principle, the conductivity of CNT increases by electron withdrawing upon exposure
of electron-withdrawing gases such as oxides of nitrogen, phosphorus trichloride,
sulfur dioxide, oxygen, etc., and the conductivity decreases by electron donation
upon exposure to electron-acceptor gases like hydrogen sulfide, ammonia, hydrazine, methanol, ethanol, etc. Moreover, the main limitation with pristine CNT is its
poor sensitivity for minimum charge transfer with analyte molecules and lack of
selectivity toward target vapor due to the weak specific interactions (Hatchett and
Josowicz 2008).
10.2.2 Organic Nanomaterials as Receptor
The organic materials are made from (or extracted from) plants or animals, and the
nano-dimensional organic chemicals are included in this category. The most common organic nanomaterials are nanosized polymers, small organic molecules, etc.
Organic polymers are the most widely used versatile materials in the globe due to
some serious advantages over the other materials like flexibility, tailorability, processability, environmental stability, low cost, light weight, etc. Polymers are million
times bigger than the normal molecules, i.e., macromolecules due to repetitive union
(mer unit or repeating unit) of a large number of a reactive small molecule in a
regular sequence. The simplest example is polyethylene, where ethylene moiety is
the “mer unit” In general, the properties of polymers depend on their chemical
composition, molecular structure, molecular weight, molecular weight distribution,
and morphology.
Optical sensing principle based on colorimetric, fluorescence, or luminescence
effects and on changes in light refraction/propagation on the polymer surface in
presence of gaseous analyte has been demonstrated (Adhikari and Kar 2010).
However, those optical sensing principles are commonly used for bulk polymer
sensing layer as there is no need to introduce the nano-dimensional polymer in order
to influence those properties.
Semiconducting organic nanomaterials especially the conjugated polymer semiconductor should be introduced as sensing layer using almost similar sensing
principle. As the conducting polymer interacts with gaseous species, the p-type
conjugated polymer donates electrons to the gaseous analyte, and n-type conjugated
polymer accepts electrons from the gaseous analyte. Subsequently, the hole conductivity of conjugated polymer increases when it donates electron to the analyte, and
conductivity decreases when it accepts electron from the analyte. For that reason, the
conductivity of conjugated polymers like polythiophene, polypyrroles, polyaniline,
and their derivatives decreases upon exposure to nucleophilic gases such as hydrogen sulfide, ammonia, hydrazine, methanol, ethanol, etc. On the other hand, the
opposite effect shown by the electrophilic gases like oxides of nitrogen, phosphorus
trichloride, sulfur dioxide, oxygen, etc. having higher electron affinity than the
conducting polymer increases the number of charge carriers in the conducting
polymer. Alternatively, the pollutants can interact with the polymer by some type
356
P. Kar
of electron-withdrawing gases such as oxides of nitrogen, phosphorus trichloride,
sulfur dioxide, oxygen, etc., and the conductivity decreases by electron donation
upon exposure to electron-acceptor gases like hydrogen sulfide, ammonia, hydrazine, methanol, ethanol, etc. Moreover, the main limitation with pristine CNT is its
poor sensitivity for minimum charge transfer with analyte molecules and lack of
selectivity toward target vapor due to the weak specific interactions (Hatchett and
Josowicz 2008).
10.2.2 Organic Nanomaterials as Receptor
The organic materials are made from (or extracted from) plants or animals, and the
nano-dimensional organic chemicals are included in this category. The most common organic nanomaterials are nanosized polymers, small organic molecules, etc.
Organic polymers are the most widely used versatile materials in the globe due to
some serious advantages over the other materials like flexibility, tailorability, processability, environmental stability, low cost, light weight, etc. Polymers are million
times bigger than the normal molecules, i.e., macromolecules due to repetitive union
(mer unit or repeating unit) of a large number of a reactive small molecule in a
regular sequence. The simplest example is polyethylene, where ethylene moiety is
the “mer unit” In general, the properties of polymers depend on their chemical
composition, molecular structure, molecular weight, molecular weight distribution,
and morphology.
Optical sensing principle based on colorimetric, fluorescence, or luminescence
effects and on changes in light refraction/propagation on the polymer surface in
presence of gaseous analyte has been demonstrated (Adhikari and Kar 2010).
However, those optical sensing principles are commonly used for bulk polymer
sensing layer as there is no need to introduce the nano-dimensional polymer in order
to influence those properties.
Semiconducting organic nanomaterials especially the conjugated polymer semiconductor should be introduced as sensing layer using almost similar sensing
principle. As the conducting polymer interacts with gaseous species, the p-type
conjugated polymer donates electrons to the gaseous analyte, and n-type conjugated
polymer accepts electrons from the gaseous analyte. Subsequently, the hole conductivity of conjugated polymer increases when it donates electron to the analyte, and
conductivity decreases when it accepts electron from the analyte. For that reason, the
conductivity of conjugated polymers like polythiophene, polypyrroles, polyaniline,
and their derivatives decreases upon exposure to nucleophilic gases such as hydrogen sulfide, ammonia, hydrazine, methanol, ethanol, etc. On the other hand, the
opposite effect shown by the electrophilic gases like oxides of nitrogen, phosphorus
trichloride, sulfur dioxide, oxygen, etc. having higher electron affinity than the
conducting polymer increases the number of charge carriers in the conducting
polymer. Alternatively, the pollutants can interact with the polymer by some type
356
P. Kar