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Title: Chemical Sensors


1
Chemical Sensors
  • Chapter 8

2
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.

3
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.

4
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)

5
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.

6
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.

7
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.

8
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.

9
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).

10
Electrochemical sensors
  • These are different names for the same properties
    since voltage, current and resistance are related
    by Ohms 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.

11
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).

12
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

13
Metal-oxide sensors
  • Typical sensor CO sensor shown in Figure 8.1a.
  • Consists of a heater and a thin layer of SnO2

14
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.

15
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.

16
Metal-oxide sensors - operation
  • Conductivity of the oxide can be written as

?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.
17
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

a is a constant defined by the material and
construction and a an experimental quantity for
the gas. P is the concentration.
18
Response of a metal-oxide sensor
19
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).

20
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

21
Ethanol/ CO sensor
22
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.

23
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.

24
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.

25
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.

26
Solid electrolyte oxygen sensor
27
Solid electrolyte sensor - operation
  • The potential across the electrodes is

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.
28
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!

29
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

30
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

31
Diffusion-controlled current limiting oxygen
sensor
32
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.

33
Oxygen sensor for molten metals
34
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.

35
MOS chemical sensors
  • Example, by simply replacing the metal gate in
    Figure 8.6 with palladium, the MOSFET becomes a
    hydrogen sensor

36
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.

37
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.

38
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).

39
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.

40
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.

41
Glass membrane sensors
  • By far the oldest of the ion-selective
    electrodes,
  • Used for pH sensing from the mid-1930s 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.

42
pH sensor
43
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.

44
pH probe with reference electrode
45
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

46
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

47
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).

48
Soluble inorganic salt membrane sensors for
fluoride
49
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.

50
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.

51
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.

52
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.

53
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.

54
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.

55
Gel-immobilized enzyme membrane sensor
56
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.

57
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.

58
Ion-sensitive field effect transistor ISFET
59
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.

60
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.

61
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).

62
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.

63
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.

64
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.).

65
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).

66
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.

67
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.

68
Catalytic sensor (pelistor) using a catalyst layer
69
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.

70
Catalytic sensors with reference pelistor
71
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
    (100LEL corresponds to 5 methane in air)

72
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.

73
Thermal conductivity sensor
74
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.

75
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.

76
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.

77
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.

78
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

? 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. Alog(P0/P) is the absorbance where P0
is the incident and P the transmitted light
intensity.
79
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.

80
Reflection optical sensor
81
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.

82
Evenescent field sensing
83
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.

84
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.

85
Reflection pH sensor
86
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.

87
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.

88
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.

89
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.

90
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.

91
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

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.
92
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.

93
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).

94
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.

95
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.

96
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.

97
SAW mass sensor
98
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)

99
Coatings and analytes for SAW sensors
100
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

101
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

102
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.

103
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.

104
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

105
Humidity and moisture sensors
  • e0 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

106
Humidity and moisture sensors
  • The capacitance of a parallel plate capacitor is
    C?A/d
  • This establishes a relation between capacitance
    and relative humidity

C0 is the capacitance of the capacitor in vacuum
(C0??A/d). This relation is linear at any given
pressure and temperature.
107
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

108
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.

109
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.

110
Capacitive moisture sensor
111
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.

112
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).

113
Resistive moisture sensor
114
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

115
Thermally conductive moisture sensor
116
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.

117
Thermally conductive moisture sensor
118
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).

119
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.

120
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

121
Optical dew point temperature sensor
122
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 sensors 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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