Food Biochemistry and Food Processing (2 edition)

BLBS102-c45 BLBS102-Simpson March 21, 2012 14:38 Trim: 276mm X 219mm Printer Name: Yet to Come

866 Part 8: Food Safety and Food Allergens

Substrate

Product

Enzyme

e-

Transducer

Output signal

Figure 45.6.Schematic diagram of an enzyme-transducer biosensor. When the redox enzyme goes through its catalytic cycle (going from an

oxidised 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−.

monitoring the quality of agricultural produce. Furthermore, the

possibility of translating these methodologies onto portable mi-

crodevices will permit ‘on-site’ analysis to be performed in a

rapid, reliable and sensitive manner.

Electrochemical Sensors

Electrochemical sensors have also been used extensively to de-

tect analytes of interest in agricultural produce. These platforms

are based on four different transducer types, namely amperomet-

ric, impedimetric, potentiometric and conductimetric (Conroy

et al. 2009, Byrne et al. 2009). The biorecognition element in

these sensors is in direct contact with a transducer, and the re-

sulting signal that is generated is converted from a biochemical

signal to an electrical signal. Similar to optical immunosensors,

antibodies and enzymes are commonly used for biorecognition

purposes. Enzymes that belong to the oxidoreductase class (en-

zyme classification (EC) 1) are frequently selected as they alter-

nate between oxidised and reduced states that can be measured

electrochemically and, therefore, can be exploited in these ana-

lytical devices.

Equation 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. 45.6). 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, such as ferrocene, are used (Fig. 45.7).

e-

Substrate

Product

Transducer

R

R

Electrical signal

Enzyme

Figure 45.7.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 oxidised

and reduced forms, respectively, of an electron mediator.