1. Beijing National Laboratory for Molecular Sciences Laboratory of Polymer Physics and Chemistry Institute of Chemistry Chinese Academy of Sciences Beijing 100190 P. R. China
2. University of Chinese Academy of Sciences Beijing 100049 P. R. China
3. School of Dentistry The University of Queensland Brisbane 4006 Australia
4. Department of Chemical and Biomolecular Engineering University of California Los Angeles CA 90066 USA
5. Department of Nuclear Medicine Peking University People's Hospital Beijing 100044 P. R. China