Biomedical Engineering Reference
In-Depth Information
as the change in measured current. h ese silicon nanowire FETs are used
to detect very low concentrations (~100 pM) of dopamine. DNA-aptamers
were functionalized on the biosensors. h e sensor is not only very sensitive,
it is also has high specii city and is able to detect dopamine from ascorbic
acid, catechol, phenethylamine, tyrosine, epinephrine, and norepinephrine.
h is sensor was also applied to monitor DA release under hypoxic stimu-
lation from living PC12 cells. h e real-time recording of the exocytotic
DA induced by hypoxia reveals that the increase in intracellular Ca2+ that
is required to trigger DA secretion is dominated by an extracellular Ca2+
inl ux, rather than the release of intracellular Ca2+ stores (Li
et al.
, 2013).
8.4
Acoustic Wave-based Mass Sensors
Another totally dif erent label-free measuring technique that is commonly
used in biosensors is acoustic-wave based mass sensing. h is device employs
piezoelectric acoustic waves, or traveling waves in a piezoelectric layer or
crystal. Piezoelectricity is a phenomenon where mechanical strain in the
material is generated by an applied electrical i eld or vice versa. h e applied
electric i eld can be in the form of oscillating sinusoidal electrical signals,
and in such case, the induced mechanical strains in the material will be in
the form of standing or travelling waves. h e acoustic waves can travel either
on the surface of the piezoelectric material (surface acoustic waves - SAW) or
through the piezoelectric layer (bulk acoustic waves - BAW). Both modes of
propagation SAW and BAW can be used for biological sensing.
For biological applications the acoustic wave-based MEMS devices are
integrated in a microl uidic system and the sensing area is coated with a
biospecii c layer. When a bioanalyte interacts with this sensing layer, physi-
cal, chemical, and/or biochemical changes are produced. Typically, mass
and viscosity changes of the biospecii c layer can be detected by analyzing
changes in the acoustic wave properties such as velocity, attenuation and
resonant frequency of the sensor. An important advantage of the acous-
tic wave biosensors is simple electronic readout that characterizes these
sensors. h e measurement of the resonant frequency or time delay can be
performed with high degree of precision using conventional electronics.
8.5
Bulk Acoustic Wave Sensors
Bulk acoustic wave resonators have been widely used for mass detec-
tion in the form of quartz crystal microbalances (QCM). As shown in
