Biomedical Engineering Reference
In-Depth Information
9.4.1.1 Amperometric Transducers
Voltammetric biosensors are based on measuring a current with a voltage change.
The subtechnique in voltammetry is the amperometric biosensors where the current
at a fixed potential gives a steady value with time. In the amperometric category,
a biosensing element is typically coupled to an amperometric electrode and as the
biosensing element reacts with the substrate, a current is produced that is correlated
to the analyte concentration. The electrode potential is used to drive an interfacial
redox reaction and the current resulting from that reaction is measured. The current
flowing is directly proportional to the analyte concentration.
Amperometric transducers are more versatile than potentiometric devices.
Amperometric detection is based on measuring the oxidation or reduction of an
electroactive compound at the working electrode (sensor). The rate of the analyte
reaction is monitored by the variation of the current. The measured signal is a
variation in the current, at constant potential, depending on the variation in the
reactive species concentration: the relation is linear. In continuous operation there
is a risk of surface poisoning; nonsteady-state measurements are preferred to avoid
this problem. Sensitivity is greater for an amperometric sensor (LOD of about 10
−8
M compared with 10
−6
M for a potentiometric device). Typically, Ag/AgCl or a
saturated calomel electrode (SCE) is used as the reference electrode so that revers-
ible oxidation/reduction occurs at a fixed potential at the reference electrode. The
applied potential is an electrochemical driving force that causes the oxidation or
reduction reaction. With large Cl
−
concentration, the Ag/AgCl reaction produces a
stable potential. According to the Faraday's law of electrochemistry, the amount of
substance consumed or produced at one of the electrodes in an electrolytic cell is
directly proportional to the amount of electricity that passes through the cell. The
current response,
I
,
dn
I F
d
⎛⎞
=
(9.6)
⎜
⎝⎠
where
F
is Faraday's constant,
z
is the valency of the reagent, and
dn/dt
is the oxida-
tion or reduction rate [mols]. Equation (9.4) can also be derived from (3.17). The
reaction rate depends on both the rate of electron transfer at the electrode surface
and analyte mass transport.
EXAMPLE 9.4
A group of researchers are trying to develop an electrochemical biosensor with a require-
ment of detecting a constant current of 50 mA for 1 hour. If the valency of the analyte is
+2, calculate the minimum number of moles of analyte required to be present.
Solution: Rearranging (9.6),
1
dk
Idt
=
zF
Integrating over the time period,
