Page 66 - Organic Electronics in Sensors and Biotechnology

Scaling Effects in Organic Transistors and Transistor-Based Chemical Sensors 43

(hydrogen acceptor) and significantly increase their interaction with
(including the time that the analyte is held in the grain boundary) and
their percolation depth into the grain boundaries of P3HT.
In an unpublished work, it was demonstrated that the incorpo-
122
ration of receptor molecules not only can improve the sensitivity of
chemical sensing, but also can show different magnitude and rate of
response to the analyte molecules with different alkyl chain lengths
or different polar strengths. This work also indicated that different
receptor molecules differentiate the same group of analyte molecules
in different manners. For example, for some analyte-semiconductor
combinations, the receptor contributing three hydrogen bonds appears
better in sensitivity enhancement than that contributing two hydro-
gen bonds. Furthermore this work showed that the OTFTs modified
with a molecular-cage receptor can make distinctions between the size
of the analyte molecule as well as the molecular position of the func-
tional group (based on rate of change of the response and the extent
of the response change). Devices modified with small molecule recep-
tors showed the similar ability to be refreshed as unmodified OTFT
sensors. The drain current of a receptor-modified OTFT sensor, how-
ever, did not fully recover to the original level, probably due to the
strong and prolonged receptor/analyte binding. These devices also
had lower mobilities than the neat OTFTs, due to the presence of the
relatively unconductive receptors in the grain boundaries. A better
understanding of the relationship between sensing response and
mobility adjustment to receptor quantity, which has not been studied
to date, will alleviate these issues to a large extent. Furthermore, the
added flexibility which receptors offer in tuning the responses of
organic semiconductors to various analytes allows for the selection of
the semiconductors that possess the highest mobility as the most
common host material making up a channel in an array of OTFT
sensors. In spite of these challenges, it is still our prediction that fab-
ricating an array of OTFTs with specific receptor incorporated, in
combination with the circuitry for pattern recognition, could lead to
an electronic nose. Small molecule receptors seem to be a very prom-
ising direction to pursue when attempting to further enhance the
selectivity and sensitivity of the analyte/semiconductor interaction.
These enhancements would reduce the need for full fingerprint pat-
tern recognition and could do so without greatly increasing device
fabrication complexity.

1.3 The Unified Picture of Scaling Behavior of Charge
Transport and Chemical Sensor
The vapor sensing behavior of nanoscale organic transistors is differ-
ent from that of large-scale devices due to the fact that electrical trans-
port in an OTFT depends on its morphological structure and interface
properties, and thereby analyte molecules are able to interact with