Overview

The scanning electrochemical microscope (SECM) was introduced in 1989¹ as an instrument that could examine chemistry at high resolution near interfaces.  By detecting reactions that occur at a small electrode (the tip) as it is scanned in close proximity to a surface, the SECM can be employed to obtain chemical reactivity images of surfaces and quantitative measurements of reaction rates.  Numerous studies with the SECM have now been reported from a number of laboratories all over the world, and the instrument has been used for a wide range of applications, including studies of corrosion, biological systems (e.g., enzymes, skin, leaves), membranes, and liquid/liquid interfaces.²  Trapping and electrochemical detection of single molecules with the SECM has also been reported.

The CHI920C Scanning Electrochemical Microscope consists of a digital function generator, a bipotentiostat, high resolution data acquisition circuitry, a three dimensional nanopositioner, and a sample and cell holder. Diagrams for the SECM and cell/sample holder are shown below. The three dimensional nanopositioner has a spatial resolution down to nanometers but it allows a maximum traveling distance of 50 millimeters. The potential control range of the bipotentiostat is ± 10 V and the current range is ± 250 mA. The instrument is capable of measuring current down to sub-picoamperes.

In addition to SECM imaging, other modes of operation are available for scanning probe applications: Surface Patterned Conditioning, Probe Scan Curve, and Probe Approach Curve, Surface Interrogation SECM. Surface Patterned Conditioning allows user to edit a pattern for surface conditioning by controlling the tip at two different potentials and durations. The Probe Scan Curve mode allows the probe to move in the X, Y, or Z direction while the probe and substrate potentials are controlled and currents are measured. The probe can be stopped when the current reaches a specified level. This is particularly useful in searching for an object on the surface and determining approach curves. The Probe Approach Curve mode allows the probe to approach the surface of the substrate. It is also very useful in distinguishing the surface process, using PID control. The step size is automatically adjusted to allow fast surface approach, without letting the probe touching the surface. Constant height, constant current, potentiometric, and impedance modes are available for SECM imagine and probe scan curve.

The CHI920C is designed for scanning electrochemical microscopy, but many conventional electrochemical techniques are also integrated for convenience, such as CV, LSV, CA, CC, DPV, NPV, SWV, ACV, SHACV, i-t, DPA, DDPA, TPA, SSF, STEP, IMP, IMPE, IMPT, and CP. When it is used as a bipotentiostat, the second channel can be controlled at an independent constant potential, to scan or step at the same potential as the first channel, or to scan with a constant potential difference with the first channel. The second channel works with CV, LSV, CA, DPV, NPV, DNPV, SWV, and i-t.

The CHI920C SECM is an upgrade from the CHI900B/CHI910B SECM. The stepper motor positioner now has a resolution of 8 nanometers with 50 mm travel distance. CHI920C will be the stepper motor positioner with a combination of closed loop 3-dimensional piezo positioner. The closed-loop piezo control allows improved linearity and reduced hysteresis of the piezo devices.

The other improvements for the CHI920C over the CHI900B/910B include faster data acquisition (1M Hz 16-bit), higher current (250mA), faster CV (1000V/s at 0.1 mV potential increment), and ac impedance measurements.

1. A. J. Bard, F.-R. F. Fan, J. Kwak, and  O. Lev, Anal. Chem. 61, 132 (1989); U.S. Patent No. 5,202,004 (April 13, 1993).

2. A. J. Bard, F.-R. Fan,  M. V. Mirkin, in Electroanalytical Chemistry, A. J . Bard, Ed., Marcel Dekker, New York, 1994, Vol. 18, pp 243-373.

Specifications

Nanopositioner: X, Y, Z resolution: 1.6 nm with Piezo positioner, closed loop control, 8 nm with stepper motor positioner X, Y, Z total distance: 5 cm Bipotentiostat: Probe Potential: ± 10 V Substrate Potential: ± 10 V Compliance Voltage: ± 12 V 3- or 4-electrode configuration Reference electrode input impedance: 1e12 ohm Current Sensitivity: 10-12 A/V to 10-1 A/V Maximum Current: ± 250 mA External signal recording channel ADC Resolution: 16-bit @ 1 M Hz Galvanostat: Current range:    ± 250 mA Other Features: Real Time Absolute and Relative Distance Display Real Time Probe and Substrate Current Display Dual channel measurements for CV, LSV, CA, DPV, NPV, SWV, i-t Cell control: purge, stir, knock Automatic potential and current zeroing Current low-pass filters, covering 8-decade frequency range, Automatic and manual setting RDE control output:  0-10 V (corresponding to 0-10000 rpm) Flash memory for quick software update Serial port or USB port selectable for data communication Digital CV simulator, user defined mechanisms Impedance simulator and fitting program

Experimental Parameters: CV and LSV scan rate:  0.000001 to 10,000 V/s Potential increment during scan: 0.1 mV @1000 V/s CC and CA pulse width:  0.0001 to 1000 sec True integrator for CC DPV and NPV pulse width:  0.001 to 10 sec SWV frequency: 1 to 100 kHz ACV frequency: 0.1 to 10 kHz SHACV frequency: 0.1 to 5 kHz IMP frequency: 0.00001 to 100 kHz  (extend to 1M Hz for impedence 10-1000 ohm) IMP amplitude: 0.00001V to 0.7V RMS Automatic potential and current zeroing Automatic and manual iR compensation Current low-pass filters, covering 8-decade frequency range, Automatic and manual setting RDE control output:  0-10 V (corresponding to 0-10000 rpm) Flash memory for quick software update Serial port or USB port selectable for data communication

Techniques

Scanning Probe Techniques SECM Imaging (SECM) : constant height, constant current, potentiometric modes, and impedance modes Probe Approach Curves (PAC) Probe Scan Curve (PSC) : constant height, constant mode, potentiometric and impedance modes Surface Patterned Conditioning (SPC) Surface Integration SECM (SISECM) Sweep Techniques Cyclic Voltammetry (CV) Linear Sweep Voltammetry (LSV) Tafel Plot (TAFEL) Step and Pulse Techniques Staircase Voltammetry (SCV) Chronoamperometry (CA) Chronocoulometry (CC) Differential Pulse Voltammetry (DPV) Normal Pulse Voltammetry (NPV) Differential Normal Pulse Voltammetry (DNPV) Square Wave Voltammetry (SWV)

AC Techniques AC Voltammetry (ACV) Second Harmonic AC Voltammetry (SHACV) AC Impedance (IMP) Impedance versus Potential (IMPE) Impedance versus Time (IMPT) Galvanostatic Techniques Chronopotentiometry (CP) Chronopotentiometry with Current Ramp (CPCR) Multi-Current Steps (ISTEP) Potentiometric Stripping Analysis (PSA) Other Techniques Amperometric i-t Curve (i-t) Differential Pulse Amperometry (DPA) Double Differential Pulse Amperometry (DDPA) Triple Pulse Amperometry (TPA) Integrated Pulse Amperometric Detection (IPAD) Bulk Electrolysis with Coulometry (BE) Hydrodynamic Modulation Voltammetry (HMV) Sweep-Step Functions (SSF) Multi-Potential Steps (STEP) Electrochemical Noise Measurements (ECN) Open Circuit Potential - Time (OCPT) Various Stripping Voltammetry Potentiometry

Principles and Applications

• Electrode surface studies
• Corrosion
• Biological samples
• Solid dissolution
• Liquid/liquid interfaces
• Membranes

I. Operational Principles of SECM

As in other types of scanning probe microscopes, SECM is based on the movement of a very small electrode (the tip) near the surface of a conductive or insulating substrate.¹ In amperometric SECM experiments, the tip is usually a conventional ultra-microelectrode (UME) fabricated as a conductive disk of metal or carbon in an insulating sheath of glass or polymer. Potentiometric SECM experiments with ion-selective tips are also possible.²

In amperometric experiments, the tip current is perturbed by the presence of the substrate. When the tip is far (i.e. greater than several tip diameters) from the substrate, as shown in Fig. 1A, the steady-state current, i_T,∞, is given by

          i_T,∞  = 4nFDCa

where F is Faraday's constant, n is the number of electrons transferred in the tip reaction (O + ne → R), D is the diffusion coefficient of  species O, C is the concentration, and a is the tip radius. When the tip is moved toward the surface of an insulating substrate, the tip current, i_T, decreases because the insulating sheath of the tip blocks diffusion of O to the tip from the bulk solution. The closer the tip gets to the substrate, the smaller i_T becomes (Fig 1B). On the other hand, with a conductive substrate, species R can be oxidized back to O. This produces an additional flux of O to the tip and hence an increase in i_T (Fig. 1C). In this case, the smaller the value of d, the larger i_T will be, with i_T → ∞ as d → 0, assuming the oxidation of R on the substrate is diffusion-limited. These simple principles form the basis for the feedback mode of SECM operation.

When the tip is rastered in the x-y plane above the substrate, the tip current variation represents changes in topography or conductivity (or reactivity). One can separate topographic effects from conductivity effects by noting that over an insulator i_T is always less than i_T,∞, while over a conductor i_T is always greater than i_T,∞. 

In the feedback mode of the SECM operation as stated above, the overall redox process is essentially confined to the thin layer between the tip and the substrate. In the substrate-generation/tip-collection (SG/TC) mode (when the substrate is a generator and the tip is a collector), the tip travels within a thin diffusion layer generated by the substrate electrode._1b,3 There are some shortcomings which limit the applicability of the SG/TC mode if the substrate is large: (1). the process at a large substrate is always non-steady state; (2). a large substrate current may cause significant iR-drop; and (3). the collection efficiency, i.e., the ratio of the tip current to the substrate current, is low. The tip-generation/substrate-collection (TG/SC) mode is advisable for kinetic measurements, while SG/TC can be used for monitoring enzymatic reactions, corrosion, and other heterogeneous processes at the substrate surface.

II. Applications

A. Imaging and positioning

A three-dimensional SECM image is obtained by scanning the tip in the x-y plane and monitoring the tip current, i_T, as a function of tip location.

A particular advantage of SECM in imaging applications, compared to other types of scanning probe microscopy, is that the response observed can be interpreted based on fairly rigorous theory, and hence the measured current can be employed to estimate the tip-substrate distance. Moreover, SECM can be used to image the surfaces of different types of substrates, both conductors and insulators, immersed in solutions. The resolution attainable with SECM depends upon the tip radius. For example, Fig. 2 shows one SECM image of a filtration membrane obtained with a 2-μm-diameter Pt disk tip in Fe(CN)₆^4- solution. Average pore diameter is ca. 10 μm. An image demonstrating the local activity of an enzymatic reaction on a filtration membrane is shown in Fig. 9 as described below.

B. Studies of heterogeneous electron transfer reactions

SECM has been employed in heterogeneous kinetic studies on various metal, carbon and semiconductor substrates.⁴ In this application, the x-y scanning feature of  SECM is usually not used. In this mode, SECM has many features of UME and thin layer electrochemistry with a number of advantages. For example, the characteristic flux to an UME spaced a distance, d, from a conductive substrate is of the order of DC/d, independent of the tip radius, a, when d < a. Thus, very high fluxes and thus high currents can be obtained.  For example, the measurement of the very fast kinetics of the oxidation of ferrocene at a Pt UME has been carried out.^4e Five steady-state voltammograms obtained at different distances are shown in Fig. 3, along with the theoretical curves calculated with the values of the kinetic parameters extracted from the quartile potentials. The heterogeneous rate constant, k^o, obtained (3.7 ± 0.6 cm/sec) remains constant within the range of experimental error, while the mass-transfer rate increases with a decrease in d.

C. Studies of homogeneous chemical reactions

As mentioned above, the TG/SC (with small tip and substrate) mode of SECM, in the same manner as the rotating ring disk electrode (RRDE), is particularly suitable  for the studies of homogeneous chemical kinetics.^1b,5 The SECM approach has the advantage that different substrates can be examined easily, i.e., without the need to construct rather difficult to fabricate RRDEs, and higher interelectrode fluxes are available without the need to rotate the electrode or otherwise cause convection in the solution. Moreover, in the TG/SC mode, the collection efficiency in the absence of perturbing homogeneous chemical reaction is near 100%, compared to significantly lower values in practical RRDEs. Finally, although transient SECM measurements are possible, most applications have involved steady-state currents, which are easier to measure and are not perturbed by factors like double-layer charging and also allow for signal averaging. For example, the reductive coupling of both dimethyl-fumarate (DF) and fumaronitrile (FN) in N,N-dimethyl formamide has been studied with the TG/SC mode.^5a Fig. 4 shows tip and substrate steady-state voltammo-grams for the TG/SC regime. Comparable values of  both of the plateau currents indicated that the mass transfer rate was sufficiently fast to study the rapid homogeneous reaction. From the approach curves of both tip and substrate currents (Fig. 5) obtained at various FN concentrations, a rate constant k_c  = 2.0 (± 0.4) x 10⁵ M^-1s^-1 was determined for the dimerization reactions.

D. Characterization of thin films and membranes

SECM is also a useful technique for studying thin films on interfaces. Both mediated and direct electrochemical measurements in thin films or membranes can be carried out. For example, polyelectrolytes, electronically conductive polymers, passivation films on metals and dissolution processes have been investigated by SECM.⁶ A unique type of cyclic voltammetry, called tip-substrate cyclic voltammetry (T/S CV), has been used to investigate the electrochemical behavior of an Os(bpy)₃²⁺-incorporated Nafion film.^6a T/S CV involves monitoring the tip current vs. the substrate potential (ES) while the tip potential (ET) is maintained at a given value and the tip is held near the substrate. The substrate CV (i_S vs. ES) of an Os(bpy)₃²⁺-incorporated Nafion film covering a Pt disk electrode in Fe(CN)₆^3- solution only shows a wave for the Os(bpy)₃^2+/3+ couple (Fig. 6B), indicating the permselectivity of the Nafion coating.  Fig. 6A shows the corresponding T/S CV curves.  When the tip is far from the substrate, i_T is essentially independent of ES.  When the tip is close to the substrate (d = 10 μm), either negative or positive feedback effects are observed, depending on the oxidation state of the Os(bpy)₃^2+/3+ couple in the Nafion.  When ES is swept positive of the Os(bpy)₃^2+/3+ redox wave, a positive feedback effect is observed due to the regeneration of Fe(CN)₆^3- in the solution gap region because of the oxidation of Fe(CN)₆^4- by Os(bpy)₃³⁺ at the solution-film interface. When ES is negative of the redox wave, the film shows negative feedback behavior, since the Os(bpy)₃²⁺ formed is unable to oxidize tip-generated Fe(CN)₆^4- back to Fe(CN)₆^3-.

E. Liquid-liquid interfaces

One of the most promising applications of SECM is the study of charge transport at the interface between two immiscible electrolyte solution (ITIES).⁷ Unlike conventional techniques, SECM allows for the studies of both ion and electron transfer at the interface.  For example, uphill electron transfer, in which an electron is transferred uphill from a redox couple with a higher standard reduction potential in one phase to another redox couple having a lower standard reduction potential in a second immiscible phase has been demonstrated using the system TCNQ (in 1,2-dichroloethane (DCE))/ferrocyanide (in water).^7c  Fig. 7 shows the approach curve obtained as the UME approaches the interface when the system contains supporting electrolytes with no partitioning ions such as tetraphenylarsonium (TPAs⁺). However, the reverse electron flow for the same redox reaction can be induced by employing TPA⁺ as a potential-determining ion as shown in Fig. 8.  The driving force for this reverse electron transfer is the imposition of an interfacial potential difference by the presence in solution of TPA⁺ in both phases (Δ_owφ = -364 mV).  Note that the detection of reverse electron flow in this case could not be done using the method commonly used for studies of  the ITIES, e.g., cyclic voltammetry. Since the ITIES is not polarizable in the presence of TPA⁺ in both phases, any attempt to impose externally a potential across the interface with electrodes in two phases would result in interfacial ion transfer and a current flow.  The SECM approach does not suffer from this interference.  Charge transfer processes across the ITIES with or without membranes have also been studied.

F. Probing patterned biological systems

SECM has been actively employed to probe artificially or naturally patterned biological systems.⁸  Both amperometric and  potentiometric techniques with ion-selective tips can be used.  A direct test of the SECM's ability to image an enzymatic reaction over a localized surface region^8a is shown in Fig. 9.  Glucose oxidase (GO) hydrogel was filled inside small, well-defined pores of polycarbonate filtration membranes.  The buffered assay solution contained a high concentration of D-glucose as well as two redox mediators, methyl viologen dication (MV²⁺) and neutral hydroquinone (H₂Q). Fig. 9a shows an image obtained with a tip potential of -0.95 V vs. a silver quasi reference electrode (AgQRE) where MV²⁺ was reduced to MV⁺. Since MV⁺. does not react with reduced GO at the hydrogel-filled region, a negative feedback current was obtained. However, with the tip potential changed to 0.82 V, where hydroquinone was oxidized to p-benzoquinone by reduced GO, an increased tip current was observed (Fig. 9b). This positive feedback current over the hydrogel region indicates a significant catalytic feedback of the hydroquinone and provides a direct image of the local enzymatic reaction.

G. Fabrication

The SECM can be used to fabricate microstructures on surfaces by deposition of metal or other solids or by etching of the substrate.⁹ Two different approaches have been used, the direct mode^9a,b and the feedback mode^9c. Typically, in the direct mode, the tip, held in close proximity to the substrate, acts as a working electrode (in deposition reactions) or as the counterelectrode(in etching processes). The feedback mode of fabrication utilizes the same arrangement as in SECM imaging.

The tip reaction is selected to generate a species that reacts at the substrate to promote the desired reaction, i.e., deposition or etching.  For example, a strong oxidant, like Br₂, generated at the tip can etch the area of the substrate, e.g., GaAs, directly beneath the tip.^9d The mediator reactant is chosen to be one that reacts completely and rapidly at the substrate, thus confining the reaction to a small area on the substrate and producing features of area near that of the tip.  Small tip size and close tip-substrate spacing are required for high resolution.

III. References

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