1. Inorganic Materials Science, MESA+ Institute for Nanotechnology & Faculty of Science and Technology, University of Twente, P.O. Box 217, 7500 AE, Enschede, The Netherlands, Aquamarijn Micro Filtration B.V. Berkelkade 11, NL 7201 JE, Zutphen, The Netherlands, and Laboratory of Biosensors, MESA+ Institute for Nanotechnology & Faculty of Electrical Engineering, Mathematics and Information Technology, University of Twente, P. O. Box 217, 7500 AE, Enschede, The Netherlands