STABILISATION OF CARBON NANOTUBE SUSPENSIONS - Carbon Nanotubes: From Bench Chemistry to Promising Biomedical Applications - page 4

a b c

Figure 1.3 A schematic representation of how drugs can interact with CNTs. (a) A
bundle of CNTs can act as a porous matrix encapsulating drug molecules between the
grooves of individual CNTs. (b) Drug moieties can be attached to the exterior of a CNT
either by covalent bonding to the CNT wall or by hydrophobic interaction. (c) The
drug can be encapsulated within the internal nanochannel of a CNT. Reproduced from
Foldvari and Bagonluri 10 with permission. See also Colour Insert.

luorescein isothiocyanate (FITC) can also be attached to CNTs for probing
their way to the cell nucleus. CNTs can also act as controlled-release systems
for drugs by releasing the loaded drugs for a long period of time. In this way
CNTs can be used multifunctionally for drug delivery and targeting.

1.2 FUNCTIONALISED CNTS FOR DRUG DELIVERY

From a pharmaceutical perspective, solubility of CNTs in a biological milieu is
essential for biocompatibility, and therefore CNTs must be dispersed before
they are incorporated in therapeutic formulations. CNT dispersions should
also be uniform and stable to ensure that accurate data can be obtained in
vivo .
The main obstacle in the application of CNTs in drug delivery is that
pristine CNTs (non-functionalised) are inherently hydrophobic and hence
have poor solubility in most solvents compatible with the aqueous-based
biological milieu. CNTs also have a tendency to aggregate to form large
bundles which also contribute to their inability to form stable suspensions in
aqueous solutions. 11 The aggregation of the CNTs is a result of van der Waals
(VDW) attractive forces, hydrophobic interactions and π stacking between
individual CNTs. 12 VDW attractions supersede any existing electrostatic or
steric repulsive forces that may render these suspensions thermodynamically
unstable. 13
To overcome this barrier and render CNTs more hydrophilic, CNTs can
be structurally modiied by functionalisation with different functional groups
through adsorption, electrostatic interaction or covalent bonding of different
molecules. 2 The two main approaches adopted for CNT modiication are the
covalent and non-covalent modiication. Covalent modiication is when the