Typical superparamagnetic nanospheres range from nanometers to a few micrometers in diameter and are made of a polystyrene matrix containing ferrite particles (Fe3O4). The nanospheres are chemically similar to standard latex or polystyrene beads, enabling easy coating with antibodies by passive adsorption or covalent coupling via carboxyl or amino groups.
Figure 1. Structure of superparamagnetic nanospheres. The nanospheres are polystyrene beads containing ferrite (Fe3O4) particles.
Superparamagnets
Although ferrite (or magnetite) is normally ferromagnetic, in the form of nanoparticles in the nanospheres it is superparamagnetic.
Paramagnetism is magnetism that occurs only in the presence of an externally applied magnetic field. Unlike ferromagnetic materials, superparamagnetic materials do not retain any significant amount of magnetization in the absence of an externally applied magnetic field, and thus do not form aggregates.
When a magnetic field is applied to the superparamagnetic nanospheres, the ferrite particles align with it. This locally increases the strength (amplitude) of the applied magnetic field. Once the field is removed, Brownian motion removes the alignment of the ferrite particles and the nanoparticles become demagnetised.
Figure 2. Behaviour of superparamagnetic nanospheres in magnetic fields. The ferrite crystals align with the magnetic field, locally increasing the amplitude of the field. Neither hysteresis nor residual magnetization is observed.
Exposure to external magnetic fields does not alter the paramagnetic properties of the nanospheres. This means that the nanospheres, and the tests using them, will not be damaged if they are exposed to a magnetic fields e.g. during transportation or storage.
Paramagnetic particles can be easily manipulated during manufacture utilising magnets. This removes the need for centrifugations, saving time, energy and laboratory space.