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 123

gave an odour response while bare devices did not, thus suggesting that
ssDNA layer increased the binding afinity for these molecules to the device,
with a concomitant increase in sensor response. Interestingly, diverse odours
elicited different current responses from ssDNA/SWCNT-FET sensors, but
the responses were highly reproducible across different devices and speciic
to the base sequence of the ssDNA used. Moreover, the signal-to-noise levels
of the measurements indicated that detection of concentrations less than 1
ppm should be possible and, even more encouraging, it was observed that
the ssDNA chemical recognition layer was reusable through at least 50 cycles
without refreshing or regeneration. Therefore, such indings represent a
signiicant progress towards the realisation of an effective sensor array for
electronic olfaction.

SWCNTs+DMMP
SWCNTs+ssDNA+DMPP

SWCNTs+DNT
SWCNTs+ssDNA+DNT

Figure 5.19 (a) Change in the device current when sarin simulant DMMP is applied
to SWCNT-FETs before and after ssDNA functionalisation. (b) Sensor response to DNT.
Reproduced from Staii et al. 70 with permission. See also Colour Insert.

DNA oligonucleotides can also represent a valuable tool able to identify
the interaction of small-molecule ligands targeting speciic proteins or
protein receptors. Wu et al. 71 showed that the oligonucleotides not only act
as coding sequences for the linked organic molecules but offer immediate
signal amplification via polymerase chain reactions (PCRs). A unique
advantage is that ssDNA, once covalently conjugated to a small molecule, is
protected from degradation by exonuclease I (Exo I), provided that the small
molecules are bound to their protein targets, as shown in Fig. 5.20a. Most
probably, such terminal protection is due to steric hindrance of the bound
protein molecule, which prevents Exo I from approaching and cleaving the
phosphodiester bond adjacent to the 3 ′ terminus. At the same time, it also
enables the exploitation of a sensitive electrochemical biosensor strategy
for detecting the binding of small molecules to proteins (Fig. 5.20b). The
readout system is based on COOH-terminated, long-chain alkanethiols, such