Page 99 - Organic Electronics in Sensors and Biotechnology

76 Cha pte r T w o

the size dependence of NP properties may be exploited to tune the sen-
sor performance level, through a proper choice of NP morphology and
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structure. Frequently, metal NPs are synthesized in the presence of
organic capping agents, thus giving rise to core-shell structures in which
an inorganic core is surrounded and stabilized by an organic shell; 152
the latter may play a crucial role in driving the final NP properties,
including its sensing performance. The present section deals with FET
devices based on the use of gold core–quaternary ammonium shell NPs.
The use of this transition metal was due to its peculiar reactivity toward
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nitrogen oxide target analytes, and will be discussed in detail in the
next sections. In fact, the active layer of such FET devices is not purely
organic: it is a hybrid nanocomposite film. The analytical results
reported in the following show that the organic capping agent plays a
key role in preserving the catalytic activity of the Au nanophases and
significantly influences the properties of the active layer.
Gold nanoparticles (Au-NPs) have attracted a great interest in the
last decades because of their unique chemical and physical proper-
ties, but also for the wide range of potential applications (optics, 154
sensing, electronics, catalysis, etc.). As catalyst active material,
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Au-NPs have not fulfilled the initial expectations because of their
substantially low catalytic activity due to their completely filled d-band.
Nevertheless, recently, it has been found that a relatively high number
of chemical reactions can be catalyzed by gold structures in the
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nanometer range. In a recent report, the catalytic activity of Au-NPs
of different size was investigated in the reduction of aromatic nitro
compounds. The authors showed the strict relationship between nan-
oparticle size and the reaction rate for a wide range of particle diam-
eters: in particular, the bigger the particles, the slower the reaction. 159
The first study on Au-NPs based gas sensor was reported by Wohltjen
and Snow. They reported that, using a catalytic thin film composed
by metallic NPs stabilized by an organic thiol, a good sensibility in
the detection of toluene, tetrachloroethene, 1-propanol, and water
can be reached. The electrical conductivity of the particle film showed
a strong dependence on the core size and the thickness of the organic
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capping layer. A great number of applications of Au-NPs as sensing
layer have since been published, particularly based on the use of core-
shell NPs stabilized by thiols 160–161 and/or with several different func-
162
tionalities, self-assembled coatings from Au-NPs and dendrimers, 163
gold nanotriangles, etc. A promising feature of Au-NPs based sens-
164
ing devices was their ability to perform a selective detection of target
analytes, the sensitivity of such systems being affected by the particle
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size. Subsequently, other studies were published on further devel-
opments of these concepts, focusing on the introduction of functional
groups in the organic layer in order to tune the sensor toward various
organic vapours. 160, 162, 165–166
In the last years, several types of NO sensors have been devel-
x
oped, most based on changes of conductance of different metal-oxide