Biomedical Engineering Reference
In-Depth Information
11.2.1 Electrochemical biosensors
The most common electrochemical biosensors utilise amperometric and
potentiometric transducers,
1
where the detection of the analyte is converted
to a current or voltage, respectively. Immobilised enzymes catalyse reac-
tions in the presence of the analyte being detected. This produces a sub-
stance, such as hydrogen peroxide or NADH, which then generates a
current through oxidation or reduction and the amount of analyte detected
can be quantifi ed. The best example of this is the glucose oxidase enzyme,
which has been most extensively studied for use in glucose biosensors.
Glucose oxidase catalyses a reaction that produces hydrogen peroxide from
glucose, in the presence of oxygen:
2
Glucose
+
O
2
→
gluconic acid
+
H
2
O
2
The original glucose enzyme electrode designed by Clark and Lyons in
1962
3
then used an oxygen electrode to measure the amount of oxygen
that had been consumed by the reaction. It used the fact that the oxygen
concentration in a solution was proportional to the current resultant
from the reduction of oxygen at the cathode. This was the original
glucose enzyme electrode, from which the entire fi eld of biosensors
developed.
Another method of monitoring this reaction is to measure the amount
of hydrogen peroxide produced, by setting up a working electrode with a
positive potential (
+
0.5-
+
0.8 V) against a silver-silver chloride reference
electrode:
1
H
2
O
2
=
>
O
2
+
2H
+
+
2e
−
Working electrodes for these types of biosensors are typically platinum,
gold, carbon and metallised carbon,
1
but the choice depends on a number
of factors, including the reactivity to the substrate being measured and the
requirements of the specifi c application in terms of cost, mechanical proper-
ties and reproducibility. The detection of liberated hydrogen peroxide is
the preferred method of measurement in amperometric enzyme biosen-
sors.
4
Other types of enzyme have been used in enzyme biosensors, includ-
ing glucose dehydrogenase as an alternative in glucose biosensors, but
oxidase-based enzymes are most widely used. Cholesterol biosensors use
cholesterol oxidase or cholesterol esterase,
5
while lactate sensors use mainly
lactate oxidase.
4
The immobilisation of the enzyme on the electrode is critical to the suc-
cessful operation of amperometric-type biosensors. Enzymes are trapped
under dialysis membranes and inside polymer layers of various types.
Cross-linking agents, avidin-biotin binding and carbon composite matrices
are also used for this purpose.
1
These immobilisation techniques must be
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