To overcome the lack of tumor specificity and low solubility in water, Hwu et al. incorporated paclitaxel by its thiol terminal onto colloidal GNPs through ligands exchange experiments (Fig. 10d) [131]. A flexible PEG spacer was used to increase biocompatibility. Incorporation of the phosphodiester moiety in paclitaxel-containing nanoparticles enhanced the cancer-targeting capability. Dephosphorylation reactions in cancer cells liberated paclitaxel molecules from nanoparticles.