Electrochemical sensors based on carbon nanotubes - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 488

1130

1595

410

612 765

a

1328

1591

b
c

1575

400 800 1200

Raman shift (cm 1 )

a

b

c

d
e

0 400 800 1200 1600 2000

Raman shift (cm 1 )

FIGURE 15.23 Raman spectra of (a) ds-calf thymus DNA/PDDA/SWNTs, (b) PDDA/SWNTs,
(c) oxidized SWNTs, (d) calf thymus DNA, and (e) PDDA. Inset: Magnifi cation of the Raman spectra of
DNA/PDDA/SWNTs, PDDA/SWNTs, and oxidized SWNTs. (Reprinted with permission from [55].
Copyright (2005) American Chemical Society.)

Poly(diallyldimethylammonium) (PDDA), a positively charged polyelectrolyte, and
DNA, a negatively charged counterpart, are alternatively deposited on the water-soluble
oxidized SWNTs. A purifi ed DNA/PDDA/SWNT composite-modifi ed electrode is
used as a DNA sensor. Raman spectroscopy was used to monitor the individual steps
of the LBL fabrication process and to characterize the resulting DNA/PDDA/SWNT
samples [55]. Figure 15.23 shows typical Raman spectra at different steps of the
LBL fabrication process. The disorder-induced D band at 1328 cm 1 and the tangen-
tial stretch G band at 1580 cm 1 are observed for oxidized SWNTs, PDDA/SWNTs,
and DNA/PDDA/SWNTs. The PDDA/SWNTs and DNA/PDDA/SWNTs show tan-
gential G bands upshifted 16 and 22 cm 1 , respectively, relative to that of bare oxi-
dized SWNTs at 1575 cm 1 . Similar upshifts in the tangential mode have also been
reported in SWNTs/polymer composites which indicate the presence of polymer mol-
ecules wrapped around the sidewall of SWNTs [151-152]. The intensities of the typi-
cal disorder-induced D band at 1328 cm 1 for both PDDA/SWNTs and DNA/PDDA/
SWNTs are markedly increased compared to the intensity of the same band in oxi-
dized SWNTs owing to the overlapping Raman features from added layers of PDDA
and/or DNA on oxidized SWNTs. Raman spectra in the range from 200 to 1350 cm 1
also have four very weak Raman scattering peaks at 1130, 765, 615, and 410 cm 1
for DNA/PDDA/SWNTs (inset). These peaks originated from Raman features of both
PDDA and DNA confi rmed the stepwise layer-by-layer preparation of the DNA/
PDDA/SWNT composite.