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