Ferromagnetic Nanoparticles Used in New Immunoassays

Immunoassays have been developed based on the peroxidase activity of ferric oxide (Fe3O4) magnetic nanoparticles (MNPs).

The nanoparticles are active catalysts, and the catalytic activity is greater in smaller particles. In addition, iron oxide nanoparticles are more stable over a variety of temperatures and varying levels of acidity, giving them an additional advantage over the traditional horseradish peroxidase that is used in the enzyme-linked immunosorbant assay (ELISA).

Most immunoassays are ELISAs. Because it can evaluate the presence of an antigen or antibody in a sample, the ELISA is commonly used to test for human immunodeficiency virus (HIV), hepatitis B, and West Nile Virus. The test basically involves an antigen attached to a solid surface. When the antibody is washed over the surface, it binds to the antigen. The antibody is then linked to an enzyme, usually a peroxidase, which reacts with certain substrates, resulting in a change in color. The evolution of immunoassays has continued with developments such as the fluorimetric immunoassay.

Prof. Xiyun Yan from the National Laboratory of Biomacromolecules in the Institute of Biophysics of the Chinese Academy of Sciences (CAS; Beijing, PR China), and colleagues showed that iron oxide nanoparticles are intrinsically active catalysts for oxidation reactions.

"The magnetic properties allow separation of the target from a complex solution, and the peroxidase assay can then be used to detect and quantify the amount of the target molecule,” said Prof. Yan. According to the CAS scientists, magnetic ELISA assays are already widely used for laboratory and diagnostic purposes. However, the difference when using the intrinsic activity of the MNPs compared to the traditional ELISA, is that there is no need for the binding step with a secondary, enzyme-counjugated antibody.

Prof. Yan and her collaborators have already developed two novel immunoassays (one for hepatitis B and the other for myocardial infarction) that exploit this new discovery. Using the appropriate antibody, the nanoparticles capture the target molecule in solution and then a magnet is used to isolate the nanoparticles. With the simple addition of a peroxidase-sensitive dye, the solution changes from clear to blue in the presence of the nanoparticles.

According to the scientists, the stability, ease of production, and versatility of the iron oxide nanoparticles makes them a powerful tool for a wide range of potential applications in clinical diagnostics, biotechnology, and environmental chemistry.

"The MNP assay would be worth developing commercially, because it is cheaper, faster, more direct, and potentially more sensitive,” said Prof. Yan. "… we are already collaborating with a company to develop the MNP-based immunoassay and to apply MNPs for the treatment of wastewater. The magnetic properties allow separation of the target from a complex solution and the peroxidase assay can then be used to detect and quantify the amount of the target molecule.”