The authors acknowledge financial support from National Natural Science Foundation of China ( 21272088 , 21472059 and 21402057 ) and the Program for Academic Leader in Wuhan Municipality (201271130441). The work was also supported by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, Ministry of Education, and the Natural Science Foundation of Hubei Province ( 2013CFB207 ) and financially supported by self-determined research funds of CCNU from the colleges’ basic research and operation of MOE ( CCNU14A05009 and CCNU14F01003 ).