1. Department of Chemical Engineering, University College London, Torrington Place, London WC1E 7JE, U.K.
2. Biophysics Group, Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, U.K.
3. UCL Healthcare Biomagnetic and Nanomaterials Laboratories, 21 Albemarle Street, London W1S 4BS, U.K.
4. Department of Physics, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece
5. School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan