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

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

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

4. 4 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 Chemical sensors:

5. 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. 6 An example of biochemical sensor

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

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

9. 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. 10 Analytical potentiometric signal is equally divided between the ISE and the reference electrode.

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

12. 12 Schematic view of the equilibrium between sample

13. 13 Classification and Mechanism (1) Phase boundary potential i z i (membrane) ⇌ i z i (sample) Under equilibrium conditions  i M =  i W which is equivalent to a i W and a i M are the ion activities in the sample and membrane phases

14. 14 with

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

16. 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, K p, 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 ClO 4 - > SCN - > I - > NO 3 - >Br - > Cl - > HCO 3 - > SO 4 2- > F -

17. 17 (3) Neutral-ionophore-based ISEs I + (Membrane) + L(membrane) ⇌ IL + (membrane) The formation constant, , is given by  = a IL M /(a I M a L M ) the charge balance in the membrane phase R T = [I + ] M + [IL + ] M ⇌ [IL + ] M [L] M = L T - [IL + ] M ⇌ L T - R T a I L =  IL M R T /   L M (L T - R T )

18. 18

19. 19 (4) Charged-ionophore-based ISEs

20. 20

21. 21

22. 22 Outline Introduction 1 Potentiometric sensors 2 Amperometric sensors 3 Voltametric sensors 4

23. 23 Conceptual drawing of three electrode amperometric electrochemical sensor and potentiostat When the information is obtained from measurement of current, that is in amperometric sensors, the role of the Ohm’s Law becomes immediately apparent. 1. Introduction

24. 24 2. Amperometric titrations A. Simple amperometric titration I V V equiv i ii iii iv Forms of amperometric titration Two electrodes: a redox indicator electrode & a reference electrode A fixed potential difference

25. 25 B. Biamperometric titration Two redox electrodes Non-reference electrode App: a reversible system before or after the endpoint i ii iii I V V equiv. R 1 + O 2 ⇌ O 1 + R 2 i Both 1 and 2 are rev. ii only 2 is rev. iii only 1 is rev.

26. 26 C. Amperometric titrations with double hydrodynamic electrode generator A±n 1 e →B solution B+X → products Detector B±n 2 e → C (or A) I gen I det N0N0 N0N0 M  2/3 N0N0 J. Electroanal. Chem., 1983, 144, 211 N ’ = 0.035

27. 27 3. Membrane and membrane-covered electrodes Enzyme & Microb. Tech. 1998, 23, 10–13

28. 28 L. C. Clark Jr., Trans. Am. Soc. Artif. Intern. Organs, 1956, 2, 41 The Clark electrode for determination of dissolved oxygen Amperometric sensors for dissolved gases Gases dissolved in aqueous phase: O 2, NO, halothane, CO 2 Gas phase: H 2 S, HCN, CO, NO, NO 2, Cl 2

29. 29 4. Modified electrodes A. Chemical modification {chemical bonding). B. Adsorption C. Electroadsorption D. Plasma( 等离子体 )

30. 30 Examples of modifiers for amperometric sensors Bard and Faulkner, 2001, pp. 584–585

31. 31 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

32. 32 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 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 i = nFAv A

33. 33 Talanta, 2006, 69, 259–266

34. 34 Principles of pre-concentration techniques MethodPreconcentration step Determination step Measure- ment AStripping voltammetry Potential control Potential control I vs. t (or I vs. E) BAdsorptive stripping voltammetry Adsorption (with or without applied potential) Potential control I vs. t (or I vs. E) CPotentiometric stripping analysis Potential control Reaction with Oxidant or reductant in solution E vs t DStripping Chron- opotentiometry Potential control Current controlE vs t

35. 35 e.g. Determination step in stripping techniques I E Ip  cIp  c E p → species A(anodic) I E Ip  cIp  c E p → species B(cathodic)

36. 36 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

37. 37 Example 1. Direct Oxidation of Glucose Oxidase reactions take place in the enzyme layer Schematic diagram of a simple amperometric biosensor Ag anode: 4Ag + 4Cl - ⇌ 4AgCl + 4e - Pt cathode: O 2 + 4H + + 4e - ⇌ 2H 2 O

38. 38 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 TCNQH 2

39. 39 Substrate(2H) + FAD-oxidase ⇌ Product + FADH 2 -oxidase (a) biocatalyst FADH 2 -oxidase + O 2 ⇌ FAD-oxidase + H 2 O 2 electrode H 2 O 2 ⇌ O 2 + 2H + + 2e - (b) biocatalyst FADH 2 -oxidase + 2 Ferricinium + ⇌ FAD-oxidase + 2 Ferrocene + 2H + electrode 2 Ferrocene ⇌ 2 Ferricinium + + 2e - (c) FADH 2 -oxidase ⇌ FAD-oxidase + 2H + + 2e -

40. 40 Example 2. Ethanol Electrodes

41. 41 Example 3. Urea Electrodes

42. 42 Some of Common Enzyme Electrodes

43. 43 Gas Sensors Some potentiometric gas sensors

44. 44 Example 4. CO 2 Sensors

45. 45 Example 5. O 2 Sensors

46. 46 Outline Introduction 1 Potentiometric sensors 2 Amperometric sensors 3 Voltametric sensors 4

47. 47 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

48. 48 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

49. 49 Typical example of real-time measurement using the system Novel PLM-voltammetric Coupling Devices for Trace Metal Speciation Proc. ECS 2003, Paris

50. 50 voltammetric determination of acetaminophen, aspirin and caffeine Electroch. Acta, 2010, 55, 8638–8648

51. 51 AdSDPV curves obtained for the oxidation of ACOP, ASA and CF at equal concentrations of each: (1) blank, (2) 2.91 × 10 −7, (3) 2.89 × 10 −6, (4) 7.62 × 10 −6, (5) 1.78 × 10 −5, (6) 2.56 × 10 −5, (7) 3.10 × 10 −5, (8) 4.08 × 10 −5, (9) 5.32 × 10 −5 and (10) 6.27 × 10 −5 M.