1. 일시 : 2013. 1. 16 (4:00 p.m. ~ 7:00 p.m.) 강사 : 건국대학교 화학과 권성중
<연구장비 엔지니어 양성사업>
Basic Principles of Scanning Electrochemical Microscopy (SECM), experiment and application
일시 : (4:00 p.m. ~ 7:00 p.m.)
강사 : 건국대학교 화학과 권성중

2. Content introduction of SECM Basic principles of SECM
SECM history
SECM experimental set-up
SECM tip fabrication and characterization
Basic principles of SECM
Feedback mode
Approaching curve
Operation of SECM
Application of SECM
Determination of kinetics parameters of heterogeneous electron transfer
High resolution etching and imaging
Imaging electrochemical activity by generation-collection modes
Biological system

3. Scanning ElectroChemical Microscopy (SECM)
Scanning electrochemical microscopy (SECM) is a technique within the broader class of scanning probe microscopy (SPM)
Initial characterization of the technique was credited to University of Texas electrochemist, Allen J. Bard, in 1989
("Scanning Electrochemical Microscopy. Introduction and Principles," Allen J. Bard, Fu-Ren F. Fan, Juhyoun Kwak, and Ovadia Lev, Anal. Chem., 1989, 61(2), )
It measures the local electrochemical behavior of liquid/solid, liquid/gas and liquid/liquid interfaces
Control and monitor redox reactions with high spatial resolution

4. Comparison with STM
Comparison of Scanning Tunneling Microscopy (STM) and Scanning Electrochemical Microscopy (SECM)
Tip
d ≤ 10 Ǻ
Substrate
STM
Insulated Tip
variable d
Substrate e- R O
SECM

5. Electrochemical Cell Configuration
Tip (working electrode) : typically dia. ~ 10 µm
Substrate : disk electrode or something else
Reference & counter electrode

6. SECM
Instrument & electrochemical cell

7. SECM tip Ultramicroelectrode (UME) : SECM tip
a working electrode used in a three electrode system
The small size of UME give them relatively large diffusion layers and small overall currents
at least one dimension (the critical dimension) smaller than 25 μm
Platinum, or gold electrodes with a radius of 5 μm are commercially available
SECM tip
Tip is prepared by sharpening UME
The tip will be sharpened such that the
RG (ratio of the outer glass diameter to the
diameter of the wires) is between 2 and 10
RG ~3

8. Tip Fabrication and Characterization
UME (10 μm Pt) fabrication
Take a clean and dry Pyrex (borosilicate) capillary tube (at least 15 cm long to manipulate in the flame, inner diameter 1 mm, outer diameter 2 mm)
The end of a glass capillary will be sealed such that a conical shape using a gas oxygen flame
A straight 10 μm Pt wire will be inserted and positioned at the bottom of the sealed capillary
<“Scanning Electrochemical Microscopy”, Bard A. J.; Mirkin M. V. (eds.), Marcel Dekker Inc, N.Y., 2001, Chapter 3, p. 75.>

9. Tip Fabrication and Characterization
UME (10 μm Pt) fabrication
It will be put under vacuum for 30 minutes
The capillary will slowly be sealed onto the wire using a heated Nickel-Chromium resistor coil
The sealed wire will then be electrically connected to a larger wire using a conducting silver epoxy (placed in the oven at 120 °C overnight to cure the epoxy)

10. Tip Fabrication and Characterization
UME (10 μm Pt) sharpening
The glass at the end of the electrode will be ground off with sandpaper until the Pt wire is exposed
Successively finer sandpaper and alumina solutions on micropolishing cloths will then be used to smoothly polish the electrode surface

11. Tip Fabrication and Characterization
UME (10 μm Pt) sharpening
The glass at the end of the electrode will be ground off with sandpaper until the Pt wire is exposed
Successively finer sandpaper and alumina solutions on micropolishing cloths will then be used to smoothly polish the electrode surface
The electrode will be sharpened such that the RG is between 2 and 10.

12. Diffusion layer of UME
Comparison of diffusion layer at (dia. 3 mm) Pt electrode and Pt UME (dia. 25 μm)
electrode
Linear diffusion
UME
Hemispherical diffusion

13. Cyclic Voltammogram of UME
Comparison of cyclicvoltammograms of 1 mM FcMeOH at (dia. 3 mm) Pt electrode and Pt UME (dia. 25 μm)
< Figure. Steady state voltammogram at a 3 mm Pt disk electrode and a 25 μm Pt disk UME in a solution of approximately 1 mM ferrocenemethanol in 0.1 M KCl electrolyte. The potentials are given with respect to Ag/AgCl.>
The steady state current, Iss for a UME
n : number of electrons
F : Faraday’s constant
D : diffusion coefficient (for ferrocenemethanol D= 7*10-6 cm2/s)
C : bulk concentration
a : the radius of the electrode (in cm).
Iss = 4nFDaC

14. Basic Principle of SECM
Feedback mode
No feedback
Positive feedback mode
Negative feedback mode
Other mode (Generation/Collection mode)
Generation/collection mode
Tip Generation – Substrate Collection mode
Substrate Generation – Tip Collection mode
Large substrate
Small substrate
Shielding or competitive mode

15. Feedback mode IT,∞ = 4nFDaC* conductive substrate IT > IT,∞
Conductive substrate : Positive feedback
Insulating substrate : Negative feed back
SECM tip
No feedback
Steady-state current R O
IT,∞ = 4nFDaC*
SECM tip
Positive feedback R O
conductive substrate
IT > IT,∞
SECM tip
Negative feedback R O
Insulating substrate
IT > IT,∞

16. Approaching curve at positive feedback mode
In a bulk solution, the oxidized species is reduced at the tip, producing a steady-state current that is limited by hemispherical diffusion – no feedback
IT,∞ O R
As the tip approaches a conductive substrate in the solution, the reduced species formed at the tip is oxidized at the conductive surface, yielding an increase in the tip current and creating a regenerative - positive feedback
IT > IT,∞
conductive substrate

17. Experimental data
Raw data needs fitting to the theoretical approaching curves for the calculation of distance between tip and substrate

18. Fitting Approach Curves with the Theoretical equations
You will need to use calculus software to process the approach curves. Microsoft Excel® or Origin® are very suitable for this purpose. Thus, you must export the curves as .txt files from the menu File/Convert to Text….
The raw data contains two columns: one with the tip current values (iT) and the other with the distances from the scan initial point (dexp). You need to normalize these values according to eqs. (1) and (2) to make them compatible with the theoretical equations for conductive (eq. 3) and insulating (eq. 4) substrates (see Table A for the parameter values).
𝑖 𝑇 → 𝐼= 𝑖 𝑇 𝑖 𝑇,∞ (1)
𝑑 𝑒𝑥𝑝 → 𝐿= 𝑑 𝑎 = ( 𝑑 𝑜 − 𝑑 𝑒𝑥𝑝 ) 𝑎 (2)
𝐼=𝐴+ 𝐵 𝐿 +𝐶 𝑒 (𝐷/𝐿) (3)
where iT,∞ is the tip current, a is the UME tip radius, d is the tip-substrate distance, do is the distance from the scan first point to the substrate surface. In addition, the experimental value of I must be corrected by an RG-dependant factor. Note that the only unknown parameter, which will require adjustment in the fitting process, is do.

19. Fitting parameters Table A
Parameter values for Euqation (3) (positive feedback) at different RG values RG A B C D
1.1
1.5
2.0
5.1 10

20. Calculation of distance
a = 5 µm
Feedback effect appears from ~ 20 µm
I=IT/IT,∞ ~ 2 or 3 when d = a

21. Approaching curve at negative feedback mode
In a bulk solution, the oxidized species is reduced at the tip, producing a steady-state current that is limited by hemispherical diffusion – no feedback
IT,∞ O R
As the tip approaches a insulating substrate in the solution, as the oxidized species cannot be regenerated and diffusion to the electrode is inhibited as a result of physical obstruction - negative feedback
Insulating substrate
IT < IT,∞
Hindered diffusion

22. Experimental data
Raw data needs fitting to the theoretical approaching curves for the calculation of distance between tip and substrate

23. Fitting Approach Curves with the Theoretical equations
𝑖 𝑇 → 𝐼= 𝑖 𝑇 𝑖 𝑇,∞ (1)
𝑑 𝑒𝑥𝑝 → 𝐿= 𝑑 𝑎 = ( 𝑑 𝑜 − 𝑑 𝑒𝑥𝑝 ) 𝑎 (2)
𝐼= 1 (𝐴+ 𝐵 𝐿 +𝐶 𝑒 (𝐷/𝐿) ) (4)
An additional parameter to consider when probing insulating surfaces is the electrode sheath diameter, rg, since it contributes to the physical obstruction of diffusion.

24. Fitting parameters Table A
Parameter values for Euqation (4) (negative feedback) at different RG values RG A B C D
1.1
1.5
2.0 10

25. Calculation of distance
a = 5 µm
Feedback effect appears from ~ 20 µm
I=IT/IT,∞ can reach almost 0
=> better approaching than positive feedback mode

26. RG value and approaching curves
Approaching curve dependency on RG value.
=> Use positive feedback mode when the RG is large
< Figure. Calculated current-distance curves for negative feedback (curves 1-4 ) and positive feedback (curves 5-8) for different RG value ( RG value ; 1 : 1.5 , 2 : 5.1, 3: 10.2 , 4 : 1002, 5 : 1.5 , 6: 5.1 , 7: 10.2 , and 8 : 1002 >
Angew. Chem. Int. Ed. 2007, 46, 1–35.

27. RG value and approaching
RG should be small for better approaching

28. When the tip touch the substrate
Distorted approaching curves are obtained when the tip touch the substrate
Be careful for the tip breaking

29. Topographical image
Surface topology scan using negative feedback mode.

30. Imaging of Insulator/Conductor Surface

31. Is the substrate conducting? Insulating?
How does one distinguish conductive and insulating regions from the tip current?

32. Generation/Collection mode
Substrate : generation (or collection) electroactive speices
Tip : collection (or generation) electroactive speices
SG / TC
SG / TC
TG / SC O R O R O R
Large substrate
Small substrate
substrate
Measure iT and is
Measure iT and is
Measure iT and is
Low to medium
collection efficiency
steady state is
Low collection efficiency
No steady state is
High collection efficiency
steady state is

33. SG / TC mode Substrate : generation electroactive speices
Tip : collection electroactive speices

34. TG/SC experiment for ORR
Typical chronoamperometric response of a platinum substrate during a modified TG/SC experiment (Oxygen Reduction Reaction)
Tip off
Tip on
Tip off
iT = -7.1x10-8 A
ES = 0.2 V vs HRE
dT-S = 10 μm

35. TG/SC mode Tip Pt UME ( dia.= 10 µm) Substrate Pt UME ( dia.= 10 µm)
Experiment with 2 aligned UME using modified TG/SC mode (FcMeOH, or Oxygen Evolution Reaction)
Tip Pt UME ( dia.= 10 µm) R R R R O R R R R R O R
R = FcMeOH or water
Substrate Pt UME ( dia.= 10 µm)

36. 2D scan of SECM
X-Y plane scan
Tip
Substrate

37. Axis profile Axis profile Approaching curve Tip substrate CV substrate
Cyclic voltammetry
Approaching curve
Tip
substrate CV
substrate
Tip

38. Shielding (or competitive) mode
Substrate : electrochemical reaction
Tip : electrochemical reaction
SECM tip O R
substrate
Competition between
Tip and substrate
Same reaction at tip and substrate

39. Shielding effect of NP collision
Diffusion layer investigation by SECM
- Time dependent collision frequency H
0.8 V applied Pt
dia. 10 μm
(RG~10)
dia. 2 mm
J. Am. Chem. Soc. 2012, 134(16), 7102.

40. Experimental - operation
Experimental setup – electrode connection

41. CHI900b operating software interface
Probe control for the initial positioning

42. Techniques and parameters

43. 1D & 2D scanning

44. Application of SECM Example SECM Applications Imaging
Ultramicroelectrode Shape Characterization
Heterogeneous Kinetics
Homogenous Kinetics
Biological Systems
Liquid/Liquid Interfaces
Membranes and Thin Films
Surface Reactions
Sensors
Semiconductor Surfaces
Electrochemistry in Small Volumes
Fabrication
Kinetics of electrochemical reactions; fuel cells.
Using Tip scan sample for decrease in molecular oxygen to image a live cell.

45. Heterogeneous kinetics
Kinetics study
Effect of heterogeneous kinetics on SECM approach curve
Rate constants (cm/s)
A. 1
B. 0.5
C 0.1
D 0.025
E 0.015
F 0.01
G 0.005
H 0.002 I
Biosensors and Bioelectronics 2003, 18,

46. Local generation of reagents
Deposition
Direct mode
Copper needles generated in direct mode.
Deposition of a 1-mm-wide line of silver onto gold
Phys. Chem. Chem. Phys. 2005, 7, 3185 – 3190.
J. Electrochem. Soc. 2000, 147, 586 – 591.

47. Interface study Interface study
Principal methods for inducing and monitoring interfacial processes with SECM:
feedback mode
induced transfer
double potential step chronoamperometry.
Physiol. Meas. 2006, 27, R63–R108.

48. Biological system – liquid/gas interface
monolayer study
SECM-induced transfer of oxygen across a
1-octadecanol monolayer at the air/water interface
J. Phys. Chem. B 2004, 108, 3801–3809.

49. Biological system – liquid/liquid inferface
electron transfer (ET) rate at the interface
Long-Range Electron Transfer through a Lipid Monolayer at the Liquid/Liquid Interface
J. Am. Chem. Soc. 1997, 119, 10785–10792

50. Transport pathways in membrane
Transportation rate determine
Examining permeability with SECM. Transport of species by convection, diffusion or migration can be detected as an increase in the transport-limited current at a UME.
determine the rate of convection of an electrolyte solution containing hexacyanoferrate(II) ions
Langmuir 1995, 11, 3959–3963
.J. Chem. Soc. Faraday Trans.1995, 91, 1407–1410.
Analytica Chimica Acta 1999, 385,

51. Electrocatalysis investigation
Screening activity
Electrocatalysis investigation
Imaging the electrocatalytic activity for Oxygen Reduction of Platinum and Gold disks by the modified TG-SC mode

52. Screening activity – fuel cell catalyst
Electrocatalysis investigation
SEM image of typical ternary catalyst Pd-Au-Co array
B. C. SECM TG/SC mode image of ORR activity of catalyst
biased at 200mV and 750 mV.
J. Am. Chem. Soc., 2005, 127, 357.

53. Screening activity – fuel cell catalyst
Electrocatalysis investigation
Pt-Pd-Au combinations for ORR activity for fuel cell cathodes

54. Biological system – imaging
Cell imaging
Topographic images of cell.
Anal. Chem., 2005, 77, 1111.

55. Biological system – living cell
The SECM measurement for a living cell.
Biosensors and Bioelectronics 2003, 18,

56. Biological system – living cell
Schematic diagrams of the SECM experiments with four different types of mediator regeneration
PNAS 2000, 97(18), 9855–9860.

57. Biological system – membrane
Living cell membrane
A schematic detailing some of the ways in which mediators of differing hydrophobicities may be utilised to interrogate living cells
(A) Positive feedback is observed as the hydrophobic reduced species generated at the tip diffuses through the cell membrane and is re-oxidised by a different intracellular redox centre confined within the boundaries of the cell. (B) Positive feedback is observed as a result of a self-exchange electron transfer reaction. Hydrophobic mediator diffuses to the cell membrane where it is re-oxidised by ET occurring across the cell membrane. (C) Schematic diagram illustrating the potentiometric measurement of the pH profile around a single cell.
Biosensors and Bioelectronics 2003, 18,

58. Biological system – cell metabolism
Cellular response for drug
Cellular response to menadione in the presence or absence of MRP1 blocker MK571
Menadione
thiodione
PNAS 2004, 101(51),
PNAS 2012, 109(29), 11522–11527.

59. Sensor – DNA sensing DNA sensor
the influence of coulomb interaction
on the diffusion of anions (An−) towards a DNA-modified chip surface.
Electrostatic repulsion between An− and the phosphate groups at the
backbone of the immobilised DNA strands hinders diffusion of the redox
species to the underlying Au-surface.
Biosensors and Bioelectronics 2004, 20, 925–932.

60. Sensor – immunosensing
immunosensor
Assembly of the sandwich structure on the streptavidin-coated substrate
and SECM detection.
Analytica Chimica Acta 2006, 558, 110–114.