1. University of Freiburg – IMTEK, Department of Microsystems Engineering, Laboratory for Chemistry and Physics of Interfaces, Georges-Koehler-Allee 103, D-79110 Freiburg, Germany, University of California Santa Barbara, Department of Chemistry and Biochemistry, Santa Barbara, California 93106, and University of California, Berkeley Bioengineering Department, 306 Stanley Hall, Berkeley, California 94720-1762