NTHU ESS5841
F. G. Tseng
The Principles and Applications of Micro Transducers
Spring/2001, 5-2, p1
Lecture 5-2 Chemical and Biological Transducers II –
Chemical and Bio Sensors
4. Acoustic wave sensors
 Using the property changes ( velocity, amplitude,
frequency, etc…) of propagate acoustic wave for
detection. Chemically sensitive layer is often applied to
enhance the interaction with the measurand. Acoustic
wave can also be used for mixing and moving fluid.
 Can be generated by piezoelectric (most popular, like
ZnO,
quartz),
magnetostrictive,
electrostrictive,
photothermal, etc…
 Four major types of waves:
TSM (Thickness shear mode)
SAW (Surface acoustic wave)
FPW (Flexural plate wave)
APM (Acoustic plate mode)
 Self resonant mode can be very sensitive to adsorbed or
otherwise bound molecules on their surfaces. For a
NTHU ESS5841
F. G. Tseng
The Principles and Applications of Micro Transducers
Spring/2001, 5-2, p2
SAW device, Sauerbrey equation for frequency shift:
f  kf02
m
A
k: device dependent constant, f0: natural resonant
frequency without loading. m: mass change. A: area of
membrane.
 Can be used to viscoelastic sensing, acoustoelectric
sensing, liquid density, and viscosity.
a. Viscoelastic sensing: changes in resonant
frequency due to polymer mechanical modulus
effects (like swelling). This can be used to sense
vapor.
b. Acoustoelectric sensing: The bound charges
generated by acoustic wave have a electric field to
cause motion of charge carries and dipoles in the
adjacent medium, thus affect the wave’s velocity
and attenuation (frequency does not change)
c. Liquid density and viscosity can affect wave
frequency and quality factor, respectively.
5. Biosensors
 Biological molecules are used to perform an intermediate
transduction between the desired measurand through
selectivity of bio-molecule interactions.
Enzyme
(protein), antibody (protein), polysaccharide, or nucleic
acid are used. The sensing devices can be those
discussed in the chemical sensor section, including ISFET,
microelectrodes, microcalorimeter, acoustic wave device,
etc…
NTHU ESS5841
F. G. Tseng
The Principles and Applications of Micro Transducers
Spring/2001, 5-2, p3
 Four possible modes of physically coupling the
biomolecules to the basic sensor
i. membrane entrapment ( case specific)
ii. physical adsorption (gentle binding, less distortion
of the conformation of molecules, but easily effected
by pH, temperature)
iii. matrix entrapment (similar to membrane type)
iv. covalent bonding (most permanent bonding, but
may chemically modify the binding sites and
denature the molecules)
NTHU ESS5841
F. G. Tseng
The Principles and Applications of Micro Transducers
Spring/2001, 5-2, p4
 the biological material used need to consider the
following:
i. material is constrained for the life time of
bio-sensors
ii. contact with analyte solution
iii. by-products can be diffused away
iv. biomaterials are not inactivated by the
immobilization layer.
a. Resonant Biosensors
Acoustic wave or other resonant sensors can be used
as biosensors. A sensitivity molecule is attached to
the surface of the resonator and the binding of the
analyte molecules adds mass causing frequency shift.
b. Optical-detection biosensors
 Optical diffraction: antibody-antigen binding area shows
diffraction grating. Using latex or colloidal gold to
increase the effective size of the bound analyte to amplify
signal.
 Electrochemiluminescence
(ECL):when
electrical
current is passed through an appropriate mixture of
compounds, light is emitted at an amplitude related to
the concentration of a iluminescence cell for the detection
of fluorescently labeled samples
 Fiber optical biosensors.
c. Thermal-detection biosensors
 Calorimeters: The heat of reaction for each enzyme
can be measured and is related to its concentration.
d. ISFET biosensors (CHEMFETs)
NTHU ESS5841
F. G. Tseng
The Principles and Applications of Micro Transducers
Spring/2001, 5-2, p5
e. Electrochemical –detection biosensors
 Cyclic voltammetry has been used to detect DNA
hybridization and DNA-binding drugs. The anodic
peak current is a linear relationship with the
concentration of the samples.
 Amperometric biosensor:
glucose sensor as an example: Using polyvinyl
alcohol (PVA) matrix to apply the enzyme glucose
oxidase (GOD) to a Pt electrode.
Thick-film
Ag/AgCl counterelectrode for reference. Pt held at
700mV and oxidize hydrogen peroxide to release
current.
glu cos e  O2  gluconolac tone  H 2O2
H 2O2  O2  2 H   2e _
The glucose concentration can be determined by the
current or pH.
f. CMOS-compatible biosensor
Screen printing: screened printed polymer membrane
NTHU ESS5841
F. G. Tseng
The Principles and Applications of Micro Transducers
Spring/2001, 5-2, p6
containing bio-molecules on CMOS wafer.
Screen
printed silver epoxy is also applied for the interconnection.
g. Others
Using whole cell as part of sensors
SPR (Surface Plasma Resonance)