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Applied Electrochemistry

Applied Electrochemistry. Dept. Chem. & Chem. Eng. Lecture 14 Electrochemical sensors. Dept. Chem. & Chem. Eng. 1. Introduction. 2. 4. Potentiometric sensors. Voltametric sensors. 3. Amperometric sensors. Outline. Chemical sensors :.

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Applied Electrochemistry

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  1. Applied Electrochemistry Dept. Chem. & Chem. Eng.

  2. Lecture 14 Electrochemical sensors Dept. Chem. & Chem. Eng.

  3. 1 Introduction 2 4 Potentiometric sensors Voltametric sensors 3 Amperometric sensors Outline

  4. Chemical sensors: small devices that can be used for direct measurement of the analute in the sample matrix. characteristics (1) responding continuously and reversibly (2) Does not perturb the sample

  5. Contruction of chemical sensors A transduction element covered with chemical or biochemical recognition layer Target analyte interact with the recognition layer and change the resulting from interactions to electrical signals

  6. An example of biochemical sensor

  7. Electrochemical sensors Electrochemical sensors is a subclass of chemical sensors in which electrode is used as a transduction elements

  8. 1 Introduction 2 4 Potentiometric sensors Voltametric sensors 3 Amperometric sensors Outline

  9. Miniaturization of potentiometric sensors. (A) Conventional ion-selective electrode (ISE) with reference electrode connected to the field-effect transistor amplifier. (B) The electrical connection between ISE and the amplifier is made shorter. (C) Electrical connection is eliminated and the ISE membrane is placed directly at the input of the amplifier, thus forming an ISFET (D)

  10. Analytical potentiometric signal is equally divided between the ISE and the reference electrode.

  11. Principle reference electrode 2 || sample solution | membrane | inner solution || reference electrode 1 activity in the sample solution of analyte I unique for the analyte Charge

  12. Schematic view of the equilibrium between sample

  13. Classification and Mechanism (1) Phase boundary potential izi (membrane) ⇌ izi (sample) Under equilibrium conditions iM = iW which is equivalent to aiW and aiM are the ion activities in the sample and membrane phases

  14. with

  15. (2) Ion-exchanger-based ISEs(ion-selective electrodes) Membrane compositions and selectivity coefficients of ion-exchanger-based ISEs

  16. The salt-extraction process can be defined as I+(water) + X-(water) ⇌ I+(membrane) + X-(membrane) Also, the equilibrium reaction can be quantified by salt-partitioning constant, Kp, as defined by Thus, the concentration of the aqueous anion in the cation-selective membrane doped with anionic sites is negligible in the charge balance in the membrane phase [I+]M = [R-]M + [X-]M⇌ [R-]M selectivity of anion-exchanger-based ISEs follows ClO4- > SCN- > I- > NO3- >Br- > Cl- > HCO3- > SO42- > F-

  17. (3) Neutral-ionophore-based ISEs I+(Membrane) + L(membrane) ⇌ IL+(membrane) The formation constant, , is given by  = aILM/(aIMaLM) the charge balance in the membrane phase RT =[I+]M + [IL+]M⇌ [IL+]M [L]M =LT - [IL+]M⇌ LT- RT aIL = ILM RT/LM(LT - RT)

  18. (4) Charged-ionophore-based ISEs

  19. 1 Introduction 2 4 Potentiometric sensors Voltametric sensors 3 Amperometric sensors Outline

  20. 1. Introduction When the information is obtained from measurement of current, that is in amperometric sensors, the role of the Ohm’s Law becomes immediately apparent. Conceptual drawing of three electrode amperometric electrochemical sensor and potentiostat

  21. 2. Amperometric titrations A. Simple amperometric titration Two electrodes: a redox indicator electrode & a reference electrode i I ii iii iv V Vequiv A fixed potential difference Forms of amperometric titration

  22. B. Biamperometric titration R1 + O2⇌ O1 + R2 Two redox electrodes Non-reference electrode I i ii App: a reversible system before or after the endpoint iii i Both 1 and 2 are rev. ii only 2 is rev. iii only 1 is rev. V Vequiv.

  23. C. Amperometric titrations with double hydrodynamic electrode generator A±n1e →B solution B+X → products Detector B±n2e → C (or A) Idet N0 N’= 0.035 M2/3 N0 M2/3 Igen N0 J. Electroanal. Chem., 1983, 144, 211

  24. 3. Membrane and membrane-covered electrodes Enzyme & Microb. Tech. 1998, 23, 10–13

  25. The Clark electrode for determination of dissolved oxygen Amperometric sensors for dissolved gases Gases dissolved in aqueous phase: O2, NO, halothane, CO2 Gas phase: H2S, HCN, CO, NO, NO2, Cl2 L. C. Clark Jr., Trans. Am. Soc. Artif. Intern. Organs, 1956, 2, 41

  26. 4. Modified electrodes A. Chemical modification {chemical bonding). B. Adsorption C. Electroadsorption D. Plasma(等离子体)

  27. Examples of modifiers for amperometric sensors Bard and Faulkner, 2001, pp. 584–585

  28. Processes that can occur at a modified electrode. (1) heterogeneous reduction process; (2) successive transfer of electron between reduced molecules Q (5), until the transfer to A at the surface (3); (4) diffusion of A into the film and its reaction with Q; (6) direct penetration of A through the pinhole to the substrate electrode

  29. 5. Increase in sensitivity: pre-concentration techniques principle a. application of a pulse waveform and a.c. voltammetry b. utilization of a pre-concentration step that accumulates the electroactive species on the electrode surface i = nFAvA process a. Deposition or adsorption of the species on the electrode b. Change to an inert electrolyte medium c. Reduction/oxidation of the species that was accumulated at the electrode

  30. Talanta, 2006, 69, 259–266

  31. Principles of pre-concentration techniques

  32. e.g. Determination step in stripping techniques A(anodic) B(cathodic) I I Ip c Ip c Ep→species Ep→species E E

  33. the choice of which technique and experimental protocol to use depends on various factors • The concentration range of the species to be determined • Possible interferences to its exact determination, i.e. matrix composition • The accuracy and precision necessary • The quantity of sample • The required speed with which an answer is required

  34. Example 1. Direct Oxidation of Glucose Oxidase reactions take place in the enzyme layer Ag anode:  4Ag + 4Cl- ⇌ 4AgCl + 4e- Pt cathode:    O2 + 4H+ + 4e- ⇌ 2H2O Schematic diagram of a simple amperometric biosensor

  35. NAD+ nicotinamide adenine dinucleotide (a) Ferrocene (e5-bis-cyclopentadienyl iron), the parent compound of a number of mediators. (b) TMP+, the cationic part of conducting organic crystals. (c) TCNQ.-, the anionic part of conducting organic crystals. It is a resonance-stabilised radical formed by the one-electron oxidation of TCNQH2

  36. Substrate(2H) + FAD-oxidase ⇌ Product + FADH2-oxidase (a) biocatalyst FADH2-oxidase + O2 ⇌ FAD-oxidase + H2O2 electrode H2O2 ⇌ O2 + 2H+ + 2e- (b) biocatalyst FADH2-oxidase + 2 Ferricinium+ ⇌ FAD-oxidase + 2 Ferrocene + 2H+ electrode 2 Ferrocene ⇌ 2 Ferricinium+ + 2e- (c) FADH2-oxidase ⇌ FAD-oxidase + 2H+ + 2e-

  37. Example 2. Ethanol Electrodes

  38. Example 3. Urea Electrodes

  39. Some of Common Enzyme Electrodes

  40. Gas Sensors Some potentiometric gas sensors

  41. Example 4. CO2 Sensors

  42. Example 5. O2 Sensors

  43. 1 Introduction 2 4 Potentiometric sensors Voltametric sensors 3 Amperometric sensors Outline

  44. Def. voltammetric systems are produced commercially for the determination of specific species that are of interest in industry and research. These devices are sometimes called electrodes but are, in fact, complete voltammetric cells and are better referred to as sensors. These sensors can be employed for the analysis of various organic and inorganic analytes in various matrices

  45. Scheme of voltametric electrochemical sensor PLM (Permeation Liquid Membrane) and voltammetric detector (WE: working electrode, MAE: micro auxiliary electrode).  Novel PLM-voltammetric Coupling Devices for Trace Metal Speciation   Proc. ECS 2003, Paris

  46. Typical example of real-time measurement using the system  Novel PLM-voltammetric Coupling Devices for Trace Metal Speciation   Proc. ECS 2003, Paris

  47. voltammetric determination of acetaminophen, aspirin and caffeine Electroch. Acta, 2010, 55, 8638–8648

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