1. Key Laboratory of Green Chemistry and Technology of Ministry of Education; College of Chemistry and State Key Laboratory of Polymer Materials Engineering; Sichuan University; Chengdu 610064 P. R. China
2. Department of Chemistry and Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration & Reconstruction; Hong Kong University of Science and Technology; Clear Water Bay Kowloon 999077 Hong Kong SAR
3. Department of Physics; North Carolina State University; Raleigh NC 27695 USA