1. Shenzhen Key Laboratory of Polymer Science and Technology College of Materials Science and Engineering Shenzhen University Shenzhen 518060 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 (HKUST) Clear Water Bay, Kowloon Hong Kong 999077 P. R. China
3. Department of Physics Chinese University of Hong Kong New Territories Hong Kong 999077 P. R. China