Electrochemical sensors based on carbon nanotubes - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 487

wavenumbers after the treatment. The change in the radial breathing modes indicates
the covalent functionalization of SWNTs on the sidewall [141].
15.4.1.5 Raman spectroscopy of self-assembled carbon nanotubes
One interesting development in the carbon nanotube-based electrochemical sensor is
the ability to self-assemble the CNT to other types of nano materials such as gold and
silver nanoparticles or to a polymer surface. The enhancement of Raman signals of car-
bon nanotubes through the adsorption on gold or silver substrate has been also reported
[142-146].
With the aid of a bi-functionalized reagent (terminated with pyrenyl unit at one end
and thiol group at the other end), gold nanoparticles were self-assembled onto the sur-
face of solubilized carbon nanotubes [147]. Raman spectrum of the gold nanoparticle
bearing CNTs is enhanced possibly due to charge transfer interactions between nano-
tubes and gold nanoparticles.
Self-assembling of CNTs normally results in vertically aligned CNTs on the substrate
surface. The self-aligned SWNT can be confi rmed by polarized Raman spectra. Diao
et al. [49] reported the fabrication and characterization of chemically assembled SWNTs
(ca-SWNTs), which are constructed by the combination of a self-assembling procedure
and a surface condensation reaction between the ß NH 2 group on the gold substrate and
the ß COOH group on SWNT. Polarized Raman spectra at the cross-section of Au sub-
strate with aligned SWNTs show much stronger signals when the incident light is paral-
lel to the CNT axis than when perpendicular to the nanotube axis.
15.4.1.6 Raman spectroscopy of CNT composites
CNT composites have also been used to develop CNT-based sensors. Raman spectros-
copy has been used to study the CNT-polymer composite concerning the orientation of
nanotubes in polymers and CNT-polymer interactions. Poulin [148] and Frogley [149]
have performed a thorough study of nanotube alignment in polymers using polarized
Raman spectroscopy, and have compared a large amount of experimental results with
existing models. Generally, the interaction between nanotubes and polymers is fea-
tured by peak shifts or broadenings [134, 150].
Conducting polymers have been extensively used to make CNT composite modifi ed
electrode because of their unique electrochemical properties. Zhang et al. [40] studied the
doping level of CNT in PPy using Raman spectra. The Raman spectrum only shows PPy
bands since CNTs were completely coated with the polymer. A strong peak at 1583 cm 1
is assigned to the backbone stretching mode of C ¨ C bonds, and the peaks at 1377 and
1083 cm 1 to the ring stretching and the N-H in-plane deformation of the oxidized
(doped) species of the PPy, respectively. The corresponding band of the neutral species
at 1048 cm 1 is relatively weak indicating a relatively high level of doping of PPy.
Recently, a layer-by-layer (LBL) technique has been introduced into the fabrica-
tion of CNT-modifi ed electrode and received a great deal of interest. He et al. [55]
reported a fabrication of DNA-wrapped carbon nanotubes using the LBL technique.