A variety of functional polymer chains prepared by RAFT were directly grafted onto 5, 10, and 20 nm gold nanoparticles (AuNPs). The polymer shell coating the AuNPs was densely packed because of the strong binding between the trithioester groups on the polymer chain-ends and gold. It was found that due to the densely packed nature of the shell the polymer chains were significantly stretched compared to their usual Gaussian coil conformation in water. This was even evident for polymer chains where ionic repulsion between neighboring chains should be significant. Therefore, with such high grafting densities the surface properties and size of the hybrid nanoparticles should be the only contributing factors in cellular uptake in epithelial Caco-2 cells. This study has provided valuable insight into the effects of charge and size of NPs for the application of NPs in the delivery of therapeutic agents across the intestine. Our results showed that the negatively charged AuNPs were taken up by the cells with greater efficiency than the neutral AuNPs, most probably due to binding with membrane proteins. The positively charged AuNPs as expected gave the greatest uptake efficiency. Interestingly, the uptake for PNIPAM-AuNPs (hydrophobic coating at 37 degrees C) increased from approximately 2% efficiency after a 30 min incubation to 8% after 2 h, and was much greater than the negative or neutral AuNPs. We believe that this was due to the interplay between the hydrophobic nature of the NPs and their increased size.