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 : 2013. 1. 16 (4:00 p.m. ~ 7:00 p.m.) 강사 : 건국대학교 화학과 권성중

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

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), 132 - 138.) It measures the local electrochemical behavior of liquid/solid, liquid/gas and liquid/liquid interfaces Control and monitor redox reactions with high spatial resolution

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

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

SECM Instrument & electrochemical cell

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

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.>

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)

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

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.

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

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

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

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,∞

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

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

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.

Fitting parameters Table A Parameter values for Euqation (3) (positive feedback) at different RG values RG A B C D 1.1 0.5882629 0.6007009 0.3872741 -0.869822 1.5 0.636860 0.6677381 0.3581836 -1.496865 2.0 0.6686604 0.6973984 0.3218171 -1.744691 5.1 0.72035 0.75128 0.26651 -1.62091 10 0.7449932 0.7582943 0.2353042 -1.683087

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

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

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

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.

Fitting parameters Table A Parameter values for Euqation (4) (negative feedback) at different RG values RG A B C D 1.1 1.1675164 1.0309985 0.3800855 -1.701797 1.5 1.0035959 0.9294275 0.4022603 -1.788572 2.0 0.7838573 0.877792 0.424816 -1.743799 10 0.4571825 1.4604238 0.4312735 -2.350667

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

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.

RG value and approaching RG should be small for better approaching

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

Topographical image Surface topology scan using negative feedback mode.

Imaging of Insulator/Conductor Surface

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

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

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

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

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)

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

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

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

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.

Experimental - operation Experimental setup – electrode connection

CHI900b operating software interface Probe control for the initial positioning

Techniques and parameters

1D & 2D scanning

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.

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 0.0001 Biosensors and Bioelectronics 2003, 18, 1379-1383.

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.

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.

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.

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

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, 223-240.

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

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.

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

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

Biological system – living cell The SECM measurement for a living cell. Biosensors and Bioelectronics 2003, 18, 1379-1383.

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

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, 1379-1383.

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), 17582-17587. PNAS 2012, 109(29), 11522–11527.

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.

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