Electrochemical sensors based on carbon nanotubes - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 489

90

88

86

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1715

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1574

3426

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1166

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4000 3500 3000 2500 2000 1500 1000 500

Wavenumber/cm 1

FIGURE 15.24 FTIR spectrum of acid-treated MWNT. (Reprinted with permission from [72].
Copyright (2004) Elsevier.)

15.4.2 FTIR of CNT-based sensors
Since FTIR is a powerful tool to identify functional groups, it has been extensively used
to characterize CNT-based sensors which normally require the modifi cation of nano-
tubes. For instance, acid (HNO 3 /H 2 SO 4 ) and ultrasonic treatments, which are common
procedures to purify CNTs, are known to shorten and introduce oxygen-containing
groups (such as the carboxylic group) to CNTs. This can be easily confi rmed by the
FTIR spectrum [72, 154, 155]. The FTIR spectrum of acid-treated MWNTs shows the
peaks at 1715 cm 1 (carboxylic) and 1574 cm 1 (carboxylate). The peaks at 3426 and
1166 cm 1 are attributed to stretching vibrations of ß OH and C ß OH, respectively
(Fig. 15.24).
As mentioned earlier in this section, peroxytrifl uoroacetic acid (PTFAA) can be
used to modify SWNTs with carboxylic groups [56]. FTIR was used to characterize
the modifi ed SWNTs and the results are compared with that from thermal oxidation
(used to remove amorphous carbon) by heating the catalyst-free SWNTs in air. The
thermally oxidized SWNTs show a band at 1744 cm 1 (originated from the formed car-
boxylic groups) in addition to the modes at 1570 cm 1 (in-plane E 1u mode of SWNTs
[156, 157]) and at 1165 cm 1 . Figure 15.25 shows the IR spectra of the SWNTs after
the PTFAA treatment. The band at 1733 cm 1 in the PTFAA-treated sample (Fig.
15.25b) is from carboxylic groups and/or ester groups. The shift of the carboxylic band
from 1744 cm 1 for the air-oxidized sample to 1733 cm 1 for the PTFAA-treated sam-
ple (Fig. 15.25b) is due to the formation of a hydrogen bond between the carboxylic
groups ( ß COOH) suggesting abundance in ß COOH groups after the PTFAA oxida-
tion [158]. The bands at 1660 cm 1 and 1448 cm 1 are assigned to the C ¨ O stretching
mode in quinone groups, while the bands at 1281 cm 1 and 855 cm 1 are the evidence