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Carbon nanotubes (CNTs) have been widely investigated as one of the most promising nanomaterials in biomedical applications. Functionalization of CNTs with poly(ethylene glycol) diamine (PEG) is a recognized methodology with good solubility and biocompatibility. In this study, PEG conjugated (PEGylated) multi-walled CNTs (MWCNTs) were prepared and labeled with fluorophore FITC. Using fluorophore-conjugated PEGylated MWCNTs, we monitored their accumulation in transformed cancer cells and in normal cells. The intracellular accumulation of PEGylated MWCNTs was studied under confocal microscope and transmission electron microscope. At the same concentration and exposure time, PEGylated MWCNTs entered all three of the cancer cell lines tested much more efficiently than the three primary human fibroblast lines. This different cell-penetration efficiency was observed both in separate cell culture separate exposure conditions and in co-culture and co-exposure conditions. This study highlights that the cell-penetration efficiency of PEGylated MWCNTs is dependent on cell types, and possibly due to different metabolic rates in different cell types. Furthermore, the intracellular accumulation of PEGylated MWCNTs did not impair membrane integrity, and the treated cells remained normal morphology, indicating good biocompatibility of PEGylated MWCNTs. This study suggests that PEGylated MWCNTs can be developed as potential drug carrier with its intrinsic higher preference to tumor cells than normal cells.