Page 329 - Organic Electronics in Sensors and Biotechnology

306 Cha pte r Ei g h t

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shown ambipolar transport properties in OFETs. Ambipolar trans-
port is also a necessary condition for light-emitting transistors, 30–31
and some of the highly luminescent polymers such as MDMO-PPV
and F8BT have been shown to be not only ambipolar transport mate-
rials but also useful materials for light-emitting devices. 32–33 There is
probably a large number of organic semiconductors with unknown
ambipolar characteristics. One of the latest examples is the function-
alized heptacene as reported by Chun et al. 34
Self-assembled monolayer (SAM) modified gate dielectrics also
influence the morphology and electronic properties, including ambi-
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polar transport of all the aforementioned semiconductors. SAMs
bring an improvement in the charge injection by interposing an
appropriately oriented dipole layer between the contact and the sem-
36
iconductor in OFETs. Some of the commonly used SAMs in OFETs
are shown in Fig. 8.4.

8.1.2 DNA
For practical use of DNA as an electroactive material in organic elec-
tronics, natural DNA, fish waste, e.g., salmon sperm, which is nor-
mally a waste product of the salmon fishing industry, are attractive.
Although there is a wealth of knowledge on the nature of the trans-
port properties on synthetic DNA, in this chapter we focus only on
DNA materials derived from salmon milt. The reader may also note that
there is an ongoing debate on the insulating, 37–42 semiconducting, 43-44
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highly conducting, as well as superconducting nature of transport
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in DNA molecules.
The DNA used for our research in optoelectronic devices was
purified DNA provided by the Chitose Institute of Science and Tech-
47
nology (CIST). The processing steps involved first the isolation of
the DNA from frozen salmon milt and roe sacs through a homogeni-
zation process. It then went through an enzymatic treatment to
degrade the proteins by protease. The resulting freeze-dried purified
DNA has a molecular weight ranging from 500,000 to 8,000,000 Da
with purity as high as 96% and protein content of 1 to 2%. The molec-
ular weight of the DNA provided by CIST is on average greater than
8,000,000 Da. If necessary, the molecular weight of DNA supplied by
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CIST can be tailored and cut using an ultrasonic procedure which
gives rise to a lower molecular weight of 200,000 Da depending on
the sonication energy, as shown in Fig. 8.5. It was found that purified
DNA is soluble only in aqueous media; the resulting films are water-
sensitive and have insufficient mechanical strength, so they are not
compatible with typical fabrication processes used in polymer-based
devices. It has also been observed that many particulates are present
in the DNA films. Therefore, additional processing steps are per-
formed to render DNA more suitable for organic device fabrication
with better film quality. From the knowledge of stoichiometric