Dr. Marc Madou Class IV. Microfabrication of electrochemical sensors Winter 2011 BIOMEMS.

Dr. Marc Madou

Class IV. Microfabrication of electrochemical sensorsWinter 2011

BIOMEMS

Contents

Ion selective electrodes (ISE’s) 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 ISN’t FET (potentiometric)

Ion selective electrodes (ISE’s)

Inner reference electrode/ inner solution/membrane/analyte (external solution)/external reference electrode

Ag/AgCl Ag/AgCl

analyte

Inner solution

Inner solution

Membrane (e.g. potassium, sodium, pH, etc.)

Frit

Ion selective electrodes (ISE’s) 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)

Ion selective electrodes (ISE’s)

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 > than that of the sensor!)

Very high impedance can make the measurement noisy. The smaller the sensor the bigger this problem becomes.

Ion selective electrodes (ISE’s)

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)

Ion selective electrodes (ISE’s)

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 ISE’s 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 ISE’s lend themselves well to miniaturization and cost reduction (it is much more difficult to miniaturize a glass pH electrode)

Ion selective electrodes (ISE’s)

Evaporated Ag film

Chloridized Ag i.e. AgCl

Hydrogel on reference electrodes with internal electrolytes

Insulator layer

Ion selective membrane

By making ISE’s 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)

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.

1. CO 2 + H2 O ⇔ H2CO-3

⇔ H+ + HCO- 3 ⇔ 2H+ + CO2-2

2. NH3 + H2 O ⇔ NH+4 + OH-

3. H2 S + H2 O ⇔ HS- + H3O+

aH + =

K aCO2

aHCO-3

Ecell = E ind - Eref (1)

As the indicator is only H+ sensitive, and the potential of the reference is aconstant (because of the constant chloride concentration in the electrolyte), we have

Ecell =K1 + 0.059logaH+ (at 25oC) (2)

CO2 penetrates through the gas permeable membrane and will react with theelectrolyte in the agar hydrogel:

CO2 + H2O = H+ + HCO3

- (3)

aH+ =K'PCO2aH2O /aHCO3

− (4)

As the activities for H2O and HCO3- are constant in the electrolyte, the

voltage of the sensor cell should be:

Ecell =K2 + 0.059logPCO2 (5)

Carbon dioxide sensor

Carbon dioxide sensor (3D)

Ir/IrOx electrode

Ag/AgCl electrode

Gas permeable membrane

Dual lumen PVC tube

epoxysilver epoxy

silver spring contact

Hydrogel

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. .

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).

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)

Glucose H2O2 + gluconic acid

Glucose oxidase (in presence of oxygen)

Pt- anode (+)

Ag cathode (-)

Immobilized glucose oxidase (e.g. in cellulose-diacetate with heparin)

Polyurethane membrane

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:

-

i

l

Anodic

Cathodic

+i

-i

+

+ 0.6 V

Urease

CO (NH2 )2 → CO2 + 2 NH3 H2O

Enzyme based sensor

A potentiometric urea sensor may consist of two pH sensors one with the enzyme coated on its 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

V

urease

IrOxIrOx

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.

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.”

Oxygen sensor

Oxygen permeable membrane

Immobilized antibody

Competition for sites on the antibody

Immunosensors

Oxygen sensor

Oxygen permeable membrane

Immobilized antibody

Antigen

Enzyme labeled antigen

Immunosensors

Oxygen sensor

Oxygen permeable membrane

Wash the unbound antigen away and add H2O2

The oygen signal is lower the higher the amount of native antigen

Oygen is formed

From ISFET to ISN’T FET

Homework

1. Design a combination glass electrode. Explain how it works.

2. Design a planar immunosensor. How could you incorporate a good reference?

3. Explain how a potentiometric CO2 sensor works.

4. List a list of reasons why the ISFET did not become a commercial success.