Fig. 30. (A) The drug delivery system that could perform real-time monitoring of drug release based on PEG-functionalized CDs. Doxorubicin (DOX) was used as a drug molecule, which was adsorbed onto the surface of CDs through π–π stacking; (B) proposed mechanism of real-time monitoring of DOX release based on the FRET process between DOX and CDs. Figure reproduced from ref. 258 with permission from Wiley, copyright 2013.