1. Contribution from the Biochip Group, MESA+ Research Institute for Nanotechnology, University of Twente, P. O. Box 217, NL-7500 AE Enschede, The Netherlands, and Department of Biomolecular Chemistry, Nijmegen Centre for Molecular Life Sciences, Institute for Molecules and Materials, Radboud University Nijmegen, P. O. Box 9101, NL-6500 HB Nijmegen, The Netherlands