Biomedical Engineering Reference
In-Depth Information
13.2
Electrochemical Behavior of CNT-Modified Electrodes
The first preliminary study of the possible applications of CNTs as electrode materials has
been reported by Britto et al. [9] in 1996. They used MWCNT for the oxidation of dopamine
(an important neurotransmitter). An MWCNT-modified electrode was constructed using
bromoform as binder, and the oxidative behavior of dopamine was examined at this elec-
trode. The cyclic voltammetric study in PBS (pH 7.4) at an MWCNT-modified electrode
revealed a characteristic two-electron oxidation of dopamine to dopaminequinone with
ideal reversibility. These results obtained in the MWCNT-modified electrode were superior
to those observed at other ordinary carbon electrodes. They also studied the electrochemi-
cal oxidation of dopamine at a CNT electrode after treating the electrode with goat's brain
homogenate. The cyclic voltammetric behavior of dopamine showed the same features as
the untreated electrode. It demonstrated the potential applications of CNT electrodes for
the determination of dopamine in vitro and in vivo.
Following the demonstration of the enhanced electroactivity of CNT-modified electrode
toward dopamine oxidation, Davis et al. [10] have examined the electrochemical proper-
ties of MWCNT-modified electrodes immobilized with redox proteins. Closed CNTs, pro-
duced by arc vaporization, were opened by treatment with either nitric acid or a
combination of nitric acid and potassium permanganate. The opened CNTs were packed
into a glass capillary in mineral oil, deionized water, nujol, or bromoform. Redox proteins
were immobilized on and within these CNTs. Well-behaved and reproducible voltammet-
ric responses were obtained with both cychrome
c
and a “blue” copper protein, azurin.
These good characteristics were mediated by hydrogen bonding interactions between
protein surface lysine and negatively charged carboxylate and phenolate of the electrode.
These results demonstrated the possible application of CNTs as biosensing devices.
Compared to the MWCNT, SWCNT is a well-defined system in terms of electronic prop-
erties. Individual SWCNT can be regarded as quantum wires. Luo et al. [11] investigated
the electrochemical and electrocatalytic behavior of SWCNT-modified glassy carbon (GC)
electrode. The carboxylic acid groups were introduced at the open ends of the SWCNT.
The nitric-acid-purified SWCNT solution was cast on a GC electrode to form a CNT-mod-
ified electrode. The CNT-modified electrode showed very stable cyclic voltammetric
behavior and favorable electrocatalytic activity toward the oxidation of biomolecules such
as dopamine and epinephrine. These results also suggested that the SWCNT-modified
electrode could be used for the construction of electrochemical biosensors to study the
electrochemistry of biosystems.
Toward the use of CNT for the construction of electrochemical biosensors, low-potential
NADH detection at the CNT-modified electrode was first reported by Musameh et al. [7]
in 2002.
-NADH is involved as a cofactor in several hundred enzymatic reactions of
NAD
+
/NADH-dependent dehydrogenase systems as shown in the scheme below in
Equation (13.1) (e.g., alcohol dehydrogenase).
ADH
Ethanol
NAD
Acetaldehyde
NADH
(13.1)
The NADH produced from the enzymatic reaction is electrochemically detected.
Problems associated with the anodic detection of NADH are the large overpotential
required for its oxidation at ordinary carbon electrode and surface fouling associated with
the accumulation of the reaction products. Therefore, considerable efforts are directed to
the development of new electrode materials that will reduce the overpotential for the
