1. Hunan Key Laboratory of the Research and Development of Novel Pharmaceutical Preparations, Changsha Medical University, Changsha 410219, P. R. China
2. Department of Chemical Engineering, University College London, Gower Street, London WC1E 6BT, U.K.
3. The Hunan Provincial University Key Laboratory of the Fundamental and Clinical Research on Functional Nucleic Acid, Changsha Medical University, Changsha 410219, P. R. China