Biosensors

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• CHEMICAL SENSOR
• PREPARED AND PRESENTED
• BY
• PWADUBASHIYI C. PWAVODI msC(20143883)
• FELIX chibuzo obi msc(20144610)
• MTHABISI MOYO(20143906)
• Erine saurel guepp(20135813)
• LECTURERALI ISIN

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INTRODUCTION

• DEFINITION
• BRIEF EXPLANATION OF CHEMICAL SENSOR
• GENERAL WORKING PRINCIPLE
• CLASSIFICATION OF CHEMICAL SENSOR
• DISCUSSION OF THE TYPES OF CHEMICAL SENSOR
• SUMMARY/CONCLUSION
• QUESTIONS FROM THE PRESENTATION

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INTRODUCTION

• Definition of the Chemical Sensor
• A chemical sensor is a device that transforms
  chemical information (composition, presence of a
  particular element or ion, concentration,
  chemical activity, partial pressure.) into an
  analytically useful signal. The chemical
  information, mentioned above, may originate from
  a chemical reaction of the analyte or from a
  physical property of the system investigated.
  They can have applications in different areas
  such as medicine, home safety, environmental
  pollution and many others.

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WORKING PRINCIPLE OF THE CHEMICAL SENSOR

• Chemical sensors usually contain two basic
  components connected in series a chemical
  (molecular) recognition system (receptor) and a
  physicochemical transducer. In the majority of
  chemical sensors, the receptor interacts with
  analyte molecules. As a result, its physical
  properties are changed in such a way that the
  appending transducer can gain an electrical
  signal.
• Receptor The function of the receptor is
  fulfilled in many cases by a thin layer which is
  able to interact with the analyte molecules,
  catalyze a reaction selectively, or participate
  in a chemical equilibrium together with the
  analyte. The receptor layer can respond
  selectively to particular substances or to a
  group of substances.

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WORKING PRINCIPLE OF THE CHEMICAL SENSOR

• The term molecular recognition is used to
  describe this behavior. Among the interaction
  processes, the most important for chemical
  sensors are adsorption, ion exchange and
  liquid-liquid extraction. Primarily these
  phenomena act at the interface between analyte
  and receptor surface.
• Transducer Nowadays, signals are processed
  almost exclusively by means of electrical
  instrumentation. Accordingly, every sensor should
  include a transducing function, i.e. the actual
  concentration value, a non-electric quantity must
  be transformed into an electric quantity,
  voltage, current or resistance. Some of them
  develop their sensor function only in combination
  with an additional receptor layer. In other
  types, receptor operation is an inherent function
  of the transducer.

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DIAGRAM OF CHEMICAL SENSOR
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CLASSIFICATION OF CHEMICAL SENSOR

• Chemical sensors may be classified according to
• The operating principle of the transducer.
• The type of substance either, chemicl,biochemical
  or physical( in terms of optical) it is sensing
  from its vicinity.
• And they types include the folowing
• Optical devices transform changes of optical
  phenomena, which are the result of an interaction
  of the analyte with the receptor part.
• Electrochemical devices transform the effect of
  the electrochemical interaction analyte -
  electrode into a useful signal.
• . Electrical devices based on measurements, where
  no electrochemical processes take place, but the
  signal arises from the change of electrical
  properties caused by the interaction of the
  analyte.

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CLASSES OF THE CHEMICAL SENSOR

• 4. Mass sensitive devices transform the mass
  change at a specially modified surface into a
  change of a property of the support material.
• 5. Magnetic devices based on the change of
  paramagnetic properties of a gas being analysed.
  These are represented by certain types of oxygen
  monitors.
• 6. Thermometric devices based on the measurement
  of the heat effects of a specific chemical
  reaction or adsorption which involve the analyte.
  for example in the so called catalytic sensors
  the heat of a combustion reaction or an enzymatic
  reaction is measured by use of a thermistor.
• 7. Other physical properties as for example X-,
  p- or r- radiation may form the basis for a
  chemical sensor in case they are used for
  determination of chemical composition.

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THERMOMETRIC SENSOR(CATALYTIC SENSOR)

• A thermometric sensor can be set up easily by
  coating the surface of a thermometer with a
  catalytic layer. If the catalysed reaction has a
  considerable heat effect, then the reaction heat
  is preferably released locally at the active
  surface.Thermistors are micro thermometers useful
  as a basis for thermometric chemical sensors.
  They mainly consist of a semiconductor body with
  a temperature-dependent conductivity. As an
  example, a hydrogen sensor is created by coating
  a thermistor with a thin layer of black platinum
  Hydrogen traces in the air burn in the catalytic
  area. The reaction heat causes the temperature to
  rise

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• The resulting temperature difference compared to
  ambient temperature can be measured in terms of
  resistance change. This difference depends on the
  hydrogen content in the air. Other combustible
  gases like hydrogen sulphide or carbon monoxide
  can be analysed by means of the same arrangement,
  but with different catalysts. It is essential to
  find a catalyst as selective as
  possible.Thermistors are available in numerous
  sizes and shapes. Common forms are balls with
  diameters as small as 0.1mm or thin films on a
  substrate.

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CATALYTIC SENSOR(PELLISTOR)

• A catalyst is a chemical or substance that
  increases the rate of a reaction without being
  itself consumed.Heat is liberated as a result of
  a catalysed reaction.The temperature related to
  the chemical reaction is measured, using a
  calorimeter.Catalytic sensors are widely used to
  detect to detect low concentrations of flammable
  gases.
• Pellistors are used to detect the presence of
  flammable gases.Any combustible gases present
  will oxidise on the catalyst bead, raising the
  temperature of the coil.The change in resistance
  is detected by comparing with an uncatalysed
  reference sensor.

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http//www.citytech.com/technology/pellistors.asp
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Pellistor operating Modes.

• The platinum coil is embedded in a ceramic pellet
  coated with a porous catalytic metal (palladium
  or platinum). This coil acts as both the heater
  and temperature sensor (like in the Mass Flow
  Controller). When the combustible gas reacts at
  the catalytic surface, the heat evolved increases
  the temperature inside the thermal shield.
• This is raises the temperature of the platinum
  coil and thus its resistance

• www.ipc.uni-tuebingen.de/weimar/pictures/chem_sens
  or.gif

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• Pellistors have two operating modes
• Isothermal, where an electronic circuit controls
  the current in the coil required to maintain
  constant temperature.
• Non-isothermal, where the sensor is connected as
  part of a wheatstone bridge whose output voltage
  is a measure of the gas concentration.Catalytic
  sensors are widely used in the industries for
  detectation of flammable gases.

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OPTICAL CHEMICAL SENSORS.

• Optical sensors represent a group of chemical
  sensors in which electromagnetic (EM) radiation
  is used to generate the analytical signal in a
  transduction element. The interaction of this
  radiation with the sample is evaluated from the
  change of a particular optical parameter and is
  related to the concentration of the analyte.
• Typically, an optical chemical sensor consists of
  a chemical recognition phase (sensing element or
  receptor) coupled with a transduction element.
  The receptor identifies a parameter, e.g., the
  concentration of a given compound, pH, etc., and
  provides an optical signal proportional to the
  magnitude of this parameter. The function of the
  receptor is fulfilled in many cases by a thin
  layer that is able to interact with the analyte
  molecules, catalyse a reaction selectively, or
  participate in a chemical equilibrium together
  with the analyte. The transducer translates the
  optical signal produced by the receptor into a
  measurable signal that is suitable for processing
  by amplification, filtering, recording, display,
  etc.

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• The figure below shows a schematic of the basic
  components of an optical chemical sensor, namely,
  the sample (analyte), the transduction platform,
  and signal processing element (electronics)
  leading to the optical signal measurement which
  is related to the analyte concentration.

Principal stages in the operation of a Chemical
pH Sensor.
Optical sensors can be based on various optical
principles (absorbance, reflectance,
luminescence, fluorescence), covering different
regions of the spectra (UV, Visible) and allowing
the measurement not only of the intensity of
light, but also of other related properties, such
as lifetime, refractive index, scattering,
diffraction and polarization.
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• As an example, a luminescent sensor can be
  constructed by associating a sensing element,
  which emits light when in contact with a specific
  analyte, with a photodiode, which converts the
  energy of the incident light into a measurable
  signal.
• OPTICAL DETECTION PRINCIPLES
• For sensor applications only part of
  spectroscopic wavelength range is useful. From
  the practical point of view the following ways
  (Figure below) in which radiation can interact
  with an analytical sample are the most useful.
• absorption
• emission (fluorescence or phosphorescence)
• reflexion and refraction

Fig. General arrangement of spectroscopic
measurements A light reflection, B
light refraction, C light absorption, D light
emission.
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• Basically, Optical sensors employ optical
  transduction techniques and are based on reagents
  that change their optical properties on
  interaction with the analyte of interest. The
  most commonly measured optical properties are
  absorption, fluorescence intensity, and decay
  time, but in addition, reflectance, refractive
  index, light scattering, and light polarization
  have also been used as analytical parameters.A
  very good example of the Optical Chemical Sensor
  is the Optical Chemical PH Sensor.
• OPTICAL CHEMICAL PH SENSOR
• The Optical chemical pH sensors was developed in
  the last 2 years (August 2011-August 2013).
  Optical Chemical pH sensors could be
  reagent-based, namely, absorption- and
  fluorescence-based, as most optical pH sensors
  uses of colorimetric or fluorescent indicator
  dyes.

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• In more specific terms, pH is of concern in life
  sciences, food and beverage processing, soil
  examination, and marine and pharmaceutical
  research to name a few. Thus the development of
  an optical pH sensor which can be applied in real
  world applications is not trivial.
• WORKING PRINCIPLE OF THE OPTICAL CHEMICAL pH
  SENSOR.
• Free hydrogen ions do not exist in aqueous
  solution and should be described as hydronium
  ions H3O. The hydrogen ion is very small and
  possesses very high charge density. These
  characteristics promote a reaction with a water
  molecule resulting in very strong association.
  Very often, the term hydrogen ion or proton is
  used, but it has to be kept in mind that this is
  only an accepted convention. Søren Sørensen,(a
  Danish chemist) defines pH as the negative
  logarithm of hydrogen ions concentration.
• Today, the pH of a solution is defined in terms
  of a hydrogen ion H activity (sometimes called
  protons, or more correctly hydronium ions, H3O)

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• pH -log aH  where aH is the activity of the
  hydrogen ions.The negative sign assures that pH
  of most solutions is always positive. Optical
  pH sensors exploit pH indicator dyes which are
  typically weak organic acids or bases with
  distinct optical properties associated with their
  protonated (acidic) and deprotonate (basic)
  forms. The absorption (color) or fluorescence
  properties of these dyes are modified with a
  change of concentration of the hydrogen ions
  (pH). A schematic representation of absorption-
  and luminescence-based sensing is shown in the
  Figure below.

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• Schematic representation of the principle of (A)
  absorptionbased and (B) fluorescence-based pH
  sensing mechanisms.

Basically, the working Principle of the Optical
Chemical pH Sensors is passing Light through the
Analyte, then depending the the pH Concentration
of the Analyte, we will have different Degrees of
Absorption and Fluorescence of Light which are
given as Signals. These Signals are then
Processed and Amplified and digitally displayed
on a Screen. A Chemical Parameter (pH
Concentration) has been Converted to an
Electrical Signal.
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Coulometric Oxygen Sensors

• A Coulometric oxygen sensor is system that
  employs two chambers with a specimen mounted as a
  sealed semi-barrier between them.
• One chamber contains oxygen while the other is
  slowly purged with a stream of a carrier gas such
  as nitrogen.

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Mechanism of the Coulometric Oxygen sensor

• As the oxygen gas permeates through the specimen
  into the carrier gas, it is transported to the
  coulometric detector where it creates an electric
  current with a magnitude that is proportional to
  the number of oxygen atoms flowing into the
  detector

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• The solid state coulometric oxygen analyzer
  measures oxygen concentration by counting the
  number of electrons flowing through its circuit.
  Coulometric oxygen sensor is not consumed when it
  is exposed to oxygen, it does not require a
  constant purge to protect the sensor when the
  analyser is not being used and its lifetime is
  not dependant on how much oxygen it is exposed
  to. Coulometric oxygen sensors are suitable when
  oxygen concentrations are low in the gas to be
  measured.

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Summary of testing Method

• The specimen is mounted as a sealed semi-barrier
  between two chambers at ambient atmospheric
  pressure. One chamber is slowly purged by a
  stream of nitrogen and the other chamber contains
  oxygen. As oxygen gas permeates through the film
  into the nitrogen carrier gas, it is transported
  to the coulometric detector where it produces an
  electrical current, the magnitude of which is
  proportional to the amount of oxygen flowing into
  the detector per unit time. Among that, oxygen
  gas is the testing gas and nitrogen gas is the
  carrier gas. Oxygen gas concentration of upper
  chamber is higher than that of lower chamber, due
  to which certain concentration difference is
  formed between two sides of specimen. During the
  permeability process, oxygen gas transmits from
  upper chamber through specimen into lower
  chamber.

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Instrument Calibration

• The oxygen sensor used in this test method is a
  coulometric device that yields a linear output as
  predicted by Faraday's Law. In principle, four
  electrons are produced by the sensor for each
  molecule of oxygen that passes into it.
  Experience has shown under some circumstances the
  sensor may become depleted or damaged to the
  extent that efficiency and response are impaired.
  For that reason, this test method incorporates
  means for a periodic sensor evaluation. standard
  film is needed in instrument calibration. Since
  the data of standard film can directly influence
  the determination of calibration coefficient Q,
  special attention should be paid to film
  preparation and calibration repeatability.

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GAS SENSORS

• Definition
• Gas sensor is a subclass of chemical sensors.
• -Gas sensor it a subclass chemical sensor that
  measures the concentration of gas in its
  vicinity. Gas sensor interacts with a gas to
  measure its concentration. Each gas has a unique
  breakdown voltage i.e. the electric field at
  which it is ionized. Sensor identifies gases by
  measuring these voltages. The concentration of
  the gas can be determined by measuring the
  current discharge in the device.

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Applications of Gas Sensor

• Process control industries
• Environmental monitoring
• Boiler control
• Fire detection
• Alcohol breath tests
• Detection of harmful gases in mines
• Home safety
• Grading of agro-products like coffee and spices

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Disadvantages

• Low gas sensitivity due to the limited
  surface-to-volume ratio
• Bulky or very heavy.
• Consume lots of power in order to increase the
  sensitivity ( 500 degree)
• Require risky high voltage to operate.

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Types of gas sensors

• Carbone dioxide gas sensor
• Carbone monoxide gas sensor
• Hydrogen gas sensor

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1-Carbone dioxide (CO2) gas sensor

• it is based on infrared light absorption (by CO2)
  principle.
• The infrared detector detects the infrared light
  which is not absorbed by CO2 between source and
  detector. And then measures the heat produced by
  the non absorbed light. A voltage is produced due
  to the increasing of temperature in the infrared
  sensor.

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2- carbone monoxide gas sensor

• It can either be battery-operated or AC powered.
• Mostly the sensor will not sound an alarm at
  lower concentrations. (e.g. 100ppm).The alarm
  will sound within a few minutes at 400 ppm.So the
  function is specific to concentration-time.
• Figure shows simple carbon monoxide sensor.

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Types of carbone monoxide gas sensors

• Semiconductor sensor
• Electrochemical sensor
• Digital sensor
• Biomimetic sensor (chem-optical or gel cell sensor

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3- hydrogen gas sensor

• Palladium is commontly used to detect hydrogen
  because palladium selectively absorbs hydrogen
  gas and forms the chemical palladium hydride.
• Types of hydrogen gas sensor
• Optical fiber hydrogen sensors
• Nanoparticle-based hydrogen microsensors
• Diode based sensor

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GAS SENSING TECHNOLOGIES

• Metal Oxide Based Gas Sensors
• Capacitance Based Gas Sensors
• Acoustic Wave Based Gas Sensors
• Calorimetric Gas Sensors
• Optical gas sensors
• Electrochemical gas sensors

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NEW TECHNOLOGIES IN GAS SENSOR

• Advanced technology today by using nanomaterials
  offers possibility to improve gas detection. we
  can have the new carbon nanotube with has one of
  the best surface-of-volume ratio which is very
  important for hight sensitivity fast response
  and low temperature.
• The recent progress in developing MEMS
  (Micro-Electro-Mechanical Systems) leads to a
  new based H2 gas sensors. These sensors couple
  novel thin films as the active layer with a MEMS
  structure known as a Micro-Hotplate. This
  coupling results in a micro H2 gas sensor that
  has several unique advantages in terms of speed,
  sensitivity, stability and amenability to large
  scale manufacture.

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The new flexible carbon nano tube
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Some Companies involved in gas sensor fabrication
and prices

• RAE system offers different types of sensors and
  prices depends on the performance of the sensor.
  ex QRAEII which can senses hydrogen sulfide
  oxygen and carbon( 176 pound),carbon monoxide and
  oxygen(132 pound) ,oxygen(88 pound per unit)..
• Barharach
• crowncon

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SUMMARY/CONCLUSION

• DEFINITION
• BRIEF EXPLANATION OF CHEMICAL SENSORS
• GENERAL WORKING PRINCIPLES
• CLASSIFICATION OF CHEMICAL SENSORS
• SUMMARY/CONCLUSION
• QUESTIONS FROM THE PRESENTATION

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QUESTIONS FROM THE PRESENTATION

1. DEFINE CHEMICAL SENSORS
2. OUTLINE ANY 5 OF THE TYPES OF CHEMICAL SENSORS WE
   HAVE
3. EXPLAIN ANY ONE OF TYPES OF THE CHEMICAL SENSOR.

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• THANK YOU FOR GOOD LISTENING

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