Biology Reference
In-Depth Information
is more di
cult but can be achieved using highly reactive species
such as fluorine, nitrenes, arylation using diazonium salts, 1,3-
dipolar cycoadditions, and addition of carbenes to name a few [86].
These methods allow the incorporation of various reactive groups
(---COOH, ---NO
2
, OH, H, and ===O) with high specificity for attach-
ment of DNA or other biomolecules. Furthermore, photochemistry
has been used to functionalize the sidewalls of MWCNTs [87]. CNTs
photoetched with azidothymidine serve as photoadducts, with a
reactive group on each photoadduct for the subsequent
in situ
synthesis of DNA oligonucleotides. This method may potentially
enable photolithographic patterning of different DNA sequences on
CNTs arrayed on genomic chips. The covalent modification of CNTs
can completely change the electronic properties of the CNTs as a
consequence of the transformation of the sp
2
hybridization of CNTs
to sp
3
hybridization. This can lead to partial loss of conjugation
affecting electron-acceptor and/or electron-transport properties. A
vastamountofworkhasbeenconductedonCNTfunctionalizationin
thelastdecadetoovercomethesechallengessincecovalentcoupling
of biomaterials to CNTs is critical to the development of biosensors
as well as bioelectronic devices. In contrast to the traditional
approach of covalent modification, noncovalent modification of the
sidewalls for sensor applications has been shown to preserve the
desired electronic and optical properties of CNTs while improving
their solubilities. The earliest work on DNA linkage to CNTs was
throughnoncovalentinteractions[88]andhascontinuedtobeused
as a nondestructive functionalization method in the construction
of field-effect transistor (FET)-based biosensors [89]. Sidewalls are
functionalized noncovalently through
stacking or hydrophobic
interactions. DNA bases interact with CNTs via
π
π
stacking on the
nanotube surface, with the hydrophilic sugar-phosphate backbone
exposed to the solvent, thereby achieving solubility in water. Zheng
et al.
[90] demonstrated DNA-assisted dispersion of CNTs in water
during sonication. Noncovalently wrapped DNA-CNTs were then
separated via ion exchange chromatography. Further, the wrapping
of SWCNTs with ssDNA was found to be sequence dependent [91].
Through a systematic search of a ssDNA library, it was found that
the selected sequence, d(GT)
n
,
n
= 10 to 45, self-assembles into
a highly ordered structure around individual nanotubes in such
