1. Institute of Biophysics, Bulgarian Academy of Sciences, Acad. G.Bonchev Str. Bl.21, BG-1113 Sofia, Bulgaria, Biosensors group, IMEC, Kapeldreef 75, B-3001 Leuven, Belgium, CNRS UMR 8612, 92296 Châtenay, France, and LURE, Université Paris-Sud, F-91898 Orsay, France