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Chemical Sensors

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					Chemical Sensors

    Chapter 8
                  Introduction
   Chemical sensors are very different
   Sensing is usually based on sampling
   Sample is allowed to react in some fashion with
    elements of the sensor
   Usually an electric output is produced
   Transduction can be multi-stage and complex
   In some sensors, a complete analysis of the
    substance occurs
   In others a direct output occurs simply due to the
    presence of the substance.
                     Introduction
   Chemical sensing is quite common
   Used in industry for process control and for
    monitoring, including monitoring for safety.
   Important role in environmental protection
   Tracking of hazardous materials
   Tracking natural and man made occurrences
       pollution,
       waterways infestation
       migration of species
       weather prediction and tracking.
                 Introduction
   In sciences and in medicine - sampling of
    substances such as oxygen, blood, alcohol
   In the food industry for monitoring food safety
   Military has been using chemical sensors at
    least since WWI to track chemical agents used
    in chemical warfare
   Around the home and for hobbies (CO detection,
    smoke alarms, pH meters)
                Classification
   Direct and indirect output sensors
   Direct sensor: the chemical reaction or the
    presence of a chemical produces a measured
    electrical output.
   Example: the capacitive moisture sensor – the
    capacitance of a capacitor is directly
    proportional to the amount of water present
    between its plates.
                  Classification
   Indirect (also called complex) sensor relies on a
    secondary, indirect reading of the sensed stimulus.
   Example: optical smoke detector. An optical sensor such
    as a photoresistor is illuminated by a source and
    establishes a background reading.
   Smoke is “sampled” by allowing it to flow between the
    source and sensor and alter the light intensity, its
    velocity, its phase or some other measurable property.
   Some chemical sensors are much more complex than
    this and may involve more transduction steps. In fact,
    some may be viewed as complete instruments or
    processes.
                   Approach
   Avoid a rigid classification
   Concentrate on chemical sensors that are most
    important from a practical point of view while
   Try to cover most principles involved
   Steer clear of most chemical reactions and the
    formulas associated with them,
   Replace these by physical explanations that
    convey the process and explain the results
    without the need for analytic chemistry.
                          Approach
   Will start with the class of electrochemical sensors.
       Includes those sensors that convert a chemical quantity directly
        into an electrical reading and follows the definition above for
        direct sensors.
   The second group studied are those sensors that
    generate heat and the heat is the sensed quantity.
       These sensors, just like the thermo-optical sensors in chapter 4
        are indirect sensors as are the optical chemical sensors.
   Following these are some of the most common sensors
    such as pH and gas sensors.
   Humidity and moisture sensors are included here even
    though their sensing is not truly chemical but because
    the sensing methods and materials relate to chemical
    sensors.
      Electrochemical sensors
   Expected to exhibit changes in resistance
    (conductivity) or changes in capacitance
    (permittivity) due to substances or reactions.
   These may carry different names.
   Potentiometric sensors do not involve current –
    measurement of capacitance and voltage.
   Amperimetric sensors rely on measuring current
   Conductimetric sensors rely on measurement of
    conductivity (resistance).
        Electrochemical sensors
   These are different names for the same
    properties since voltage, current and resistance
    are related by Ohm’s law.
   Electrochemical sensors include a large number
    of sensing methods, all based on the broad area
    of electrochemistry. Many common sensors
    including fuel cells, surface conductivity sensors,
    enzyme electrodes, oxidation sensors and
    humidity sensors belong to this category.
          Metal-oxide sensors
   Rely on a very well known property of metal
    oxides at elevated temperature to change their
    surface potential, and therefore their conductivity
    in the presence of various reducible gases such
    as ethyl alcohol, methane and many other
    gases, sometimes selectively sometimes not.
   Metal oxides that can used are oxides of tin
    (SnO2), zinc (ZnO), iron (Fe2O2), zirconium
    (ZrO2), titanium (TiO2) and Wolfram (WO3).
   These are semiconductor materials and may be
    either p or n type (with preference to n –type).
           Metal-oxide sensors
   Fabrication is relatively simple
   May be based on silicon processes or other thin
    or thick film technologies.
   The basic principle is that when an oxide is held
    at elevated temperatures, the surrounding gases
    react with the oxygen in the oxide causing
    changes in the resistivity of the material.
   The essential components are the high
    temperature, the oxide and the reaction in the
    oxide
           Metal-oxide sensors
   Typical sensor: CO sensor shown in Figure
    8.1a.




   Consists of a heater and a thin layer of SnO2
            Metal-oxide sensors
   Construction:
   A silicon layer is first created to serve as temporary
    support for the structure.
   Above it an SiO2 layer is thermally grown.
   This layer can withstand high temperatures.
   On this a layer of gold is sputtered and etched to form a
    long meandering wire.
   The wire serves as the heating element by driving it with
    a sufficiently high current.
   A second layer of SiO2 is deposited.
             Metal-oxide sensors
   Then the SnO2 oxide is sputtered on top and patterned
    with grooves on top to increase its active surface.
   The original silicon material is etched away to decrease
    the heat capacity of the sensor.
   The sensing area can be quite small – 1-1.5 mm2.
   The device is heated to 300 C to operate but, because
    the size is very small and the heat capacity small as well,
    the power needed is typically small, perhaps of the order
    of 100 mW.
Metal-oxide sensors - operation
   Conductivity of the oxide can be written as:
                        = 0 + kPm
       0 is the conductivity of the tin oxide at 300C, without
      CO present
      P is the concentration of the CO gas in ppm (parts per
      million),
      k is a sensitivity coefficient (determined experimentally
      for various oxides)
      m is an experimental value - about 0.5 for tin oxide.
Metal-oxide sensors - operation
   Conductivity increases with increase in
    concentration as shown in Figure 8.1b.
   Resistance is proportional to the inverse of
    conductivity so that it may be written as
                     R = aPa
      a is a constant defined by the material and construction
      and
       a an experimental quantity for the gas.
      P is the concentration.
Response of a metal-oxide
        sensor
Metal-oxide sensors - operation
   The response is exponential (linear on the log
    scale)
   A transfer function of the type shown in
    Figure 8.1b must be defined for each gas
    and each type of oxide.
   SiO2 based sensors as well as ZnO sensors
    can also be used to sense CO2, touluene,
    benzene, ether, ethyl alcohol and propane
    with excellent sensitivity (1-50ppm).
Metal-oxide sensors - Variations
   A variation of the structure above is shown in
    Figure 8.2.
   It consists of an SnO2 layer on a ferrite
    substrate.
   The heater here is provided by a thick layer of
    RuO2, fed through two gold contacts.
   The resistance of the very thin SnO2 (less
    than about 0.5 m) is measured between two
    gold contacts.
   This sensor, which operates as described
    previously is sensitive to ethanol and carbon
    monoxide
Ethanol/ CO sensor
     Metal-oxide sensors - notes
   The reaction is with oxygen
   Any reducible gas (a gas that reacts with
    oxygen) will be detected.
   Lack of selectivity - common problem in metal
    oxide sensors. To overcome it,
       Select temperatures at which the required gas reacts
       The particular gas may be filtered.
   These sensors are used in many applications
    form CO and CO2 detectors to oxygen sensors
    in automobiles.
     Metal-oxide sensors - notes
   Example: oxygen sensors in automobiles use a TiO2
    sensor built as above in which resistance increases in
    proportion to the concentration of oxygen.
   This is commonly used in other application such as
    oxygen in water (for pollution control purposes).
   The process can also be used to determine the amount
    of available organic material in water by first evaporating
    the water and then oxygenating the residue to determine
    how much oxygen is consumed using an oxygen sensor.
   The amount of oxygen is then an indication of the
    amount of organic material in the sample.
        Solid elecrolyte sensor
   Another important type of sensor is the solid
    electrolyte sensor
   Has found significant commercial application
   Most often used in oxygen sensors, including
    those in automobiles.
   Principle: a galvanic cell (battery cell) is built
    which produces an emf across two electrodes
    based on the oxygen concentrations at the two
    electrodes under constant temperature and
    pressures.
          Solid elecrolyte sensor
   A solid electrolyte capable of operating at high
    temperatures is used
   Usually made of zirconium dioxide (ZrO2) and Calcium
    oxide (CaO) in a roughly 90% 10% ratio
   It has high oxygen ion conductivity at elevated
    temperatures (above 500C).
   The electrolite is made of sintered ZrO2/ CaO powder
    which makes it into a ceramic material.
   The inner and outer electrodes are made of platinum
    which act as catalysts and absorb oxygen. The structure
    is shown in Figure 8.3 for an exhaust oxygen sensor in a
    car engine.
Solid electrolyte oxygen sensor
       Solid electrolyte sensor -
               operation
   The potential across the electrodes is
                                 1
                     emf = RT lnpO2
                           4F pO22


      R is the gas constant (=8.314 J/K/mol),
      T is the temperature (K)
      F is the Faraday constant (=96487 C/mol).
      P1 is the concentration of oxygen in the exhaust,
      P2 the concentration of oxygen in the atmosphere, both
      heated to the same temperature.
    Solid electrolyte sensor - use
   Used to adjust the fuel ratio at the most efficient
    rate at which pollutants (NOx and CO) are
    converted into nitrogen (N2), carbon dioxide
    (CO2) and water (H2O), all of which are natural
    constituents in the atmosphere and hence
    considered non-pollutants
   Usually fuel is enriched to achieve full
    combustion of pollutants
   A PASSIVE SENSOR!
Solid electrolyte sensor - use as
        an active sensor
   Many engines operate in a much leaner mode
    (for better fuel efficiency),
   The solid electrolyte sensor is not sufficiently
    sensitive (the amount of oxygen in the exhaust is
    high and the reading of the electrolytic cell is
    insufficient).
   The solid electrolyte sensor is modified to act as
    a passive sensor
Solid electrolyte sensor - use as
        a passive sensor
   A solid electrolyte between two platinum electrodes as
    shown in Figure 8.4. are used, but:
   A potential is applied to the cell.
   This arrangement forces (pumps) oxygen across the
    electrolyte and a current is produced proportional to the
    oxygen concentration in the exhaust.
   The current is then a measure of the oxygen
    concentration in the exhaust
   This sensor is called a diffusion oxygen sensor or the
    diffusion-controlled limiting current oxygen sensor.
   Operates similar to charging a battery
Diffusion-controlled current
  limiting oxygen sensor
Oxygen sensor for molten metal
   Important in oxygen sensing in production of steel and
    other molten materials
   The quality of the final product is a direct result of the
    oxygen in the process. The sensor is shown in Figure
    8.5.
   The molybdenum needle keeps the device from melting
    when inserted in the molten steel.
   A potential difference is developed across the cell
    (between the molybdenum and the outer layer).
   The voltage is measured between the inner electrode
    and outer layer through an iron electrode dipped into the
    molten steel.
   The voltage developed is directly proportional to the
    oxygen concentration in the molten steel.
Oxygen sensor for molten
        metals
     The MOS chemical sensor
   Use of the basic MOSFET structure commonly
    used in electronics, as a chemical sensor.
   The basic idea: the classical MOSFET transistor
    in which the gate serves as the sensing surface.
   Advantage: a very simple and sensitive device is
    obtained which controls the current through the
    MOSFET.
   The interfacing of such a device is simple and
    there are fewer problems (such as heating,
    temperature sensing, etc.) to overcome.
        MOS chemical sensors

   Example, by simply replacing the metal
    gate in Figure 8.6 with palladium, the
    MOSFET becomes a hydrogen sensor
         MOS chemical sensors
   Palladium absorbs hydrogen and its potential changes
    accordingly.
   Sensitivity is down to about 1 ppm.
   Similar structures can sense gases such as H2S and
    NH3.
   Palladium mosfets (Pd-gate MOSFET) can also be used
    to measure oxygen in water, relying on the fact that the
    absorption efficiency of oxygen goes down in proportion
    to the amount of oxygen present.
   We shall say much more about the MOSFET sensor in
    the subsequent section on PH sensing since these have
    been very successful in this capacity.
         Potentiometric sensors
   A large subset of electrochemical sensors
   Principle: electric potential develops at the
    surface of a solid material immersed in solution
    containing ions that exchange at the surface.
   The potential is proportional to the number or
    density of ions in the solution.
   A potential difference between the surface of the
    solid and the solution occurs because of charge
    separation at the surface.
          Potentiometric sensors
   The contact potential, analogous to that used to set up a
    voltaic cell cannot be measured directly.
   If a second electrode is provided, an electrochemical cell
    is setup and the potential across the two electrodes is
    directly measurable.
   To ensure that the potential is measured accurately, and
    therefore that the ion concentration is properly
    represented by the potential, it is critical that the current
    drawn by the measuring instrument is as small as
    possible (any current is a load on the cell and therefore
    reduces the measured potential).
         Potentiometric sensors
   For a sensor of this type to be useful, the
    potential generated must be ion specific – that
    is, the electrodes must be able to distinguish
    between solutions.
   These are called ion-specific electrodes or
    membranes.
   The four types of membranes are:
   Glass membranes, selective for H+, Na+ and
    NH4+ and similar ions.
          Potentiometric sensors
   Polymer-immobilized membranes: In this type of
    membrane, an ion-selective agent is immobilized
    (trapped) in a polymer matrix. A typical polymer is PVC
   Gel-immobilized enzyme membranes: the surface
    reaction is between an ion specific enzyme which in turn
    is either bonded onto a solid surface or immobilized into
    a matrix - mostly for biomedical applications
   Soluble inorganic salt membranes: either crystalline or
    powdered salts pressed into a solid are used. Typical
    salts are LaF3 or mixtures of salt such as Ag2S and AgCl.
    These electrodes are selective to F, S and Cl and
    similar ions.
     Glass membrane sensors
   By far the oldest of the ion-selective electrodes,
   Used for pH sensing from the mid-1930’s and is
    as common as ever.
   The electrode is a glass made with the addition
    of sodium (Na2O) and aluminum oxide (Al2O3),
   Made into a very thin tube-like membrane.
   This results in a high resistance membrane
    which nevertheless allows transfer of ions
    across it.
   The basic method of pH sensing is shown in
    Figure 8.7a.
pH sensor
                    pH sensor
   Consists of the glass membrane electrode on the left
    and a reference electrode on the right.
   The reference electrode is typically an Ag/AgCl
    electrode in a KCl aqueous solution or a saturated
    Calomel electrode (Hg/Hg2Cl2 in a KCl solution).
   The reference electrode is normally incorporated into
    the test electrode so that the user only has to deal
    with a single probe as shown in Figure 8.7b.
   The sensor is used by first immersing the electrode
    into a conditioning solution of Hcl (0.1.mol/liter) and
    then immersing it into the solution to be tested. The
    electric output is calibrated in pH.
   A sensor of this type responds to pH from 1 to 14.
pH probe with reference
      electrode
       Glass membrane sensors
   Modifications of the basic configuration, both in
    terms of the reference electrode (filling) as well
    as the constituents of the glass membrane lead
    to sensitivity to other types of ions as well as to
    sensors capable of sensing dissolved gas in
    solutions, particularly ammonia but also CO2,
    SO2, HF, H2S and HCN
          Soluble inorganic salt
           membrane sensors
   Based on soluble inorganic salts which undergo
    ion-exchange interaction in water and generate
    the required potential at the interface.
   Typical salts are the lanthanum fluoride (LaF3)
    and silver sulfide (Ag2S).
   The membrane may be either
       a singe crystal membrane,
       a sintered disk made of powdered salt
       a polymer matrix embedding the powdered salt
       each has its own application and properties
          Soluble inorganic salt
           membrane sensors
   The structure of a commercial sensor used to
    sense fluoride concentration in water is shown
    next
   The sensing membrane, made in the form of a
    thin disk grown as a single crystal.
   The reference electrode is created in the internal
    solution (in the case: NaF/NaCl at 0.1 mol/liter).
   The sensor shown can detect concentrations of
    fluoride in water between 0.1 and 2000 mg/l.
   This sensor is commonly used to monitor
    fluoride in drinking water (about 1mg/l).
   Soluble inorganic salt
membrane sensors for fluoride
          Soluble inorganic salt
           membrane sensors
   Membranes may be made of other materials
    such as silver sulfide.
   The latter is easily made into thin sintered disks
    from powdered material and may be used in lieu
    of the single crystal.
   Other compounds may be added to affect the
    properties of the membrane and hence
    sensitivities to other ions.
   This leads to selective sensors sensitive to ions
    of chlorine, cadmium, lead and copper and are
    often used to sense for dissolved heavy metals
    in water.
      Polymeric salt membranes
   Polymeric membranes are made by use of
    a polymeric binder for the powdered salt
   About 50% salt and 50% binding material.
   The common binding materials are PVC,
    polyethylene and silicon rubber.
   In terms of performance these membranes
    are quite similar to sintered disks.
         Polymer-immobilized
        ionophore membranes
   A development of the inorganic salt membrane
   Ion-selective, organic reagents are used in the
    production of the polymer by including them in
    the plasticizers, particularly for PVC.
   A reagent, called ionophore (or ion-exchanger)
    is dissolved in the plasticizer (about 1% of the
    plasticizer).
   This produces a polymer film which can then be
    used as the membrane replacing the crystal or
    disk in sensors.
Polymer-immobilized ionophore
         membranes
   The construction of the
    sensor is simple
   Shown in Figure 8.9 and
    includes an Ag/AgCl
    reference electrode.
   The resulting sensor is a
    fairly high resistance
    sensor.
Polymer-immobilized ionophore
         membranes
   A different approach to building
    polymer-immobilized ionophore
    membranes is shown in Figure
    8.10.
   It is made of an inner platinum wire
    on which the polymer membrane is
    coated
   The wire is protected with a coating
    of paraffin.
   This is called a coated wire
    electrode.
   To be useful a reference membrane
    must be added.
      Gel-immobilized enzyme
            membranes
   Similar in principle to polymer immobilized
    ionophore membranes
   A gel is used and the ionophore is replaced by
    an enzyme which is selective to a particular ion.
   The enzyme, (a biomaterial) is immobilized in a
    gel (polyacrylamide) and held in place on a
    glass membrane electrode as shown in Figure
    8.11.
   The choice of the enzyme and the choice of the
    glass electrode define the selectivity of the
    sensor.
Gel-immobilized enzyme
  membrane sensor
        Gel-immobilized enzyme
          membrane sensors
   Gel sensors exist for the sensing of a variety of
    important analytes including urea glucose, L-
    amino acids, penicillin and others.
   The operation is simple; the sensor is placed in
    the solution to be sensed which diffuses into the
    gel and reacts with the enzyme.
   The ions released are then sensed by the glass
    electrode.
   These sensors are slow in response because of
    the need for diffusion but they are very useful in
    analysis in medicine including blood and urine.
    The Ion-sensitive field effect
          transistor ISFET
   Also called the ChemFet
   Essentially a MOSFET in which the gate has
    been replaced by an ion-selective membrane.
   Any of the membranes above may be used -
    most often the glass and polymeric membranes
   In its simplest form, a separate reference
    electrode is used but the reference electrode
    may be easily incorporated within the gate
    structure as shown in Figure 8.12.
Ion-sensitive field effect
    transistor ISFET
      Ion-sensitive field effect
          transistor ISFET
   The gate is then allowed to come in
    contact with the sample to be tested
   The drain current is measured to indicate
    the ion concentration.
   The most important use of this device is
    measurement of pH
   Available commercially.
     Thermo-chemical sensors
   A class of sensors that rely on the heat
    generated in chemical reactions to sense the
    amount of particular substances (reactants).
   There are three sensing strategies, each leading
    to sensors for different applications.
       sense the temperature rise due to the reaction
       catalytic sensor used for sensing of flammable gases.
       measures the thermal conductivity in air due to the
        presence of a sensed gas.
    Thermisotor based chemical
             sensors
   Principle: sense the small change in
    temperature due to the chemical reaction.
   A reference temperature sensor is usually
    employed to sense the temperature of the
    solution
   The difference in temperature is then related to
    the concentration of the senses substance.
   The most common approach is to use an
    enzyme based reaction (enzymes are highly
    selective - so that the reaction can be
    ascertained - and because they generate
    significant amounts of heat).
     Thermisotor based chemical
              sensors
   A typical sensor is made by
    coating the enzyme directly
    on the thermistor.
   The thermistor itself is a
    bead thermistor which
    makes for a very compact
    highly sensitive sensor.
   The construction is shown in
    Figure 8.13.
     Thermisotor based chemical
              sensors
   Used to sense concentration of urea and glucose, each
    with its own enzyme (urease or glucose enzymes).
   The amount of heat generated is proportional to the
    amount of the substance sensed in the solution.
   The temperature difference between the treated
    thermistor and the reference thermistor is then related to
    the concentration of the substance.
   A thermistor can measure temperature differences as
    low as 0.001C but most are less sensitive than that
   Overall sensitivity depends on the amount of heat
    generated.
            Catalytic sensors
   True calorimetric sensors:
   A sample of the (gas) analyte is burned
   The heat generated in the processed is
    measured through a temperature sensor.
   This type of sensor is very common
   Main tool in detection of flammable gases such
    as methane, butane, carbon monoxide and
    hydrogen, fuel vapors such as gasoline as well
    as flammable solvents (ether, acetone, etc.).
              Catalytic sensors
   Principle: sampling of air containing the flammable gas
    into a heated chamber
   Combusts the gas to generate heat.
   To speed up the process, a catalyst is used.
   The temperature sensed is then indicated as a
    percentage of flammable gas in air.
   The simplest form of a sensor is to use a platinum coil
    through which a current is passed.
   The platinum coil heats up due to its own resistance and
    serves as a catalyst for hydrocarbons (this is the reason
    why it is the active material in a catalytic converter in
    automobiles).
             Catalytic sensors
   The released heat raises the temperature of the
    coil.
   This resistance is then a direct indication of the
    amount of flammable gas in the sampled air.
            Catalytic sensors
   Better catalysts are palladium and rhodium
   One such sensor, called a “pellistor” (the name
    comes from Pellister who discovered the
    process), is shown in Figure 8.14.
   It uses the same heater and temperature
    sensing mechanism (platinum coil)
   Uses a palladium catalyst either external to the
    ceramic bead or embedded in it.
Catalytic sensor (pelistor)
  using a catalyst layer
              Catalytic sensors
   The second is better because there is less of a
    chance of contamination by noncombustible
    gases (called poisoning – which reduce
    sensitivity).
   Advantage: operate at lower temperatures
    (about 500C as opposed to about 1000C for
    the platinum coil sensor).
   A sensor of this type will contain two beads, one
    inert (serving as reference) and one sensing
    bead, in a common sensing head shown in
    Figure 8.15.
   This generates a reaction in a few seconds.
Catalytic sensors with
  reference pelistor
           Catalytic sensors -
              application
   Used in mines to detect methane and in industry
    to sense solvents in air.
   The most important issue is the concentration at
    which a flammable gas explodes.
   This is called the lower explosive limit (LEL),
    below which a gas will not ignite.
   For methane for example, the LEL limit is 5% (by
    volume, in air).
   A methane sensor will be calibrated as % of LEL
    (100%LEL corresponds to 5% methane in air)
    Thermal conductivity sensor
   Does not involve any chemical reaction
   Uses the thermal properties of gases for detection.
   A sensor of this type is shown in Figure 8.16.
   It consists of a heater set at a given temperature (around
    250C).
   The heater looses heat to the surrounding area,
    depending on the gas with which it comes in contact.
   As the gas concentration becomes higher a larger
    amount of heat is lost compared to loss in air and the
    temperature of the heater as well as its resistance
    diminish.
Thermal conductivity sensor
    Thermal conductivity sensor
   This change in resistance is sensed and
    calibrated in terms of gas concentration.
   Unlike the previous two types of sensors, this
    sensor is useful for high concentrations of gas.
   It can be used for inert gasses such as nitrogen,
    argon and carbon dioxide as well as for volatile
    gases.
   The sensor is in common use in industry and is
    a useful tool in gas chromatography in the lab.
     Optical chemical sensors
   Transmission, reflection and absorption
    (attenuation) of light in a medium, its velocity
    and hence its wavelength are all dependent of
    the properties of the medium.
   These can all serve as the basis of sensing
    either by themselves or in conjunction with other
    transduction mechanisms and sensors.
   For example, the optical smoke detector uses
    the transmission of light through smoke to detect
    the presence of smoke.
      Optical chemical sensors
   Other substances are sensed in this way, sometimes by
    adding agents to, for example, color the substance
    tested.
   More complex mechanisms are used to obtain highly
    sensitive sensors to a variety of chemical conditions.
   In many optical sensors, use is being made of an
    electrode which, when in the substance being tested,
    changes some optical property of the electrode.
   An electrode of this type is called an “optode” in parallel
    with “electrode”.
   The optode has an important advantage in that no
    reference is needed and it is well suited for use with
    optical guiding systems such as optical fiber.
      Optical chemical sensors
   Other options for opto-chemical sensing are the
    properties of some substances to fluoresce or
    phosphoresce under optical radiation.
   These chemiluminescence properties can be senses and
    used for indication of specific materials or properties.
   Luminescence can be a highly sensitive method
    because the luminescence is at a different frequency
    (wavelength) than the frequency (wavelength) of the
    exciting radiation.
   This occurs more often with UV radiation but can occur
    in the IR or visible range as well and is often used for
    detection.
    Optical chemical sensors
   Optical sensing mechanisms rely at least in part on
    absorption of light by the substance through which it
    propagates or on which it impinges.
   This absorption, is governed by the Beer-Lambert
    law, stated as follows:

                            A = bM
      is the absorption coefficient characteristic of the medium
    [103cm2/mol],
    b is the path length [cm] traveled and
    M is the concentration in [mol/l].
    A=log(P0/P) is the absorbance where P0 is the incident and P the
    transmitted light intensity.
        Optical chemical sensor
   The simplest sensors are the reflectance
    sensors
   Rely on the reflective properties of a membrane
    or substance to infer a property of the
    substance.
   In many of these sensor a fiber optic cable or an
    optical waveguide are used.
   The basic structure is shown in Figure 8.23.
   A source of light (LED, white light, laser)
    generates a beam which is conducted through
    the optical fiber to the optode.
Reflection optical sensor
         Optical chemical sensor
   The optical properties of the optode are altered by the
    substance to which it reacts
   The reflected beam is then a function of the
    concentration of the analyte or its reaction products in
    the optode.
   It is also possible to separate the incident and reflected
    beams by separate optical guides but usually this is not
    necessary.
   An alternative way of sensing is to use an uncladded
    optical fiber so the light is lost through the walls of the
    fiber.
   This is called an evanescent loss and depends on what
    is in contact with the walls of the fiber.
Evenescent field sensing
        Optical chemical sensor
   In this type of sensor the coupling to the optode
    is through he walls of the fiber rather than its
    end.
   This also means that rather than reflection, the
    transmission through the fiber is measured.
   The transmitted wave is then dependent on the
    amount of light absorbed in the optode and
    therefore a function of the analyte in the optode.
             Optical pH sensor
   pH sensing can be done optically by using
    special optodes which change color with change
    in pH.
   In these systems, only about one pH unit on
    either side of the pH of the optode (before the
    analyte interaction) can be sensed.
   This span is sufficient for some applications in
    which the range is narrow.
   A sensor of this type is shown in Figure 8.25.
Reflection pH sensor
              Optical pH sensor
   A hydrogen permeable membrane is used in which
    phenol red is immobilized on polyacrylamide
    microspheres.
   The membrane is a dialysis tube (cellulose acetate)
   The optode is attached to the end of an optical fiber.
   When immersed the analyte, diffuses into the optode.
   Phenol red is known to absorb light at a wavelength of
    560 nm (yellow-green light).
   The amount of light absorbed depends on pH and hence
    the reflected light will change with pH.
   The difference between the incident and reflected
    intensities is then related to pH.
             Optical pH sensor
   A similar sensor uses the fluorescent properties
    of HPTS (a weak acid).
   This substance fluoresces when excited by UV
    light at 405 nm.
   The intensity of fluorescence is then related to
    the pH.
   This material is particularly useful since its
    normal pH is 7.3 so that measurements around
    the neutral point can be made and in particular
    in physiological measurements.
               Optical pH sensor
   Optodes can also be used to sense ions.
   Metal ions are particularly easy to sense because they
    can form highly colored complexes with a variety of
    reagents.
   These reagents are embedded in the optode and the
    reflectivity properties are then related to concentration of
    the metal ions.
   Fluorescence is also common in metal ions, a method
    that is used extensively in analytical chemistry, primarily
    by use of UV light, with fluorescence in the visible range.
   These methods have been used to sense a variety of
    other ions including oxygen in water, penicillin and
    glucose in blood and others.
                Mass sensors
   Detect the changes in the mass of a sensing
    element due to absorption of an analyte.
   Masses involved in absorption are minute
   A method must be found that will be sensitive to
    these minute mass changes.
   Mass sensors are also called microgravimetric
    sensors.
   In a practical sensor it is not possible to sense
    this change in mass and therefore indirect
    methods must be used.
                   Mass sensors
   This is done by using piezoelectric crystals such as
    quartz
   Setting them into oscillation at their resonant frequency
    (see chapter 7).
   This resonant frequency is dependent on the way the
    crystal is cut and on dimensions but once these have
    been fixed, any change in mass of the crystal will change
    its resonant frequency.
   The sensitivity is generally very high - of the order of 10
    g/Hz and a limit sensitivity of about 10g.
   Since the resonant frequency of crystals can be very
    high, the change in frequency due to change in mass is
    significant and can be accurately measured digitally.
                    Mass sensors
   An equivalent approach can be taken with SAW
    resonators which,
   They can resonate at even higher frequencies than
    crystals and hence offer higher sensitivities.
   The shift in resonant frequency can be written as:
                           f = f0Smm
      f0 is the base resonant frequency
      Sm is a sensitivity factor that depends of the crystal (cut,
      shape, mounting, etc.)
       m is the change in mass.
               Mass sensors
   The mass due to the analyte may be
    absorbed directly into the crystal (or any
    piezoelectric material) or in a coating on the
    crystal.
   Simple and efficient sensors.
   Selectivity is poor since crystals and coatings
    tend to absorb more than one species
    confounding discrimination between species.
   A basic requirement is that the process be
    reversible, that is, the absorbed species must
    be removable (by heating) without any
    hysteresis.
       Mass sensors - humidity
              sensing
   The most common analyte is water vapor
   A mass humidity sensor is made by coating the
    crystal by a thin layer of hygroscopic material
   There are many hygroscopic materials that may be
    used including polymers, gelatins, silica, fluorides.
   The moisture is removed after sensing by heating.
   A sensor of this type can be quite sensitive but its
    response time is slow.
   It may take many seconds (20-30sec) for sensing
    and many more for regeneration (30-50 sec).
         Mass sensors - notes
   The method is very useful and has been
    applied to sensing of a large variety of gases
    and vapors, some being sensed at room
    temperatures, some at elevated
    temperatures.
   The main difference between sensing one
    gas or another is in the coating, in an attempt
    to make the sensor selective.
   The applications are mostly in sensing of
    noxious gases and in dangerous substances
    such as mercury.
           Mass sensors - notes
   Sensing of sulfur dioxide (mostly due to burning of coal and
    fuels) is by amine coatings which react with sulfur dioxide.
    Sensitivities as low as 10 ppb are detectable.
   When detecting ammonia (for application in environmental
    effects of waste water and sewage), the coating is ascorbic acid
    or pyridoxine hydrochloride (and some similar compounds) with
    sensitivities down to micrograms/kg.
   Hydrocarbon sulfide is similarly detected by using acetate
    coatings (silver, copper, lead acetates as well as as others).
   Mercury vapor is sensed by the use of gold as a coating since
    the two elements form an amalgalm that increases the mass of
    the gold coating.
   Other applications are in sensing hydrocarbons, nitro-toluenes
    (emitted by explosives) and gases emitted by pesticides,
    insecticides and other sources.
          SAW mass sensors
   A SAW mass sensor is made as a delay line
    resonator, as we have seen in chapter 7.
   The delay line itself is now coated with the
    specific reactive coating for the gas to be
    sensed.
   This is shown in Figure 8.17.
   To operate, air containing the gas is sampled
    (drawn above the membrane) and the resonant
    frequency measured.
SAW mass sensor
            SAW mass sensors
   Can be used to sense solid particles such as pollen or
    pollutants by replacing the membrane with a sticky
    substance.
   The problem then would be the regeneration – cleaning
    the surface for the next sampling.
   The choice of coating determines the selectivity of the
    sensor. Table 8.1 shows some sensed substances and
    the appropriate coatings.
   Sensitivities of saw resonators can be much higher than
    crystal resonators with limit sensitivities of approximately
    10-15g. Sensitivities expected are of the order of 50
    Hz/Hz. (25 kHz shift for a 500 Mhz resonator)
 Coatings and analytes for SAW
            sensors

Table 8.1. Some sensed substances and the coatings used for that purpose.
Compound                     Chemical coating               SAW mterial
SO2                          TEA                            Lithium Niobate
H2                           Pd                             Lithium Niobate, Silicon
NH3                          Pt                             Quartz
H2S                          WO3                            Lithium Niobate
Water vapor                  Hygroscopic material           Lithium Niobate
NO2                          PC                             Lithium Niobate, Qu7artz
NO2, NH3, SO2, CH4           PC                             Lithium Niobate
Explosives vapor, drugs      Polymer                        Quartz
SO2, methane                 none                           Lithium Niobate
TEA=Trithanolamine, PC=Phthalocyamine.
    Humidity and moisture sensors
   SAW sensors is indicated are common sensors
   There are however other methods of sensing
    humidity
   All involve some type of hygroscopic medium to
    absorb water vapor.
   These can take many forms - capacitive,
    conductive and optical are the most common
    Humidity and moisture sensors
   The terms humidity and moisture are not
    interchangeable.
   Humidity refers to the water content in gases such as in
    the atmosphere.
   Moisture is the water content in any solid or liquid.
   Other important, related quantities are
        dew point temperature
        absolute humidity and
        relative humidity.
   These are defined as follows:
Humidity and moisture sensors
   Relative humidity is the ratio of the water
    vapor pressure of the gas (usually air) to the
    maximum saturation water vapor pressure in
    the same gas at the same temperature.
   Saturation is that water vapor pressure at
    which droplets form. The atmospheric
    pressure is the sum of the water vapor
    pressure and the dry air pressure.
   Relative humidity is not used above the
    boiling point of water (100C) since the
    maximum saturation above that temperature
    changes with temperature.
Humidity and moisture sensors
   Dew-point temperature is the temperature at
    which relative humidity is 100%. This is the
    temperature at which air can hold maximum
    amount of moisture. Cooling below it creates
    fog (water droplets), dew or frost.
   Absolute humidity is defined as the mass of
    water vapor per unit volume of wet gas in
    grams/cubic meter [g/m3].
Humidity and moisture sensors
   The simplest moisture sensor is capacitive
    sensor
   It relies on the change in permittivity due to
    moisture.
   The permittivity of water is rather high (800 at
    low frequencies).
   Humidity of course is different than liquid
    water and hence the permittivity of humid air
    is either given in tables as a function of
    relative humidity or may be calculated from
    the following empirical relation:
Humidity and moisture sensors
           = 1 + 211 P + 48Ps H 1060
                  T        T

     0 is permittivity of vacuum,
     T is the absolute temperature [K],
     P is the pressure of moist air [mm Hg],
     H is the relative humidity [%]
     Ps is the pressure of saturated water vapor
      at the temperature T [mm Hg]
    Humidity and moisture sensors
   The capacitance of a parallel plate capacitor is
    C=A/d
   This establishes a relation between capacitance
    and relative humidity:
                                             21148Ps
                  C = C0 + C0211P106 + C0              106H
                              T                 T2

       C0 is the capacitance of the capacitor in vacuum
       (C0=A/d).
    This relation is linear at any given pressure and temperature.
Humidity and moisture sensors
   In more practical designs, means of
    increasing this capacitance are used.
   Use a hygroscopic material between the
    plates both to increase the capacitance at no
    humidity and to absorb the water vapor.
    (hygroscopic polymer films.
   The metal plates are made of gold. In a
    device of this type the capacitance is
    approximated as:
                       C = C0 + C0ahH
     where ah is a moisture coefficient
    Humidity and moisture sensors
   Method assumes that the moisture content in the
    hygroscopic polymer is directly proportional to
    relative humidity and that
   As the humidity changes, the moisture content
    changes (that is, the film does not retain water).
   Under these conditions the sensing is
    continuous but, as expected, changes are slow
    and
   A sensor of this type can sense relative humidity
    from about 5% to 90% at an accuracy of 2-3%.
    Humidity and moisture sensors
   In a parallel plate capacitor the film must be thin
   Moisture can only penetrate from the sides.
   It is therefore slow to respond to changes in
    moisture because of the time it takes for
    moisture to penetrate throughout the film.
   A different approach is shown in Figure 8.18.
   Here the capacitor is flat and built from a series
    of interdigitated electrodes to increase
    capacitance.
Capacitive moisture sensor
    Capacitive moisture sensors
   The hygroscopic dielectric may be made of SiO2 or
    phosphorosilicate glass.
   The layer is very thin to improve response.
   Because the sensor is based on silicon, temperature
    sensors are easily incorporated as are other
    components such as oscillators.
   The capacitance of the device is low and therefore it
    will be used as part of an oscillator and the frequency
    sensed.
   However, the permittivity of the dielectric is frequency
    dependent (goes down with frequency).
   This means that frequency cannot be too high,
    especially if low humidity levels are sensed.
    Resistive moisture sensors
   Humidity is known to change the resistivity
    (conductivity) of some nonconducting
    materials.
   This can be used to build a resistive sensor.
   A hygroscopic conducting layer and two
    electrodes are provided.
   The electrodes will be interdigitated to
    increase the contact area, as shown in
    Figure 8.19.
   The hygroscopic conductive layer must have
    a relatively high resistance which goes down
    with humidity (actually absorbed moisture).
Resistive moisture sensor
    Resistive moisture sensors
   Materials that can be used for this purpose include
    polystyrene treated with sulfuric acid and solid
    polyelectrolytes
   A better structure is shown in Figure 8.20. It operates
    as above but the base material is silicon.
   An aluminum layer is formed on the silicon (highly
    doped so its resistivity is low).
   The aluminum layer is oxidized to form a layer of
    Aluminum oxide which is porous and hygroscopic.
   An electrode of porous gold is deposited on top to
    create the second contact and to allow moisture
    absorption in the Al2O3 layer
Thermally conductive moisture
           sensor
         Thermally conductive
           moisture sensors
   Humidity may also be measured through thermal
    conduction
   Higher humidity increases thermal conduction.
   This sensor however senses absolute humidity
    rather than relative humidity.
   The sensor makes use of two thermistors
    connected in a differential or bridge connection
    (bridge connection is shown in Figure 8.21.
Thermally conductive moisture
           sensor
    Thermally conductive moisture
               sensor
   The thermistors are heated to an identical temperature
    by the current through them so that the output is zero in
    dry air.
   One thermistor is kept in an enclosed chamber as a
    reference and its resistance is constant.
   The other is exposed to air and its temperature changes
    with humidity.
   As humidity increases, its temperature drops and hence
    its resistance increases (for NTC thermistors).
   At saturation the peak is reached. Above that the output
    drops again (Figure 8.21b).
         Optical humidity sensor
   By measuring the ambient temperature t, and
    then evaluating the dew point temperature DPT,
    RH is calculated from Eq. (8.1).
   The basic idea is to use a dew point sensor.
   The latter is built as shown in Figure 8.22.
   The sensors is based on detecting the dew point
    on the surface of a mirror.
   To do so, light is reflected off the mirror and the
    light intensity monitored.
          Optical humidity sensor
   A Peltier cell is used to cool the mirror to its dew point.
   When the dew point temperature is reached, the
    controller keeps the mirror at the dew point temperature.
   The reflectivity now drops since water droplets form on
    the mirror (the mirror fogs up).
   This temperature is measured and is the dew point
    temperature in Eq. (8.1).
   Although this is a rather complex sensor and includes
    the reference diodes for balancing, it is rather accurate,
    capable of sensing the dew point temperature at
    accuracies of less than 0.05C
Optical dew point temperature
           sensor
Mass/SAW resonator dew point
     temperature sensor
   The same measurement can be done with the
    mass sensor described in the previous section.
   The resonant frequency of a crystal, covered with
    a water-selective coating is used and its resonant
    frequency sensed while the sensor is cooled.
   At the dew point, the sensor’s coating is saturated
    and the frequency is the lowest.
   Equally well, a SAW mass sensor may be used
    with even higher accuracy.
   The heating/cooling is achieved as in Figure 9.22
    by use of a Peltier cell.

				
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