1. Physikalisches Institut and Center for Nanotechnology, CeNTech, University of Münster, Mendelstrasse 7, 48149 Münster, Germany, Biophysical Engineering Group, Faculty of Science and Technology and MESA+ Institute for Nanotechnology, University of Twente, 7500 AE Enschede, The Netherlands, and Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, 3012 Bern, Switzerland