Carbon
nanotubes (CNTs) have attractive electrical,
optical, thermal, and mechanical properties,
which have been described in detail in reviews
and books [1-4]. Over the last decade, interest
in CNTs has increased dramatically (Fig.
1). However, there are two critical factors
limiting their practical application. One
such factor is the need to mass-produce
CNTs at low cost. At present, catalytic
chemical vapor deposition (CCVD) methods,
particularly the floating catalyst method,
are considered to be the most optimum methods
for large-scale production of CNTs [5,6].
The post-synthesis process including repeated
acid reflux and annealing cycles is generally
used to purify the as-grown CNTs. The other
factor limiting CNTs’ practical application
is their potential toxicity and biological
incompatibility; clear conclusions have
not yet been reached regarding either of
these concerns [7-12]. Although this critical
“CNT safety issue” requires time-consuming
biological studies, some promising studies
have reported effective means to improve
the biocompatibility of CNTs [11,12]. For
example, high-temperature, thermally treated
CNTs have shown excellent biocompatibility
compared with that of as-grown CNTs [11].
Multi-walled carbon nanotubes (MWCNTs) doped
with nitrogen would be more biocompatible
than non-doped MWCNTs or other types of
CNTs, such as single-walled carbon nanotubes
and double-walled carbon nanotubes, suggesting
that MWCNTs could be modified to lower their
toxicity [12].
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