Environmental Engineering Reference
In-Depth Information
molecules were detected from the step-like changes in resistance caused by the change
in local carrier concentration of graphene due to the interaction of gas molecules. An FET
device for the detection of hydrogen gas using RGO is also reported.
162
For this, holes were
introduced to RGO (with different edge-to-plane ratios) via electrochemical deposition of
metal NPs. A change in the electrical conductance of the composite was seen as differ-
ent concentrations of H
2
gas was introduced into the FET. Lu et al.
163
proposed an RGO-
based NH
3
sensor. They found that the sensing behavior is highly dependent on the gate
voltage (Vg). This was explained on the basis of the ambipolar transport of RGO and the
Vg-induced change in the graphene work function. The coulomb interaction between NH
3
and the FET can also lead to such a behavior.
A large number of graphene-based FETs for the sensitive detection of chemical and bio-
logical entities are reported. Zhang and coworkers
164
developed a mercury detection device
by self-assembling 1-octadecanethiol monolayers on graphene and constructing FET using
this alkanethiol-modiied graphene. The detection limit of the device was reported to be
10 ppm. A graphene-based FET biodevice with single bacterium detection capability was
proposed by Mohanty and Berry.
165
They proposed that the attachment of a single-bacterium
generated ~1400 charge carriers in a p-type chemically modiied graphene resulting in a
huge increase in current leading to the detection (Figure 34.4a and b). This FET was also
used as a label-free DNA sensor. Huang and coworkers
166
also constructed a bacterial sen-
sor from CVD graphene through noncovalent interaction. In addition, the sensor was used
to evaluate glucose-induced metabolic activities of the bound
E. coli
bacteria in real time as
well. Ohno et al.
167
recently fabricated a graphene-based FET from single-layer graphene
obtained from micromechanical cleavage for chemical and biological sensing applications.
The FET was able to detect the changes in solution pH with a lowest detection limit (signal-
to-noise ratio = 3) of 0.025. Using the same setup, they were able to detect different charge
types of biomolecules owing to their isoelectric point. Choi et al.
168
recently developed an
electrical sensor for DNA. Highly water-soluble graphene sheets were synthesized via a
microwave-assisted sulfonation strategy and the interactions of the functionalized gra-
phene with DNA induced sensitive electrical changes leading to the label-free detection
of DNA. Zhang et al.
169
recently reported the fabrication of a GO-polypyrene (PPr) com-
posite ilm prepared by the electrochemical codeposition of GO and PPr using propylene
carbonate as the organic electrolyte. They used this ilm as a chemoresistor-type volatile
organic vapor sensor, and a normalized sensitivity of 9.87 × 10
−4
ppm
−1
was reported for
toluene in this study. The mechanism of the response was also proposed. The response
of the sensor to different contaminants indicated that contaminants with aromatic π elec-
trons (such as toluene) and lone pairs (like chloroform) would show more response. This
indicated that the dipolar electrostatic forces are important in the sensing process. In addi-
tion, a very low response was observed in the case of nonpolar hexane, pointing to the
importance of molecular polarizability in controlling the sensitivity. Hence, the following
mechanism was proposed. The absorption of polarizable analyte on PPr chains under the
applied bias generated an induced dipole moment leading to the change in coniguration
of conjugated PPr chains into a more stretched planar structure. This resulted in enhanced
electronic coupling between the monomer units of the PPr chains, increasing the overall
conductivity of the polymer phase. It was also proposed that the large pyrene rings of
the PPr chains allow enhanced adsorption toluene through π-π interactions, leading to
higher sensitivity.
169
Myung et al.
170
recently devised a novel label-free polypeptide-based
graphene-NP hybrid biosensor to detect the enzymatic activity and ultralow concentra-
tions of speciic enzymes. Change in electrical hysteresis of the hybrid by enzyme interac-
tion was used as signal for detection.
