Single Wall Carbon Nanotubes Enter Cells by Endocytosis and Not Membrane Penetration

Carbon nanotubes are increasingly being tested for use in cellular applications. Determining the mode of entry is essential to control and regulate specific interactions with cells, to understand toxicological effects of nanotubes, and to develop nanotube-based cellular technologies. We investigated cellular uptake of Pluronic copolymer–stabilized, purified ~145 nm long single wall carbon nanotubes (SWCNTs) through a series of complementary cellular, cell-mimetic, and in vitro model membrane experiments. SWCNTs localized within fluorescently labeled endosomes as measured by fluorescence lifetime imaging microscopy. Confocal Raman spectroscopy showed a dramatic reduction in SWCNT uptake into cells at 4 °C compared with 37 °C. Electrochemical impedance spectroscopy and Langmuir monolayer film balance measurements showed that Pluronic-stabilized SWCNTs associated with membranes but did not possess sufficient insertion energy to penetrate through the membrane. SWCNTs associated with vesicles made from plasma membranes but did not rupture the vesicles. These measurements, combined, demonstrated that Pluronic-stabilized SWCNTs did not penetrate the membrane via non-specific physical interactions but entered cells via endocytosis, and association of SWCNTs to membrane likely increased uptake. These results suggest that specific targeting of SWCNTs for use in therapeutics should focus on endocytosis as the preferential method for the cellular uptake.