Nano-Bio Integration - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 234

a b

DNA

CNT

Fig. 9.1 ( a ) CNTs that incorporate DNA molecules. ( b ) DNA molecules wrapped around a CNT

phase in potentiometric sensors and electronic collector. Additional examples of the
properties of bionanocomposites can be found in Darder et al. ( 2007 ).
Biomaterials are reversibly tunable conductors. Hybrid structures that integrate
biological molecules and nanosized conductors include carbon nanotubes (CNTs)
that incorporate single-stranded DNA (ssDNA) or double-stranded DNA (dsDNA)
molecules wrapped around CNTs, as illustrated in Fig. 9.1 a, b, respectively. For
example, ssDNA encapsulated inside double-wall CNTs with inner diameters of
about 4 nm modify selectively their properties depending on the base sequence
( Li et al. 2010a ). More precisely, after incorporating a sequence of 30 bases, the
semiconducting ambipolar behavior of pristine double-wall CNTs changes into a
p-type or an n-type characteristic for C or G bases, respectively, while the resulting
structure remains ambipolar after encapsulating sequences of A or T bases, the
gate voltage determining whether electrons or holes transport electric charges.
The ambipolar behaviors of pristine and A- or T-encapsulated double-wall CNTs
are, however, different. The hybrid ssDNA-CNT conductors are stable, and the
change in conduction is caused by charge transfer through stacking from
the CNT to DNA bases, the direction of the charge transfer being determined by
reduction/oxidation potentials of the DNA bases. In particular, C has the lowest
reduction potential, and G has the lowest oxidation potential of all bases.
On the contrary, metal single-wall CNTs can become p-type semiconductors
if an ssDNA molecule is wrapped around them, irrespective of the homo- or
heteropolymeric nature of the base sequence ( Cha et al. 2009 ). The same -stacking
interactions between the side wall of the CNT and DNA bases render these
structures stable, the hydrophilic sugar-phosphate backbone being positioned at the
exterior. The metal-semiconductor transition takes place only in the presence of
water, which screens the negative charges in the phosphate group of DNA. In fact,
the metal-semiconductor transition is reversible at repeated hydration-dehydration
cycles, no such effect being observed for metallic CNTs without DNA wrapping.
The reason is that the phosphate groups of DNA are negatively charged only in water
and not in the dry state, and in the presence of polar water, molecules move closer
to the surface of the single-wall CNT, facilitating a charge transfer between CNT
and DNA. In addition, the wrapped DNA molecule acts as a helical perturbation to
the charge carriers in CNT, the net result of these effects being the opening of an
energy band gap. The width of the gap is estimated to be 30 meV for a (6,6) single-
walled CNT wrapped with adeno monophosphate, case in which the CNT donates