Electrochemical sensors based on carbon nanotubes - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 491

FTIR spectroscopy was used to monitor the functionalization process. In the fi rst
scheme, MWNT-P t BA shows a strong C ¨ O stretch (ca. 1730 cm 1 ), characteristic of
carbonyl groups, while only a weak absorption peak at 1730 cm 1 was found in the
spectrum of MWNT-Br. When PtBA was hydrolyzed to PAA, a wide absorption band
assigned to the hydroxyl groups around 3440cm 1 was observed in the FTIR spec-
trum. FTIR spectra of MWNTs modifi ed according to the second scheme show the
aromatic C ß H stretch at 3098 and 1580-1602 cm 1 , the aliphatic C ß H stretch of the
polymer backbones at 2921 and 2848 cm 1 , and the absorption peaks of the O ¨ S ¨ O
stretch at 1208 and 1156 cm 1 . The S ß O and C ß S stretches were observed at 663
and 641 cm 1 , respectively. A strong O ß H stretch was also observed at 3436 cm 1 for
MWNT-PSS due to H-bonding with water. The resulting polyelectrolyte-functionalized
MWNT as well as MWNT-COOH were used as templates for further surface function-
alization by the LBL self-assembly approach on the basis of their high surface charge
densities. Linear poly[2-(N,N-dimethylaminoethyl) methacrylate] (PDMAEMA) and
hyperbranched poly(sulfone amine) (HPSA) were selected as polycations. A strong
band at 1100cm 1 from the C ß O (PDMAEMA) or O ¨ S ¨ O (HPSA) absorption,
confi rmed the assemblage of polyelectrolytes.
In another study, cationic polyethyleneamine (PEI) and anionic citric acid (CA)
as well as heat treatment in NH 3 are also found to modify the nanotube surface and
change the nanotube properties [160]. FTIR spectrum of CA coated CNTs shows
absorption peaks corresponding to stretching vibration of C ¨ O (1724 and 1650 cm 1 ),
and C ß O (1392 and 1218 cm 1 ). The NH 3 -treated and PEI coated CNTs show basic
nitrogen-containing groups (e.g. amine) on the nanotubes, as suggested by the bend-
ing vibration of N ß H (1633, 1581, 1639, and 1581 cm 1 ), and stretching vibration of
C ß N (1093 cm 1 , 1164 cm 1 , and 1114 cm 1 ).
Amide bond is an effective anchor to connect CNTs to substrate surfaces. Lan et al.
[52] covalently assembled shortened multi-walled carbon nanotubes (s-MWNT) on
polyelectrolyte fi lms. The shortened MWNT is functionalized with acyl chloride in
thionyl chloride (SOCl 2 ) before self-assembling. The FTIR spectrum of self-assem-
bled MWNT (SA-MWNT) adsorbed on a CaF 2 plate modifi ed with PEI/(PSS/PEI) 2
shows two characteristic absorption peaks at 1646 cm 1 (amide I bond) and 1524 cm 1
(amide II bond) resulting from the amide bond formed between the polyelectrolyte
fi lms and s-MWNTs.
FTIR has also been used to study how the functional group is attached to the CNTs.
Khara et al. [140] functionalized single-walled carbon nanotubes through a microwave
discharge with ammonia. As it is shown in Fig. 15.26, the N ß H absorption bands at
3343 and 3198 cm 1 are much lower in frequencies than the end-functionalized open
metallic SWNT with CONH-4-C 6 H 4 (CH 2 ) 13 CH 3 showing an N ß H stretching fre-
quency of 3450 cm 1 [161]. The lowered N ß H frequency is indicative of an N ß H
directly attached to the side walls.
CNT-doped conducting polymers possess improved mechanical, chemical, and opti-
cal properties. They also provide a simple strategy for making aligned CNTs. The dis-
appearance of the characteristic peaks of carbon nanotubes in the FTIR spectrum of
polymer/CNT composite fi lms is normally an indication of perfect enwrapping of CNTs
with the deposited conducting polymer [162, 163]. Zhang et al. [40] have studied the