1. Biomedical Technologies Group, Microsystems and Nanotechnology Division, Physical Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States
2. Departments of Bioengineering, Materials Science and Engineering, and Beckman Institute, University of Illinois at Urbana-Champaign, Champaign, Illinois 61801, United States
3. Mills Breast Cancer Institute, Carle Foundation Hospital, Urbana, Illinois 61801, United States
4. Photonics and Plasmonics Group, Microsystems and Nanotechnology Division, Physical Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States