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

860 Part 8: Food Safety and Food Allergens

Table 45.1.The Recognition Elements Commonly

Used in Sensor Systems

1. Antibodies and antibody fragments: Derived by enzyme
   digestion or genetic engineering (Fab fragment, scFv
   and diabody).


2. Lectins, namely carbohydrate-binding proteins. Only
   suitable for the detection of glycosylated entities, such
   as glycoproteins.


3. Enzymes: e.g. those specific for one particular substrate
   (i.e. horseradish peroxidase for hydrogen peroxide).


4. Cell membrane receptors.


5. A living cell: eukaryotic or prokaryotic.


6. Nucleic acid-based probes: DNA and RNA or peptide
   nucleic acid.


7. Aptamers.


8. Chemically generated recognition surfaces: Including
   plastibodies (artificial antibodies or molecularly
   imprinted polymers).

Fab, fragment antigen binding; scFv, single-chain variable fragment.

A transducer is a device, such as a piezoelectric crystal or

photoelectric cell that converts input energy of one form into

output energy of another, with the output signal generated pro-

portional to the concentration of the target analyte (Luong et al.

1995). There are many different types of transducers that can

be used in biosensor-based platforms, and a selection of these is

shown in Table 45.2. Finally, a readout device typically consists

of a computer-linked monitor that presents the data in a form

that can easily be interpreted by the end-user.

Many different biosensor platforms have dedicated software

packages that can facilitate in the interpretation of biomolecu-

lar interactions. For example, Biacore, a frequently used com-

mercial optical biosensor based on surface plasmon resonance

(SPR), which is discussed in more detail later, uses Biaevalua-

tion software that presents data in the form of a sensorgram (Fig.

45.2). Here, interactions between immobilised and free entities

can be easily visualised by monitoring changes in refractive in-

dex (RI). This correlates to a change in mass imparted by the

interaction between the two binding elements (e.g. antibody and

cognate antigen), with units of measurement referred to as re-

sponse units (RU). In Biacore analysis, a response of 1000 RU

is representative of a change in resonance angle of 0.1◦,which

corresponds to an alteration in the surface coverage of the sensor

surface of approximately 1 ng/mm^2.

While Biacore is an excellent example of a biosensor that

is commonly used for the evaluation of quality of food (which

is discussed in more detail later) other biosensor platforms are

also applicable that are based on electrochemical, piezoelectric,

magnetic and thermal detection (Byrne et al. 2009). Many of

these platforms are developed ‘in-house’ and have their own

dedicated software packages for the interpretation of data, and

key examples of these novel sensors are also described later.

Finally, for a biosensor to be applicable in any field, including in

the monitoring of quality of food produce, it must have certain

characteristics. These are listed in Table 45.3.

Biacore

Biacore (GE Healthcare) uses an optical-based transducer sys-

tem for the measurement of analytes based on the principle of

SPR. SPR works on the principle of total internal reflection

(TIR), a phenomenon that occurs at the interface between two

non-absorbing materials, such as water and a solid. When a

source of light is directed at such an interface from a medium

with a higher RI to a medium of lower RI (such as light travelling

through glass and water), the light is refracted to the interface

Table 45.2.Examples of Transducers Commonly Used in Biosensor Systems

Type Example Principle of Use

Electrochemical Conductimetric Solutions containing ions conduct electricity. Depending on the reaction,

the change in conductance is measured.

Potentiometric Measurement of the potential of a cell when there is no current flowing to

determine the concentration of an analyte.

Voltammetric A changing potential is applied to a system and the resulting change in

current is measured.

Field effect

transistor-based

Field effect transistor A current flows along a semi-conductor from a source gate to a drain. A

small change in gate voltage can cause a large variation in the current

from the source to the drain.

Optical Surface plasmon resonance Surface plasmon resonance (a detailed explanation is given in this

chapter).

Thermal Calorimetry Heat exchange is detected by thermistors and related to the rate of a

reaction.

Surface acoustic

wave

Rayleigh surface wave An immobilised sample on the surface of a crystal affects the

transmission of a wave to a detector.

Piezoelectric Electrochemical quartz

crystal microbalance

A vibrating crystal generates current that is affected by a material

adsorbed onto its surface.