Page 79 - Organic Electronics in Sensors and Biotechnology

56 Cha pte r T w o

for the quality control of beverages, olive oil, explosives, pathogenic
bacteria, and many other products.
Currently, the commercially available sensors are metal-oxide or
conducting polymer-based chemiresistors and inorganic field-effect
sensors. The most widely used MOS (Metal Oxide Semiconductor)
devices are based on tin oxide resistors doped with different catalytic
metal additives such as platinum or palladium in order to modulate
its sensor response. p-type MOS devices are also available. Such
devices are based on copper oxide, nickel oxide, and cobalt oxide,
which respond to oxidizing odorants like oxygen, nitrous oxide, and
chlorine. The gas analyte is detected by means of its effect on the elec-
trical resistance of a metal-oxide semiconductor active layer.
Modern MOS devices are typically produced through sputtering
processes and are patterned with microfabrication techniques to min-
imize the device size. However, integration of multiple metal oxides,
or even the same metal oxide containing different dopants, onto the
same substrate is difficult and expensive using traditional microfab-
rication techniques, because an extensive subtractive processing is
required, and the production yield quickly drops with each addi-
tional layer of material used.
Generally MOS devices offer the advantages of low cost, a good
level of insensitivity to humidity, and an output signal that is easy to
read and process. Disadvantages, however, include high operating
temperatures, signal drift over time, limited selectivity, high power
consumption, and only modest sensitivity. For these reasons, the use
of organic active layer, such as conductive polymers (CPs) instead of
metal-oxide, is being widely investigated in chemiresistors. The sensor
response is produced when ambient vapors absorb into the polymer,
inducing physical or chemical interactions that change the conducti-
vity of the film. 53, 54 The wide range of organic material that can be
synthesized enables the fabrication of CP chemiresistors with sensi-
tivities over a broad range of organic compounds. CPs are cost-effective
and easily synthesized, and they present fast responses to a large
number of volatile analytes with low power consumption.
Nevertheless, reliability in CP-based chemiresistors is still an
issue. The biggest disadvantage of CP sensors is their sensitivity to
humidity and their susceptibility to poisoning due to irreversible
binding of vapor molecules to the sensing material. 55
Organic thin-film transistors, as discrete elements or implemented
in plastic circuits, are currently the most fascinating technology for
chemical and biological sensing. OTFTs are field-effect devices with
organic or polymer semiconductor thin film as channel materials.
They can act as multiparametric sensors, with remarkable response
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repeatability and as semi-CP-based sensing circuits. 57, 58 As a matter of
fact, transistors based on organic semiconductors are known to
exhibit gas sensor responses, and they may be able to provide a low-
cost replacement for the gas sensor arrays currently used in e-noses.