1. The Hong Kong Polytechnic University Shenzhen Research Institute, Shenzhen 518057, P. R. China
2. Department of Chemistry, Hong Kong Baptist University, Waterloo Road, Kowloon Tong, Hong Kong, P.R. China
3. Department of Physics, The Chinese University of Hong Kong, Shatin, Hong Kong SAR, Hong Kong, P.R. China
4. Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hong Hom, Hong Kong, P.R. China