1. Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul 151-742, Republic of Korea
2. School of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul National University, Seoul 151-742, Republic of Korea
3. Department of Radiology, Seoul National University College of Medicine, Seoul 110-744, Republic of Korea
4. Center for Mechanics of Solids, Structures, and Materials, Department of Aerospace Engineering and Engineering Mechanics, Texas Materials Institute, University of Texas at Austin, 210 E. 24th Street, Austin, Texas 78712, United States
5. School of Mechanical and Aerospace Engineering, Seoul National University, Seoul 151-742, Republic of Korea