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 membrane require high spatial resolution in three dimensions (3D). Fluorescence imaging along the axial dimension with nanometer resolution has been highly challenging but critical to revealing displacements in trans-membrane events. Here, utilizing a plasmonic ruler based on the sensitive distance dependence of near-infrared fluorescence enhancement (NIR-FE) of carbon nanotubes on a gold plasmonic substrate, we probe ~10 nm scale trans-membrane displacements through changes in nanotube fluorescence intensity, enabling observations of single nanotube endocytosis in 3D. Cellular uptake and trans-membrane 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 cell membrane.