Credit: © 2007 Nature Nanotechnology

Single-walled carbon nanotubes (SWNTs) have been shown to be toxic to many types of cells. However, it has not been possible to directly observe how carbon nanotubes enter cells and cause damage because imaging them in an already carbon-rich cell is challenging. Using a combination of electron microscopy techniques, researchers at the University of Cambridge in the UK now show, for the first time, the entry and fate of SWNTs in the cell.

Alexandra Porter and colleagues1 exposed macrophages — cells of the immune system — to SWNTs at a concentration of 5 mg ml-1 for up to four days. Traditionally, cells are stained with heavy metals for contrast but this would obscure the SWNT. Using low-loss, energy-filtered transmission electron microscopy (EFTEM), Porter and co-workers improved the contrast between SWNTs and cellular structures in unstained specimens. SWNTs were seen to fuse with the cell membrane before entering the cell in vesicles and finally penetrating the membrane of the nucleus and localizing there. Over time, features characteristic of cell death were observed.

Whereas traditional cytotoxicity assays measure cell viability in a large population of cells, EFTEM imaging complements these assays by clearly defining the ultrastructural characteristics of cell death in a single cell.