# Probing biological bonds by means of rotation of superparamagnetic beads

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- Technische Universiteit Eindhoven
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## Abstract

The ability of superparamagnetic beads to rotate in a rotating magnetic field yields the possibility for probing the rotational properties of biological interactions with these beads. Torques can be applied to the beads, depending on the magnitude of a permanent magnetisation present in the bead. This research focuses on developing tools to easily determine the bead's permanent magnetisation and derive functional information by investigating the beads' rotational behaviour in rotating or static magnetic fields. Homogeneous magnetic fields are generated with a newly designed magnetic quadrupole and its current regulator. Magnetic fields of 37 mT can be reached for currents of 1 A. The field has a virtually absent horizontal gradient and a vertical gradient that yields forces on the order of magnitude of the gravitational force that acts on the bead. An image analysis toolbox for Matlab has been developed to derive the orientation of the bead as function of time. This is done by image correlation with respect to the rotation of the bead. The image analysis toolbox enables the development of techniques to quickly measure the permanent magnetic moment of an unbound bead in a rotating magnetic field. These techniques have an accuracy of 10% for the determination of the permanent magnetic moment. For bound beads a harmonic potential well is introduced to describe the bond's behaviour under stress. A method to simultaneously measure the bead's permanent magnetic moment and the torsion spring constant, involved in the harmonic potential, is presented. This method, which does not require the bead to continuously rotate, was shown to have an accuracy of 10% for the determination of permanent magnetisation of an unbound bead. It was shown that a bead, in a field that instantaneously switches its direction, turns over an angle that increases for increasing field strength. The corresponding spring torsion constant was found to be about 4×10-18 Nm. The typical error on this measurement is 35%. Experiments with bound beads are done with Streptavidincoated beads that bind to Biotin-BSA incubated on the solid phase. This system showed 2 different kinds of behaviour for bound beads in a rotating field. Beads are restricted to an oscillating movement for low fields, while for higher fields an energy barrier can be overcome and beads can rotate.

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