1. Georgia Institute of Technology, School of Electrical and Computer Engineering, Atlanta, Georgia 30332, United States
2. Now at Texas Instruments, Dallas, Texas, United States 75243
3. Now at Center for Disease Control, Atlanta, Georgia, United States 30333
4. Bioengineering Laboratory, Department of Biosystems Science and Engineering, ETH Zurich, Basel, Switzerland
5. Institute of Analytical and Bio-analytical Chemistry, University of Ulm, Ulm, Germany