Page 78 - Organic Electronics in Sensors and Biotechnology

Or ganic Thin-Film Transistors for Inor ganic Substance Monitoring 55

efforts have been devoted to the development of chemical sensor
technologies with the aim of overcoming the limitations of the “non-
real-time” analytical methods. Ease of implementing in portable sys-
tems and the online monitoring of the inorganic volatile analytes are
the main claimed advantages of the different sensor categories.
Nowadays, only few chemical sensor-based breath detectors
(electrochemical transducers) are used to monitor the concentration
of NO and CO while no portable systems for H S in exhaled breath
2
are known (H S is usually measured via blood plasma analysis in the
2
laboratory). Unfortunately, these detectors do not allow the simulta-
neous monitoring of NO, CO, and H S. The detection of such gases at
2
the same time in human breath could give a more accurate assess-
ment of inflammation and oxidative stress status of the patients.
The monitoring of inorganic substances (NO , CO, and H S) in
x 2
the exhaled and inhaled air is therefore a very interdisciplinary field
of research, and the development of stable, low-cost, and portable
detectors is still a challenge. A wide range of sensors have been inves-
tigated in order to fulfill these needs, and although some of them are
sensitive and reliable, they generally suffer from low selectivity and
stability, or often require high working temperature. Examples of
such sensors are potentiometric 35–37 and amperometric sensors, 38, 39
metal-oxide sensors (TiO , SnO , ZnO, WO ), 40–47 and MOSFET-type
2 2 3
sensors. 48, 49
In this area, organic thin-film transistors (OTFTs) are a promising
alternative since they can offer reliable and reversible “analytical signal”
upon exposure to the target analyte, at room temperature. Besides,
OTFTs can be miniaturized and give quite selective responses at ppm-
ppb concentration level through the suitable choice of the sensitive
layer, allowing the detection of a wide range of organic and inorganic
compounds.

2.2 OTFT-Based Sensors: A Bird’s-Eye View
Despite an impressive list of available analytical technologies, design-
ing an inexpensive handheld or household sensor-based instrument
is still an open challenge for the worldwide scientific community. In
addition to sensitivity, selectivity, and robustness, low power con-
sumption and compact size are stringent requirements that sensor
devices have yet to fulfil. 50
Recognition of complex odors is being addressed, so far, by sensor
array-based systems called electronic noses (e-noses) that attempt to
mimic the mammalian olfactory system. 51, 52 The e-noses commer-
cially available today, however, remain well outside of the feasible
range for consumer products. The introduction of e-noses into the
consumer market could enable a plethora of applications ranging from
medical self-diagnosis kits to indoor air quality monitoring. Moreover,
at the industrial level, sensor arrays could be integrated into devices