Biomedical Engineering Reference
In-Depth Information
NADH oxidation and circumvent surface passivation. Musameh et al. prepared the CNT-
coated GC electrode by dispersing the SWCNT or MWCNT in a solution of concentrated
sulfuric acid and casting the resulting solution of 10
L on the GC electrode surface. The
CNT-modified GC electrode exhibited a substantial (490 mV) decrease in the overpoten-
tial of the NADH-oxidation reaction compared to conventional carbon electrodes, and the
oxidation started at
0.05 V (vs. Ag/AgCl). Furthermore, the CNT-modified electrode
showed extremely stable responses to NADH, with 96% of the initial activity remaining
after 60-min stirring in 2
10
4
M NADH solution compared to 20% at the bare GC-elec-
trode surface. These results indicate that the ability of CNT to promote the oxidation of
NADH along with high resistance to the surface fouling provides a useful avenue for the
construction of the CNT-based dehydrogenase biosensors.
Wang et al. [8] reported on the greatly enhanced electrocatalytic activity of CNT toward
the oxidation of hydrogen peroxide. They employed perfluorosulfonated inomer Nafion
to solubilize the single-wall and multiwall CNT. The CNT- or Nafion-modified electrode
offered a significant decrease in the overvoltage for the oxidation of hydrogen peroxide to
allow convenient low-potential amperometric detection. Although a bare GC electrode
exhibits no redox activity for H
2
O
2
over most of the potential range, the CNT-modified
electrode displays significant oxidation and reduction currents starting at around
0.20 V.
This pioneering finding opened the door for the preparation of CNT-based electrochemi-
cal oxidase enzyme biosensors (e.g., glucose biosensor) as shown in the following scheme
(Equations (13.2) and (13.3)).
Glucose
O
2
Gluconolactone
H
2
O
2
(13.2)
H
2
O
2
O
2
2H
+ 2e
(13.3)
The enzyme-liberated hydrogen peroxide is sensitively detected with a CNT-modified
electrode. Therefore, the CNT-modified electrode coimmobilized with oxidase enzyme
(e.g. glucose oxidase) can offer highly sensitive amperometric biosensors.
13.3
Enzymatic Biosensors Based on CNTs
13.3.1
Biosensors Based on the Enhanced Electrocatalytic Activity of CNT
13.3.1.1 Fabrication of CNT-Based Enzymatic Biosensors
As discussed already, CNT-based electrode permits low-potential detection as well as
remarkably enhanced electrochemical activity toward the oxidation of hydrogen peroxide
and NADH compared to the conventional GC electrode. Since there are over 200 dehy-
drogenase and 100 oxidase enzymes, CNT-based electrode offers an effective biosensing of
clinically important analytes (e.g., glucose, lactate, cholesterol, and alcohol) based on the
oxidase or dehydrogenate enzyme reactions. To utilize the remarkable properties of CNT
in such electrochemical biosensing applications, the CNT need to be properly functional-
ized and also immobilized along with enzymes in electrode surface.
The purification of CNT with sulfuric acids creates the carboxylic moieties at the ends
of the CNTs and also partially on sidewalls of CNT surface as defect sites. These carboxylic
groups in CNTs provide sites for covalent linking of CNT to amino groups of biomolecules
(or other mediators) through the carbodiimide-coupling reaction. From the perspectives of
