Biomedical Engineering Reference
In-Depth Information
Fig. 5.2
EIS system with (
a
) the redox reaction and (
b
) the blocking of the redox reaction
of the redox reaction, as shown in Fig.
5.2
b. In this case, the change in C
dl
is the
only parameter with which to detect analyte binding.
The EIS method has been used for immunoassays [
6
-
8
] and nucleic acid
detection [
8
-
11
]. It is possible to detect target biomolecules with this label-free
electrical method, but one of its disadvantages is low sensitivity. Several ampli-
fication techniques had been proposed to improve detection sensitivity, including
techniques that use enzymes [
9
,
12
], liposomes [
13
,
14
], conducting polymers [
15
],
or nanomaterials [
16
,
17
].
To achieve high sensitivity, a field-effect transistor (FET)-based biosensor has
been suggested [
18
,
19
]. Miniaturization and compatibility with complementary
metal-oxide semiconductor (CMOS) technology are additional advantages of this
type of biosensor. FET-based biosensors are reviewed in the following section.
5.2.2
FET-Based Biosensor
FET-based sensors are attractive because they allow changes in the solution pH or
the binding of analytes on the surface to be directly monitored based on changes in
the electrical properties of the target molecules.
An ion-sensitive field-effect transistor (ISFET), in which the electrical properties
change according to the pH or ionic strength of the solution, was first reported
in the 1970s [
20
]. The structure of an ISFET is shown in Fig.
5.3
. An ISFET is
essentially a FET in which a gate electrode is replaced by a reference electrode
and electrolyte solution. It is noteworthy that the gate dielectric is exposed to
the electrolyte solution. The reference electrode supplies a stable potential in the
solution and produces a channel under the gate dielectric layer.
The ISFET is not a biosensor but rather a chemical sensor. The gate dielectric of
the ISFET, which is a chemically sensitive material, changes the surface potential
of the channel depending on the H
C
ion concentration; hence, the source-to-drain
