Journal
BIOSENSORS & BIOELECTRONICS
Volume 24, Issue 7, Pages 1937-1941Publisher
ELSEVIER ADVANCED TECHNOLOGY
DOI: 10.1016/j.bios.2008.09.024
Keywords
Superparamagnetic beads; Controlled torque; Rotation; Functional biosensor; Magnetic moment; Nanoparticle size distribution
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We demonstrate that a rotating magnetic field can be used to apply a controlled torque on superparamagnetic beads which leads to a tunable bead rotation frequency in fluid. Smooth rotation is obtained for field rotation frequencies many orders of magnitude higher than the bead rotation frequency. A quantitative model is developed, based on results from a comprehensive set of experiments at different field strengths and frequencies. At low frequencies (<10 Hz), rotation is due to a small permanent magnetic moment in the bead. At high frequencies (kHz-MHz), the torque results from a phase lag between the applied field and the induced magnetic moment, caused by the non-zero relaxation time of magnetic nanoparticles in the bead. The control of torque and rotation will enable novel functional assays in bead-based biosensors. (C) 2008 Elsevier B.V. All rights reserved.
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