Biomedical Engineering Reference
In-Depth Information
become increasingly important to the assembly of surface-confined architectures or pat-
terns via molecular recognition and self-assembly. Furthermore, by means of site-selec-
tive modification (such as CNT tips [94], ends of CNTs [95,96], inner or outer walls [73]),
it is possible to modulate precisely the position of the biomolecule, which in turn enables
control of their architecture and functionality. To enable easy access for biomolecules to
CNTs in the functionalization procedure, it is advantageous to have CNTs solubilized.
12.4.1.1 Solubilization of Carbon Nanotubes
Chemical strategies to solubilize and disperse CNTs in aqueous as well as in organic solvents
overcomes a major technical barrier to their facile manipulation and processing in biological
environments. Different methods, including oxidative acid treatments [97,98], surfactant
[99,100] (such as sodium dodecyl sulphate [101-103], Triton X-100 [104], and tetraoctylammo-
nium bromide[105]) -assisted dispersion and polymer-assisted dispersion, have been widely
used to generate relatively stable CNT suspensions. The judicious selection of different meth-
ods has made it possible to adjust the solubility properties of these CNTs and to direct their
assembly into more complex structure. For example, a concentrated H
2
SO
4
/HNO
3
mixture
has been widely used to generate carboxylic groups on the ends and walls of CNTs via a
refluxing/sonication process [106]. Such heavily oxidized CNTs may then be stabilized in
aqueous suspensions, and various defects in the structure [73,107] can provide sites for cova-
lent coupling of biomaterials (Figure 12.12) through the creation of amide and ester bonds.
Derivatization of CNTs with functional organic groups will also increase their solubility
in organic [110,111] and aqueous solvents. CNTs reacted with an amine-functionalized
D
COOH
COOH
HOOC
R
R
HOOC
c
COOH
COOH
(A)
B
(B)
SWNT
A
(D)
(C)
A
(E)
FIGURE 12.12
Left: Typical defects in an SWNT: (A) five- or seven-membered rings in the C framework, instead of the normal
six-membered ring, leads to a bend in the tube, (B) sp
3
-hybridized defects (R
H and OH), (C) C framework
damaged by oxidative conditions, which leaves a hole lined with -COOH groups, and (D) open end of the
SWNT, terminated with -COOH groups. Besides carboxy termini, the existence of which has been unambigously
demonstrated, other terminal groups such as -NO
2
, OH, H, and
O are possible. Right: Functionalization pos-
sibilities for SWNTs: (A) defect-group functionalization, (B) covalent sidewall functionalization, (C) noncovalent
exohedral functionalization with surfactants, (D) noncovalent exohedral functionalization with polymers, and
(E) endohedral functionalization with, for example, C
60
. For methods B-E, the tubes are drawn in idealized fash-
ion, but defects are found in real situations. (From Hirsch, A. (2002). Functionalization of Single-Walled Carbon
Nanotubes.
Angew. Chem. Int. Ed.,
11, 1853-1859.)