handbook of modern sensors

520 17 Chemical Sensors
Fig. 17.15. Schematic diagram of an enzyme sensor.
17.5.6 Enzyme Sensors
One of the most efficient ways of achieving selectivity is by using sensors with
enzymatic layers. Enzymes are a special kind of catalyst—proteins of molecular
weight 6–4000 kDa found in living organisms. They have two remarkable properties:
(1) They are extremely selective to a given substrate and (2) they are extraordinarily
effective in increasing the rate of reactions. Therefore, they favorably contribute to
both the selectivity and the magnitude of the output signal. The maximum velocity
of the reaction is proportional to the concentration of the enzyme. A general diagram
of an enzymatic sensor is shown in Fig. 17.15 [17].
The sensing element can be a heated probe, an electrochemical sensor, or an optical
sensor. Enzymes operate only in an aqueous environment, so they are incorporated into
immobilization matrices which are gels—specifically, hydrogels. The basic operating
principle is as follows.An enzyme (a catalyst) is immobilized inside a layer into which
the substrate diffuses. Hence, it reacts with the substrate and the product is diffused
out of the layer into the sample solution. Any other species which participates in the
reaction must also diffuse in and out of the layer.
17.6 Chemical Sensors Versus Instruments
Because of the complexity of operation and numerous influences, a chemical sensor
is rarely used alone, but, rather, it is a key part of a more rigorous chemical detection
instrument. An instrument often combines sensor measurement hardware with
decision-making and control software (instructions). Most instruments employ some
form of feedback to adjust the operation based on actual conditions versus desired
conditions. Some instruments and microinstruments include components to perform
mechanical actions (e.g., pumping, filtration, and separation).
Instruments such as gas chromatographs, mass spectrometers, IR spectrometers,
and others provide the most comprehensive chemical analysis, especially when compared
to simple individual gas sensors. These instruments contain sensors calibrated
to perform a specific type of measurement or analysis as well as support circuits and
signal processing to control, minimize, and compensate for chemical signal drift and
other operation-induced errors.
Liquid and gas chromatography (LC and GC, respectively) are effective and popular
chemical analysis methods. Chromatography involves injecting a liquid or gas