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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). - PowerPoint PPT Presentation
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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.
Either a chemical reaction takes place or the charge
transport is modulated by the reaction
Electrochemical sensing always requires a closed circuit. Current
must flow to make a measurement.
Since we need a closed loop we need at least two electrodes.
These sensors are often called an electrochemical cell.
How the cell is used depends heavily on the sensitivity, selectivity and
accuracy.
Electrochemical Sensors
Potentiometric SensorsPotentiometric 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/
The reaction takes place at the cathode where electrons are “pulled” out of the electrode.
Electrochemical Cell
• 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
• E0 is the electrode potential at a standard state.
The Nernst equation gives the potential of each half
cell.
)(log 00
Re C
C
nF
RTEE 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
reaction
CHEMFET SensorsCHEMFETs are chemical
potentiometric sensors based on the Field-Effect transistors
Very popular where small size and low power
consumption is essential. (Biological and Medical
monitoring).
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.
Four types of CHEMFETs:• Ion Selective
• gas selective,
• enzyme-selective
• immuno-selective sensors.
Ion selective are the most widely used, known as ISFETs
A lot of the art of CHEMFETs is in engineering the porous layer over
the gate.
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.
As the ionic concentration varies, the surface charge density at the CHEMFET gate changes as well.
Ionic selectivity is determined by the surface complexation of the gate insulator. Selectivity of the sensor can be obtained by varying the composition of the
gate insulator.
Also add ion-selective membranes can be deposited on the top of of the gate to provide a large selection of different chemical
sensors.
A change in the surface charge density affects the CHEMFET channel conductance, which can be measured as a variation in the
drain current.
Thus a bias applied to 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 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!
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
Carbon nanotubes
• Carbon nanotubes can be grown en masse, or separated as individuals.
Nanotube forest
Nanotube (blue) lying across electrodes
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
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
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
Lecture 10
• Empty?
Lecture 11
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.
Resistive Sensors
To detect the presence of a liquid phase chemical, a sensor must be specific to that particular agent a certain
concentration.Eg. Resistive detector of hydrocarbon fuel leaks. (Bell
Corporation).
Made of silicone and carbon black composite
Polymer matrix is the sensing element.
Constructed as a very thin layer with large surface area.
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.
Sensor returns to normal conductive state when hydrocarbon is removed.
The device is reusable and can be placed underground.
Ideal for oil exploration.
Gravimetric Sensors
Measurement of microscopic amount of mass cannot be accomplished using conventional balances.
Use oscillating sensor (sometimes called 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.
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 electronically.
fSf
fm
o
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 mass value to a frequency shift.
It is extremely easy to dtermine frequency, so the sensor’s accuracy is determined by how well Sm is known.
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
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.
The tube inlet and outlet are at the side and the entire loop is designed for torsional vibration.
The mass of the actual tube is kept small so the total mass of the vibrating object is mostly that of the fluid.
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 characteristics of the material (the fluid).