1. School of Materials, University of Manchester, MSS Tower, Manchester, M13 9PL, U.K.
2. Photon Science Institute, University of Manchester, Alan Turing Building, Oxford Road, Manchester M13 9PL, U.K.
3. BioAFM Facility, Faculty of Biology, Medicine and Health, Stopford Building, University of Manchester, Oxford Road, Manchester, M13 9PT, U.K.