1. Graduate School of Materials Research, Department of Physical Chemistry, Åbo Akademi University, Porthansgatan 3-5, FI-20500 Åbo, Finland, Department of Chemistry, University of Oslo, P.O. Box 1033 Blindern, NO-0315 Oslo, Norway, and Amersham Health AS, Research Oslo, P.O. Box 4220 Nydalen, NO-0401 Oslo, Norway