Class 5

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BIOMEMS
Class V. Ion Selective Electrodes
Dr. Marc Madou
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Contents

• Ion selective electrodes (ISEs) and CO2 sensor
  (examples of potentiometric sensors)
• Oxygen sensor (based on the fuel cell principle)
• Enzyme based glucose sensor (amperometric) and
  urea (potentiometric)
• Immunosensor (amperometric)
• From ISFET to ISNt FET (potentiometric)

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Ion selective electrodes (ISEs)
Frit
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Ion selective electrodes (ISEs)

• A traditional pH measurement with a glass
  electrode is the best known potentiometric ion
  selective electrode (ISE) (e.g. a thin glass
  layer with this composition 22 Na2O, 6 CaO,
  72 SiO2)
• There is no change in the inner solution and
  there is no actual contact between inner and
  outer solution for any potentiometric probe or
  sensor
• Contact with the solution is always through the
  external reference electrode (Luggin capillary)

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Ion selective electrodes (ISEs)

• Often reference and glass electrode are combined
  in one single structure (How would you make such
  a thing ? See homework Q 1)
• The resistance (impedance) of this sensor is very
  high (glass layer) so that the input amplifier
  of the pH meter must be very high (the input
  impedance of the meter must be at least 100 gt
  than that of the sensor!)
• Very high impedance can make the measurement
  noisy. The smaller the sensor the bigger this
  problem becomes.

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Ion selective electrodes (ISEs)

• The so-called Donnan potential is established on
  both sides of the glass membrane-the potential on
  one side is kept constant through the internal
  reference solution while the other side is
  determined by the analyte solution
• For other ions than protons (cations and anions)
  other membranes are available (see e.g. LaF3 for
  F- and a wide variety of polymeric membranes)

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Ion selective electrodes (ISEs)

• An ion selective polymeric membrane is often made
  by mixing an ionophore (e.g. valinomycin, a
  natural occuring antibiotic) with PVC and a
  plasticizer (to make the rigid plastic more
  flexible)
• In these types of ISEs one sometimes does not
  use an internal reference solution at all or one
  incorporates a hydrogel to replace the aqueous
  solution . This makes the electrode easier to
  handle and store. Especially with no internal
  reference electrode drift tends to be larger!
• The polymeric ISEs lend themselves well to
  miniaturization and cost reduction (it is much
  more difficult to miniaturize a glass pH
  electrode)

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Ion selective electrodes (ISEs)

• By making ISEs planar (e.g. on a polyimide
  sheet) many sensors can be made in parallel (i.e.
  batch fabnrication). From 3D structures to 2D !
• Mass production can make them very small (e.g. 2
  by 3 mm), cheap (perhaps disposable),
  reproducible and even electronics might be
  integrated (see below under ISFETs)

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Carbon dioxide sensor

• Gases that react with water freeing or absorbing
  a proton in the electrolyte may be detected by a
  pH sensitive detector element e.g. glass or IrOx
• Example gases CO2, NH3, H2S, etc.
• A direct proportionality exists between the
  concentration of the neutral gas and the measured
  pH e.g. in the case of CO2 ( with NaHCO3 for
  internal electrolyte) i.e.

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Carbon dioxide sensor
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Carbon dioxide sensor (3D)
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Carbon dioxide sensor (MEMS version)

• A pH, CO2 and oxygen electrochemical sensor array
  for in-vivo blood measurements was made using
  MEMS techniques
• The pH and CO2 sensors are potentiometric and the
  oxygen sensor is amperometric (see further in
  this class)
• The pH sensor is an ISE based on a pH sensitive
  polymer membrane.
• The CO2 sensor is based on an IrOx pH sensor and
  a Ag/AgCl reference electrode. .

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Electrochemical oxygen sensor (fuel cell)
"Fuel cell" oxygen sensors consist of a diffusion
barrier, a sensing electrode (cathode) made of a
noble metal such as gold or silver, and a working
electrode made of a metal such as lead or zinc
immersed in a basic electrolyt (such as a
solution of potassium hydroxide). Oxygen
diffusing into the sensor is reduced to hydroxyl
ions at the cathode O2 2H2O 4e- --------
4 OH- Hydroxyl ions in turn oxidize the lead
(or zinc) anode 2Pb 4OH- ------------- 2PbO
2H2O 4e- 2Pb O2 ----------------- 2PbO
Fuel cell oxygen sensors are current
generators. The amount of current generated is
proportional to the amount of oxygen consumed
(Faraday's Law).
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Enzyme based sensor

• Enzymes are high-molecular weight biocatalysts
  (proteins) that increase the rate of numerous
  reactions critical to life itself
• Enzyme electrodes are devices in which the
  analyte is either a substrate (also called
  reactant) or a product of the enzyme reaction,
  detected potentiometrically or amperometrically
• Example glucose sensor substrate (glucose)
  diffuses through a membrane to the enzyme layer
  where glucose is converted
• Both oxygen (which is being consumed) and H2O2
  (which is being produced) can be measured
  electrochemically (in an amperometric technique),
  or the local pH change can be monitored (in a
  potentiometric measurement)

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Enzyme based sensor

• Amperometric glucose sensor based on peroxide
  oxidation,
• Plateau of limiting current is proportional to
  the peroxide concentration which in turn is
  proportional to glucose - - - typical 0.6 to 0.8
  V vs Ag cathode
• Glucose oxidase is an oxidase type enzyme,
  urease is a hydrolytic type enzyme

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Enzyme based sensor

• A potentiometric urea sensor may consist of two
  pH sensors one with the enzyme coated on aits
  surface and one without (the reference electrode)
  
• The electrode with the urease will sense a local
  pH change
• The pH difference bewteen the two electrodes is
  proportional to the urea concentration
• As an example two IrOx electrodes may be used

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Immunosensors

• Affinity pairs An enzyme/ substrate combination
  is only one example of an affinity pair, in
  nature there are many other examples of affinity
  pairs based on molecular recognition (think about
  double stranded DNA)
• Affinity pairs exhibit tremendous binding
  selectivity for each other through their
  intricate 3D molecular structures (lock and key)
  
• A much more selective affinity pair than enzyme /
  substrate pair is the antigen/antibody pair
  (AgAb) -- KA (affinity constant) values of
  106-1012 LM-1 vs 102-106 LM-1 (as a consequence
  enzyme sensors may be reversible while
  imunosensors are irreversible but much more
  selective)
• In an immunosensor one measures the concentration
  of either an antibody or an antigen by measuring
  an event triggered by the binding of an
  antigen/antibody- usually a label is involved
  (e.g. an enzyme, an isotope, a chromophore, etc.)
  , a direct detection of the binding event
  (without label) is very difficult but is being
  attempted in various research labs.

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Immunosensors

• One example of an immunosensor is an enzyme based
  immunosensor where the label is an enzyme--see
  next slide
• Typically an antigen (the same antigen we are
  trying to determine in the unknown solution) is
  labeled with an enzyme (say catalase) and added
  to the unknow sample in which the sensor is
  placed
• The labeled antigen competes with native
  (unlabeled antigen) for reaction with the
  antibody, which is immobilized on an electrode
  surface
• Unbound labeled antigen is washed off and
  substrate for the enzyme (H2O2 in the case of
  catalase) is added
• The enzyme decomposes H2O2 and the oxygen is
  picked up by the underlying oxygen sensor
• The oxygen current decreases with increasing
  concentration of the nonlabeled native antigen in
  the sample solution
• The enzyme reaction will produce many detectable
  species per bound AbAg pair, hence the name
  enzyme amplification.

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Immunosensors
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Immunosensors
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From ISFET to ISNT FET
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Homework

• Design a combination glass electrode. Explain
  how it works.
• Design a planar immunosensor. How could you
  incorporate a good reference?
• Explain how a potentiometric CO2 sensor works.
• List a list of reasons why the ISFET did not
  become a commercial success.