UNIT IV - SENSORS
Definitions for sensors and biosensors - Technical definitions: calibration, selectivity,
sensitivity, reproducibility, detection limits, response time; Introduction to
Transducers - primary and secondary types, Active and passive, Analog and Digital
transducers.
Sensors based on sensing layer - Chemical sensor - semi-conductor gas sensors, solid
electrolyte gas sensors, ion-selective electrode sensors, humidity sensors and field
effect transistor sensors.
Bio-sensors - Enzymes based, Affinity-based biosensors, Inhibition-based biosensors;
Cell-based biosensors (Membrane receptors and transporters).
Definitions for Sensors
A device that responds to a physical stimulus (such as heat, light, sound,
pressure, magnetism, or a particular motion), and transmits a resulting
impulse (as for measurement or operating a control).
Definitions for Biosensors
A biosensor is defined as a device that produces a measurable signal
proportional to the concentration of the target analyte, usually incorporating
a biological sensing element and measuring signals derived from biological
interactions.
Technical Definitions
1. Calibration
The linear region of a biosensor is obtained from a calibration curve of its
response to different inputs.
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A good calibration curve is also indicative of stability of the biosensor's
response, which should neither drift nor oscillate with time.
A biosensor having linear response can be more easily described
mathematically, than one which has a nonlinear response.
2. Selectivity
Ability to recognize a single event/condition among other events/conditions
in the same sample/signal. The selectivity of the biosensor is determined by
both the sensor and transduction method.
3. Sensitivity
The sensitivity of a sensor is measured by the change in its response as a
function of the analogous change in the quantity examined.
The sensitivity s of a biosensor is defined by the ratio.
Δ output
s=
Δinput
Where,
Δ output = change in the amplitude of output
Δ input = change in the amplitude of input
The sensitivity determines the aptness of the sensor for a particular
application.
4. Reproducibility
The method is reproducible if similar results are obtained by other scientists
at different laboratories.
5. Detection limits
Detection limit or lower limit of detection is the lowest quantity of a
substance that can be distinguished by the sensor signal from the absence of
that substance (control).
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6. Response Time
Response time is defined as the time to reach a steady state output from the
instant of variation in the input value to the sensor.
Transducers
A transducer is an electronic device that converts energy from one form to
another.
The process of converting energy from one form to another is known as
transduction.
1. Classification
The transducers may be classified in various ways such as on the basis of
electrical principles involved, methods of application, methods of energy
conversion used, nature of output signal etc
(a) Primary and Secondary Transducers
(b) Active and Passive Transducers
(c) Analog and Digital Transducers
(d) Transducers and Inverse Transducers
(a) Primary and Secondary Transducers
Some transducers consist of mechanical device along with the electrical
device.
In such transducer’s mechanical device acts as a primary transducer and
converts physical quantity into mechanical signal.
The electrical device that converts mechanical signal produced by primary
transducer into an electrical signal acts as a secondary transducer.
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For example, in case of pressure measurement, bourdon tube is a primary
sensor which converts pressure first into displacement, and then the
displacement is converted into an output voltage by an LVDT (core).
In this case LVDT is secondary transducer.
(b) Active and Passive Transducers
Active transducers - These transducers do not require any external source or
power for their operation. So they are called as self-generating type.eg) Piezo
electric crystal.
Passive transducers - do not generate electrical signals by themselves. To
obtain an electric signal for such transducers an external power source is
essential. They are also known as externally power-driven transducers. Ex.
POT, LVDT.
(c) Analog and Digital Transducers
Transducers, on the basis of nature of output signal, may be classified into
analog and digital transducers.
Analog transducer converts input signal into output signal, which is a
continuous function of time such as thermostat, strain gauge, LVDT, thermo-
couple etc.
Digital transducer converts input signal into the output signal of the form of
pulse e.g. it gives discrete output.
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Sensors Based on Sensing Layer
Chemical Sensor
A chemical sensor is a device that converts a property (physical or chemical)
of a particular analyte into a measurable signal that is proportional to the
analyte concentration.
It recognizes the analyte molecule in a selective way by transforming the
response into an analytical electrical signal.
Chemical sensors are classified into ion sensors, gas sensors and humidity
sensors according to the property of analytes.
The general characteristics of chemical sensors are,
Transform chemical quantities into electrical signals
Respond rapidly
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Maintain their activity over a long time period
Small
Cheap
Specific, i.e., they should respond exclusively to one analyte, or at least be
selective to a group of analytes.
Semi-conductor Gas Sensors
Semiconductor gas sensors are (also: metal oxide sensors, MOX) electrical
conductivity sensors.
The resistance of its active sensor layer changes upon contact with the gas to
be detected.
MOX gas sensors react all reducing and oxidizing gases and therefore not
only enable the detection of trace gases such as carbon monoxide (CO), nitric
oxides (NOx), ammonia (NH3), sulfurous gases (H2S, SO2), hydrocarbons
(CxHy), but also the analysis of complex aromas such as volatile organic
compounds (VOCs).
A high level of selectivity is achieved through a suitable chemical coating.
Depending on the material and target gas, operating temperatures between
300ºC and 900ºC are necessary to ensure the intrinsic conductivity of the
sensor.
The sensitivity level depends on the gas and ranges from a few ppb to the
percentage range.
The detection limit depends on the gas-sensitive material.
Solid Electrolyte Gas Sensors
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Ion-selective Electrode Sensors
Humidity Sensors
Field Effect Transistor Sensors
Bio-sensors
A biosensor is an analytical device which converts the biological signal into a
measurable electrical signal.
The basic principle of a biosensor is to detect this molecular recognition and
to transform it into another type of signal using a transducer.
It detects records and transmits information regarding a physiological change
or process.
It determines the presence and concentration of a specific substance in any
test solution.
Principle of Biosensor
Basic principle of biosensor involved in three elements,
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First, biological reorganization element which highly specific towards the
biological material analytes produces.
Second, transducers detect and transducers signal from biological target -
receptor molecule to electrical signal which is due to reaction occurs.
Third, after transduction signal from biological to electrical signal where its
amplification is necessary and takes place and read out in detector after
processing the values are displayed for monitor and controlling the system.
Enzymes based Bio-sensors
Enzymatic sensors are based on the catalytic chemical reaction of the enzyme
and substrate. The reaction products, the charge exchange, or the heat
generation may be the bases for the indirect transduction.
The enzyme-based biosensor is a chemical sensor in which the catalytic
property of an enzyme is utilized.
Enzyme-based biosensors can be formed by different sensing principles, such
as electrochemical, electromagnetic, optical, thermal, and gravimetric ones.
The amperometric principle has been widely used in enzyme electrodes. For
example, glucose can be measured selectively by detecting the consumption
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of oxygen or the formation of hydrogen peroxide produced by the enzymatic
reaction of glucose oxidase (GOx).
Enzyme based biosensors are used in different analyzers for quantification of
glucose (PO2 electrode), urea, creatinine etc., where the enzyme is
immobilized on the sensor.
Affinity-based Biosensor
In these, the receptor molecule binds the analyte irreversibly and non-
catalytically.
Receptor molecules are Antibodies, Nucleic acids, Hormone receptors
Affinity biosensors are based on specific chemical binding.
In immunosensors, this means the antigen-antibody reaction.
They employ the phenomenon that the immunosystem of living things
produces specific antigens against foreign objects (bacteria, viruses,
molecules, etc.) that are able to form stable complexes for biological
recognition.
In DNA sensors (also called DNA chips), the selective chemical binding is the
hybridization of molecule clusters with DNA molecules to form a double
structure.
The immunosensor is a type of chemical sensor in which the antigen-
antibody reaction is utilized so as to realize highly specific and sensitive
measurements
The antigen, Ag, and the antibody, Ab, form an antigen-antibody complex,
AgAb.
When Ag is introduced, but the amount of Ab remains unchanged, the
amount of introduced Ag can be determined by the increment of AgAb.
If the antibody Ab is immobilized and fixed on the sensor surface, formation
of AgAb will cause some change in the sensing element, which can be
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measured by many different ways such as electrochemical, acoustic,
gravimetric, and optical techniques.
The DNA sensor is a device for detecting DNA molecules having specific
base sequence.
To recognize the specific base sequence among the sample DNA, the
hybridization technique is employed.
A single strand DNA segment is hybridized to another single strand DNA
segment forming a double strand structure when the base sequence is
matched.
In order to detect DNA segment having a specific base sequence (target),
another DNA segment having a complementary base sequence (probe) is
used so that the target is hybridized with the probe and forms a hybrid.
Hybridization is a consequence of a reaction given by Probe + Target →
Hybrid
The concentration of target can be predicted by measuring the amount of
hybrid.
To detect the result of hybridization, different sensing principles have been
used such as amperometric, potentiometric, electrochemical, acoustic, and
gravimetric.
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