Sensor Used in E-nose - The Electronic Nose Artificial Olfaction Technology - page 154

(b) Potentiometric: MOSFET
In potentiometric sensors, the zero-current potential (relative to a reference)
developed at a selective membrane or electrode surface in contact with a sample
solution is related to analyte concentration. The main use of potentiometric
transducers in biosensors is as a pH electrode. Biosensors based on ion-selective
electrodes (ISE) and ion-sensitive field effect transistors (ISFET) are defined as
potentiometric biosensors. Potentiometric biosensors primary output signal is
might be due to ions gathered at the ion-selective membrane interface.
(c) Amperometric
The amperometric gas sensor (AGS) was one of the initial sensors to be used in
an E-nose design and has been included in a various sensor array-based instrument.
Amperometry is an old electroanalytical technique that encompasses coulometry,
voltametry and constant potential techniques and is widely used to identify and
quantify electroactive species in liquid and gas phases. Application of amper-
ometry to gas phase analytes involves a unique gas-liquid/solid interfacial trans-
port process. The common characteristic of all AGSs is that measurements are
made by recording the current in the electrochemical cell between the working and
counter electrodes as a function of the analyte concentration. An amperometric
sensor consists of a working, counter, and reference electrodes that are dipped in
an electrolyte. The analyte molecules diffuse into the electrochemical cell and to
the working electrode surface through a porous membrane. Then, the analyte
reacted electrochemically, i.e., oxidized or reduced, and this process, governed by
Faraday's Law, either produces or consumes electrons at the working electrode.
The amperometric class of electrochemical sensor complements the other two
classes of electrochemical sensors, i.e., potentiometric sensors that measure the
null potential at zero current, and conductometric sensors that measure changes in
impedance.
4. Gravimetric
As one of the most important sensing principles, gravimetric sensor systems are
often used for E-noses. The main subgroups are bulk acoustic wave (BAW),
surface acoustic wave or sound acoustic wave (SAW) or flexural plate wave
(FPW), or shear horizontal acoustic plate mode sensors (SH-APM). BAW and SH-
APM are commonly referred as thickness shear mode (TSM) or quartz crystal
microbalance (QMB or QCM) sensors.
Gravimetric odor sensors using acoustic wave devices which are operated by
detecting the effect of adsorbed molecules on the propagation of acoustic wave
have also been investigated for application to an E-nose [ 16 ]. Figure 6.10 shows
one type of gravimetric odor sensor used in E-nose system. Table 6.3 describes
various types of E-nose sensors and material used in manufacturing of the sensor
and operating principle of the same.