1. School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore
2. Environmental Chemistry and Materials Centre, Nanyang Environment & Water Research Institute, 1 Cleantech Loop, CleanTech One, Singapore 637141, Singapore
3. Center for Nanotechnology and Nanotoxicology, Department of Environmental Health, Harvard T.H. Chan School of Public Health, Boston, MA 02115, USA