1. School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798 Singapore
2. Center for Nanotechnology and Nanotoxicology, Department of Environmental Health, Harvard T. H. Chan School of Public Health, Boston, Massachusetts 02115, United States
3. Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), Singapore 138634, Singapore
4. Nanyang Environment & Water Research Institute, 1 Cleantech Loop, CleanTech One, Singapore 637141, Singapore