Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.
This research was sponsored by the K.C. Wong Magna Fund in Ningbo University. This work was supported by the Natural Science Foundation of Ningbo City, China (2011A610060); the Scientific Research Fund of Zhejiang Provincial Education Department (Y201120665); the Talented Person Project of Ningbo University; the Project of Social Development of Ningbo City, China (2009C50012) and the Scientific Innovation Team Project of Ningbo (2011B82014).