Temperature-dependent uptake of SWCNTs measured with confocal Raman spectroscopy. (A) Cells were exposed to SWCNTs for 45 min at either 37°C or 4°C to test for endocytosis, which is an active cellular process. At 37°C, SWCNTs localized within cells; and at 4°C, SWCNTs localized outside cells, as measured by Raman (with 40 × oil objective, data acquired in 5 μm increments). (B) Zooming into the cell (cell outline sketched with a dashed line), we observed a colocalization of SWCNT Raman signal within cells at 37°C versus excluded from cells at 4°C (with 100 × oil objective, data acquired in 1 μm increments). (C) From part C, we integrated the Raman signal over x-y and plotted as a function of z-position within the cell. At 37°C we observed increased Raman signal from -2 to 5 μm, which corresponds to a region within the cell. At 4°C, there was no increase in Raman signal. More than 25 cells were imaged and averaged for each condition.