Nanotechnology-Enabled Sensors

7.5 Nano-sensors based on Nucleotides and DNA 461
dles of DNA have a low resistance, in the order of several tens of kohm,
13 kohm as shown in Fig. 7.72. Such a structure can be used as a biosensor
by functionalizing the DNA strands or as a gas sensor (as the conductivity
of both DNA and carbon nanotubes changes upon exposure to different gas
species).
Due to the above mentioned electronic properties, DNA can be efficiently
used both in electrical and electrochemical sensing. For instance
Xu et al described an electrochemical biosensor based on a DNA modified
indium tin oxide (ITO) electrode. 165 which approaches the resistance quantum (described in Chap. 6) of
They utilized self-assembly and electrochemical
techniques to modify the ITO surface using (3-aminopropyl)
trimethoxysilane, gold nanoparticles and DNA molecules. The modified
electrode was employed to detect mifepristone (a synthetic steroid com-
pound used as a pharmaceutical) with the detection limit of 2 × 10 mol/L.
–7
The change of conductance of DNA strands, in contact with other materials,
can also be directly employed in biosensing applications. Tsia et al 166
reported on an electrical DNA sensor which was developed using a selfassembled
multilayer gold nanoparticle structure between nano-gap electrodes
(300 nm apart). Bifunctional organic molecules were utilized to
build up the gold nanoparticle monolayer on the wafer substrate. After the
hybridization of the target DNA (5'-end thiol-modified probe DNA and 3'end
thiol-modified capture DNA) a second layer of gold nanoparticles was
built up through a self-assembly process between the gold nanoparticles
and the thiol-modified end of the probe DNA (Fig. 7.73). The electrical
current through the multilayer gold nanoparticle structure was much
greater than that through monolayered gold nanoparticle structure for the
same voltage. The concentration of the target DNA in the tested sample solutions
ranged from several fM to 100 pM. The linear I-V curves of the
multilayer gold nanoparticles structures indicate that the device proposed
in this study can detect target DNA concentrations as low as 1 fM.