Biomedical Engineering Reference
In-Depth Information
14.2.1.1.4 Mixing Nanotubes with a Binder and Packing Them as a Paste Electrode
Similar to carbon-paste electrodes, matrices with CNTs have recently been one of the focuses in the
fi eld of electrochemical sensors. In the electrochemical study employing CNT-modifi ed electrodes
by Britto et al. [190], nanotubes were dispersed in bromoform as a binder material and packed into
a glass tube. The resultant electrode had randomly distributed tubes without any control over the
alignment of the nanotubes. A variety of binders such as mineral oil [191], Tefl on [192], and epoxy
resins, [193] were explored to produce CNT pastes or composites.
14.2.1.2
Coupling with Biological Molecules
After the modifi cation of electrodes with CNTs, the next key issue for the fabrication of biosensors
is to immobilize biomolecules onto the modifi ed electrodes. The simplest method is noncovalent
immobilization of an enzyme by adsorbing the enzyme physically on the electrode surface modifi ed
with CNTs; one such example has been reported by Trojanowicz et al. for a screen-printed biosensor
with OP hydrolase [194]. Streptavidin was found to adsorb on MWCNTs presumably by hydropho-
bic interactions between the nanotubes and hydrophobic domains of the proteins [195]. Chen et al.
has also reported a simple and general approach to noncovalent functionalization of the sidewalls
of SWCNTs, and subsequent immobilization of various biological molecules onto nanotubes with
a high degree of control and specifi city [196]. Noncovalent functionalization of CNTs has been
reported for selective recognition and binding of target proteins and detection of clinically impor-
tant biomolecules, such as antibodies associated with human autoimmune diseases [197].
Covalent immobilization of enzymes is another method by which enzymes and other redox
proteins can be covalently immobilized to SWCNTs functionalized with carboxylic groups. The
CNTs may be plugged into the enzymes in two ways [198]. In the fi rst strategy, native GOD was
covalently attached to the ends of the aligned tubes that allowed close approach to FAD, and direct
electron transfer was observed with a rate constant of 0.3 s
-
1
. In the second strategy, FAD was
attached to the ends of the tubes and the enzyme reconstituted around the surface-immobilized
FAD. The latter approach allowed more effi cient electron transfer to the FAD with a rate constant
of 9 s
-1
(Figure 14.7).
Glucose is one of the most reported analytes detected through enzyme-CNT electrodes. CNT-
modifi ed gold electrodes described in Ref. 185 have been used to achieve a fast electron-transfer
(i.e., in the case of GOD) between the redox active site of the enzyme, FAD, and the transducing
electrode.
CNTs have also been utilized in the development of electrochemical DNA hybridization bio-
sensors. The application of CNTs in electrochemical DNA biosensors includes two main aspects.
On one hand, using CNTs as a novel substrate not only enables immobilization of DNA molecules
but also serves as a powerful amplifi er to amplify signal transduction event of DNA hybridiza-
tion. On the other hand, CNTs can be employed as a powerful carrier to preconcentrate enzymes
or electroactive molecules for electrochemical sensing of DNA hybridization as a novel indicator
[199]. Various confi gurations of such biosensors have been described in recent years. DNA oligo-
nucleotides can be strongly adsorbed onto the exterior surface of CNTs by nonspecifi c adsorption
[200,201]. Covalent grafting of DNA oligonucleotides onto CNTs plays a more important role in
sensitive and selective DNA biosensor. Aminated or carboxylated DNA oligonucleotides were cova-
lently linked to carboxylated or aminated SWCNT-multilayer fi lms through appropriate coupling
chemistry, respectively [202].
14.2.2 P
OLYMER
14.2.2.1
Conducting Polymer Membrane
Since the discovery of high conductivity in doped polyacetylene in the 1970s [202,203], conduct-
ing polymers have attracted much interest and many of their applications including biosensors have
