Electrochemistry MAE-295

1. Electrochemistry MAE-295 Dr. Marc Madou, UCI, Winter 2012 Class V Potentiometric and Amperometric Sensors (I)

2. Table of content • Potentiometric Sensors • Amperometric Sensors • Nanosensors as electrochemical sensors [Potentiometric and Amperometric Sensors (II)]

3. Potentiometric Sensors • Potentiometric techniques are the most widely used electroanalyticalmethod: • Direct potentiometry – pH and ions (pH sensors and ion selective probes) • Indirect potentiometry: Enzyme sensors, Gas sensors • Miniaturization of Potentiometric Sensors

4. Direct PotentiometricSensors Outer tube: reference electrode (Ag/AgCl) and salt bridge (KCl) • Best know example is the pH sensor. • Combination electrodes (indicator+reference) for convenience (tube within a tube) • pH sensing component of the indicator electrode is the glass bulb, which is a thin glass membrane ~ 0.03 – 0.1 mm thick • When immersed, H+ ions from the solution enter the Si-O lattice structure of the glass membrane in exchange for Na+ Inner tube: pH indicator electrode (pH sensing membrane, Ag/AgCl reference electrode and HCl

5. Direct PotentiometricSensors • 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 • How to construct a combination electrode?

6. Direct PotentiometricpH Sensors • The glass bulb creates an electric ‘boundary’ potential across the membrane w.r.t. the internal Ag/AgCl reference electrode. This is called the Donnan potential: Where a H+ = activity of H+ (= concentration in very dilute solutions). Slope factor (2.303RT/F) is temperature dependent, pH meter must be adjusted for changes in temperature • All modern pH meters record potential (mV) and transform the voltage caused by H+ into pH units • Standard buffers (4.0, 7.0, 10.0) are used for calibration • Automatically recognize standard buffers and adjust for temperature

7. Electrochemical MethodsApplications in Environmental Analysis Direct PotentiometricpH Sensors

8. Direct PotentiometricSensors Measurement of Ions by Ion Selective Electrodes (ISEs) • Uses direct potentiometry to measure ion concentration • Membrane responds selectively to a given ion • mV reading between sensing and reference electrode

9. Direct PotentiometricSensors • There are many other types of potentiometric ion sensors or ISE’s. • The so-called Donnan potential is established on both sides of any ion selective 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

10. Direct PotentiometricSensors • 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)

11. Indirect PotentiometricSensors: Enzyme Base Potentiometric 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

12. Indirect PotentiometricSensors: Carbon Dioxide Sensor

13. Indirect PotentiometricSensors: Carbon Dioxide Sensor (3D)

14. Indirect PotentiometricSensors: 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. .

15. Miniaturization of Potentiometric Sensors • 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)

16. Miniaturziation of Potentiometric Sensors • Potentiometric sensors have been made the size of a transistor in ISFETs (almost).

17. Amperometric Sensors • Our first example of an amperometric sensors involves a "Fuel cell" oxygen sensors consisting 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).

18. Amperometric Sensors • A second example of an amperometric sensors is a simple (first generation) glucose sensor. This sensor is based on the enzyme Glucose Oxidase (GO). • 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 • Here we consider an amperometric glucose sensor where the substrate (glucose) diffuses through a membrane to the enzyme layer where glucose is converted and H2O2 is produced and electrochemically detected.

19. Anodic +i l i -  +  + 0.6 V -i Cathodic Amperometric Sensors • Amperometric glucose sensor based on peroxide oxidation, • The lateau of the 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. Other sensors can be constructed based on those enzymes.

20. Amperometric Sensors Measurement of Dissolved Oxygen • e.g. Polarographic Clark cell

21. Amperometric Sensors Measurement of Dissolved Oxygen e.g. Polarographic Clark cell O2 + 2H2O + 4e- ⇌ 4OH- (O2 reduced at gold cathode) 4Ag(s) + 4Cl-(aq) ⇌ 4AgCl(s) + 4e- (oxidation of silver at anode) • Membrane is susceptible to degradation, must be replaced if it dries out • Calibrated in air (O2), air saturated water (aerated water) or by Winkler method

22. Amperometric Sensors Measurement of Dissolved Oxygen • Calibrate the probe (in air) • Place the probe below the surface of the water • Set the meter to measure temperature and allow the temperature reading to stabilize • Switch the meter to 'dissolved oxygen‘ • For saline waters, measure electrical conductivity level or use correction feature • Re-test water to obtain a field replicate result NOTE: The probe needs to be gently stirred to aid water movement across the membrane