BIOMEDICAL APPLICATIONS IV: CARBON NANOTUBE-NUCLEIC ACID COMPLEXES FOR BIOSENSORS, GENE DELIVERY AND SELECTIVE CANCER THERAPY - Carbon Nanotubes: From Bench Chemistry to Promising Biomedical Applications - page 124

as 16-mercaptohexadecanoic acid (MHA), which form a hydrophobic self-
assembled monolayer (SAM) in aqueous solutions. SWCNT-DNA complexes
can be digested by the Exo I enzyme, which forms precipitates of “naked”
SWCNTs that adsorb onto the MHA-SAM and generate a signal through
their electron transfer properties. However, when small-molecule-binding
proteins are present, the negatively charged SWNT-wrapping ssDNA is bound
to the protein target, thus preventing the degradation of the ssDNA by Exo
I. In this case, no signal is generated because the SWCNT-DNA complexes
create a repulsion with anionic SAM and SWCNTs do not deposit on the
electrode. These results provide very useful insights into the development
of sensitive, specific and e cient platforms for quantitatively screening the
small-molecule-protein interactions, and they can be exploited, for example,
to detect the interaction of folate with its protein target, folate receptor (FR),
a biomarker that is often over-expressed in various tumours. 72-74
a
b
Figure 5.20 Terminal protection assay of small-molecule-linked ssDNA. Small-
molecule-linked ssDNA is hydrolysed successively into mononucleotides from the 3 ′
end by Exo I, while protected from the hydrolysis when the small molecule moiety
is bound to its protein target (A). SWNT-wrapping ssDNA terminally tethered to the
small molecule is degraded by Exo I, rendering SWNTs assembled on MHA-SAM, which
mediates electron transfer between electroactive species and the electrode. Protein
binding of small-molecule-linked ssDNA prevents digestion of ssDNA, precluding
adsorption of DNA-wrapped SWNTs on MHA-SAM with no redox current generated
(B). Reproduced from Wu et al. 71 with permission. See also Colour Insert.
Another example of an effective fluorescent sensing platform has been
proposed on the basis of the non-covalent conjugation of SWCNTs and dye-
labelled ssDNA. 75 CNTs were able to either quench or, in the presence of a target,
restore the fluorescence signal. To that purpose, a 23-base oligonucleotide