1. Guangxi Key Laboratory of Natural Polymer Chemistry and Physics, Nanning Normal University, Nanning, Guangxi 530001, P. R. China
2. School of Chemistry and Pharmaceutical Sciences, Guangxi Normal University, Guilin, Guangxi 541004, P. R. China
3. College of Chemistry and Chemical Engineering, Central South University, Changsha, Hunan 410083, P. R. China