1

Scanning ElectrochemicalMicroscopy (SECM)

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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

Heterogeneous reactions

Industrial applications - heterogeneous catalystscombinatorial chemistry

3

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

D

D

4

Electron transfer reactions

Electrode reaction

f

b

'Ox Redkz z

kn e 'z n z

net f b f ox b red0, 0,i

v v v k c t k c tnFA

c a f ox b red0, 0,i i i nFA k c t k c t

cf f ox 0,

iv k c t

nFA a

b b red 0,i

v k c tnFA

Forward reaction rate Backward reaction rate

5

Electron transfer reactions

Dependence of kf and kb on the interfacial potential difference

0'f 0 exp

nFk k E E

RT

0'b 0

1exp

nFk k E E

RT

Current-potential characteristic (Butler-Volmer model)

0' 0'0 ox red

1exp 0, exp 0,

nFnFi nFAk E E c t E E c t

RT RT

f ox b red0, 0,i nFA k c t k c t

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Electron transfer reactions

Exchange current

At equilibrium i = 0

0' * 0' *0 eq ox 0 eq red

1exp exp

nFnFnFAk E E c nFAk E E c

RT RT

*ox ox0,c t c *

red red0,c t c eqE E

Current-potential characteristic

0' 0'0 ox red

1exp 0, exp 0,

nFnFi nFAk E E c t E E c t

RT RT

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0' * 0' *eq ox eq red

1exp exp

nFnFE E c E E c

RT RT

Testing the equation

Electron transfer reactions

*

0 'oxeq*

red

expc nF

E Ec RT

*0 ' ox

eq *red

lncRT

E EnF c

Nernst-equation

0' * 0' *0 eq ox 0 eq red

1exp exp

nFnFnFAk E E c nFAk E E c

RT RT

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Electron transfer reactions

Exchange current

* * 0'0

0'0 o r eqx edeq

1x xpe p e

nFnFAk c E E

RT

nFnFAk c E E

RT

ic ia0 a cii i

*

0 'oxeq*

red

expc nF

E Ec RT

*

0 'oxeq*

red

expc nF

E Ec RT

*

1* * *ox0 0 ox 0 ox red*

red

ci nFAk c nFAk c c

c

* *ox redc c c 0 0i nFAk c

* 0'0 0 ox eqexp

nFi nFAk c E E

RT

Calculation of i0 starting from ic

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Scanning ElectrochemicalMicroscopy (SECM)

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Scanning probe microscopy (SPM) techniques

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Principle of scanning probe techniques

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Scanning electrochemical microscope

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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

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Planar and radial diffusion at electrodes

Concentration profiles at disk electrodes

1 s after starting a diffusion-controlled electrolysis

r0 = 3 mm

Fick‘s second law in one dimension

r0 = 300 µm r0 = 30 µmR/O

rad 20

D t

r

rad > 6 = UME

2

2

c cD

t x

rad

vert

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Planar diffusion at a conventional electrode

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?

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Current-time curves

Planar and radial diffusion at electrodes

r0 = 1.5 mm

r0 = 12.5 µm

r0 = 5 µm

*R/OR/O

Dj t nF c

t

(Cottrell-equation)

Planar diffusion

*

*R/O R/O R/OR/O

0

D nFD cj t nF c

t r

Hemispherical diffusion at UME

*R/O R/O

0

nFD cj

r

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Preparation of UME

r0rA

A

0

rRG

r

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

glass

Pt

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UME probe

Ultramicroelectrode

5 < RG < 20

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Scanning electrochemical microscope

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Approach curves

Tip far away from surface

Tip close to the surface

Current depends on distance between tip and sample

*R/O R/O 02i nFD c r

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Approach curves

d/r0

i/ i

i/ i

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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 feedbackPositive feedback

=> Constant current

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Generation-collection mode

Sample-generation/tip-collection mode (SG/TC)

Generator:Heterogeneous reaction Mass transport through a pore

Tip is scanned across the surface at constant height

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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

Generation-collection mode

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N

N

NH

N O

O

CH2

HO

HO

HO

H2CO

P

O

OO

P

O

OO

N

NN

N

NH2

O

OHOH

HH

HH

FAD

O

H

HO

OH

H

H

H

OHHOH

OH

+ O2

O

H

HO

OH

H

H

OHH

OH

O

+ H2O2

Generator: glucose oxidase

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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

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Feedback mode

Negative feedback Positive feedback

d/r0

i/ i

d/r0

i/ i

=> Topography of inactive surfaces => Reactivity of flat surfaces

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Enzyme mediated positive feedback mode

Enzyme is immobilized on surfaceEnzyme catalyzes the reduction of the oxidized species

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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-

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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

Enzyme mediated positive feedback mode

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Combination of SECM and AFM

New strategies are required to determine sample topography and reactivity independently

Samples can show variations in both reactivity and topography. Thus, it is difficult to resolve these two components with conventional SECM measurements

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

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Principle of AFM

Detection of atomic forces to monitor tip-sample distances

10-7-10-11N!

Binnig, Quate, and Gerber 1986, Phys. Rev. Lett. 56, 9

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Tip

Spring constant

l

b

h

Example

ESi = 179 GPa, l = 200 μm, w = 10 μm, t = 0.5 μm

=> k = 0.007 N/m

Size length l =100-500 µm

thickness t = 0.3-5 µm

width w = 10-50 µm

Material Si or Si3N4 (E = modulus of elasticity)

3

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wtk E

l

F k x F = 1 nN => x = 140 nm

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• Van-der-Waals forces• Coulomb forces• Repulsive forces• Hydrophobic entropic forces

Which forces can occur?

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Setup of a scanning force microscope

Mirror

PSD

PiezoScanner

LED

Cantilever with tip

sample

z-Signal

Scanning electronics

Contact mode - constant height mode

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Contact mode / constant height

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Setup of a scanning force microscope

setpoint

Control unit

Contact mode - constant force mode

Mirror

PSD

PiezoScanner

LED

Cantilever with tip

sample

z-Signal

Scanning electronics

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Preparation of SECM-AFM tips

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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

3

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wtk E

l

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Approximation of tip radius

*R/O R/O 02i nFD c r

D(IrCl63-) = 7.5 ∙ 10-6 cm2 s-1

c*(IrCl63-) = 0.01 M

i∞ = 0.8 nA

Linear sweep voltammetry

Hemispherical geometry

=> r0 = 180 nm

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Determination of the tip geometry

Cantilever deflection Approach curve

0

hb

r

h

r0

b = 1, 1.5, 2, 2.5, 3

b = 2

Contact point

Contact point

Cone-like geometry

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Experimental setup

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Imaging polycarbonate membranes

AFM image (constant force mode)

SECM image

Diffusion profile

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Imaging polycarbonate membranes

AFM image (constant force mode)

SECM image

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Experimental setup II

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Imaging polycarbonate membranes

AFM image (constant force mode)

SECM image

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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.

References

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Prof. Wolfgang Schuhmann

Anal.Chem.-Electroanalytik & Sensorik,Ruhr-University Bochum

"Microelectrochemistry – from materials to biological applications"

Wednesday, June 18, 200317.00 h

Lecture room: Biol. 5.2.38

For further information see http://www.uni-regensburg.de/GK/SP