Tuning the cellular uptake and cytotoxicity of carbon nanotubes by surface hydroxylation

Multi-walled carbon nanotubes named CNTan, CNTox, and CNTir were prepared by high temperature annealing, acid oxidation and gamma irradiation, respectively. Their morphology, metal contents and surface properties were characterized by a series of techniques. The effects of surface hydrophilicity on their cell uptake efficiency and cytotoxicity were evaluated by radio-labeling techniques and several biological assays. Results showed that the CNTs possess similar diameters, lengths and metal contents, but different surface hydrophilicity. All of these CNT samples were readily taken up by Hela cells within 2 h, and their cell uptake ratio showed a positive correlation with their surface hydrophilicity in the following order: CNTir > CNTox > CNTan. Among these CNT samples, CNTan exhibited the lowest cytotoxicity to Hela cells due to its severe agglomeration. Although the cell uptake efficiency of CNTir was greater than that of CNTox, their cytotoxicity showed no significant difference, and other factors such as serum protein coating, and incubation time could also influence their cytotoxicity. These conclusions demonstrated that surface hydrophilicity of MWCNTs could alter their cell uptake efficiency, underlining the possibility for rational design of CNT-specific surface properties for their further development in biomedical fields and a mechanistic understanding of observed CNT toxicity.