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CHEMICAL SENSORPREPARED AND PRESENTED

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PWADUBASHIYI C. PWAVODI MSC(20143883)

FELIX CHIBUZO OBI MSC(20144610)

MTHABISI MOYO(20143906)

ERINE SAUREL GUEPP(20135813)

LECTURER:ALI 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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INTRODUCTIONDefinition 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 SENSORChemical 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:

1. Optical devices transform changes of optical phenomena, which are the result of an interaction of the analyte with the receptor part.

2. Electrochemical devices transform the effect of the electrochemical interaction analyte - electrode into a useful signal.

3. . 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. CHEMICAL SENSORS

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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.

CHEMICAL SENSORShttp://www.citytech.com/technology/pellistors.asp

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Pellistor operating Modes.

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• 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_sensor.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.

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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

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Fig. General arrangement of spectroscopic measurements: A – light reflection, B – lightrefraction, 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

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• 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. CHEMICAL SENSORS

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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/CONCLUSIONDEFINITIONBRIEF EXPLANATION OF CHEMICAL

SENSORSGENERAL WORKING PRINCIPLESCLASSIFICATION OF CHEMICAL

SENSORSSUMMARY/CONCLUSIONQUESTIONS 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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