Three-dimensional imaging of single nanotube molecule endocytosis on plasmonic substrates.

Investigating the cellular internalization pathways of single molecules or single nano objects is important to understanding cell-matter interactions, and to applications in drug delivery and discovery. Imaging and tracking the motion of single molecules on cell plasma membranes require high spatial resolution in three dimensions. Fluorescence imaging along the axial dimension with nanometre resolution has been highly challenging, but critical to revealing displacements in transmembrane events. Here, utilizing a plasmonic ruler based on the sensitive distance dependence of near-infrared fluorescence enhancement of carbon nanotubes on a gold plasmonic substrate, we probe ~10 nm scale transmembrane displacements through changes in nanotube fluorescence intensity, enabling observations of single nanotube endocytosis in three dimensions. Cellular uptake and transmembrane displacements show clear dependences to temperature and clathrin assembly on cell membrane, suggesting that the cellular entry mechanism for a nanotube molecule is via clathrin-dependent endocytosis through the formation of clathrin-coated pits on the cell membrane.