Page 82 - Organic Electronics in Sensors and Biotechnology

Or ganic Thin-Film Transistors for Inor ganic Substance Monitoring 59

offers several advantages, one of which is the improvement of the
reversibility of the adsorption process thanks to the reverse bias
applied. Very recently the potentialities of the OTFT gas sensors
69
have been extended to the chiral discrimination of enantiomeric com-
pounds. A chiral bilayer OTFT gas sensor—comprising an outermost
layer with built-in enantioselective properties—has been demon-
strated to exhibit field-effect amplified sensitivity that allows differ-
ential detection of optical isomers in the tens of ppm concentration
70
range. Moreover, although response enhancement has been already
reported, 50, 67, 71–74 this is the first direct evidence of gate field-induced
sensitivity enhancement. This opens interesting perspectives for the
use of microscopic OTFTs as ultrasensitive electronic transducers that
could also operate as sensing switches.
It can be concluded that organic materials are useful for e-nose
applications for several reasons:

• The carbon backbones of the organic semiconductors make
the sensitive film more chemically active than most of their
inorganic counterparts, thus amplifying the electrical responses
and the sensitivity.
• Organic electronic materials can be readily modified using
synthetic chemistry, allowing their chemical sensitivities to
be controlled by careful design of the organic semiconductors
or by the introduction of selected functional groups.
• Organic molecules are commonly soluble at room tempera-
ture in common solvents. This is especially important for
applications such as the e-noses, where the construction of an
integrated array of different chemical sensors can be made by
printing techniques on plastic substrates.
• Detection limits and sensitivity also benefit from the signal
amplification that is inherent in transistor devices, allowing
transistor-based sensors to outperform chemiresistors even
better than amperometric and potential sensors.

2.3 Anthracene-Based Organic Thin-Film Transistors
as Inorganic Analyte Sensors
In this section, the field-effect and gas sensing properties of the OTFT
devices based on functionalized 9,10-ter-anthrylene-ethynylene oli-
gomers (D3ANT) are presented. The oligomer was characterized by
1 H-NMR, mass spectrometry, IR, UV–vis, photoluminescence (solu-
tion and solid state), and cyclic voltammetry (CV). Moreover, the
morphological and structural characterizations of the molecule have been
investigated by means of AFM, STM, and grazing incidence X-ray dif-
fraction (GIXRD) techniques. D3ANT oligomers have shown good