Part 2: Structure/Protection, Functionalization, and ApplicationThe first part of this series provided a general overview of the most common synthesis methods for generating superparamagnetic nanoparticles of only a few nanometers in size. This second part touches on the procedures necessary to protect and functionalize these nanoparticles to extend their usefulness across a great number of applications.
Structure and Protection
Raw nanoparticles, depending on their composition, are susceptible to oxidation in air or erosion in acidic or basic solutions. Additionally, unprotected magnetic nanoparticles are prone to irreversible agglomeration and spontaneous precipitation out of solution. There are two main structures for nanoparticle protection: core-shell architecture and embedded matrices.
This method is exactly what it sounds likes. A superparamagnetic core is surrounded by a protective outer shell layer, which mediates electrostatic interactions between particles and controls oxidation or prevents it. The outer layer can also be useful in biomedical applications by making the nanoparticles biocompatible and able to circulate long enough in the body to reach their target tissue. The outer shell can be made of surfactants, polymers, silica, precious metals, or oxides. Some nanoparticles are coated with a single outer shell layer, and other types they can be made into multi-layered nanoparticles with two or more functions combined.
Another method used to prevent nanoparticle agglomeration is to embed them into a polymer, carbon, or silica matrix, or attaching them to the surfaces of larger particles. This strategy can also be considered adding a ‘magnetic pigment’ to a material. This prevents the irreversible binding of nanoparticles to each other, but it also limits their free mobility, which could be undesirable based on the application at hand.
Functionalization and Application
The protective coating also plays a role in the functionalization of the nanoparticles. Coatings such as silica are useful for functionalization because they are rich in sites for the chemical attachment of functional groups, ligands, catalysts, flurophores, and drug molecules. The ability to attach specific molecules to the surfaces of superparamagnetic nanoparticles is a key component to their popularity. The versatility of superparamagnetic beads is seemingly limitless in this regard. The attachment of carboxyl or amine groups is a common strategy to adsorb and isolate nucleic acids. Specific antigens can be separated out of solution by antibody-bound magnetic nanoparticles, and when a fluorophore is incorporated the system can become a useful and rapid diagnostic tool for the presence of pathogens. Enzymes can be attached to particle surfaces to create reusable catalyst supports.
Superparamagnetic nanoparticles are also used as contrast agents in magnetic resonance imaging (MRI), as localized heating agents in hyperthermia treatment specifically targeting tumor cells, targeted drug delivery systems, and fluorescent imaging techniques. Superparamagnetic nanoparticles are easily controlled by a magnetic field and are easily functionalized for a variety of applications. More detailed information about the protection, functionalization, and application of magnetic nanoparticles is widely available in the scientific literature. Two particularly comprehensive sources are cited below.