1. Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of the Ministry of Education; College of Chemistry and Materials Science; Northwest University; Xi'an 710069 China
2. Department of Materials Science and Engineering; Faculty of Engineering; National University of Singapore; 7 Engineering Drive 1 117574 Singapore
3. Department of Anatomy; Yong Loo Lin School of Medicine; National University of Singapore 4 Medical Drive; MD10 117597 Singapore
4. Magnetic Resonance Imaging Group; Singapore Bioimaging Consortium; Agency for Science Technology and Research (A*STAR); 11 Biopolis Way, #02-02 Helios 138667 Singapore
5. State Key Laboratory of Magnetism and Beijing National Laboratory for Condensed Matter Physics; Chinese Academy of Sciences; Beijing 100190 China
6. Key Laboratory of Advanced Materials; Ministry of Education; School of Material Science and Engineering; Tsinghua University; Beijing 100084 China
7. NUS Graduate School for Integrative Sciences and Engineering; National University of Singapore; 28 Medical Drive 117456 Singapore
8. School of Materials Science and Engineering; University of New South Wales; Kensington NSW 2052 Australia