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

Table of content Potentiometric Sensors Amperometric Sensors Nanosensors 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 sensors Miniaturization of Potentiometric Sensors

Direct 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 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 Outer tube: reference electrode (Ag/AgCl) and salt bridge (KCl)

Direct Potentiometric Sensors 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 ) 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?

Direct Potentiometric pH 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

Electrochemical Methods Applications in Environmental Analysis Direct Potentiometric pH Sensors

Direct Potentiometric Sensors 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

Direct Potentiometric Sensors 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. LaF 3 for F - and a wide variety of polymeric membranes

Direct Potentiometric Sensors 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)

Indirect Potentiometric Sensors: 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 IrO x 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, CO 2 and oxygen electrochemical sensor array for in-vivo blood measurements was made using MEMS techniques The pH and CO 2 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 CO 2 sensor is based on an IrOx pH sensor and a Ag/AgCl reference electrode..

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)

Miniaturziation of Potentiometric Sensors Potentiometric 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: 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).

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 H 2 O 2 is produced and electrochemically detected.

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. -  i l Anodic Cathodic +i -i +  V

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

Amperometric Sensors Measurement of Dissolved Oxygen e.g. Polarographic Clark cell O 2 + 2H 2 O + 4e - ⇌ 4OH - (O 2 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 (O 2 ), air saturated water (aerated water) or by Winkler method

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