Electrochemical sensors are the most versatile and highly developed chemical sensors. Electrochemical sensors are a type of chemical sensor that uses an electrode to detect the concentration of an analyte based on a chemical reaction. They are characterized by their low cost, ease of manufacture, rapid analysis, small size, and ability to detect multiple elements simultaneously. They are also powerful analytical tools because of their: Superior sensitivity and selectivity, Quick response period, Simplicity in operation, and Miniaturization.

1. Electrochemi
cal Sensors
Tanvir Siddike Moin

2. Electrochemical
Sensors
Electrochemical sensors are the most versatile
and highly developed chemical sensors.
They are divided into
several types:
• Potentiometric (measure voltage)
• Amperometric (measure current)
• Conductometric (measure
conductivity)
In all these sensors, special
electrodes are used.
Sometimes the distinction between these types can be
blurred.

4. Electrochemical Sensors
Either a chemical reaction takes place or the
charge transport is modulated by the reaction
Since we need a closed loop we need at least two
electrodes.
How the cell is used depends heavily on the
sensitivity, selectivity and accuracy.
Electrochemical sensing always requires a closed
circuit. Current must flow to make a measurement.
These sensors are often called an electrochemical
cell.

5. Potentiometric Sensors
Potentiometric sensors use the effect of the concentration on the equilibrium of redox
reactions occurring at the electrode-electrolyte interface of an electrochemical cell
The redox reaction takes on the electrode
surface:
Oxidant + Ze- => Reduced
product
Z is the number of electrons involved in the
redox reaction
www.chemie.uni-greifswald.de/

6. Electrochemical Cell
The reaction takes place at the cathode where electrons are
“pulled” out of the electrode.

7. • Co is the oxidant
concentration
• CR is the Reduced
Product Concentration
• n is the number of
electrons transferred
per redox reaction
• F is the Faraday
constant
• T is the
temperature
• R is the gas
Constant
The Nernst
equation gives
the potential of
each half cell.
)
(
log 0
0
R
e
C
C
nF
RT
E
E 

Nernst Equation
In a potentiometric
sensor, two half-cell
reactions take place at
each electrode. Only
one of the reactions
should involve sensing
the species of
interest. The other
should be a well
understood reversible
and non-interfering

8. CHEMFET
Sensors
• Very popular where small size and low power consumption are
essential. (Biological and Medical monitoring).
• CHEMFETs are chemical potentiometric sensors based on the Field-
Effect transistors
• CHEMFETs are solid-state sensors suitable for batch fabrication.
• The surface field effect can provide high selectivity and sensitivity.
• These are extended gate field-effect transistors with the
electrochemical potential inserted over the gate surface.

9. Four types of
CHEMFETs:
• Ion Selective
• gas selective,
• enzyme-selective
• immuno-selective sensors.
Ion selective are the most widely used, known as
A lot of the art of CHEMFETs is in
engineering the porous layer over the
gate.

10. Ion selective
CHEMFET with a
silicon nitride
gate for measuring
pH (H+ ion
concentration.)
The sensor is given a
pH sensitivity by
exposing the bare
silicon nitride gate
insulator to the sample
solution.

11. As the ionic
concentration varies,
the surface charge
density at the CHEMFET
gate changes as well.
The surface complexation of
the gate insulator
determines ionic
selectivity. Selectivity of
the sensor can be obtained
by varying the composition

12. • Also ion-selective membranes can be deposited on the top of the gate
to provide a large selection of different chemical sensors.
• Thus, a bias applied to the drain and source of the FET results in a
current I, controlled by the electrochemical potential.
• This in turn is proportional to the concentration of the interesting
ions in solution.
A change in the surface charge density affects the
CHEMFET channel conductance, which can be measured as a
variation in the drain current.

13. A biosensor sensitive to
a particular protein or
virus can be made by
coating the electrode
with the appropriate
antibody.
Extreme care must
be taken to
electrically
isolate the signals
from the solution!

14. Carbon nanotubes
• Sheets of carbon atoms can be ‘rolled’
up into tubes of nanometer dimensions
• Layers of nanotubes have a huge surface
to volume ratio

15. Carbon nanotubes
Carbon nanotubes can be grown en masse, or separated as
individuals.
Nanotube forest
Nanotube (blue) lying
across electrodes

16. Carbon Nanotube sensors
The resistance
of the sensor
increases upon
exposure to N2
gas
www.bios.el.utwente.nl/internal/Transducers03/Volume_1/2E80.P.pdf
The Scanning Electron
Micrograph shows a bridge
made from a single nanotube.
It is linking two ‘cliffs’
made of Au and Ti.
N2 gas is blown up from the
bottom

17. CNT FET sensor
Can also make
FET sensors out
of carbon
nanotubes
A small current
in the nanotube
causes a much
larger current
in the FET
This particular
sensor responds
to light.
www.echo.nuee.nagoya-u.ac.jp/~yohno/research/cnt/qnn03_abstract_submitted.htm

18. Titanium nanotube sensors
H2 gas is ionised when it hits the walls
of the titanium nanotubes
The resulting electron current is a
measure of the amount of hydrogen
present.
www.eurekalert.org/pub_releases/2003-07/ps-tnm072903.php

19. Concentration Sensors
CONCENTRATION SENSORS
REACT TO THE
CONCENTRATION OF A
SPECIFIC CHEMICAL.
THE CONCENTRATION
MODULATES SOME PHYSICAL
PROPERTY (EG RESISTANCE
OR CAPACITANCE).
GENERALLY SPEAKING, NO
CHEMICAL REACTION TAKES
PLACE IN THE SENSOR.
OFTEN CALLED PHYSICAL
SENSORS.

20. • To detect the presence of a liquid-
phase chemical, a sensor must be
specific to that particular agent at a
certain concentration.
• Eg. Resistive detector of hydrocarbon
fuel leaks. (Bell Corporation).
• Made of silicone and carbon black
composite
• The polymer matrix is the sensing
element.
• Constructed as a very thin layer with
a large surface area.
Resistive
Sensors

21. Sensor is not susceptible to polar
solvents like water.
However hydrocarbons are absorbed by the
polymer matrix
The matrix swells and the resistivityy
increases from 10 /cm to 109 /cm
Response time is less than a second.

22. • The device is reusable and can be
placed underground.
• The sensor returns to a normal
conductive state when the hydrocarbon
is removed.
• Ideal for oil exploration.

23. Gravimetric Sensors
Measurement of
microscopic amounts of
mass cannot be
accomplished using
conventional balances.
Use an oscillating
sensor (sometimes called
an acoustic gravimetric
sensor) which measures
thin layers.
The oscillating sensor
measures the shift in
the resonant frequency
of a piezoelectric
quartz oscillator.
The resonant frequency
is a function of the
crystal mass and shape.

24. The device can be described as an
oscillating plate whose natural frequency
depends on its mass.
Adding material to
that mass would shift
the frequency which
can be accurately
measured
f
S
f
f
m
o




25. • F0 = the unloaded natural
frequency, f is the
frequency shift, m is the
added mass per unit area and
Sm is the sensitivity factor.
The numerical value of Sm
depends upon the design,
material, and operating
frequency of the sensor.
The oscillating detector
converts the mass value
to a frequency shift.
It is extremely easy to
determine frequency, so
the sensor’s accuracy is
determined by how well Sm

26. Fluid density sensors.
Several basic methods are used for
determination of fluid density
Measurement of inertial mass.
Measurement of Gravitational Mass.
Buoyant force.
Hydrostatic pressure.
Attenuation of -rays

27. Density measurement
The fluid is forced to
flow through the
sensor which has a
hollow tube.
The sensor is made of
silicon and the tube
forms a double-loop
within the device.

28. • The mass of the actual tube is kept small, so the total mass of the
vibrating object is mostly that of the fluid.
• The tube inlet and outlet are at the side and the entire loop is
designed for torsional vibration.
• The resonant frequency of the vibration is proportional to the total
mass of the tube and fluid.
• Since the volume in the tube is constant, the frequency is
proportional to the density of the fluid.
• Once again, we exploit the physical properties of the material to
directly measure the characteristics of the material (the fluid).