1. National Engineering Research Center for Biomaterials, Sichuan University, Chengdu 610064, P. R. China.
2. West China School of Pharmacy, Sichuan University, Chengdu 610041, P. R. China.
3. Department of Biomedical Engineering, The University of Texas at Austin, Austin, TX 78731, USA.
4. Huaxi MR Research Center (HMRRC), Functional and Molecular Imaging Key Laboratory of Sichuan Province, Department of Radiology, West China Hospital, Sichuan University, Chengdu 610041, P. R. China.
5. Division of Molecular Pharmaceutics and Drug Delivery, College of Pharmacy, The University of Texas at Austin, Austin, TX 78731, USA.