1. Laboratory of Molecular Imaging and Nanomedicine (LOMIN), National Institute of Biomedical Imaging and Bioengineering (NIBIB), National Institutes of Health (NIH), Bethesda, Maryland 20892, United States
2. Laboratory of Cellular Imaging and Macromolecular Biophysics, NIBIB, NIH, Bethesda, Maryland 20892, United States
3. Biomedical Engineering and Physical Science Shared Resource, NIBIB, NIH, Bethesda, Maryland 20892, United States
4. Biomedical Research Center, Korea Institute of Science and Technology, Seoul 136-791, Korea,
5. Department of Chemistry, Brown University, Providence, Rhode Island 02912, United States