Biosensor-Based Technologies for the Evaluation of Quality - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 431

e −

Product

R

Gox Enzyme

R ′

Substrate

H 2 O 2

Transducer

Fig. 20.9 A representation of a sensor with two enzymes operating together. The first enzyme catalyzes the
substrate of interest and, as a byproduct, H 2 O 2 is generated. This is then catalyzed by a peroxidase enzyme. The
redox activity of the peroxidase enzyme is detected by the transducer. Gox is glucose oxidase; H 2 O 2 is hydrogen
peroxide; R and R represent the oxidized and reduced forms, respectively, of an electron mediator; and e − is an

electron.

detected by the transducer. This in turn enables the concentration of the target analyte to be
determined.
There are several well-characterized electrochemical biosensor devices that have been
applied in the fruit and vegetable industry for the detection of pesticides, herbicides and
insecticides, organophosphates, organochlorines, and carbamates. There are many different
ways to detect the presence of a pesticide. One of these protocols involves the incorporation
of enzymes, which are directly affected by the presence of the pesticide, into an analytical
platform. Acetylcholinesterase (AChE), an enzyme present in muscles, red blood cells,
and nerve tissue that catalyzes the hydrolysis of the neurotransmitter acetylcholine to yield
choline and acetic acid, is associated with cognition in mammalian hosts. Compounds
that effect AChE activity include organophosphates and parathion. Botulinum toxin, from
the bacterial strain Clostridiumbotulinum , also suppresses the release mechanism of AChE
(Rang et al., 1998). The suppression of activity of this enzyme results in the accumulation of
acetylcholine in the host, which can cause an excessive overstimulation of the cholinergic
nerves. Death usually results from the failure of the respiratory and circulatory systems
(Timbrell, 1991). Schulze et al. (2002) developed an amperometric AChE biosensor that
was used for the detection of several carbamates and organophosphates in a number of
fruit products that were sampled from a variety of countries. The protocol involved the
printing of thick film electrodes onto sheets of polyvinylchloride and “curing” for 30 min at
90 ◦ C prior to the introduction of AChE by glutaraldehyde coupling. The activity of AChE
was analyzed by monitoring the formation of thiocholine by the enzymatic hydrolysis of
acetylcholine chloride. This sensor format permitted the detection of trace levels of these
analytes (lower than 5 g/mL), and the major benefit of this protocol is a reduced analysis

time in comparison to conventional methods.
Other examples of how electrochemical biosensors have been used to monitor the pres-
ence of “indicator molecules” include the following.
Kriz et al. (2002) used a SIRE technology-measuring principle (sensors based on injec-
tion of the recognition element) to monitor L -lactate content in baby food and tomato paste,

μ