1. College of Chemistry; Sichuan University; Chengdu 610064 P. R. China
2. Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety, Institute of High Energy Physics; Chinese Academy of Sciences; Beijing 100049 P. R. China
3. Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety; National Center for Nanosciences and Technology of China; Beijing 100190 P. R. China
4. College of Materials Science and Opto-electronic Technology; Graduate University of Chinese Academy of Sciences; Beijing 100049 P. R. China