Biomedical Engineering Reference
In-Depth Information
β-d-glucose induced by enzyme, reduction of FAD to protein-bound carrier FADH
2
(14.1), and oxi-
dation of coenzyme to hydrogen peroxide by molecular oxygen (14.2):
β-d-glucose
+
GOD (FAD)
glucose-δ-lactone
+
GOD (FADH
2
)
(14.1)
GOD (FADH
2
)
+
O
2
GOD (FAD)
+
H
2
O
2
(14.2)
The glucono-δ-lactone generated in Reaction 14.1 can be hydrolyzed to gluconic acid in the
medium of H
2
O in Reaction 14.3:
Glucose-δ-lactone
+
H
2
O
gluconic acid
(14.3)
In the above reactions, GOD (FAD) is the oxidation state of GOD; GOD (FADH
2
) is the
reduction state of GOD. In general, the overall reaction can be expressed as follows in Reac-
tion 14.4:
GOD
H
2
O
2
+
+
+
β-d-glucose
O
2
H
2
O
gluconic acid
(14.4)
Based on Reaction 14.4, there are many methods to prepare biosensors for quantitative detection
of β-d-glucose.
14.1.1.1.1 Amperomet ric Glucose Biosensor
There are three approaches to construct amperometric glucose biosensors. The fi rst approach makes
use of an oxygen electrode to measure the change of dissolved oxygen in the solution and then to
derive the content of glucose in the sample indirectly. The application of this method was limited
because of its disadvantages of low sensitivity in microdetection and fragility to partial pressure of
oxygen in air.
The second approach is mainly used to detect H
2
O
2
as given in Reaction 14.4. It is known that
H
2
O
2
may undergo oxidative electrochemical reaction and produce electrons as in Reaction 14.5:
2e
−
2H
+
H
2
O
2
−
O
2
+
(14.5)
When electrons are transferred to the surface of an electrode, ampere current is generated. Thus
the content of glucose can be determined by the measurement of current. This approach was widely
researched and used since it can be used to detect glucose with high sensitivity at low detectable
concentration. The main drawback of this method is low selectivity because the impressed potential
for H
2
O
2
oxidation is usually high (0.5-0.7 V versus SCE on platinum electrode [8] and 0.9 V versus
saturated calomel electrode (SCE) on glassy carbon electrode (GCE) [9]), so that other electroac-
tive substances that coexisted in the sample, such as ascorbic acid (AA), uric acid (UA), can also be
oxidized and generate oxidative current under such potential to bring interference to the detection.
The third method introduces the electron transfer mediator (Med, including ferrocene, potas-
sium ferricyanide, benzoquinone, quinine, and so on) on the base electrode to produce response
current. The oxidation state of Med (Med
ox
) can oxidize GOD (FADH
2
) generated in Reaction 14.1
while Med itself changes into the reduction state (Med
red
) as given in Reaction 14.6:
GOD (FADH
2
)
+
2Med
ox
GOD (FAD)
+
2Med
red
+
2H
+
(14.6)
Med
red
will be oxidized to Med
ox
on electrode, and oxidative current will be produced subse-
quently as in Reaction 14.7:
2e
−
2Med
red
−
2Med
ox
(14.7 )
