Sensor Used in E-nose - The Electronic Nose Artificial Olfaction Technology - page 159

Fig. 6.13 Polyacetylene

chain

sensitivities of 0.1 ppm (ppm), but 10-100 ppm is more usual. In general, available
CPs are polyacetylenes, polypyrroles, polythiophenes, polyterthiophenes,
polyanilines, ployfluorines, poly-3-alkylthiophenes, polytetrathiafulvalenes,
polynapthalenes, polyphenylene sulfide, poly-phenylenevinylenes, poly-3,4-ethyl-
enedioxythiophene, polyparaphenylene, polyazulene, polyparaphenylene sulfide,
polycarbazole, and polydiaminonaphthalene.
The active materials used are the polypyrroles, thiophenes, indoles, or furans
(shown in Fig. 6.14 ). The conductivity of these materials changed as they are
exposed to various types of chemicals, which bond with the polymer backbone.
This bonding may be ionic or in various cases covalent. The interaction affects the
transfer of electrons along the polymer chain, that is to say, its conductivity. A
given compound's affinity for a polymer and its effects on the polymer's con-
ductivity are strongly influenced by the counter-ions and functional groups
attached to the polymer backbone. The use of conducting polymer nano com-
posites/nano particles could greatly improve diffusion since they have much
greater exposed surface area and as a result of this the basic characteristics of a
biosensor like low detection limit get enhanced.
Conducting materials change resistance when they come in environment having
chemicals/gases. As soon as the polymer composite sensor is exposed to a vapor of
chemical/gases, some of the vapor permeates into the polymer and causes the
polymer film to expand. The vapor-induced expansion of the polymer composite
causes an increase in the electrical resistance of the polymer composite because
the polymer expansion reduces the number of conducting pathways for charge
carriers [ 19 ] (Fig. 6.15 ).
Advantages:

• It can be prepared both electrochemically and chemically.
• It can be prepared in a range of soluble and insoluble forms.
• It has unique electrical, electronic, magnetic, and optical properties.
• Compliance with micro and nanoscale fabrication.
• Compatibility with diverse range of fabrication techniques such as electro-
chemical, optical, mass-based, etc.
• Biomaterials such as enzymes, antibodies, whole cells, and nucleic acids can be
incorporated into the polymer matrix.
• Strong biomolecular interactions.
• Low detection limits.
• Enhanced sensitivity (when used as a composite material with nanoparticles).