"The effect of nitrogen-doped multiwall carbon nanotubes (CNx) on the proliferation of NIH-3T3murine fibroblasts is presented. CNTs were dispersed in distillated water and incubated with mammalian cells in order to evaluate their toxicity. Also, the influence of factors such as dosage (7 and 70 mu g/mL), exposure time (24 to 96 h), and the exposure route (before and after cell liftoff) on the cell proliferation was evaluated. When the CNx were simultaneously incubated with the cells, the control culture reached a maximum cell concentration of 1.3 x 10(5) +/- 3.4 x 10(4) cells per well at 96 h, whereas cultures with 7 mu g/mL reached a concentration of 2.6 x 10(4) +/- 5.3 x 10(3) cells. In the case of 70 mu g/mL of CNx most of the cells were dead. The CNx that were added 24 h after cell dissociation showed that live cells decreased, with a cell concentration of 9.6 x 10(4) +/- 9 x 10(3) for 7 mu g/mL and 5.5 x 10(4) +/- 9.5 x 10(3) for 70 mu g/mL, in contrast to control cultures with 1.1 x 10(6) +/- 1.5 x 10(4). The results showed that the CNx had cytotoxic effects depending on the concentration and exposure route.
