handbook of modern sensors

500 17 Chemical Sensors
17.1 Chemical Sensor Characteristics
Most chemical sensors can be described using criteria and characteristics general to all
sensors such as stability, repeatability, linearity, hysteresis, saturation, response time,
and span (see Chapter 2), but two characteristics are unique and meaningful as applied
to chemical detection. Because chemical sensors are used both for identification and
quantification, they need to be both selective and sensitive to a desired target species
in a mixture of chemical species.
Selectivity describes the degree to which a sensor responds to only the desired target
species, with little or no interference from nontarget species. Sensitivity describes
the minimal concentrations and concentration changes (then referred to as resolution)
that can be successfully and repeatedly sensed by a device. Note that for the sensors
described in the previous chapters, the term “sensitivity” is often used as a synonym
of “slope” when the transfer function of a sensor is linear. For the chemical sensors,
sensitivity is the synonym of resolution. This is a characteristic that other sensors,
like pressure and temperature, are rarely concerned with.
Therefore, one of the most important functions in the evaluation of a chemical
sensor’s performance is the qualification of its selectivity. It is common practice to
evaluate the response of a sensor only for increasing the values of activity (concentration)
to the primary target species. This is mainly due to the fact that it is more
convenient to prepare a continuously broad range of test concentrations by adding
increasing amounts of a concentrated (pure) primary species to the background sample
than vice versa. An absolutely selective sensor really does not exist and there is
commonly some interference present.
17.2 Specific Difficulties
The difficulty of developing chemical sensors versus other sensors (such as temperature,
pressure, humidity, etc.) is that chemical reactions change the sensor, often
in a way that is nonreversible. For example, electrochemical cells employing liquid
electrolytes (material that conducts electrical current via charged ions, not electrons)
lose a small amount of electrolytes with each measurement, requiring that the electrolyte
be replenished eventually or have carbonic acid forming at the gate–membrane
interface and etching components in chemical field-effect transistor (FET) sensors.
Also, unlike pressure or temperature sensors which have comparatively few conditions
under which they need to be modeled to operate, chemical sensors are often
exposed to nearly unlimited numbers of chemical combinations. This introduces interference
responses, contamination from acid attack, or, for porous film devices, the
sorption of species that cannot be removed (such as silicone on zirconia sensors),
altering their surface area and effectively changing their calibrated behavior.
For the ceramic bead-type catalytic hydrocarbon sensors, bulk platinum electrodes
and heating elements begin to evaporate at elevated (1000 ◦ C) temperatures, limiting
their life spans and their usefulness for long-term continuous monitoring [1]. This
evaporation rate is even higher in the presence of combustible gases. The loss of the