Scientific Reports 4, Article number: 6399 (2014); Published: 17 September 2014; Updated: 20 January 2015

The Acknowledgements section in this Article is incomplete.

“This work was supported by the Hong Kong General Research Fund under Grant Nos. 613211 and 623212.”

should read

“This work was supported by the Hong Kong General Research Fund (Grant Nos. 613211 and 623212) and National Natural Science Foundation of China (Grant No. 51376154)”