Acknowledgements This work was supported by the National Nature Science Foundation of China (Grant Nos. 61075015 and 61073102), the Natural Science Foundation of Hubei Province, P.R. China (Grant No. 2007ABA408), China Postdoctoral Science Foundation funded project (Grant No. 0106184022) and the Research Start-up Funds in Wuhan Polytechnic University, P.R. China (Grant No. 2011363).