handbook of modern sensors

17.4 Direct Sensors 505
voltage e. To compute conductance, the current in the circuit is measured by the I/V
converter with a reference resistor R.
Hydrogen-gas-sensing chemFETs use a palladium/nickel (Pd/Ni) film as their
gates [7]. The improved, more stable, chemFETs used for liquid sensing employ a
silver/sliver chloride hydrogel (Ag/AgCl) bridge between the silicon dioxide (SiO 2 )
gate and a selective membrane that separates the gate from the analyte (Fig. 17.5). The
selective membrane is commonly polyvinyl chloride (PVC), polyurethane, silicone
rubber, or polystyrene.
For an ion-selective chemFET the gate is replaced by or coated with a chemicalselective
electrolyte or other semiconductor material. If the ion-sensitive material is
ion penetrable, then the device is called a MEMFET, and if the membrane is ion
impenetrable, it is called a SURFET. The chemical-selective gate material alters the
potential at which the device begins to conduct and thus indicates the presence of
specific chemical species. The devices are inherently small and low in power consumption.
The gate coatings for the chemFET can be enzyme membranes (ENFET)
or ion-selective membranes (ISFET). Ion-selective membranes produce a chemical
sensor, and enzyme membranes can produce a biochemical sensor. The enzyme membrane
is made from polyaniline (PANIE) and is, itself, created using a voltammetric
electrochemical process to produce this organic semiconductor.
17.4.3 Electrochemical Sensors
The electrochemical sensors are the most versatile and better developed than any other
chemical sensors. Depending on the operating mode, they are divided into sensors
which measure voltage (potentiometric), those which measure electric current (amperometric),
and those which rely on the measurement of conductivity or resistivity
(conductometric). In all of these methods, special electrodes are used, where either
a chemical reaction takes place or the charge transport is modulated by the reaction.
A fundamental rule of an electrochemical sensor is that it always requires a closed
circuit; that is, an electric current (either dc, or ac) must be able to flow in order to
make a measurement. Because electric current flow essentially requires a closed loop,
the sensor needs at least two electrodes, one of which is called a return electrode.
It should be noted, however, that even if, in the potentiometric sensors, no flow of
current is required for the voltage measurement, the loop still must be closed for the
measurement of voltage.
The electrodes in these sensing systems are often made of catalytic metals such as
platinum or palladium or they can be carbon-coated metals. Electrodes are designed to
have a high surface area to react with as much of the analyte as possible, producing the
largest measurable signal. Electrodes can be treated (modified) to improve their reaction
rates and extend their working life spans. The working electrode (WE) is where
the targeted chemical reactions take place (Fig. 17.6). The electrical signal is measured
with respect to a counter or auxiliary electrode (AE) which is not intended to be
catalytic, and in the case of three-electrode systems, a third reference electrode (RE)
is employed to measure and correct for electrochemical potentials generated by each
electrode and the electrolyte. The third electrode improves operation by correcting
for error introduced by a polarization of the working electrode. Newer electrochem-