recently because they possess attractive properties which could see potential use in catalysis including nanomaterials-based catalysts, biomedicine and tissue specific targeting, magnetically tunable colloidal photonic crystals, micro fluidics, magnetic resonance imaging, magnetic particle imaging, data storage, environmental remediation, nanofluids, optical filters, defect sensor and cation sensors.

These nanoparticles can be synthesized in several ways (e.g., chemical and physical) with controllable sizes enabling their comparison to biological organisms from cells (10–100 μm), viruses, genes, down to proteins (3–50 nm). The optimization of the nanoparticles size, size distribution, agglomeration, coating, and shapes along with their unique magnetic properties prompted the application of magnetic nanoparticles in diverse fields.

Magnetic Nanoparticles are highly stable, shape-controlled and narrow sized. These nanoparticles can be synthesized by several popular methods, including co-precipitation, micro emulsion, thermal decomposition, solvothermal, sonochemical, microwave assisted, chemical vapor deposition, combustion synthesis, carbon arc, laser pyrolysis etc.

Properties of Magnetic Nanoparticles

Magnetic nanoparticles (MNPs) are those nanoparticles (NPs) that show some response to an applied magnetic field. Nanotechnology allows physicists, chemists, material scientists and engineers to synthesize systems with nano sizes where the classic laws of physics are different at that small scale.

As the size of the particle decreases, the ratio of the surface area to the volume of the particle increases. For nanoparticles, this ratio becomes significantly large causing a large portion of the atoms to reside on the surface compared to those in the core of the particle. The large surface-to-volume ratio of the nanoparticles is the key factor for the novel physical, chemical, and mechanical properties compared to those of the corresponding bulk material. The physical properties include the optical, electric and magnetic properties

Magnetic effects are caused by movements of particles that have both mass and electric charges. These particles are electrons, holes, protons, and positive and negative ions. A spinning electric-charged particle creates a magnetic dipole, so-called Magneton.

Magnetic properties such as the Curie (TC) or Néel (TN) temperatures, saturation magnetization, remnant magnetization, blocking temperature and the coercivity field (HC) are found to be different than those for the bulk material.

The two main features that dominate the magnetic properties of nanoparticles and give them various special properties are: (a) Finite-size effects (single-domain or multi-domain structures and quantum confinement of the electrons); (b) Surface effects, which results from the symmetry breaking of the crystal structure at the surface of the particle, oxidation, dangling bonds, existence of surfactants, surface strain, or even different chemical and physical structures of internal -core and surface- shell parts of the nanoparticles.

Several magnetic effects could also result from the finite size effect of nanoparticles. These could include: (a) The existence of randomly oriented uncompensated surface spins. (b) The existence of canted spins. (c) The existence of a spin-glass-like behavior of the surface spins. (d) The existence of a magnetically dead layer at the surface. (e) The enhancement of the magnetic anisotropy which results from surface anisotropy.

When the size of single-domain particles further decreases below a critical diameter, the coercivity becomes zero, and such particles become super paramagnetic. Superparamagnetism is caused by thermal effects. In superparamagnetism particles, thermal fluctuations are strong enough to spontaneously demagnetize a previously saturated assembly; therefore, these particles have zero coercivity and have no hysteresis. Nanoparticles become magnetized in the presence of an external magnet, but revert to a nonmagnetic state when the external magnet is removed.

Several properties of magnetic nanoparticles must be attained:

(a) The magnetic nanoparticles should be biocompatible and non-toxic.

(b) The magnetic nanoparticles are preferred to be sufficiently small (10–50 nm). This will have several advantages:
(i) The nanoparticles will preserve their colloidal stability and resist aggregation if their magnetic interaction is reduced. This can be achieved if their magnetism disappears after removal of applied magnetic field. This superparamagnetism behavior is only achievable under certain particle size and above the blocking temperature.
(ii) The dipole-dipole interactions scale as r 6 (r is the radius of the particle). Hence, the dipolar interactions become very small when the particle size becomes very small. This will serve to minimize particle aggregation when the field is applied.
(iii) Decreasing size means larger surface area for certain volume (or mass) of the particle. The efficiency of coating (and also the attachment of ligands) will improve leading to even more resistance to agglomeration, avoidance of biological clearance and better targeting.
(iv) Being very small, the particles can remain in the circulation after injection and pass through the capillary systems of organs and tissues avoiding vessel embolism.
(v) The magnetic particles will be stable in water at pH = 7 and in a physiological environment.
(vi) Precipitation due to gravitation forces can be avoided with small particles.

(c) The magnetic particles must have a high saturation magnetization. This is an important requirement for two reasons:
(i) The movement of the particles in the blood can be controlled with a moderate external magnetic field.
(ii) The particles can be moved close to the targeted pathologic tissue.

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