1. Joint School of National University of Singapore and Tianjin University, International Campus of Tianjin University, Binhai New City 350207, China
2. Department of Chemistry, Faculty of Science, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore
3. School of Science and Technology, Nottingham Trent University, Clifton Lane, Nottingham NG11 8NS, U.K.