Biology Reference
In-Depth Information
to another DNA probe immobilized on a gold electrode. In this
sandwich assay the amplified electrochemical response was due to
the ferrocene-catalyzed reduction of H
2
O
2
.
14.4.3
Progress toward CNT-Based Sensors for DNA
Detection
The integration of CNTs into large-scale assemblies or circuits
requires precise control in the placement of the carbon nanotubes.
Several groups have used DNA to direct the assembly of CNTs
between metal electrodes to form a simple circuit. For example,
Hazani
et al.
[110]assembledamonolayerofprobethiol-terminated
ssDNA on two neighboring gold contacts. SWCNTs were modified
at the ends with complementary DNA sequences. Hybridization
between the complementary strands and the immobilized probes
resulted in bridging the gold contacts by the SWCNT. Current-
voltage (
I
-
V
) curves were measured on electrode pairs using both
complementaryandnoncomplementaryDNA-SWCNTstobridgethe
electrodes. The currents measured from the noncomplementary
interactions were an order of magnitude smaller compared to
currents measured from covalently bridged electrodes. Taft
et al.
[111]demonstratedthatselectivecouplingofDNAtoeithertheends
or sidewalls of CNTs was highly specific and based on the DNA-CNT
linkage scheme. Either amine-terminated DNA was immobilized
on CNTs through free carboxyl groups, or pyrene-modified DNA
was immobilized noncovalently through hydrophobic interactions.
Complementary DNA was immobilized on gold particles and
hybridized with the CNT-DNA probes (covalently or noncovalently
attached). SEM images showed that the gold particles were bound
primarily at the ends of the CNTs when covalent functionalization
of probe DNA was used, or were located at the sidewalls when
probe DNA was attached noncovalently. Similarly, transmission
electron microscopy (TEM) was used to verify the binding of gold
nanoparticles modified with complementary strands of DNA to
probe DNA attached to SWCNTs [112]. Two methods for attaching
DNA to SWCNTs in either aqueous solution or in organic solvent
were developed and both methods resulted in selective attachment
of the DNA-modified gold particles at the tips of SWCNTs. These
methods for binding DNA provide versatility for modification of
