Electrochemical sensors based on carbon nanotubes - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 469

Vertically aligned CNTs can also be obtained by using the self-assembly strategy.
To perform self-assembling, the end tips of shortened CNTs fi rst need to be modifi ed
with functional groups (such as ß COOH, ß SH); then the nanotubes are attached to a
substrate (gold, polymer, etc.) through a chemical reaction. AFM images of electrodes
prepared in this way normally show vertically aligned CNTs in bundles. In one exam-
ple, Gooding et al. [47] and Yu et al. [48] modifi ed the gold electrode with vertically
aligned SWNTs through a self-assembling approach. SWNTs were fi rst shortened by
sonicating in a 3:1 v/v solution of concentrated sulfuric acid (98%) and concentrated
nitric acid (70%) for 4 h. The shortened tubes were fi ltered and washed with Milli Q
water to bring the pH to greater than 5. Then the shortened SWNT was dispersed with
a concentration of 0.2 mg ml 1 in dimethylformamide containing 0.5 mg/mL of dicy-
clohexyl carbodiimide (DCC) to convert the carboxyl groups at the ends of the short-
ened SWNT into active carbodiimide esters. The substrate gold electrode which was
previously modifi ed with cysteamine was placed in the nanotube solution for 4 h. The
ß COOH group at the end tip of SWNTs then forms amide bonds with the amines
group of the cysteamine and results in SWNTs vertically aligned to the electrode sur-
face. Microperoxidase MP-11 is then attached to the end tip of CNTs by incubating
in the enzyme solution. The resulting electrode shows a CV with E 1/2 at 390 mV (vs

Ag/AgCl) for the redox reaction of MP-11. In a similar manner, Diao et al. [49] also
prepared vertically aligned CNTs on a gold substrate and examined the electrochemical
properties of the resulting electrode. The procedure is schematically shown in Fig. 15.5,
and Fig. 15.6 is the AFM image of the modifi ed electrode.
Liu et al. [50] have prepared aligned SWNTs by self-assembling nanotubes end
functionalized with thiol groups on a gold substrate. The procedure is schematically
shown in Fig. 15.7. The shortened SWNTs with carboxyl-end terminals formed by
acid oxidation were fi rst functionalized with thiol-containing alkyl amines through
the amide linkage. The self-assembly of aligned SWNTs was obtained by incubating
and ultrasonicating the gold substrate with the thiol-functionalized SWNT suspension.

amide
bond

NH 2 NH 2 NH 2 NH 2 NH 2

DCC in DMF

50C, 12 h

Au

O C C O O CC O

NH 2 NH 2 NH NH NH 2 NH NH NH 2 NH 2

Au

FIGURE 15.5 Schematic representation of the formation of SWNT assemblies. (Reprinted with permis-
sion from [49]. Copyright (2002) Wiley VCH Verlag GmbH & Co. KG.)