Charge Transport and Electrical Switching in Composite Biopolymers - Biomaterials: Physics and Chemistry - page 231

general terms, built up of space charges trapped within the sample due to presence of
defects create a field which is large enough to cause flow of mobile charge carriers. This
phenomenon is sometimes called coulomb blockade (Tang et al., 2005). At high electric
fields, charges are injected by Fowler-Nordheim tunnelling and subsequently trapped. As a
result, electrostatic barrier character of the structure is modified and so is its resistance.
The most insightful switching mechanism in biomolecules is the redox process and the
formation of charge transfer complex through donor-acceptor coupling. Aviram et al. (1988)
suggested that electron- proton motion within hemiquinones molecules that comprised of
catechol and o-quinone, molecules between two contacts switch the molecules to low
impedance (ON) state due to the formation of semiquinones fee radicals. When an electron
is injected into the molecules from the metal contact, it is gained by an electron acceptor
molecule. An electron donor molecule then transfers the electron to the opposite contact
thus allowing flow of charge.
The present chapter discusses structural characteristics (by use of Fourier Transform Infra-Red
spectroscopy and Atomic Force Microscopy). Electrical conduction in cuticular membranes of
Nandi flame ( Spathodea campanulata , P. Beav) seeds hereafter referred just as cuticles. Fig. 1
shows the cuticle also presented still attached to the seed. The cuticles are thin (about 2 µm),
translucent and very light. They are adapted to wind dispersion of the seeds.
4. Structural characterization
4.1 Fourier transform infra Red (FTIR) spectroscopy
Fig.2 shows the Fourier Transform Infra Red (FTIR) spectroscopy of the pristine cuticle. The
samples were first annealed at 350K for 12 hrs before measurement. The wide band at 3348
cm -1 which has been observed in many other cuticular membranes (Bykov, 2008) is assigned
to O-H stretching vibration. It is caused by presence of alcoholic and phenolic hydroxyl
groups involved in hydrogen bonds. Methylene is the most repeated structural unit in the
cutin biopolyester (Jose, et al., 2004) and these shows up in the spectra band around 2300
cm -1 . The band at 2916 cm is assigned to C-H asymmetric and symmetric stretching
vibrations of methoxyl groups. Absorption around 1604 cm -1 and 1427 cm -1 are assigned to
the stretching of C = C bonds and the stretching of benzenoid rings. Absorption bands in the
range 1300-1150 cm -1 are related to asymmetric vibration of C-O-C linkages in ester to esters
or phenolic groups. Fig. 3 show the infra red (IR) spectra of cuticle compared with the
spectra of other biopolymers.
Fig. 1. Thin and translucent cuticle attached to the Nandi flame seed