Electrochemistry MAE-212

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

Table of contentPotentiometric SensorsAmperometric SensorsNanosensors as electrochemical sensors [Potentiometric and Amperometric Sensors (II)]

Potentiometric Sensors Potentiometric techniques are the most widely used electroanalytical method:Direct potentiometry pH and ions (pH sensors and ion selective probes)Indirect potentiometry: Enzyme sensors, Gas sensorsMiniaturization of Potentiometric Sensors

3Direct Potentiometric Sensors

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 thickWhen 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 HClOuter tube: reference electrode (Ag/AgCl) and salt bridge (KCl)Direct Potentiometric SensorsA 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 sensorHow to construct a combination electrode?

Direct Potentiometric pH SensorsThe 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 unitsStandard buffers (4.0, 7.0, 10.0) are used for calibrationAutomatically recognize standard buffers and adjust for temperature

Electrochemical MethodsApplications in Environmental Analysis

Direct Potentiometric pH SensorsDirect Potentiometric SensorsMeasurement of Ions by Ion Selective Electrodes (ISEs)

Uses direct potentiometry to measure ion concentration Membrane responds selectively to a given ionmV reading between sensing and reference electrode

Liquid phase: plastic or rubber film impregnated with a complex organic molecule that acts as an ion-carrier8Direct Potentiometric SensorsThere are many other types of potentiometric ion sensors or ISEs.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 solutionFor other ions than protons (cations and anions) other membranes are available (see e.g. LaF3 for F- and a wide variety of polymeric membranes

Direct Potentiometric SensorsAn 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)

Indirect Potentiometric Sensors: Enzyme Base Potentiometric SensorA 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 changeThe pH difference bewteen the two electrodes is proportional to the urea concentrationAs an example two IrOx electrodes may be used

Indirect Potentiometric Sensors:Carbon Dioxide Sensor Indirect Potentiometric Sensors: Carbon Dioxide Sensor (3D)

Indirect Potentiometric Sensors: Carbon dioxide sensor (MEMS version)

A pH, CO2 and oxygen electrochemical sensor array for in-vivo blood measurements was made using MEMS techniquesThe 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. .

Miniaturization of Potentiometric SensorsBy 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)

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

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).

17Amperometric SensorsA 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 itselfEnzyme electrodes are devices in which the analyte is either a substrate (also called reactant) or a product of the enzyme reaction, detected potentiometrically or amperometricallyHere 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.

Amperometric SensorsAmperometric 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 cathodeGlucose oxidase is an oxidase type enzyme, urease is a hydrolytic type enzyme. Other sensors can be constructed based on those enzymes.

-ilAnodicCathodic+i-i++ 0.6 V

Amperometric SensorsMeasurement of Dissolved Oxygen

e.g. Polarographic Clark cell

20Amperometric SensorsMeasurement 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 outCalibrated in air (O2), air saturated water (aerated water) or by Winkler method

21Amperometric Sensors

Measurement of Dissolved Oxygen

Calibrate the probe (in air)Place the probe below the surface of the waterSet the meter to measure temperature and allow the temperature reading to stabilizeSwitch the meter to 'dissolved oxygenFor saline waters, measure electrical conductivity level or use correction featureRe-test water to obtain a field replicate resultNOTE: The probe needs to be gently stirred to aid water movement across the membrane

22

Vurease

IrOxIrOx

Ecell = E ind - Eref (1)

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

EMBED Equation.2 (at 25oC)(2)

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

CO2 + H2O = H+ + HCO3-(3)

EMBED Equation.2 (4)

As the activities for H2O and HCO3- are constant in the electrolyte, the voltage of the sensor cell should be:

EMBED Equation.2 (5)

Evaporated Ag film

Chloridized Ag i.e. AgClHydrogel on reference electrodes with internal electrolytes

Insulator layer

Ion selective membrane

Glucose H2O2 + gluconic acidGlucose oxidase (in presence of oxygen)

Pt- anode (+)

Ag cathode (-)

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

Polyurethane membrane