Synthesis and Characterization of Iron Oxide Nanoparticles

Synthesis and Characterization of Iron Oxide Nanoparticles

John M. Melnyczuk (Clark Atlanta University, USA) and Soubantika Palchoudhury (Yale University, USA)
DOI: 10.4018/978-1-4666-5824-0.ch004
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Iron oxide nanoparticles show great promise in bio-applications like drug delivery, magnetic resonance imaging, and hyperthermia. This is because the size of these magnetic nanoparticles is comparable to biomolecules and the particles can be removed via normal iron metabolic pathways. These nanoparticles are also attractive for industrial separations and catalysis because they can be magnetically recovered. However, the size, morphology, and surface coating of the iron oxide nanoparticles greatly affect their magnetic properties and biocompatibility. Therefore, nanoparticles with tunable characteristics are desirable. This chapter elaborates the synthesis techniques for the formation of iron oxide nanoparticles with good control over reproducibility, surface and magnetic properties, and morphology. The well-known co-precipitation and thermal decomposition methods are detailed in this chapter. The surface modification routes and characterization of these nanoparticles are also discussed. The chapter will be particularly useful for engineering/science graduate students and/or faculty interested in synthesizing iron oxide nanoparticles for specific research applications.
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Recently iron oxide nanoparticles (NPs) have attracted tremendous attention as candidates for magnetic resonance imaging (MRI) contrast enhancer, drug delivery, and magnetic fluid hyperthermia (Mahmoudi, Sant, Wang, Laurent, & Sen, 2011). Superparamagnetic magnetite and maghemite based MRI contrast agents like Feridex, Combidex, Resovist, and CLIO have been approved by the Food and Drug Administration (FDA) (Bin Na, Chan Song, & Hyeon, 2009). These dextran-coated iron oxide NPs are used to dephase the proton magnetic moments of water surrounding the tumor site to induce a darker contrast compared to healthy tissues (Pankhurst, Connolly, Jones, & Dobson, 2003). The contrast agents can be targeted to the tumor and are less cytotoxic compared to the commonly used gadolinium chelates (Qiao, Yang, & Gao, 2009). Iron oxide NPs can be passively phagocytosed by the reticuloendothelial cells or can be actively targeted to the biomarkers on tumor cells with specific binding ligands (C. Sun, Lee, & Zhang, 2008). The large surface and tunable coating of the iron oxide NPs allow surface conjugation of anti-tumor drugs like doxorubicine (Xing et al., 2012). This targeted drug delivery enables accumulation of therapeutically relevant drug doses at the tumor site for effective chemotherapy. When the iron oxide NPs at the tumor site are subjected to an alternating magnetic field, the magnetic moments reorient. This magnetic energy is dissipated as heat at the tumor and elevates the local temperature (42-45 °C) to selectively kill the cancer cells (Al-Saie et al., 2011). The healthy tissues surrounding the tumor are protected in this form of treatment called the magnetic fluid hyperthermia (Laurent, Dutz, Hafeli, & Mahmoudi, 2011). However, iron oxide NPs with good reproducibility as well as tunable size and surface properties need to be synthesized for the above applications (LaConte et al., 2007). For example, superparamagnetic iron oxide NPs (< 15 nm) do not aggregate in the presence of a magnetic field and show a good blood circulation time. Therefore, these NPs are ideal for MRI and hyperthermia applications. The surface coating on the iron oxide NPs is also a major influence on the proton relaxation rates in MRI. The NPs with completely hydrophilic surfactant coatings are most suitable as MRI contrast agents because they are accessible to the surrounding water protons (Smolensky, Park, Berguo, & Pierre, 2011).

This chapter will focus on the reproducible synthesis of iron oxide NPs of varying size, morphology, and surfactant coating. The widely used co-precipitation and thermal decomposition routes will be elaborated. In explaining these synthetic techniques emphasis will be given to the conditions required for different shapes of iron oxide NPs. The synthesis of iron oxide nanospheres, nanocubes, nanoplates, nanoflowers, nanoworms, and nanowhiskers will be detailed. The methods to characterize the size, morphology, crystal phase, and magnetic properties of the NPs via transmission electron microscopy (TEM), x-ray diffraction (XRD), and alternating gradient field magnetometry (AGM) will be covered next. Finally, the surface coating and biocompatibility of the iron oxide NPs will be addressed. The iron oxide NP products from the thermal decomposition method are coated with hydrophobic ligands. Surfactant exchange and polymer encapsulation routes will be used for aqueous phase transfer of such NPs.

Key Terms in this Chapter

Ligand Exchange: Method of partially or completely replacing the hydrophobic coating layer (ligand) on nanoparticles with hydrophilic molecules to render the particles water-soluble.

Iron Oxide Nanoparticle: Small particles of iron oxide with at least one dimension < 100 nm (1 nm = 10 -9 m).

Heat-Up Method: Heating up the reactants in an organic solvent to form the iron oxide nanoparticles.

Co-Precipitation: Reacting Fe 2+ and Fe 3+ salts in aqueous solution to form iron oxide nanoparticles.

Nanoplates: Plate-shaped iron oxide nanoparticles.

Nanoflowers: Iron oxide nanoparticles with flower-like morphology.

Polymer Encapsulation: Coating the hydrophobic nanoparticles with an additional polymer layer to render them hydrophilic.

Nanowhiskers: Rice whisker-like iron oxide nanoparticles.

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