Biomedical Engineering Reference
In-Depth Information
Liquid
Quartz
Electrodes
Thickness shear
oscillation
Figure 8.4
Schematic of bulk acoustic wave resonator.
Figure 8.4, the structure of the QCM is fairly simple, the quartz disc is
sandwiched between two electrodes (ot en gold) and the top electrode
is exposed to the biological sample. When oscillating voltage is applied
between the electrodes, the oscillating electric i eld generates mechan-
ical acoustic waves propagating through the substrate. h e mechanical
waves resonate at a specii c resonance frequency,
f
0
. h e resonance fre-
quency is dependent on the thickness of the piezoelectric layer. When
biological samples are placed on top of the gold electrode, the samples
dampen the mechanical acoustic waves, creating a shit in resonant fre-
quency,
Df
.
h e Sauerbrey equation correlates the changes of the resonant frequency
of an acoustic wave resonator with the mass deposited on it. h e acoustic
wave propagating on a piezoelectric substrate is generated and received using
IDTs. In the case of a biosensor resonator, the cell to be analyzed or the anti-
body layer for protein marker detection are added on the IDTs. h is will
cause a shit of the resonant frequency due to the increasing of mass, where
f
i
and
f
o
are the are the resonant frequencies before and at er loading the sensor.
In Equation 8.3, the Sauerbrey equation is dei ned as
2
2
f m
m
0
62
f
2.26 10
f
(8.3)
0
A
A
qq
where
Δ
f = f
0
-f
i
From Eq 8.3, the change Δf of the resonant frequency of the piezoelec-
tric crystal is directly proportional to the mass loaded on the acoustic wave
resonator, where
Δm
is expressed in g and
Δf
and
f
0
in Hz (Skládal, 2003).
Due to the simple structure of the piezoelectric resonator, which
requires only electrodes and the piezoelectric layer, they are very easy to
