Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

BIOSENSOR-BASED TECHNOLOGIES 429
Product
e −
Enzyme
Output signal
Substrate
Transducer
Fig. 20.7 Schematic diagram of an enzyme transducer biosensor. When the redox enzyme goes through its
catalytic cycle (going from an oxidized to reduced state and back to its resting state), the redox action of the
enzyme is detected by the transducer and the change in electrical state is recorded as a change in the output signal.
An electron is represented by e − .
consist of enzymes, antibodies, and other cellular components. Enzymes that belong to the
oxidoreductase class (enzyme classification EC 1) are commonly selected for use because
they alternate between oxidized and reduced states, which can be measured electrochemically,
and can therefore be exploited in analytical devices. Equation (20.1) illustrates the
generation of an electron through the redox cycling of an enzyme. When the enzyme is located
in close proximity to the surface of the transducer, electron transfer can occur directly
(as shown in Fig. 20.7). However, as is the case with many naturally occurring enzymes,
they are surrounded by a layer of carbohydrate or lipid. This increases the distance that
electrons have to traverse, thus causing a decrease in the signal recorded by the transducer.
In situations where this occurs, electron mediators are used (Fig. 20.8). These mediators are
compounds that are also electroactive, and a common example of an electron mediator is
ferrocene. Table 20.5 illustrates the characteristics that are favorable in choosing a specific
mediator (Cassidy et al., 1998). Equation (20.2) illustrates the reaction of the mediator and
the subsequent generation of an electron resulting from the electrochemical cycling of the
e −
Product
R
Enzyme
Electrical signal
Substrate
R′
Transducer
Fig. 20.8 An enzyme-based electrochemical biosensor with an electron mediator. The mediator shuttles the
electron (e − ) from the enzyme to the surface of the transducer where it is converted from a chemical signal to an
electrical signal. R and R ′ represent the oxidized and reduced forms, respectively, of an electron mediator.