Nanoparticles have attracted the attention of researchers because of the fact that the mechanical, chemical, electrical, optical, magnetic, electro-optical and magneto-optical properties of these particles are different from their bulk properties. There are numerous areas where nanoparticles can be utilized. Nanoparticle of particular interest is from iron oxide. Nanoparticles of iron(III) oxide are biocompatible, non-toxic, chemically active on their surface, and paramagnetic at particle sizes above a critical limit of about five nanometers. Iron oxide nanoparticles have been widely investigated in applications ranging from bio-imaging to bio-sensing due to their unique magnetic properties.
Synthesis methods
Synthesis methods available in the literature indicate the following:

Micro fluidic system

Seed-mediated growth method

Zone confinement method

Matrix-mediated method

Templates

Other methods
Micro fluidic system
Recently, micro fluidic systems have been utilized for synthesis of nanoparticles, which have the advantages of automation for mixing, transporting and reacting with well-controlled reactions, and high particle uniformity. It allows for a rapid and efficient approach to accelerate and automate the synthesis of the iron oxide nanoparticles as compared with traditional methods. The micro fluidic system uses micro-electro-mechanical-system technologies to integrate a new double-loop micro mixer, two micro pumps, and a micro valve on a single chip. When compared with large-scale synthesis systems with commonly-observed particle aggregation issues, successful synthesis of dispersed and uniform iron oxide nanoparticles has been observed within a shorter period of time (15 min). The size distribution of these iron oxide nanoparticles is superior to that of the large-scale systems without requiring any extra additives or heating.
Seed-mediated growth method
Iron oxide nanoparticles can be synthesized with tunable size distribution and magnetic properties by seed-mediated growth method. The distribution of size gradually becomes narrow with time via the intra-particle ripening process and Oswald ripening process. The monodispersed iron oxide nanoparticles with sizes between 5–10 nm can be fabricated using this method by varying the experimental parameters. The magnetic nanoparticles show well-defined super paramagnetic and blocking temperature due to the size effects.
Zone confinement method
Iron oxide nanoparticles can be prepared using chemical synthesis methods. Nanoparticles with a narrow size distribution can be synthesized using zone confinement methods such as nano emulsions and a novel flow injection synthesis. The flow injection method consists of the precipitation of iron oxide nanoparticles in a continuous or segmented flow in a capillary reactor under laminar flow with tailored mean particle size from 3 up to 12 nm. High phase purity and magnetization can be achieved. These methods allowed the preparation of nanoparticles.
Matrix-mediated method
IO nanoparticles can be synthesized via a novel matrix-mediated method using polyvinyl alcohol (PVA).Iron oxide nanoparticles can also be synthesized using a freshly-made or recycled 1-butyl-3-methylimidazolium bis(triflylmethylsulfonyl)imide ([BMIM][Tf2N]) ionic liquid (IL). Iron pentacarbonyl (Fe(CO)5), which dissolves in [BMIM][Tf2N] is thermally decomposed and subsequently oxidized to form iron oxide nanoparticles. These nanoparticles with a narrow size distribution separate out automatically from the imidazolium-based ionic liquid mixtures. The solvent-recyclable process of making size-controlled nanoparticles will have a broad impact on the application of imidazolium-based ionic liquids in the synthesis of nanomaterials.
Templates
Natural polymers like chitosan and starch have been employed as templates for the preparation of iron oxide nanoparticles. The templates offer selective binding sites for Fe (II) under aqueous conditions. Controlled drying and subsequent removal of the template backbone enables the synthesis of spatially separated iron oxide nanoparticles.
Surface modification
Surface of iron oxide NPs could be modified by organic materials or inorganic materials, such as polymers, biomolecules, silica, metals, etc.Surface modification of magnetic nanoparticles by organic surfactants is known to provide them with solubility in organic solvents (ferro fluids), which undoubtedly is an important property in several applications and studies. The surfactants provide them with excellent stability and solubility in organic solvents like toluene or chloroform. Furthermore, by adding the appropriate surfactant or altering the temperature of the aqueous phase at the initial stage of the reaction a size control of the nanoparticles can be achieved within the range of 6–18 nm. The presence of capping agents or high reaction temperatures favours the formation of smaller nanoparticles.
Enhancement of properties
Iron oxide nanoparticle addition to polypyrrole (PPy) increases its thermal stability. The electrical conductivity of the nanocomposites increased greatly upon the initial addition (20 wt%) of iron oxide nanoparticles. However, a higher nanoparticle loading (50 wt%) decreased the conductivity as a result of the dominance of the insulating iron oxide nanoparticles.
Hybrid bio-organic-inorganic nanostructures
According to published reports, organized bio-inorganic and hybrid bio-organic-inorganic nanostructures consisting of iron oxide nanoparticles and DNA complexes have been formed using several methods. These methods are based on biomineralization, interfacial and bulk phase assembly, ligand exchange and substitution, Langmuir-Blodgett technique, DNA templating and scaffolding. Interfacially formed planar DNA complexes with water-insoluble amphiphilic polycation or intercalator Langmuir monolayers can be prepared and deposited on solid substrates to form immobilized DNA complexes. Those complexes can then be used for the synthesis of organized DNA-based iron oxide nanostructures. Planar net-like and circular nanostructures of magnetic Fe3O4 nanoparticles can be obtained via interaction of cationic colloid magnetite nanoparticles with preformed immobilized DNA/amphiphilic polycation complexes of net-like and toroidal morphologies. Iron oxide nanoparticles can be generated in immobilized DNA complexes via redox synthesis with various iron sources of biological (ferritin) and artificial (FeCl3) nature.
Super paramagnetic iron oxide nanoparticles
According to published reports super paramagnetic iron oxide nanoparticles (SPION) stabilized by alginate (SPION-alginate) have been developed as a contrast agent to improve the sensitivity of magnetic resonance imaging (MRI) in the detection of hepatocellular carcinoma (HCC)Super paramagnetic iron oxide nanoparticles (SPION) with appropriate surface chemistry have been widely used experimentally for numerous in vivo applications such as magnetic resonance imaging contrast enhancement, tissue repair, immunoassay, detoxification of biological fluids, hyperthermia, drug delivery and in cell separation, etc. All these biomedical and bioengineering applications require that these nanoparticles have high magnetization values and size smaller than 100 nm with overall narrow particle size distribution, so that the particles have uniform physical and chemical properties. In addition, these applications need special surface coating of the magnetic particles, which has to be not only non-toxic and biocompatible but also allow a target able delivery with particle localization in a specific area. To this end, most work in this field has been done in improving the biocompatibility of the materials, but only a few scientific investigations and developments have been carried out in improving the quality of magnetic particles, their size distribution, their shape and surface in addition to characterizing them to get a protocol for the quality control of these particles. Nature of surface coatings and their subsequent geometric arrangement on the nanoparticles determine not only the overall size of the colloid but also play a significant role in biokinetics and biodistribution of nanoparticles in the body. The types of specific coating, or derivatization, for these nanoparticles depend on the end application and should be chosen by keeping a particular application in mind, whether it is aimed at inflammation response or anti-cancer agents. Magnetic nanoparticles can bind to drugs, proteins, enzymes, antibodies, or nucleotides and can be directed to an organ, tissue, or tumor using an external magnetic field or can be heated in alternating magnetic fields for use in hyperthermia.
Requirement for practical applications
For practical application, the nanoparticles must have combined properties of high magnetic saturation, stability, biocompatibility, and interactive functions at the surface.
Practical applications
Surface functionalized magnetic iron oxide nanoparticles (NPs) are a kind of novel functional materials, which have been widely used in the biotechnology and catalysis.Tiny particles of iron oxide could become tools for simultaneous tumor imaging and treatment, because of their magnetic properties and toxic effects against brain cancer cells. They find wide use in biomedical applications. It can be used as contrast agents in magnetic resonance imaging, in labeling of cancerous tissues, magnetically controlled transport of pharmaceuticals, localized thermotherapy (where the tissue is labeled by iron oxide nanoparticles, then heated by application of AC field to particles), and preparation of ferrofluids.