HETEROGENEOUS NANOSTRUCTURES FOR BIOMEDICAL DIAGNOSTICS - Bio-Inspired and Nanoscale Integrated Computing - page 439

have been demonstrated as a sensitive chemical sensor for NO 2 and NH 3 [33]. In 2 O 3
functions as the n-type semiconductor as a result of oxygen vacancy doping. In
this context, the redox property of LDL cholesterol permits the nanosensors to
accumulate or deplete electrons.
Indium oxide nanowires were synthesized through the chemical vapor
deposition by laser ablating InAs as a target in a high pressurized quartz furnace.
This process was performed on a silicon-silicon dioxide substrate that was
patterned with 10 nm monodispersed gold nanoparticles. Individual In 2 O 3 nano-
wires at 10 nm width were then sonicated into isopropyl alcohol suspension,
followed by their deposition onto a degenerately doped silicon wafer coated with
500 nm SiO 2 . Between the source and drain electrodes at 500 m m apart, the
individual transistor contained multiple nanowires/nanotubes as the conductive
channel. Therefore, the overall transistor and sensing characteristics were aver-
aged over an ensemble of nanowires or nanotubes, and the device-to-device
variation is significantly suppressed [32].
In 2 O 3 represents a wide band gap transparent conductor with a direct band
gap of about 3.6 eV and an indirect band gap of about 2.6 eV [34]. With these
properties, In 2 O 3 nanowires were selected to test whether LDL particles in the
oxidized and reduced states modify the conductivity of the filed effect transistor
based on In 2 O 3 nanowires devices.
Selective detection of oxLDL was established by functionalization of the
nanowire/nanotube surfaces with antibodies specific for LDL species. First, the
nanowire/nanotube transistor surface was decorated with a layer of poly (ethylene
imine) (PEI) and poly (ethylene glycol) (PEG). The surface chemistry was
achieved via (a) the attachment of the antibodies to the PEI/PEG surface, and
(b) interaction of the carboxyl group at the Y-end of the antibodies and the
primary amines available in PEI (Fig. 16.11).
Figure 16.12 demonstrates the feasibility of developing In 2 O 3 nanowires to
detect redox state of protein molecules. Two factors may account for the increased
electron concentration in the nanowires. The first is that the amino groups carried
by the ApoB-100 protein in LDL particles may function as reductive species and
hence, donate electrons to the nanowires. The second concomitant factor is due to
positive charges carried by the amino groups, which can function as a positive gate
bias to our nanowires, thus leading to the enhanced carrier concentration [35, 36].
Rouhanizadeh et al. reported a In 2 O 3 nanowire-based FET as an emergent sensor
to detect oxLDL [36]. Using both the I D -V DS (current versus drain-source voltage)
and I D -V GS (current versus gate-source voltage), the investigators found that the
sample which contained more oxidized LDL particles and consequently more free
electrons, increased the conductivity of the nanowire-based FET. This increase in
conductivity was distinct from the presence of ferrocytochrome-C.
16.4.3. Microscale Shear Stress Sensors
The emerging MEMS systems provide a spatial resolution comparable to the size
of the endothelial cells (EC) and temporal resolution in the kHz range to