1. Center for Bio/Molecular Science and Engineering, Code 6900, U.S. Naval Research Laboratory, Washington, D.C. 20375, Department of Biology, University of Virginia, Charlottesville, Virginia 22901, Division of Engineering, Brown University, Providence, Rhode Island 02912, Electronic Science and Technology Division, Code 6812, U.S. Naval Research Laboratory, Washington, D.C. 20375, and College of Science, George Mason University, Fairfax, Virginia 22030