Page 206 - Analytical Electrochemistry 2d Ed - Jospeh Wang

6-3 SOLID-STATE DEVICES 191

Periodic calibration is desired for addressing slow drifts. This is usually accom-
plished by exposure to samples with known oxygen content, for example, with air
assumed 20.93% O . The response lime of the electrode is generally larger when
2
changing from a high P to a low P , compared with a change in the opposite
O 2 O 2
direction.
Membraneless oxygen sensors based on solid-state technology have also been
reported. For example, coverage of a Y 2 O 3 -doped ZrO 2 disk with porous platinum
electrodes results in a selective sensor, based on the coupling of the oxygen
reduction process and the preferential transport of the oxide ion product through
vacancies in the doped crystal (63). For this purpose, one of the platinum electrodes
is exposed to the unknown gas while the second one is exposed to the reference gas.
Such potentiometric sensors commonly operate at high temperatures, and are widely
used in the automotive industry for controlling the ratio of air=fuel (with an annual
worldwide market exceeding 150 million dollars).
Other useful gas sensors include the potentiometric ammonia (64) or hydrogen
cyanide probes (65), and amperometric carbon monoxide (66) and nitrogen dioxide
(67) devices. The hydrogen cyanide probe is an example of a modern device that
relies on changes in the conductivity of electropolymerized ®lm (polyaniline) in the
presence of a given gas.

6-3 SOLID-STATE DEVICES

The integration of chemically sensitive membranes with solid-state electronics has
led to the evolution of miniaturized, mass-produced potentiometric probes known as
ion-selective ®eld-effect transistors (ISFETs). The development of ISFETs is a
logical extension of coated-wire electrodes (described in Section 5-2.4). The
construction of ISFETs is based on the technology used to fabricate microelectronic
chips. Ion-selective ®eld-effect transistors incorporate the ion-sensing membrane
directly on the gate area of a ®eld-effect transistor (FET) (Figure 6-19). The FET is a
solid-state device that exhibits high input impedance and low output impedance and
is therefore capable of monitoring charge build-up on the ion-sensing membrane. As
the charge density on this membrane changes because of interaction with the ions in
solution, a drain current ¯ows between the source and drain of the transistor. The
increased voltage needed to bring the current back to its initial value represents the
response. (This is commonly accomplished by placing the ISFET in a feedback
loop.) From the standpoint of change in drain current as a result of change in activity
of the ion of interest, the ISFET response is governed by the same Nernstian
relationship (and the selectivity limitation) that characterizes conventional ion-
selective electrodes.
Such sensors utilizing solid-state electronics have signi®cant advantages. The
actual sensing area is very small. Hence, a single miniaturized solid-state chip could
contain multiple gates and be used to sense several ions simultaneously. Other
advantages include the in-situ impedance transformation and the ability for
temperature and noise compensation. While the concept of the ISFET is very