S canning E lectro c hemical M icroscopy (SECM)

1. Scanning Electrochemical Microscopy (SECM)

2. Heterogeneous reactions Industrial applications - heterogeneous catalysts combinatorial chemistry Heterogeneous reactions Aristoteles: „corpora non agunt nisi fluida seu soluta“ Compounds that are not fluid or dissolved, do not react J. B. Karsten (1843): „Philosophy of Chemistry“ The reaction of two heterogeneous, solid, and under certain conditions reactive compounds can only occur if one of them can be transformed into a fluid induced by the interaction between the two compounds at a given temperature or due to pressure increased temperature, which then will induce the fluid state in the other compound.“

3. D D Reactions at interfaces • Assumptions • Mass transport is limited to diffusion • Diffusion constants are equal for both, educt and product • Adsorption, desorption, and reaction are not distinguished

4. Forward reaction rate Backward reaction rate Electron transfer reactions Electrode reaction

5. Dependence of kf and kb on the interfacial potential difference Current-potential characteristic (Butler-Volmer model) Electron transfer reactions

6. Current-potential characteristic Electron transfer reactions Exchange current At equilibrium i = 0

7.  Nernst-equation Electron transfer reactions Testing the equation

8. ia ic Calculation of i0 starting from ic Electron transfer reactions Exchange current

9. ScanningElectrochemical Microscopy (SECM)

10. Scanning probe microscopy (SPM) techniques

11. Principle of scanning probe techniques

12. Scanning electrochemical microscope

13. Ultramicroelectrodes (UME) Essential concept At least in one dimension (the “characteristic dimension”), the size of the electrode surface is smaller than the diffusion length of the redox active species (during the time period of the experiment) Spherical or hemispherical UME Disk UME Cylindrical UME Band UME

14. Planar and radial diffusion at electrodes Fick‘s second law in one dimension Concentration profiles at disk electrodes 1 s after starting a diffusion-controlled electrolysis r0 = 3 mm r0 = 30 µm r0 = 300 µm rad > 6 = UME vert rad

15. Spherical diffusion at an UME Planar and radial diffusion at electrodes Chronoamperometric experiments Applying a constant potential E diffusion controlled transport of the electroactive species monitoring the time-dependent current that depends on the concentration gradient How does the concentration gradient of cR/O change? Planar diffusion at a conventional electrode

16. Hemispherical diffusion at UME Planar diffusion (Cottrell-equation) Planar and radial diffusion at electrodes Current-time curves r0 = 5 µm r0 = 12.5 µm r0 = 1.5 mm

17. r0 rA Preparation of UME Pt glass Melting of wires into glass tubes => large RG-values Pulling wire-glass tube with a pipet puller => decrease of RG value Etching of platinum wires and isolation with electrodeposition paint

18. UME probe Ultramicroelectrode 5 < RG < 20

19. Scanning electrochemical microscope

20. Approach curves Tip far away from surface Tip close to the surface Current depends on distance between tip and sample

21. i/ i i/i d/r0 Approach curves

22. Modi of SECM Generation-collection mode (GC) Sample-generation/tip-collection mode (SG/TC) Tip-generation/sample-collection mode (TG/SC) => Constant height Feedback mode (FB) Negative feedback Positive feedback => Constant current

23. Generation-collection mode Sample-generation/tip-collection mode (SG/TC) Tip is scanned across the surface at constant height Generator: Heterogeneous reaction Mass transport through a pore

24. Generation-collection mode Disadvantages Diffusion layer larger than the tip => determines lateral resolution Electrical isolation of SECM-tip limits diffusion of educts to the generator In case of large generator areas a continuously increasing background signal is observed due to the formation of product Advantages In the beginning of the measurement no background signal occurs as there is no product produced

25. Generator: glucose oxidase FAD

26. Generator: glucose oxidase -D-Glucose + Glucoseoxidase/FAD  Glucono--Lacton + Glucoseoxidase/FADH2 Glucoseoxidase/FADH2 + O2  Glucoseoxidase/FAD + H2O2 Oxidation of H2O2 at the Pt-UME H2O2 2 H+ + 2 e- + O2

27. Feedback mode Positive feedback Negative feedback i/i i/i d/r0 d/r0 => Reactivity of flat surfaces => Topography of inactive surfaces

28. Enzyme mediated positive feedback mode Enzyme is immobilized on surface Enzyme catalyzes the reduction of the oxidized species

29. Oxidation of [Fe(CN)6]4-at the Pt-UME [Fe(CN)6]4- [Fe(CN)6]3-+ e- Enzyme mediated feedback mode: glucose oxidase -D-Glucose + Glucoseoxidase/FAD  Glucono--Lactone + Glucoseoxidase/FADH2 Glucoseoxidase/FADH2 + O2 Glucoseoxidase/FAD + H2O2 Glucoseoxidase/FADH2 + [Fe(CN)6]3- Glucoseoxidase/FAD + [Fe(CN)6]4-

30. Enzyme mediated positive feedback mode Disadvantages Redox mediator has to be a cofactor of the enzyme, which limits the possible enzymes to oxidoreductases As the mediator concentration is rather low, the signals are also small Enzymes need to be immobilized on inactive surfaces. Active surfaces would lead to a large background signal, larger than that of the enzyme The probe-sample distance has to be small => possible damage of the UME Advantages Lateral resolution is better than in GC mode

31. Combination of SECM and AFM Samples can show variations in both reactivity and topography. Thus, it is difficult to resolve these two components with conventional SECM measurements New strategies are required to determine sample topography and reactivity independently A) Addition of a second electroactive marker to provide information on the topography of the sample B) Vertical tip position modulation C) Shear force damping of the UME => Absolute sample-tip-distance is not known Combination of SECM and AFM

32. Principle of AFM Binnig, Quate, and Gerber 1986, Phys. Rev. Lett. 56, 9 Detection of atomic forces to monitor tip-sample distances 10-7-10-11N!

33. F = 1 nN => x = 140 nm Tip Sizelength l =100-500 µm thickness t = 0.3-5 µm width w = 10-50 µm Material Si or Si3N4 (E = modulus of elasticity) Spring constant Example ESi= 179 GPa, l = 200 μm, w = 10 μm, t = 0.5 μm => k = 0.007 N/m

34. Which forces can occur? • Van-der-Waals forces • Coulomb forces • Repulsive forces • Hydrophobic entropic forces

35. Mirror PSD LED Cantilever with tip sample z-Signal Scanning electronics Piezo Scanner Setup of a scanning force microscope Contact mode - constant height mode

36. Contact mode / constant height

37. Mirror PSD LED Cantilever with tip sample setpoint z-Signal Control unit Scanning electronics Piezo Scanner Setup of a scanning force microscope Contact mode - constant force mode

38. Preparation of SECM-AFM tips

39. Characterization of SECM-AFM tips Spring constant Example EPt= 17 GPa , l = 1200 μm, w = 200 μm, t = 5 μm => k = 0.06 N/m

40. Approximation of tip radius Linear sweep voltammetry i∞ = 0.8 nA Hemispherical geometry D(IrCl63-) = 7.5 ∙ 10-6 cm2 s-1 c*(IrCl63-) = 0.01 M => r0 = 180 nm

41. Determination of the tip geometry Approach curve Cantilever deflection Contact point b = 2 Contact point b = 1, 1.5, 2, 2.5, 3 Cone-like geometry r0 h

42. Experimental setup

43. Imaging polycarbonate membranes AFM image (constant force mode) Diffusion profile SECM image

44. Imaging polycarbonate membranes AFM image (constant force mode) SECM image

45. Experimental setup II

46. Imaging polycarbonate membranes AFM image (constant force mode) SECM image

47. References Bard, A. J., Faulkner, L. R. (2001) Electrochemical methods. Fundamentals and applications. John Wiley & Sons, Inc., New York Kranz, C., Wittstock, Wohlschläger, H. Schuhmann, W. (1997) Imaging of microstructured biochemically active surfaces by means of scanning electrochemical microscopy. Electrochimica Acta, 42, 3105-3111. Macpherson, J. V., Unwin, P. R. (2000) Combined scanning electrochemical-atomic force microscopy. Anal. Chem. 72, 276-285 Macpherson, J. V., Jones, C. E., Barker, A.L., Unwin, P. R. (2002) Electrochemical imaging of diffusion through single nanoscale pores. Anal. Chem. 74, 1841-1848.

48. Prof. Wolfgang Schuhmann Anal.Chem.-Electroanalytik & Sensorik, Ruhr-University Bochum "Microelectrochemistry – from materials to biological applications" Wednesday, June 18, 2003 17.00 h Lecture room: Biol. 5.2.38 For further information see http://www.uni-regensburg.de/GK/SP