BIOMEDICAL APPLICATIONS VI: CARBON NANOTUBES AS BIOSENSING AND BIO-INTERFACIAL MATERIALS - Carbon Nanotubes: From Bench Chemistry to Promising Biomedical Applications - page 192

moiety of isoalloxazine can self-assemble into ribbons on the CNTs' surface.
The assembly imparts eficient individualisation and chirality selection to
CNTs. In other words, CNTs function as a template platform to select a speciic
isoalloxazine ribbon structure.
CNTs are also sensitive to the structure of lipid molecules. Different lipid
derivatives may form ring, helix or double helix structures on CNTs. 34 Triton
X-100, adsorbed onto the CNT surface through π-staking interactions, leads
a full coating without any organisation, while octadecyltrimethylammonium
bromide, with a long carbon atom aliphatic chain, may form a regular helix
structure on CNTs, suggesting the CNTs' capability to distinguish different
types of compounds.
7.2.2.2.4 Electric probes
As a sensitive nanoscale structure with outstanding electronic properties, CNTs
have often been explored to be used as electric probes. Guo et al. 35 reported
that CNTs can be used as probes to detect the mismatch of a DNA duplex. The
CNTs provided two electric poles and a single DNA duplex bridging a CNT gap
(Fig. 7.5). The nano-structure can function as a nano-transistor as well as a
nano-resistor. How well the DNA duplex matched could be easily read through
the source-drain current ( I SD ) value. A well-matched DNA duplex showed low
resistance, while a mismatch increased the resistance by hundreds of times.
Therefore, such nano-devices could provide biochemical information for
DNA reorganisation and detection. Similar results were reported by Roy and
coworkers. 36 They used the carbon tips to detect the integrity of double-strain
(ds)DNA. The dsDNA bridged the gap between two CNT tips and showed
an obvious conductivity; on the contrary, conductivity decreased to a very
low level after destroying of the double-strand structure. Again this study
demonstrated that SWNTs can be employed as nano-electrodes to detect the
conformation and structure of DNA chains.
Other studies on DNA detection using CNTs were performed by many
groups. Cai and collaborators 37 reported the ability to detect DNA through
a CNT-enhanced electrochemical DNA biosensor. They covalently bonded
oligonucleotides with CNTs. When the CNT-ssDNA formed hybridisation
with target DNA strands, daunomycin could intercalate in the DNA duplex
and increased the electrochemical response. Then the CNTs functioned as
molecular wires and accepted electrons from the redox intercalator through
the dsDNA. Li et al. 38 produced a similar nano-device to detect targeting DNA
chains, but they designed the detection method on the basis of Ru(bpy) 3 2+ -
mediated guanine oxidation. It seems that CNTs are particularly attractive for
scientists involved in DNA detection and identiication.