Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

BIOSENSOR-BASED TECHNOLOGIES 421
Table 20.2
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 semiconductor 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 SPR Surface plasmon resonance (a detailed explanation is
given in the chapter)
Thermal Calorimetry Heat exchange is detected by thermistors and related to
the rate of a reaction
Surface acoustic wave Rayleigh surface wave An immobilized 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
the change in signal on an analytical chip surface over time resulting from the interaction
between bound and mobile entities. A sample sensorgram for Biacore is illustrated in Fig.
20.2. The units of measurement used in the sensorgram are referred to as response or resonance
units (RU). A response of 1,000 RU represents a change in the resonance angle of
0.1 ◦ , and is equal to a change in the surface coverage of the chip of ∼1 ng/mm 2 .
The ideal characteristics of a biosensor are listed in Table 20.3. There is immense
potential to broaden the scope and use of biosensors in many fields, such as in the monitoring
of quality of postharvest fruit and vegetable produce. A review of relevant sensor systems,
including the available commercial SPR instruments, was compiled by Leonard et al. (2003).
There are many other SPR-based instruments in the market, such as the Spreeta system. This
instrument is more cost-effective than other biosensors, but, as reported in the literature,
it is less sensitive when used with real samples compared to larger instruments (Setford
et al., 1999) such as Biacore. Biacore uses an optical-based transducer system for the
measurement of analytes based on the principle of surface plasmon resonance (SPR). SPR
works on the principle of total internal reflection (TIR), a phenomenon that occurs at the
interface between two nonabsorbing materials such as water and a solid. When a source of
light is directed at such an interface from a medium with a higher refractive index (RI) to a
medium of lower RI (such as light traveling through glass and water), the light is refracted to
the interface (Markey, 2000). When the light is above a particular angle of incidence, no light
is refracted across the interface and TIR occurs. Even though the incident light is reflected
back from the interface, an electromagnetic field (called an evanescent wave) penetrates
a distance of the order of one wavelength traveling into the less optically dense medium.
The Biacoresystem has a limit of detection of ∼10 RU (which is ∼10 pg/mm 2 ) and has
an operating temperature range of 4–40 ◦ C for the Biacore 2000 model (Hashimoto, 2000).
Additional instruments, such as the Biacore Q, have major potential for quality control