Nanotechnology-Enabled Sensors

128 Chapter 3: Transduction Platforms
transform it into a gas sensor. In this case, the shift in operational frequency,
f, after adding the conductive layer is given by: 65
f
=
f
0
2
k 1
2 ⎛σ
1+
⎜
⎝ σ
SH
OR
⎞
⎟
⎠
2
,
(3.92)
where f0 is the operational frequency, k 2 is the electromechanical coupling
coefficient, σSH is the sheet conductivity of the sensitive layer, σOR is the
product of SAW mode velocity and substrate permittivity. Obviously, a
SAW device with a higher electromechanical coupling coefficient generates
a larger frequency shift in response to gases.
The point at which the maximum frequency shift can be obtained is calculated
from the slope of the graph which is obtained by plotting f in
Eq. (3.92) against σSH, as can be seen in Fig. 3.49. Maximum sensitivity is
obtained when σOR and σSH are equal.
Frequency change
Film sheet conductivity, σSH
Fig. 3.50 Maximum response occurs when sheet conductivity value is matched to
product of SAW mode velocity and substrate permittivity.
For sensing in liquid media, SH-SAW modes are preferred as they suffer
much less attenuation when liquids come in contact with the propagating
medium. 66 As the movements of particles are shear horizontal, not
normal to the sensing surface, the contact liquid cannot damp the movements,
except in cases where the liquid has high viscousity. This makes