Biomedical Engineering Reference
In-Depth Information
field associated with the coil drives the piezoelectric effect in the device. The
configuration possesses a number of important advantages.
It is not necessary to operate the device with a metal electrode in place and
with electrical connections. This renders advantages in terms of flow-
through design.
Crucially, it is possible to operate the sensor at ultra-high frequencies (e.g.
1 GHz) via bulk acoustic wave overtones. This leads to higher analytical
sensitivity.
It is possible to tune the device with ease to specific frequencies, which
could potentially lead to important interfacial chemical information.
The surface chemistry for biomolecule attachment involves SiO
2
which is a
much higher developed area of chemistry than is the case for binding to
metals such as gold.
d
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t
3
n
g
|
1
d
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.
The system has been applied to the detection of human immunodeficiency virus
(HIV) based antibodies and bacteria.
83
1.4.2.3 Piezoelectric Surface-launched Acoustic Wave
Biosensors
A number of devices in this category have been described in the literature
together with a rather limited number of applications to bioanalytical
chemistry, at least as compared with the array of those connected to the TSM.
'Surface-launched' in this context means that acoustic waves are generated in a
piezoelectric substrate by transducers that are placed on the surface of the
material. Particle movement, which is generally detected by separate
tranducer(s) imposed on the same surface, is often restricted to the 'near'
surface of the substrate, unlike the case for the TSM discussed previously. For
an introduction to several of these devices see ref. 84.
The first device to be considered is the surface acoustic or Rayleigh wave
(SAW) sensor which is, conventionally, an extremely common component of
microelectronic and integrated circuitry. We introduce this device since it is
very much the forerunner of the SAWs in use in the sensor field. In this
structure an interdigital transducer (IDT) is fabricated on the piezoelectric
substrate (e.g. ST-cut quartz) as shown in Figure 1.18. The effect of this
arrangement is to create acoustic waves near the surface of the substrate which
possess a wavelength far smaller than the overall thickness of the structure. As
seen in Figure 1.18, the particle motion of a Rayleigh wave contains both shear
and compressional components and travels in an elliptical path. In the delay-
line configuration, the IDT to the right side of the device detects the arrival of
the acoustic particle motion. (There are other designs of the SAW structure.)
The chemistry related to sensing processes is conducted on the area between the
IDTs and it is generally the case that the electrodes have to be isolated from
liquid if the device is operated in that medium, in a similar fashion to that
described above for FETs. However, earlier work has shown unequivocally
