1. Center for Fluorescence Spectroscopy, Department of Biochemistry and Molecular Biology, University of Maryland School of Medicine, 725 West Lombard Street, Baltimore, Maryland 21201, and Institute of Biomaterials and Biomedical Engineering and Department of Materials Science & Engineering, University of Toronto, Rosebrugh Building 407 and 421, 4 Taddle Creek Road, Toronto, ON, M5S 3G9 Canada