Nanotechnology-Enabled Sensors

Input
IDTs
Shear movement of particles
Substrate
Fig. 3.49 Basic layout of a Love mode SAW device.
3.6 Acoustic Wave Transducers 127
SAW devices can be utilized for sensing various physical and chemical
parameters including temperature, acceleration, force, pressure, AC/DC
high voltages, electric fields, magnetic fields, ionic concentration, gas
flow, vapour concentration, viscosity, and in biosensing. 51
SAW devices can be employed as affinity sensors through the addition
of a mass sensitive layer on their active surface. The following equation
describes the frequency shift, Δf, as a result of added mass as a coated thin,
isotropic, non-conducting film on the active area of the device (Fig.
3.48): 64,65
⎡ μ ⎛ λ + μ ⎞⎤
Δf
=
, (3.91)
4 2
2
2
( k1
+ k2
) f0
h ρ – k2
hf0
⎢ ⎜ ⎟⎥
⎣V
⎝ λ + 2μ
r ⎠⎦
Layer
Output
IDTs
The trapped waves
Confined near the surface
where f0 is the operational frequency, k1 and k2 are material constants of
the substrate, Vr is the Rayleigh wave velocity in the piezoelectric substrate,
h is the added coating thickness, ρ is the added coating density, λ is
the Lamb constant for the added coating, and µ is the shear modulus of the
added coating. It can be seen from the equation that there is a linear relationship
between the thickness of the added coating and the frequency
shift. The frequency shift per mass added describes the sensitivity of such
a SAW device.
SAW devices can also be used to monitor changes in conductivity of a
sensing layer. Many metal oxides and conductive polymers change their
conductivity in response to different oxidizing or reducing gas species.
Depositing such materials over the active area of the SAW device can