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

Contents u Ion selective electrodes (ISE’s) and CO 2 sensor (examples of potentiometric sensors) u Oxygen sensor (based on the fuel cell principle) u Enzyme based glucose sensor (amperometric) and urea (potentiometric) u Immunosensor (amperometric) u From ISFET to ISN’t FET (potentiometric)

Ion selective electrodes (ISE’s) Frit

Ion selective electrodes (ISE’s) u 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% Na 2 O, 6% CaO, 72% SiO 2 ) u 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 u Contact with the solution is always through the external reference electrode (Luggin capillary)

Ion selective electrodes (ISE’s) u Often reference and glass electrode are combined in one single structure (How would you make such a thing ? See homework Q 1) u 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!) u Very high impedance can make the measurement noisy. The smaller the sensor the bigger this problem becomes.

Ion selective electrodes (ISE’s) u 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 u For other ions than protons (cations and anions ) other membranes are available (see e.g. LaF 3 for F - and a wide variety of polymeric membranes)

Ion selective electrodes (ISE’s) u 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) u 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! u 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) u 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 ! u 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 u 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 IrO x u Example gases: CO 2, NH 3, H 2 S, etc. u A direct proportionality exists between the concentration of the neutral gas and the measured pH e.g. in the case of CO 2 ( with NaHCO 3 for internal electrolyte) i.e.

Carbon dioxide sensor

Carbon dioxide sensor (3D)

Carbon dioxide sensor (MEMS version) u A pH, CO 2 and oxygen electrochemical sensor array for in-vivo blood measurements was made using MEMS techniques u The pH and CO 2 sensors are potentiometric and the oxygen sensor is amperometric (see further in this class) u The pH sensor is an ISE based on a pH sensitive polymer membrane. u The CO 2 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: O 2 + 2H 2 O + 4e OH- Hydroxyl ions in turn oxidize the lead (or zinc) anode: 2Pb + 4OH PbO + 2H 2 O + 4e- 2Pb + O PbO 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 u Enzymes are high-molecular weight biocatalysts (proteins) that increase the rate of numerous reactions critical to life itself u 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 u Example : glucose sensor substrate (glucose) diffuses through a membrane to the enzyme layer where glucose is converted u Both oxygen (which is being consumed) and H 2 O 2 (which is being produced) can be measured electrochemically (in an amperometric technique), or the local pH change can be monitored (in a potentiometric measurement)

Enzyme based sensor u Amperometric glucose sensor based on peroxide oxidation, u 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 u Glucose oxidase is an oxidase type enzyme, urease is a hydrolytic type enzyme: -  i l Anodic Cathodic +i -i +  V

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

Immunosensors u 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) u Affinity pairs exhibit tremendous binding selectivity for each other through their intricate 3D molecular structures (lock and key) u A much more selective affinity pair than enzyme / substrate pair is the antigen/antibody pair (AgAb) -- K A (affinity constant) values of LM -1 vs LM -1 (as a consequence enzyme sensors may be reversible while imunosensors are irreversible but much more selective) u 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 u One example of an immunosensor is an enzyme based immunosensor where the label is an enzyme--see next slide u 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 u The labeled antigen competes with native (unlabeled antigen) for reaction with the antibody, which is immobilized on an electrode surface u Unbound labeled antigen is washed off and substrate for the enzyme (H 2 O 2 in the case of catalase) is added u The enzyme decomposes H 2 O 2 and the oxygen is picked up by the underlying oxygen sensor u The oxygen current decreases with increasing concentration of the nonlabeled native antigen in the sample solution u The enzyme reaction will produce many detectable species per bound AbAg pair, hence the name “enzyme amplification.”

Immunosensors

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 CO 2 sensor works. 4. List a list of reasons why the ISFET did not become a commercial success.