1. UCL Healthcare Biomagnetics and Nanomaterials Laboratories, 21 Albemarle Street, London W1S 4BS, United Kingdom
2. Institute of Materials Research and Engineering, Agency for Science, Technology and Research, Innovis, Singapore 138634 Singapore
3. Biophysics Group, Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT, United Kingdom