1. Faculty of Health and Medical Sciences, University of Surrey, Guildford, GU2 7XH, U.K.
2. Nanoelectronics Centre, Advanced Technology Institute, University of Surrey, Guildford, GU2 7XH, U.K.
3. Institute of Biophysics, Johannes Kepler University, Altenberger Strasse 69, A-4040 Linz, Austria
4. Université de Toulouse, UPS/INP/CNRS, Institut Carnot CIRIMAT, 118 route de Narbonne, 31062 Toulouse, Cedex 9, France