The present study was sponsored by the National Natural Scientific Foundation of China (grant no. 81400117; Beijing, China), the Hunan Provincial Natural Science Foundation of China (grant no. 2015JJ4043; Changsha, China), China Postdoctoral Science Foundation (grant no. 2014M562115; Beijing, China) and the returned personnel Initial Funding of University of South China (grant no. 2014XQD46; Hengyang, China).