1. Faculty of Bioresource Sciences, Akita Prefectural University, Kaidobata-Nishi 241-438, Shimoshinjo-Nakano, Akita-shi, Akita 010-0195, Japan.
2. School of Chemistry and Environment, South China Normal University, Guangzhou 510631, P.R. China. E-mail: zhizeng@scnu.edu.cn
3. Corresponding author. Phone: +81-18-872-1500. Fax: +81-18-872-1676. E-mail: zhangh@akita-pu.ac.jp
4. Institute of Optoelectronic Materials and Technology, South China Normal University, Guangzhou 510631, P.R. China.