1. Department of Biotechnology, Lund University, P.O. Box 124, SE−22100 Lund, Sweden, Analytische Chemie, Elektroanalytik & Sensorik, Ruhr-Universität Bochum, Universitätsstrasse 150, D-44780 Bochum, Germany, and Department of Electrical Measurements, Lund Institute of Technology, P.O. Box 118, SE−22100 Lund, Sweden