Chemistry Reference
In-Depth Information
More precisely, in each single nanotube it is possible to distinguish two regions:
the tips and the sidewalls. The tips resemble a hemisphere, the reason why they are
expected to undergo typical fullerene reactions with comparable reactivity. On the
other hand, the side walls are characterized by carbon atoms presenting lower
pyramidalization angles and therefore exhibiting different chemical behavior.
The way the tube is wrapped also influences the chemical reactivity, rendering
metallic CNTs more reactive than the semiconductor analogues as a consequence of
slightly less aromaticity of the former [
52
].
The diameter is another important element affecting reactivity: smaller tubes
present more enhanced pyramidalization angles and orbital misalignment, being
therefore more susceptible to chemical modification.
Finally, the presence of structural defects in the hexagonal pattern plays a
fundamental role in the covalent attachment of molecules. Indeed, the insertion of
the pentagon-heptagon pair significantly increases the local curvature of the lattice,
creating preferential points for addition reactions.
4.1.1 Derivatization of CNTs Following Oxidation
Oxidation of CNTs has been typically used to remove the traces of metallic
catalysts employed in the fabrication process. The purification process occurs in
strong acidic and oxidative conditions, such as mixtures of concentrated acids
combined with heating or sonication, and affords shortened open tubes decorated
with oxygenated functions, predominant on the tips [
53
]. A completely different
way to produce short and open tubes has been described by our groups using a
planetary mill in solvent-free conditions. The process avoids the use of strongly
acidic conditions and produces highly dispersible SWCNTs that can be efficiently
functionalized in a variety of synthetic ways [
54
].
Among the groups introduced on the tubular grid (carbonyl, carboxyl, hydroxyl,
etc.), carboxylic functions are particularly exploited as anchor points for amidation
and esterification reactions that are easily carried out by means of acyl chloride
intermediates or carbodiimide derivatives, commonly 1-ethyl-3-(3-dimethylami-
nopropyl)carbodiimide (EDC) and hydroxybenzotriazole (HOBt) (Fig.
15
).
Several examples of amidation reactions on shortened tubes are reported in the
literature, leading to highly soluble and dispersible derivatives suitable for a great
number of applications. Haddon and co-workers were pioneers of this approach,
since they first exploited amidation of the carboxylic functions to conjugate
nanotubes with several solubilizing agents [
55
]. Among these, the conjugation of
SWCNTs with poly(aminobenzene sulfonic acid) (PABS) and polyethylene glycol
(PEG) to form water-soluble graft copolymer was particularly successful, yielding
highly-loaded SWCNTs (30% of PABS and 71% of PEG, estimated by thermogra-
vimetric analysis) which resulted in a final water solubility of 5 mg/mL [
56
]
(Fig.
16
).
The general synthetic pathway introduced by Hamon et al., involving acylation
of the carboxylic groups via an acyl chloride intermediate, was followed by several
