Investigating the cellular internalization pathways of single molecules or single nano objects isimportant to understanding cell–matter interactions, and to applications in drug delivery anddiscovery. Imaging and tracking the motion of single molecules on cell plasma membranesrequire high spatial resolution in three dimensions. Fluorescence imaging along the axialdimension with nanometre resolution has been highly challenging, but critical to revealingdisplacements in transmembrane events. Here, utilizing a plasmonic ruler based on the sensitivedistance dependence of near-infrared fluorescence enhancement of carbon nanotubes on a goldplasmonic substrate, we probe ~10nm scale transmembrane displacements through changesin nanotube fluorescence intensity, enabling observations of single nanotube endocytosis inthree dimensions. Cellular uptake and transmembrane displacements show clear dependencesto temperature and clathrin assembly on cell membrane, suggesting that the cellular entrymechanism for a nanotube molecule is via clathrin-dependent endocytosis through theformation of clathrin-coated pits on the cell membrane.
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