Fig. 3. Structures of Oct-Dox (a) and Doxo-S-S-Fol (b). (c) The schematic illustration of the cancer therapy mechanism of Doxo-S-S-Fol. The fluorescence emission and cytotoxicity of the prodrug (Doxo-S-S-Fol) are quenched (off) in the extra-cellular milieu. Upon target-specific internalization, enhanced fluorescence emission and cytotoxicity (on) occur due to cleavage of the disulfide bond by GSH. Adapted from ref. 63. Copyright© 2011 American Chemical Society.