of the metabolites are essential biomolecules critical for the growth and survival of the

biological system known as primary metabolites. The most important primary metabolite

produced by plants is glucose. In animals, anabolic processes may generate primary me­

tabolites such as peptides, chemical messengers, nucleic acids, and hormones. On the other

hand, secondary metabolites are not critical for the growth and survival of the biological

system. They may be produced in response to a particular condition. These metabolites may

be a by-product of other metabolic activities or storage biomolecules. Secondary metabo­

lites are most abundant in plants.

Over the years, the application of metabolites in diagnostic tools is increasing.

Metabolites are used as biomarkers for various diseases that affect humans; thus, their

detection and quantification are important. The detection of metabolites is also vital in the

food industry, forensic investigation, and drug discovery. Several methods have been

reported for detecting metabolites, such as nuclear magnetic resonance spectroscopy,

mass spectrometry, and liquid chromatography coupled with mass spectrometry.

Methods such as immunohistochemistry, enzyme-linked immunosorbent assays (ELISA),

and immunochromatography are applicable for the detection of some biomarkers. This

chapter focuses on biosensors using conducting polymers or a composite consisting of

conducting polymers as one of the constituents.

Conducting polymers are organic polymers with unique optical and electrical char­

acteristics composed of conjugated carbon chains of alternating single and double bonds.

The highly delocalized, polarized, and electron-dense bonds account for the optical and

electrical behavior of conducting polymers. The common examples of conducting poly­

mers that have gained lots of interest due to their unique properties include polyaniline

(PANi), polypyrrole (PPy), polyacetylene (PA), polythiophene (PTH), poly(3,4-ethylene

dioxythiophene), and polyfuran. Conducting polymers are ranked among the most used

materials to modify the surface of the working electrodes in electrochemical sensors. This

is due to their unique properties, such as high electrocatalytic activity, flexibility, scal­

ability, corrosion resistance, and they can be custom-made for a particular need. They also

have improved mechanical strength, biocompatibility, and environmental stability.

19.2 Classification of Conducting Polymers

19.2.1 Intrinsically and Extrinsically Conducting Polymers

Conducting polymers can be classified as intrinsically or extrinsically polymers

(Figure 19.1). Intrinsically conducting polymers are polymers that have a backbone made

up of a conjugated system; the conjugated system is responsible for the conductance of

the polymer. Intrinsically conducting polymers can be further grouped into polymers

with π-electron backbone and doped conducting polymer. Conducting polymers with a

conjugated π-electron backbone are defined as types of conducting polymers that contain

conjugated π-electron backbone, which is responsible for their electrical properties. This is

due to the double bonds and lone pair of electrons present. Under the influence of an

electrical field, the conjugated π-electrons of the polymer become excited and then

transported through the polymer. Moreover, due to the overlapping of conjugated pi-

electrons, valance and conduction bands develop throughout the polymer’s backbone,

contributing to the conductivity of the polymer.

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Bioelectronics