Synthesis of Magnetic Nanoparticles

Part I: Synthesis

Magnetic nanoparticles have risen in popularity in medical and biotechnology fields over the past decade. These tiny nanometer-sized particles are superparamagnetic, which means they can be magnetized by an externally applied magnetic field and quickly returned to a non-magnetic state once the field is removed. They are easy to manipulate, making them perfect for biomagnetic separation processes and a variety of other applications. There are many options to consider when choosing a magnetic nanoparticle for an experiment or therapeutic goal. A general understanding of the synthesis, protection, functionalization, and application of magnetic nanoparticles is a good place to start. In the first part of this series we focus on the synthesis of magnetic nanoparticles.

Synthesis of magnetic nanoparticles

In order to accurately predict the behavior of superparamagnetic nanoparticles in a particular situation it is important that the particles be as similar to each other in size, shape, surface defects, and composition as possible. A large size distribution makes it difficult to predict the behavior of the particles. This is one major reason why new synthesis methods are constantly being explored. There are many different methods to create superparamagnetic nanoparticles of less than 15 nm in size. These methods vary from each other in precursor chemicals, reaction conditions, speed, control over particle size distribution, and solvency of the final product. The ability for magnetic nanoparticles to enter into an aqueous solution is particularly important for biological applications. The good news is that with so many different synthesis methods available it is easier than ever to purchase the appropriate magnetic nanoparticles for a particular application.  Magnetic nanoparticles can be made from a variety of materials: iron oxide magnetite (Fe3O4) and its oxidized form maghemite (Fe2O3), pure metals Fe and Co, alloys such as CoPt3 and FePt, and others such as MnFe2O4, MgFe2O4, and CoFe2O4.

Four major synthesis methods are currently used:

1. Co-precipitation
• Iron oxide nanoparticles are commonly formed this way from iron ions. The reaction takes place in a salt solution and requires the addition of a base. The size, shape, and composition of the nanoparticles can be tuned by altering reaction temperature, salt composition and concentration, free ion ratio, and pH. Once the recipe is optimized it is easy to produce consistent batches of nanoparticles. However, it is challenging to control particle size distribution. The synthesis is fast and the nanoparticles produced are water-soluble.
2. Thermal decomposition
• High heats are needed in this process whereby an initial reactant (commonly an organometallic compound) is chemically decomposed in the presence of organic solvents containing stabilizing surfactants. This method offers good control over shape and size of the nanoparticles.
3. Microemulsion
• The most accessible example of a microemulsion is a mixture of oil and water. This synthesis technique takes advantage of the kinetics of liquid droplets in a microemulsion—microdroplets continually colliding together and breaking apart again. This technique requires a large amount of organic solvent, is difficult to scale up to produce a large number of particles at a time, and has very little control over size and shape of the produced nanoparticles
4. Hydrothermal synthesis
• In hydrothermal synthesis there are three major components. At high temperatures a phase transfer and separation reaction occurs at the interfaces between a solid phase, a liquid phase, and a water-ethanol solution. This technique produces uniform sizes of nanoparticles that are soluble in water.

More technical information about these techniques is available in the scientific review that inspired this blog post. A nice summary table of these techniques can be found there as well.