Biosensors for pesticides - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 40

Antibody immobilization Antigen immobilization Ag-hapten immobilization

Ab Ab-enzyme conjugate Ag Ag-enzyme conjugate Ag-hapten

FIGURE 2.9

Methods for modifi cation of transducer surface in immunosensor development.

agents in a number of immunoassays and immunosensors. Immunoassays have been
widely used in clinic. Its main disadvantage is the need for a long incubation time,
which implies several hours from sample collection to results generation, so it is dif-
fi cult to use this technique in alarm station or process control. Immunosensors are
regarded as devices able to partially circumvent these drawbacks. Immunosensors con-
sist of a bioactive surface (usually an antibody immobilized on a sensing surface) and
a transducer system able to generate a physical signal when the immunochemical reac-
tion takes place [63]. Two competitive enzyme-linked immunosorbent assay (ELISA)
approaches have been followed in the development of immunosensors by immobilizing
either antibody or antigen/hapten. The fi rst is the most common approach (Fig. 2.9)
[64]. The main advantages of this option are the economy of not using expensive anti-
bodies and the reduction of assay steps. The technique of immobilization of the antigen
or hapten, even though less employed, has the advantage that the regeneration process
can be performed without loss of activity of the immobilized reagent. Another impor-
tant issue to be considered in immunosensor development is the choice of immobiliza-
tion support/immobilization method [65]. The immobilization method/matrices used in
immunosensor development are discussed in later sections.
The determination of an analyte at g l 1 levels requires the use of extraction,

cleanup, and preconcentration processes [66]. This is usually accomplished using
organic solvents or combining them with solid-phase extraction. Most offi cial methods
of residue analysis are based on these methodologies employing pure organic solvents
such as methanol, acetone, acetonitrile, or hexane [67, 68]. Hence the biosensor/immu-
nosensor should be stable to perform analysis even in trace amounts of organic sol-
vents. The fl ow immunosystems have proven their stability and compatibility in high
concentrations of organic solvents and also observed improved antigen-antibody bind-
ing [69, 70]. Penalva et al. [70] reported that the performance of monoclonal antibod-
ies is more stable than polyclonal antibodies in organic solvents.

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