Biomedical Engineering Reference
In-Depth Information
The best known acoustic biosensors are QCM, surface acoustic wave (SAW), and
cantilever biosensors [
49
,
50
]. QCM-based biosensors were the first mass-sensitive
biosensors reported and are still the most commonly used acoustic biosensors.
They use thickness shear modes of the piezoelectric substrate and hence are bulk
acoustic wave devices [
50
,
52
]. In contrast, mechanical waves of SAW biosensors
propagate along the surface of the piezoelectric substrates. This allows higher
resonance frequencies than with QCM devices, which is beneficial, because the
mass sensitivity increases with increasing frequency [
53
]. Cantilever biosensors
originate from the cantilevered sharp tips used in atomic force microscopy. If they
are operated in the dynamic resonant mode, mass increase on the cantilever surface
leads to a decrease of the resonant frequency, similar to QCM biosensors. If they
are operated in the static deflection mode, mass increase causes a deflection of the
cantilever which can be measured, e.g., optically, or, provided that a piezoresistive
coating was used, electrically [
54
,
55
]. Aside from miniaturization and parallel-
ization of the devices, ongoing research is focused on other nanomechanical res-
onators, e.g., trampoline-like resonators, as a future development aiming at higher
mass sensitivity of the resulting biosensors [
51
].
2.1.5 Thermal Transduction Principles
Thermal transduction relies on the principles of calorimetry, which requires the
measurement of temperature changes. Thermal biosensors typically use thermis-
tors, i.e., temperature-dependent resistors, as the detector unit. Similar to the term
''enzyme electrode'' used for the first amperometric biosensor, the first thermal
biosensor was called an ''enzyme thermistor''. Thermal transduction offers the
possibility of label-free detection of small molecules, but measurements using
enzyme labels have also been reported. Still, thermal biosensors have rarely been
used for biosensor measurements. One reason for this may be the difficulty to deal
with nonspecific temperature changes, e.g., arising from dilution or solvation
effects, requiring additional means, such as reference channels [
56
,
57
]. More
recent research suggests the use of thermopiles instead of thermistors as this would
simplify the ambient temperature control of the system and hence the miniaturi-
zation of the device [
58
].
2.1.6 Magnetic Transduction Principles
Magnetic transduction principles for biosensors usually require magnetic nanopar-
ticles as labels, because the analytes to be detected are typically nonmagnetic. The
most commonly used magnetic effect for signal transduction in biosensors is giant
magnetoresistance. Magnetoresistance is the change in the resistance of a material
due to the application of a magnetic field. If alternating layers of ferromagnetic and
nonmagnetic metals are used, this change is much greater than expected owing to
quantum interferences resulting in giant magnetoresistance [
59
-
61
]. However,
