Scanning Electrochemical Microscopy (SECM)

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

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

Electron transfer reactions Electrode reaction Forward reaction rate Backward reaction rate

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

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

Electron transfer reactions Testing the equation Nernst-equation 

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

Scanning Electrochemical Microscopy (SECM)

Scanning probe microscopy (SPM) techniques

Principle of scanning probe techniques

Scanning electrochemical microscope

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

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

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

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

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

UME probe Ultramicroelectrode 5 < RG < 20

Scanning electrochemical microscope

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

Approach curves d/r0 i/ i

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

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

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

Generator: glucose oxidase FAD

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

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

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

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-

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

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

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 10-7-10-11N!

Tip 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) Spring constant Example ESi = 179 GPa, l = 200 μm, w = 10 μm, t = 0.5 μm => k = 0.007 N/m F = 1 nN => x = 140 nm

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

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

Contact mode / constant height

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

Preparation of SECM-AFM tips

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

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

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

Experimental setup

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

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

Experimental setup II

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

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.

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