Biology Reference
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a way that the electrostatics of the DNA-CNT hybrid depends on
tube diameter and electronic properties. This assembly enabled
improved metal from semiconducting tube separation and also
diameter-dependent separation. DNA can also enter the internal
cavities of CNTs; and electrophoretic transport through a single
MWCNT cavity has been imaged through fluorescence microscopy
[92]. The main disadvantage of noncovalent interactions, however,
is their lack of specificity, and in some cases, denaturing of the
biomoleculeupon adsorption.
14.4.2
CNTs for Electrochemical DNA Sensing
Carbon nanotubeelectrodes have beenused forthe electrochemical
characterization of DNA. Wang
et al
. [93] conducted voltammetric
studies on the electrochemical oxidation of guanine and adenine
residues in DNA at SWCNT-modified electrodes. Compared to
nonmodified electrode materials, the electrochemical response
corresponding to the oxidation peaks was greatly enhanced at
the modified electrode. Guo
et al
. [94] covalently attached both
single-stranded and double-stranded calf thymus DNA molecules
onto MWCNT-modified gold electrodes and characterized electro-
chemical differences by cyclic voltammetry and electrochemical
impedance analysis. This was done using both a redox indicator
[Fe(CN)
3
6
/Fe(CN)
4
6
]andanelectrochemicalintercalator(ethidium
bromide). Both of these studies suggested that further application
of CNT-modified electrodes might be exploited for detecting DNA
hybridization.
Baker
et al.
[95] were the first to report the formation of
DNA-SWCNT adducts in solution for DNA hybridization. The
DNA-SWCNT complexes were synthesized by reacting oxidized
SWCNTs with thionyl chloride and ethylenediamine to form amine-
terminated sites, shown in Fig. 14.16. The amines were further
reacted with succinimidyl 4-(
N
-maleimidomethyl)cyclohexane-1-
carboxylate (SMCC) forming maleimide groups which reacted with
thiol-modified DNA. To confirm covalent attachment and to test
the accessibility of the DNA-modified SWNTs, hybridization studies
were conducted using fluorescently labeled DNA oligonucleotide
targets.Shortlythereafter,theuseofcarbodiimide-assistedcoupling
