Magnetic Beads - Basics and Application
Authors: Ruffert Christine 
Published: February 2020

The time-averaged magnetization of single-domain magnetic particles without a magnetic field is zero, when their magnetic energy K·(4/3)pr
3 is smaller than about ten times the thermal energy kBT. In this expression, the parameter K describes the magnetic anisotropy constant of the magnetic material [1], r is the particle radius of a supposed spherical particle and kB
the Boltzmann constant [2]. The vanishing magnetization in this specific case is called superparamagnetism.
At room temperature, the thermal energy kBT is 4.0·10-21 J and the maximum radius r for a spherical particle of iron featuring superparamagnetic properties amounts to about 6 nm [3]. Due to their spherical shape, such small superparamagnetic particles are referred to magnetic beads. A typical magnetic bead is composed of a magnetic core (or at least magnetic material containing) and a non-magnetic coating, which is conventionally designed for selectively binding a biomaterial of interest, e.g. cells or proteins. Iron oxides, such as magnetite (Fe3O4) or maghemite (γ-Fe2O3) are more stable against oxidation and thus preferentially used as magnetic material instead of iron. The magnetization curve of an ensemble of magnetic beads is hysteresis-free (at not too high frequencies). As a consequence, the superparamagnetic particles can be manipulated by a magnetic field with the desired sample attached to the surface, but they do not agglomerate when the magnetic field is removed. This means, the magnetic interaction can be easily be switched on and off, which has a high benefit when used as carrier for bioanalytical applications. Other advantages of using small magnetic nanoparticles are the minimum disturbance of the attached biomolecules [4] and a large surface-to-volume ratio for chemical binding.
Meanwhile, magnetic beads can be considered as a state-of-the-art tool for the separation or manipulation of various biomolecules. Other applications beside magnetic separation are immuno-assays, magnetic resonance imaging, drug delivery, and hyperthermia [5].
As a “unconventional” example for the use of magnetic beads, a magnetic approach for the capture of Pt nanoparticles from a colloidal aqueous solution is presented. The basic idea is making use of the well-known strong avidin-biotin interaction forces on the one hand and the high affinity of thiol as a functional group to bind the Pt nanoparticles on the other hand. Magnetic beads, surface-functionalized with monomeric avidin, were bound to biotin-PEG-thiol serving as intermediate linker. The successful attachment of Pt nanoparticles to the surface of avidin-coated magnetic beads via biotin-PEG-thiol was proven by TEM analysis as well as zeta potential measurements.
Since nanoparticles of the platinum group (predominantly Pt and Pd) are widely used as catalyst for example in pharmaceutical processes and the natural resources are strongly limited, a recovery of spent catalysts is of high industrial impact. Industrial methods do not employ a magnetic approach and suffer from a loss of the spent catalyst or other disadvantages. The further goal of the investigations on Pt nanoparticles and magnetic beads is a re-separation and thus recovery of the precious nanoparticles for further use. First investigations on the separation of magnetic beads and Pt nanoparticles after successful attachment have been carried out employing four different methods. To accomplish a quantitative measurement of the separated amount of Pt, inductively-coupled plasma-optical emission spectroscopy (ICP-OES) was applied. First achievements indicate a successful release of the Pt nanoparticles from the magnetic beads with various levels according to the respective methods.
Since magnetic beads are commonly used in biotechnology, the approach to attach Pt can be considered as an unconventional. Vice versa, there are several methods available to extract precious metals from a liquid phase, while a magnetic approach is missing among the standard technologies. The concept is thus new with respect to magnetic beads as well as from the nanoparticle separation perspective. The developed technology can easily be transferred to Pd, featuring similar chemical properties.
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