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Phys Rev Lett. 2009 Jan 16;102(2):028302. Epub 2009 Jan 13.

Torsional stiffness of single superparamagnetic microspheres in an external magnetic field.

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DNA Motors Group, BIOTEChnology Center, University of Technology Dresden, D-01062 Dresden, Germany.

Erratum in

  • Phys Rev Lett. 2012 May 18;108(20):209902.


We used magnetic tweezers to measure the torsional stiffness of single micrometer-sized superparamagnetic spheres as a function of the applied magnetic field. By investigating the axial fluctuations of DNA-bound microspheres, we found that considerable rotational microsphere fluctuations can occur. Quantitative noise analysis allowed us to determine the rotational stiffness of individual microspheres, which was found to saturate at high magnetic fields. The saturation can be qualitatively explained considering the properties of the magnetic nanoparticles within the microsphere. Consequences for spatial resolution limits in single-molecule magnetic tweezer experiments and usage of DNA mechanics as a sensitive probe in magnetometry are discussed.

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