Nanotechnology-Enabled Sensors

118 Chapter 3: Transduction Platforms
1
α =
,
⎛ kT ⎞⎛
Cs
⎞
⎜2.
3 ⎟ + 1
2 ⎜
⎟
⎜ ⎟
⎝ q ⎠⎝
β s ⎠
(3.85)
where βs is the surface buffer capacity (the ability of the oxide surface to
deliver or take up protons), and Cs is the differential double-layer capacitance.
Eq. (3.85) describes that as α approaches 1, the maximum sensitivity
of 59 mV for one unit of pH (at 298°K) can be obtained. α = 1 is reached
for the large values of surface buffer capacity βs and/or low values of Cs.
For α < 1 a reduction in sensitivity is expected. It is obvious that if the ion
concentration at the surface [H + ]s changes with the bulk ion concentration
the sensitivity will decrease. [H + ]s should be independent of the bulk pH,
which translates as a surface with a large buffer capacity.
Many biosensors based on the chemical reactions on the surface of IS-
FETs are enzyme based (enzyme based sensors will be described in
Chap. 7), which are called Enzyme field efect transistors (ENFET). By
applying an enzyme-entrapping membrane on top of the ISFET gate, the
device can perform as an enzyme sensor.
Considerable effort has been made to develop different types of ENFETs
since the introduction of an ENFET for penicillin. 31 The performance of
ENFET biosensors is greatly affected by the integration mechanism of the
enzymes with the ISFET. Immobilizing enzymes on the gate surface of
ISFETs by cross-linking with bovine serum albumin and glutaraldehyde is
one of the most popular ways to establish a sensitive layer that can be employed
to create cross-linked enzyme matrices. 32-36 To extend the dynamic
range and/or increase the selectivity, additional membranes can be
added. 37-41 Another method for ENFET preparation is the entrapment of
enzymes with polymers. 42,43
3.6 Acoustic Wave Transducers
Acoustic wave transducers employ piezoelectric materials in which
mechanical waves are launched. As described in Chap. 2, piezoelectric
phenomenon occurs in crystals, which do not have a center of symmetry.
Applying stress to such crystals deforms their lattices and electrical fields
are produced, and vice versa. It is believed that the first acoustic wave device
was developed in 1921 when Cady utilized a quartz resonator for stabilizing
electronic oscillators. 44