1. Scanning Electrochemical
Microscopy (SECM)

2. 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.“
Heterogeneous reactions
Industrial applications - heterogeneous catalysts
combinatorial chemistry

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

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

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

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

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

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

9. Scanning Electrochemical
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 = 300 µm
r0 = 30 µm
rad > 6 = UME
vert
rad

15. 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
Spherical diffusion at an UME

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

17. Preparation of UME Pt glass r0 rA
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. Approach curves
d/r0
i/ i

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

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

29. 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-
Oxidation of [Fe(CN)6]4- at the Pt-UME
[Fe(CN)6]4-  [Fe(CN)6]3- + e-

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

33. Tip Size length l =100-500 µm thickness t = 0.3-5 µm
width w = 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 = N/m
F = 1 nN => x = 140 nm

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

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

36. Contact mode / constant height

37. Setup of a scanning force microscope
Mirror
PSD
Piezo
Scanner
LED
Cantilever
with tip
sample
z-Signal
Scanning electronics
setpoint
Control unit
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
Cantilever deflection
Approach curve
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,
Macpherson, J. V., Unwin, P. R. (2000) Combined scanning electrochemical-atomic force microscopy. Anal. Chem. 72,
Macpherson, J. V., Jones, C. E., Barker, A.L., Unwin, P. R. (2002) Electrochemical imaging of diffusion through single nanoscale pores. Anal. Chem. 74,

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
For further information see