Page 100 - Organic Electronics in Sensors and Biotechnology

Or ganic Thin-Film Transistors for Inor ganic Substance Monitoring 77

(SnO , TiO , WO ) film or organic materials during adsorption on
2 2 3
167
NO . In the intense research for the design of innovative gold-based
x
sensors, important efforts were aimed to the detection of NO , due to
x
a particularly strong affinity of NO to a gold surface underlined by
x
Lu and coworkers as a support of the idea of Au sensing layers for
168
NO detection. More recently, Langmuir-Schaeffer layers of thiol-
x
stabilized Au-NPs have been used by Hanwell and coworkers for
NO monitoring at room temperature. The authors pointed out the
169
2
strong influence on the sensor performance of both the particle size
and the composition of outer functional groups.
In 2001, an innovative field-effect gas sensor based on a thermally
evaporated nanostructured Au film was proposed for the detection of
NO . The sensor showed a very low sensitivity to interferents such
170
2
as H and CO and a preferential detection of NO with respect to NO;
2 2
better results have been shown to correspond to thinner gate layers
with smaller Au grain sizes. The high sensitivity of this type of sensor
could be explained by the large surface area arising from the adsorp-
tion on nanometer-size particles. Two studies addressed the effects of
particle size on sensor features such as sensitivity. 165, 171 Baratto and
coworkers have proposed the use of Au-doped microporous silicon
layers for selective and sensitive sensing of NO . As a result, they
x
obtained a device response to NO that was comparable to that to
172
NO . Steffes and coworkers have pointed out the improvement in
2
the sensing properties of In O toward NO by adding finely distrib-
2 3 2
173
uted gold nanoparticles. Langmuir-Schaeffer layers of thiol-stabilized
Au-NPs have proved to be sensitive to NO at concentration levels of
2
0.5 ppm, sensor performance being influenced by the composition of
outer functional groups bound to the thiol stabilizer and the particle
169
size. Noteworthy, recent studies on the NO /Au system demon-
x
strated that a slow response and/or recovery affected the perform-
ance of sensing devices. 169, 170 This evidence can be interpreted in terms
of the peculiar interaction between the NO molecules and the Au-
2
NPs surface, leading to residual polarization phenomena in the
170
active gates. Finally, a device was proposed by assembling a gold
nanostructured film on the top of zinc oxide nanowires. This sensor
was sensitive to both reducing (methanol) and oxidizing (nitrogen
dioxide) gases at high temperature. These examples show that such
174
inorganic-organic hybrid systems enable highly selective detection of
different compounds.
In our laboratory, electrochemically synthesized Au-NPs were used
as active gate materials in FET sensor for the NO monitoring. 175–176
x
2.4.2 New Materials
As mentioned, Au-NPs are of great academic and industrial interest,
due to their possible application in several fields, such as optics, 154
177
157
catalysis, medicine, microelectronics 156 and sensor technology. 155