TOXICITY OF CARBON NANOTUBES - Carbon Nanotubes: From Bench Chemistry to Promising Biomedical Applications - page 229

In general, shorter tubes can be easily obtained from controlled acidic
conditions, which are known to cut the tubes mainly in correspondence of the
tips and sidewall defects. The tubes collected at the end of the process present
lengths that are inversely proportional to the oxidising times (Table 8.2). We
conirmed previous data indicating that long sonication time in strong acidic
environment results in excessive cutting, leading to loss of tubular structure
of SWCNTs. 35,36 In fact SWCNTs treated beyond 9 hours resulted in too small
fragments, while MWCNTs preserved their structure for much longer time
(beyond 24 hours).

Table 8.2 Length of different CNTs oxidised by treatment with strong acids for
different time intervals

Time of
Sonication (h)

Length of MWCNTs (nm) Length of

SWCNTs (nm)

0 1,000-2,000 1,000-5,000

3 600-800 800-1,000

6 250-350 150-200

9 150-250 50-100

12 100-150 -

24 50-100 -

Surprisingly, there was no remarkable difference between toxicity of
SWCNTs and MWCNTs, even though their structure was much different after 9
hours. This result suggests that high aspect ratio is not a favourable parameter
in enhancing cell viability, since cells were shown to prefer fragmented tubes
(Fig. 8.2).

b

a

Figure 8.2 Percentage of cell viability of MCF7, HL-60 and U937 cells after 24 hours
exposure to oxidised CNTs (20 μg/mL) for different time periods: 3, 6, 9, 12 and 24
hours. Length-dependent cytotoxic effect was observed for both SWCNTs (A) and
MWCNTs (B). Reproduced from Nayak et al. 7 with permission.