Iron oxide nanoparticles

Iron oxide nanoparticles are iron oxide particles with diameters between about 1 and 100 nanometers. The two main forms are magnetite (Fe3O4) and its oxidized form maghemite (γ-Fe2O3). They have attracted extensive interest due to their superparamagnetic properties and their potential applications in many fields (although Co and Ni are also highly magnetic materials, they are toxic and easily oxidized). Iron oxide nanoparticles are iron oxide particles with diameters between about 1 and 100 nanometers. The two main forms are magnetite (Fe3O4) and its oxidized form maghemite (γ-Fe2O3). They have attracted extensive interest due to their superparamagnetic properties and their potential applications in many fields (although Co and Ni are also highly magnetic materials, they are toxic and easily oxidized). Applications of iron oxide nanoparticles include terabit magnetic storage devices, catalysis, sensors, superparamagnetic relaxometry (SPMR), and high-sensitivity biomolecular magnetic resonance imaging (MRI) for medical diagnosis and therapeutics. These applications require coating of the nanoparticles by agents such as long-chain fatty acids, alkyl-substituted amines and diols. They have been used in formulations for supplementation. Magnetite has an inverse spinel structure with oxygen forming a face-centered cubic crystal system. In magnetite, all tetrahedral sites are occupied by Fe3+ and octahedral sites are occupied by both Fe3+ and Fe2+. Maghemite differs from magnetite in that all or most of the iron is in the trivalent state (Fe3+) and by the presence of cation vacancies in the octahedral sites. Maghemite has a cubic unit cell in which each cell contains 32 O ions, 21​1⁄3 Fe3+ ions and 2​2⁄3 vacancies. The cations are distributed randomly over the 8 tetrahedral and 16 octahedral sites. Due to its 4 unpaired electrons in 3d shell, an iron atom has a strong magnetic moment. Ions Fe2+ have also 4 unpaired electrons in 3d shell and Fe3+ have 5 unpaired electrons in 3d shell. Therefore, when crystals are formed from iron atoms or ions Fe2+ and Fe3+ they can be in ferromagnetic, antiferromagnetic or ferrimagnetic states. In the paramagnetic state, the individual atomic magnetic moments are randomly oriented, and the substance has a zero net magnetic moment if there is no magnetic field. These materials have a relative magnetic permeability greater than one and are attracted to magnetic fields. The magnetic moment drops to zero when the applied field is removed. But in a ferromagnetic material, all the atomic moments are aligned even without an external field. A ferrimagnetic material is similar to a ferromagnet but has two different types of atoms with opposing magnetic moments. The material has a magnetic moment because the opposing moments have different strengths. If they have the same magnitude, the crystal is antiferromagnetic and possesses no net magnetic moment. When an external magnetic field is applied to a ferromagnetic material, the magnetization (M) increases with the strength of the magnetic field (H) until it approaches saturation. Over some range of fields the magnetization has hysteresis because there is more than one stable magnetic state for each field. Therefore, a remanent magnetization will be present even after removing the external magnetic field. A single domain magnetic material (e. g. magnetic nanoparticles) that has no hysteresis loop is said to be superparamagnetic. The ordering of magnetic moments in ferromagnetic, antiferromagnetic, and ferrimagnetic materials decreases with increasing temperature. Ferromagnetic and ferrimagnetic materials become disordered and lose their magnetization beyond the Curie temperature T C {displaystyle T_{C}} and antiferromagnetic materials lose their magnetization beyond the Néel temperature T N {displaystyle T_{N}} . Magnetite is ferrimagnetic at room temperature and has a Curie temperature of 850 K. Maghemite is ferrimagnetic at room temperature, unstable at high temperatures, and loses its susceptibility with time. (Its Curie temperature is hard to determine). Both magnetite and maghemite nanoparticles are superparamagnetic at room temperature.This superparamagnetic behavior of iron oxide nanoparticles can be attributed to their size. When the size gets small enough (<10 nm), thermal fluctuations can change the direction of magnetization of the entire crystal. A material with many such crystals behaves like a paramagnet, except that the moments of entire crystals are fluctuating instead of individual atoms. Furthermore, the unique superparamagnetic behavior of iron oxide nanoparticles allows them to be manipulated magnetically from a distance. In the latter sections, external manipulation will be discussed in regards to biomedical applications of iron oxide nanoparticles. Forces are required to manipulate the path of iron oxide particles. A spatial uniform magnetic field can result in a torque on the magnetic particle, but cannot cause particle translation; therefore, the magnetic field must be a gradient to cause translational motion. The force on a point-like magnetic dipole moment m due to a magnetic field B is given by the equation: In biological applications, iron oxide nanoparticles will be translate through some kind of fluid, possibly bodily fluid, in which case the aforementioned equation can be modified to: