Reducing the Toxicity of Carbon Nanotubes and Fullerenes Using Surface Modifi cation Strategy - Management of Natural Resources in a Changing Environment - page 178

Toxicology Comparison of CNTs With and Without
Substitution/Modifi cation

It was explained that infl ammatory and granuloma responses were associated
with long straight CNT fi bres, having a structural aspect ratio similar to long fi bre
asbestos (Poland et al. 2008 ; Drew 2009 ). In case of CNTs, size is an important factor
to control toxicity, which can be reduced by reducing their length below 5 m.

Curled- tangled CNT fi bres are considered to be less pathogenic than long straight
CNTs (Poland et al. 2008 ). Long fi bre CNTs have a high potential to be retained in
the pleural membrane and to cause diseases such as mesothelioma (Donaldson et al.
2009 ). Short CNT fi bres (<5

ʼ

m) will transit through the parietal pleural stomata.
However, long CNT fi bres (>15

ʼ

ʼ m) can reach the pleura and cause genotoxicity

and infl ammation.
Unmodifi ed CNTs may be classifi ed as durable and biopersistent than modifi ed
one. Drew ( 2009 ) suggested that all biopersistent CNTs, or aggregates of CNTs, of
pathogenic fi bre dimensions could be considered as potential fi brogenic and meso-
thelioma hazards. As per recent studies, unfunctionalized carbon nanotubes show
potent cytotoxicity toward alveolar macrophages, HacaT cells, and HEK293 cells.
SWCNTs present in the lungs of mice produce respiratory impairment, damage
DNA in the aorta, increase aortic plaques, induce atherosclerotic lesions in arteries
of the heart and retard bacterial clearance. Carcinogenic and fi brogenic behaviour of
MWCNTs in the intraperitoneal cavity of mice has been discussed in vivo studies
(Poland et al. 2008 ). Dutta et al. ( 2007 ) suggested that the uptake of proteins can
be signifi cantly reduced by a coating on the surface of CNTs. It was found that
SWCNT sample became less cytotoxic with sidewall functionalisation of SWCNTs.
This functionalisation was also considerably less cytotoxic than surfactant-stabilised
SWCNTs (Sayes et al. 2006 ).
It is noticeable in Table 3 that functionalisation of CNTs reduces the toxicity in
most of the cases except acid functionalization. Cytotoxic response of human dermal
fi broblast cells was also found to be dependent on the degree of functionalisation of
single-walled carbon nanotubes. Carbon nanotubes modifi ed with suitable biomimetic
polymers are found to be biocompatible while the uncoated carbon nanotubes lead
to cell death. Studies support that functionalization of MWNTs reduces toxicity and
improves biocompatibility of cells in vitro. The excretion rate of SWCNTs after
injection in mice was investigated in a study. Ammonium functionalised SWCNTs
(diam. 1 nm, length 300-1,000 nm) were rapidly excreted through the renal route
(Singh et al. 2006 ), whilst polyethylene glycol functionalised SWCNTs (dimensions
not reported) persisted for 4 months within the liver and spleen in mice (Schipper
et al. 2008 ). It is diffi cult to conclude that the rate of excretion was affected by
functionalisation because of variation in experimental conditions. The other two
studies used functionalised CNTs to investigate different toxicity properties: acid
functionalised SWCNTs showed increased cardiopulmonary toxicity in mice (Tong
et al. 2009 ) and carboxylic functionalised MWCNTs did not affect the immune
responses in human dendritic cells in vitro (Wang et al. 2009 ). The selected toxicity
results have been compared and summarized in Table 3 .