Intracellular glutathione (GSH) concentrations are substantially higher than extracellular concentrations, making selective intracellular release possible. GSH can trigger the release of payload from the nanoparticle surface inside the cancer cells. In a recent study, Rotello et al. linked a hydrophobic dye (HSBDP) to GNPs as a model of drug. GSH-mediated release was detected in HepG2 cells by enhancement of the fluorescence signal (Fig. 10a). In control experiments, low HSBDP release was found in mouse embryonic fibroblast cells which had 50% lower intracellular GSH levels than HepG2 cells.