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).

Applications of iron oxide nanoparticles include terabit magnetic storage devices, catalysis, sensors, 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.

Structure

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, 21¹⁄₃ Fe3+
ions and 2²⁄₃ vacancies. The cations are distributed randomly over the 8 tetrahedral and 16 octahedral sites.^[1][2]

Magnetic properties

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.^[3]

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.^[3]

A single domain magnetic material (ex: 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 $''T''_C$ and antiferromagnetic materials lose their magnetization beyond the Néel temperature $T_N$. 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.^[3] This superparamagnetic behavior of iron oxide nanoparticles can be attributed to their size. When the size gets small enough (<20 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.^[3]

Synthesis

The preparation method has a large effect on shape, size distribution, and surface chemistry of the particles. It also determines to a great extent the distribution and type of structural defects or impurities in the particles. All these factors affect magnetic behavior. Recently, many attempts have been made to develop processes and techniques that would yield ‘monodisperse colloids’ consisting of nanoparticles uniform in size and shape.

Coprecipitation

By far the most employed method is coprecipitation. This method can be further divided into two types. In the first, ferrous hydroxide suspensions are partially oxidized with different oxidizing agents. For example, spherical magnetite particles of narrow size distribution with mean diameters between 30 and 100 nm can be obtained from a Fe(II) salt, a base and a mild oxidant (nitrate ions).^[4] The other method consists in ageing stoichiometric mixtures of ferrous and ferric hydroxides in aqueous media, yielding spherical magnetite particles homogeneous in size.^[5] In the second type, the following chemical reaction occurs:

2Fe3+
+ Fe2+
+ 8OH−
-→ Fe₃O₄ + 4H₂O

Optimum conditions for this reaction are pH between 8 and 14, Fe3+
/Fe2+
ratio of 2:1 and a non-oxidizing environment. Being highly susceptibile to oxidation, magnetite (Fe₃O₄) is transformed to maghemite (γFe₂O₃) in the presence of oxygen:^[1]

2Fe₃O₄ + O₂ → 2γFe₂O₃

The size and shape of the nanoparticles can be controlled by adjusting pH, ionic strength, temperature, nature of the salts (perchlorates, chlorides, sulfates, and nitrates), or the Fe(II)/Fe(III) concentration ratio.^[1]

Microemulsions

A microemulsion is a stable isotropic dispersion of 2 immiscible liquids consisting of nanosized domains of one or both liquids in the other stabilized by an interfacial film of surface-active molecules. Microemulsions may be categorized further as oil-in-water (o/w) or water-in-oil (w/o), depending on the dispersed and continuous phases.^[2] Water-in-oil is more popular for synthesizing many kinds of nanoparticles. The water and oil are mixed with an amphiphillic surfactant. The surfactant lowers the surface tension between water and oil, making the solution transparent. The water nanodroplets act as nanoreactors for synthesizing nanoparticles. The shape of the water pool is spherical. The size of the nanoparticles will depend on size of the water pool to a great extent. Thus, the size of the spherical nanoparticles can be tailored and tuned by changing the size of the water pool.^[6]

High-temperature decomposition of organic precursors

The decomposition of iron precursors in the presence of hot organic surfactants results in samples with good size control, narrow size distribution (5-12 nm) and good crystallinity; and the nanoparticles are easily dispersed. For biomedical applications like magnetic resonance imaging, magnetic cell separation or magnetorelaxometry, where particle size plays a crucial role, magnetic nanoparticles produced by this method are very useful. Viable iron precursors include Fe(Cup)
3,Fe(CO)
5, or Fe(acac)
3 in organic solvents with surfactant molecules. A combination of Xylenes and Sodium Dodecylbenezensulfonate as a surfactant are used to create nanoreactors for which well dispersed iron(II) and iron (III) salts can react.^[1]

Biomedical Applications

Magnetite and maghemite are preferred in biomedicine because they are biocompatible and potentially non-toxic to humans. Iron oxide is easily degradable and therefore useful for in vivo applications. Results from exposure of a human mesothelium cell line and a murine fibroblast cell line to seven industrially important nanoparticles showed a nanoparticle specific cytotoxic mechanism for uncoated iron oxide.^[7] Solubility was found to strongly influence the cytotoxic response. Labelling cells (e.g. stem cells, dendritic cells) with iron oxide nanoparticles is an interesting new tool to monitor such labelled cells in real time by magnetic resonance tomography.^[8]

References

1. Laurent, Sophie; Forge, Delphine; Port, Marc; Roch, Alain; Robic, Caroline; Vander Elst, Luce; Muller, Robert N. (2008). "Magnetic Iron Oxide Nanoparticles: Synthesis, Stabilization, Vectorization, Physicochemical Characterizations, and Biological Applications". Chemical Reviews 108 (6): 2064–110. doi:10.1021/cr068445e. PMID 18543879.
2. Buschow, K.H.G., ed. (2006). Hand Book of Magnetic Materials. Elsevier.
3. Teja, Amyn S.; Koh, Pei-Yoong (2009). "Synthesis, properties, and applications of magnetic iron oxide nanoparticles". Progress in Crystal Growth and Characterization of Materials 55: 22. doi:10.1016/j.pcrysgrow.2008.08.003.
4. Sugimoto, T (1980). "Formation of uniform spherical magnetite particles by crystallization from ferrous hydroxide gels*1". Journal of Colloid and Interface Science 74: 227. doi:10.1016/0021-9797(80)90187-3.
5. Massart, R.; Cabuil, V.J.Chem.Phy.1987, 84,967.
6. Laughlin, R (1976). "An expedient technique for determining solubility phase boundaries in surfactant?water systems*1". Journal of Colloid and Interface Science 55: 239. doi:10.1016/0021-9797(76)90030-8.
7. Brunner, Tobias J.; Wick, Peter; Manser, Pius; Spohn, Philipp; Grass, Robert N.; Limbach, Ludwig K.; Bruinink, Arie; Stark, Wendelin J. (2006). "In Vitro Cytotoxicity of Oxide Nanoparticles: Comparison to Asbestos, Silica, and the Effect of Particle Solubility†". Environmental Science & Technology 40 (14): 4374. Bibcode:2006EnST...40.4374B. doi:10.1021/es052069i.
8. Bulte, Jeff W. M.; Kraitchman, Dara L. (2004). "Iron oxide MR contrast agents for molecular and cellular imaging". NMR in Biomedicine 17 (7): 484–499. doi:10.1002/nbm.924.