Surface chemistry and aspect ratio mediated cellular uptake of Au nanorods

Gold nanorods (Au NRs) have been recognized as promising materials for biomedical applications, like sensing, imaging, gene and drug delivery and therapy, but their toxicological issues are still controversial, especially for the Au NRs synthesized with seed-mediated method. In this study, we investigated the influence of aspect ratio and surface coating on their toxicity and cellular uptake. The cellular uptake is highly dependent on the aspect ratio and surface coating. However, the surface chemistry has the dominant roles since PDDAC-coated Au NRs exhibit a much greater ability to be internalized by the cells. The present data demonstrated shape-independent but coating-dependent cytotoxicity. Both the CTAB molecules left in the suspended solution and on the surface of Au NRs were identified as the actual cause of cytotoxicity. CTAB can enter cells with or without Au NRs, damage mitochondria, and then induce apoptosis. The effects of surface coating upon toxicity and cellular uptake were also examined using Au NRs with different coatings. When Au NRs were added into the medium, the proteins were quickly adsorbed onto the Au NRs that made the surface negatively charged. The surface charge may not directly affect the cellular uptake. We further demonstrated that the amount of serum proteins, especially for BSA, adsorbed on the Au NRs had a positive correlation with the capacity of Au NRs to enter cells. In addition, we have successfully revealed that the cationic PDDAC-coated Au NRs with an aspect ratio of 4 possess an ideal combination of both negligible toxicity and high cellular uptake efficiency, showing a great promise as photothermal therapeutic agents.