Nano-Bio Integration - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 235

about 0:2e to the biological molecule, the resulting local electric field breaking the
cylindrical symmetry of the structure.
Even the transport properties of semiconducting single-walled CNTs are modi-
fied when ssDNA wraps around them ( Li et al. 2010b ). At low temperatures, up to
80 K, the hybrid material displayed single-electron tunneling features explained by
the formation of a series of quantum dots by wrapping with an ssDNA consisting of
a sequence of 30 C bases. The size of the quantum dots is in the 25-30 nm range, and
their Coulomb charging energies are about 0.1 eV, the wrapping length having no
noticeable effect on the size of the quantum dots. At room temperature, the charge-
transfer process between CNT and ssDNA modifies the threshold voltage of a FET
structure with this material as channel from 30 V for the pristine p-type CNT to
0 V for the wrapped nanotube.
Recently, it was found that when multiwalled CNTs were complexed with the
cytoskeletal protein tubulin, the resulting hybrid assemblies had different nanotube
morphologies depending on the concentration of tubulin: at low concentration,
disorganized structures form, whereas petal-like and flower-like conformations
are obtained at tubulin concentrations between 10 nM and 10M, and 100M,
respectively ( Dinu et al. 2009 ). The flower-like patterns are typically composed of
4-6 petals, the latter exhibiting on mica surfaces lengths of 19 ˙ 4:9m, widths of
4:8 ˙ 0:7m, and heights of 0:3 ˙ 0:1m.
It is instructing to specify that not only biological molecules wrap around CNTs,
but CNTs with moderate flexibility (single-walled CNT and thin multiwalled CNT,
with diameters of 30 nm) can also wound around bacteria such as Streptococcus
mutans and immobilize them, having possible applications in eradicating oral
pathogens at the nanoscale ( Akasaka and Watari 2009 ). The CNT affinity toward
bacteria originates in van der Waals forces.
DNA molecules not only change the electronic properties of CNTs but can also
help to sort single-walled CNTs in a mixture according to their chirality ( Tu et al.
2009 ). More than 20 short DNA sequences have been identified that bind selectively
to and allow the chromatographic purification of a CNT with a specific chirality
from a mixture containing 12 species. The periodic purine-pyrimidine pattern in
the recognition sequences of the DNA forms a two-dimensional sheet resulting
from hydrogen bonding between adjacent strands that encircle the nanotube, the
resulting stable structure resembling a three-dimensional barrel. The selectivity is
more pronounced in short DNA sequences, of about 10 mer, in which the contour
length of the backbone becomes comparable to the van der Waals circumference of
a single-walled CNT. Even low-abundant species can be purified with this method.
For example, 90% purity of single-walled CNTs with chiralities (6,5), (7,5), (7,6),
and (10,2) can be achieved by the DNA sequences .TAT/ 4 , .AT T/ 4 AT, .GTT/ 3 G,
and .TAT T/ 2 TAT, respectively, in a 0.1 M NaCl dispersion solution.
ssDNA molecules can also assist in the preparation of stable suspensions of
graphene in water up to concentrations as high as 2:5mg mL 1 ( Patil et al. 2009 ).
As for CNTs, DNA binds to the graphene surface via stacking interactions
such that the sugar-phosphate backbone is positioned away from the surface and
creates a hydrophilic outer layer that prevents sedimentation for several months.