Magnetic Beads - Basics and Applications

摘要

This contribution is about small magnetic particles (so-called "beads") starting from the synthesis, presenting the characteristic features, and discussing the physics and forces involved when these particles are exposed to an external magnetic field. Finally, various conventional applications and an unconventional approach for the use such beads are described. Magnetic beads are small particles with a spherical shape and diameters ranging from the nano to the micro scale. Various types of such beads are commercially available. The approach of embedding ferromagnetic nanoparticles of only a few tens or hundreds of nanometers, which is on the magnetic single-domain scale in a non-magnetic matrix (made of e.g. polystyrene or silica) creates small particles with superparamagnetic properties. Superparamagnetic nanoparticles have a zero magnetization in the absence of a magnetic field, which is attributed to the ratio of the magnetic anisotropy energy and the thermal energy. The magnetic anisotropy energy depends on the orientation of the particles relative to the applied magnetic field and is proportional to the magnetic volume of the particle, which is typically smaller than the thermal energy. The nanoparticles therefore essentially behave as non-magnetic particles in the absence of a magnetic field. This feature entails that magnetic beads can be manipulated applying magnetic forces, but do not agglomerate in the absence of an external magnetic field since they have no permanent magnetic moment due to the dimension of the embedded particles. The large surface-to-volume ratio of magnetic beads lends itself to a further use, i.e. for chemical bonding of target molecules. Meanwhile, magnetic beads have been well established as tool for the specific attachment of biomolecules for detection and quantification purposes. This contribution presents some basics on magnetic bead fabrication and use, which can be considered as state of the art in the area of biotechnology and biomedicine. Finally, a rather unconventional application of magnetic beads is presented comprising the magnetic manipulation of metal nanoparticles for catalyst recovery.