TOXICITY OF CARBON NANOTUBES - Carbon Nanotubes: From Bench Chemistry to Promising Biomedical Applications - page 221

modiications at the surface of SWCNTs reduced or even eliminated the
complement activation, but further investigations are necessary to conirm
this observation.
In our case, both surface charges and functionalisation on the tubes have
been evaluated on the basis of the evidence that surface charges might alter
blood-brain-barrier integrity, with concomitant modulation of toxic effects, 21
although some cationic liposomes exhibited improved cell uptake while
lowering toxicity. 22 No remarkable effects were observed for the presence of
charges on the surface in comparison with uncharged CNTs. Conversely, in
agreement with earlier studies done in this regard, 23 we demonstrated that
CNTs, once functionalised, caused less cell damage. Moreover, it was observed
that toxicity inversely correlated with the length of water-soluble chains
(e.g., polyethylene glycol [PEG]) attached (Fig. 8.1), since PEG decreases the
hydrophobic character of CNTs and promotes their dispersion in aqueous
solvents. Therefore, as the number of ethylene oxide groups increased,
cell viability showed a better correlation with water dispersibility. As
expected, longer PEG-CNTs showed a better proile than shorter tri-ethylene
glycol ones (TEG-CNTs) (this was statistically signiicant, with p < 0.05).
Interestingly, raw (unfunctionalised) SWCNTs displayed higher cell viability
than MWCNTs, but such difference became less obvious after oxidation (not
statistically signiicant, with p > 0.05), so that PEG-MWCNTs displayed similar
cell loss to PEG-SWCNTs ( p > 0.05). Conversely, pristine CNTs, being the most
hydrophobic, had the worst cytotoxic proile. This result is in accordance
with the previous result pertaining to more hydrophobic molecules leading
to greater cell toxicity due to their higher propensity to interact with the
hydrophobic membrane. 24

f -SW CNTs

f -MW CNTs

100

90 100

90

80

80

PEG

70

PEG
TEG
COOH
Pristine

70

60

60

50 TEG

50

40

40

30 COOH

30

20 20

Pristine

10 10

0

0

10 μg/ml 20μg/ml 30μg/ml 10 μg/ml 20μg/ml 30μg/ml

Concentr ation μg/ml

Concentr ation μg/ml

Figure 8.1 Percentage of cell viability of MCF-7 cells after 24 hours exposure to
Pristine, oxidised, TEG and PEG functionalised CNTs ( f- SWCNTs on the left and f-
MWCNTs on the right) at three concentrations: 10 μg/mL, 20 μg/mL and 30μg/
mL. Dose-dependent cytotoxic effect was observed for all the samples ( p < 0.05).
Reproduced from Nayak et al. 7 with permission.