Biomedical Engineering Reference
In-Depth Information
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a
*DWHELDV9
b
c
Fig. 12.3
Basic characteristics of a CNT-based transducer and the mechanism of CNT-based sen-
sors.
a
Gating effect of a top-gate CNT-FET under different source-drain voltages. The insets show
the optical image (
left
) and the cross-sectional structure (
right
) of a fabricated CNT device where
S, D, and G are source, drain, and gate electrodes, respectively.(Adapted with permission from
Lee et al. [
5
])
b
Schematic diagram depicting the theoretical model for CNT-based sensors. The
[A], [B], and [AB] represent the concentration of analytes in bulk solution, the surface density of
binding sites, and the surface density of adsorbed analytes, respectively. It is assumed that analytes
A adsorb to the binding sites B following the Langmuir isotherm process. Then, adsorbed analytes
AB generate the sensor response, ΔG/G
0
.
c
Graph showing the typical response of CNT-based sen-
sors. (Adapted with permission from Lee et al. [
26
])
12.2
Carbon Nanotube-Based Sensor Transducer
12.2.1
Mechanism of Carbon Nanotube Transistor-Based
Transducers
In this section, various sensing mechanisms and applications of CNT-based biosen-
sors will be discussed. Figure
12.3a
shows gate voltage dependent electrical charac-
teristics with the schematics of a typical CNT-field effect transistor (CNT-FET) [
5
].
A typical CNT-FET is composed of source, drain and gate electrodes as shown in
the left inset, and the source and drain electrodes are connected by CNTs (the right
inset in Fig.
12.3a
). The gating effect of a CNT-FET on different biases (Fig.
12.3a
)
shows a decrease in a source-drain current as the gate voltage is swept from nega-
tive to positive, which is a typical p-type behavior.
Figure
12.3b
and
c
illustrate the
mechanism
and the
theoretical model
of CNT-
based sensors, respectively [
26
]. In this model, it is assumed that analytes A bind to
the finite number of binding sites B on solid substrates. Then, the binding between
A in the bulk solution and B follows the Langmuir isotherm model. Thus, bound
analytes and free analytes in solution maintain an equilibrium state in general. It
should be noted that the binding sites of CNT sensors vary depending on sensor
types. For example, the binding sites of CNT-based gas sensors are usually the bare
surfaces of CNTs [
27
,
28
]. In the case of CNT-based biosensors, specific receptor
molecules fixed on CNT surfaces work as binding sites [
29
]. In Fig.
12.3b
,
c
,
[A]
,
[B]
,
[AB]
, and
[B]
max
represent
the concentration of analytes in the bulk solution
,
the surface density of the binding sites on CNTs
,
the surface density of adsorbed
analyte molecules
, and
the maximum surface density of the binding sites on CNTs
,
respectively. The surface density of adsorbed analytes can be expressed by the fol-
lowing Langmuir isotherm equation:
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