1. Instituto de Física, Universidad Nacional Autónoma de México, Apartado Postal 20-364, 01000 Ciudad de México, México
2. CEMES-CNRS, University of Toulouse 3 Paul Sabatier, 31400 Toulouse, France
3. Laboratory of Biosensors and Bioelectronics, Institute for Biomedical Engineering, ETH Zürich, 8092 Zürich, Switzerland