Biosensors Based on Odorant Binding Proteins - Bioelectronic Nose: Integration of Biotechnology and Nanotechnology - page 169

Fig. 10.2 Clark oxygen
electrode. The reaction
chamber is separated from the
electrodes by a semiperme-
able membrane, which per-
mits oxygen to diffuse from
the reaction buffer into the
potassium chloride solution
that bathes the electrodes: a
platinum cathode and a silver
anode. A voltage is applied
between the electrodes and
the resulting current (in the
µA range) is proportional to
the concentration of oxygen
Fig. 10.3 Randles equivalent
circuit. Impedance measure-
ments are often fitted to the
Randles equivalent circuit,
where R ct is the charge-
transfer resistance, C d is the
differential capacitance, R s is
the solution-phase resistance
and Z w is the Warburg diffu-
sion element
case of catalytic receptor elements, mainly enzymes that convert the target analyte
into a detectable product.
Biosensors based on impedance, can be also classified as electrochemical trans-
ducers. Impedance measurements involve application of a small sinusoidal alternate
current (AC) voltage probe and determination of the current response. Impedance
measurements are often fitted to the Randles equivalent circuit shown in Fig. 10.3 ,
where R ct is the charge-transfer resistance, C d is the differential capacitance, R s is
the solution-phase resistance and Z w is the Warburg diffusion element [ 14 ]. The
Randles circuit describes the Faradaic impedance. In this case, it is necessary to
have a redox species in solution where it is possible to monitor the charge transfer
resistance. Impedance sensors detect a change in one of these equivalent circuit
parameters due to the direct interactions of the target analyte with the probe.
Optical sensors rely on the optical transduction of the signal and comprise ultra-
violet, visible and infrared spectrophotometry in transmission or reflectance modes.
The relationship between the incident light intensity and the transmitted radiation
is given by the Beer-Lambert law. Optical methods have been used classically to
determine analyte concentrations. Properties like absorption, refractive indices,
fluorescence, phosphorescence, chemiluminescence, etc., can be used in order to