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.