Scanning electrochemical microscopy
Scanning electrochemical microscopy is a technique within the broader class of scanning probe microscopy that is used to measure the local electrochemical behavior of liquid/solid, liquid/gas and liquid/liquid interfaces. Initial characterization of the technique was credited to University of Texas electrochemist, Allen J. Bard, in 1989.
Since then, the theoretical underpinnings have matured to allow widespread use of the technique in chemistry, biology and materials science. Spatially resolved electrochemical signals can be acquired by measuring the current at an ultramicroelectrode tip as a function of precise tip position over a substrate region of interest. Interpretation of the SECM signal is based on the concept of diffusion-limited current. Two-dimensional raster scan information can be compiled to generate images of surface reactivity and chemical kinetics.
The technique is complementary to other surface characterization methods such as surface plasmon resonance,
electrochemical scanning tunneling microscopy, and atomic force microscopy in the interrogation of various interfacial phenomena. In addition to yielding topographic information, SECM is often used to probe the surface reactivity of solid-state materials, electrocatalyst materials, enzymes and other biophysical systems.
SECM and variations of the technique have also found use in microfabrication, surface patterning, and microstructuring.
History
The emergence of ultramicroelectrodes around 1980 was pivotal to the development of sensitive electroanalytical techniques like SECM. UMEs employed as probes enabled the study of quick or localized electrochemical reactions. The first SECM-like experiment was performed in 1986 by Engstrom to yield direct observation of reaction profiles and short-lived intermediates. Simultaneous experiments by Allen J. Bard using an Electrochemical Scanning Tunneling Microscope demonstrated current at large tip-to-sample distances that was inconsistent with electron tunneling. This phenomenon was attributed to Faradaic current, compelling a more thorough analysis of electrochemical microscopy. The theoretical basis was presented in 1989 by Bard, where he also coined the term Scanning Electrochemical Microscopy. In addition to the simple collection modes used at the time, Bard illustrated the widespread utility of SECM through the implementation of various feedback modes. As the theoretical foundation developed, annual SECM-related publications steadily rose from 10 to around 80 in 1999, when the first commercial SECM became available. SECM continues to increase in popularity due to theoretical and technological advances that expand experimental modes while broadening substrate scope and enhancing sensitivity.Principles of operation
Electric potential is manipulated through the UME tip in a bulk solution containing a redox-active couple. When a sufficiently negative potential is applied, is reduced to at the UME tip, generating a diffusion-limited current. The steady-state current is governed by the flux of oxidized species in solution to the UME disc and is given by:where iT,∞ is the diffusion-limited current, n is the number of electrons transferred at the electrode tip, F is Faraday's constant, C is the concentration of the oxidized species in solution, D is the diffusion coefficient and a is the radius of the UME disc. In order to probe a surface of interest, the tip is moved closer to the surface and changes in current are measured.
There are two predominant modes of operation, which are feedback mode and collection-generation mode.
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. 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 loop. The opposite effect is observed when probing insulating surfaces, as the oxidized species cannot be regenerated and diffusion to the electrode is inhibited as a result of physical obstruction as the tip approaches the substrate, creating a "negative" feedback loop and decreasing the tip current. An additional parameter to consider when probing insulating surfaces is the electrode sheath diameter, rg, since it contributes to the physical obstruction of diffusion.The change in tip current as a function of distance d can be plotted as an "approach curve" as shown.
Due to the rate dependent nature of SECM measurements, it is also employed to study electron-transfer kinetics.
Collection-generation modes
Another mode of operation that is employed is tip generation/substrate collection. In TG/SC mode, the tip is held at a potential sufficient for an electrode reaction to occur and "generate" a product while the substrate is held at a potential sufficient for the electrode product to react with or be "collected" by the substrate. The reciprocal to this method is substrate generation/tip collection, where the substrate acts to generate a species that is measured at the tip. Both TG/SC and SG/TC variations are also categorized as "direct" modes.Two currents are generated: the tip current, iT, and the substrate current, iS. Since the substrate is generally much larger than the tip, the efficiency of collection, iS/''iT, is 1 if no reactions occur during the transfer of tip-generated species to the substrate. As the distance between tip and substrate, d'', decreases, the collection efficiency, iS/''iT'', approaches 1.
Alternating Current (ac)-SECM
In ac-SECM a sinusoidal bias is applied to the dc bias of the SECM probe allowing the impedance of a sample to be measured, as is the case in electrochemical impedance spectroscopy. Unlike dc-SECM techniques ac-SECM does not require the use of a redox mediator. This is particularly advantageous for measurements where the redox mediator could affect the chemistry of the system under study. Examples include corrosion studies where a redox mediator may act to inhibit or enhance the rate of corrosion, and biological studies where a redox mediator may be toxic to the living cell under study.In ac-SECM the feedback response measured is dependent on both the sample type and the experimental conditions. When a sample is insulating the measured impedance will always increase with decreasing probe to sample distance. This is not the case for a conductive sample however. For a conductive sample measured in a high conductivity electrolyte, or measured with a low ac frequency, decreasing the probe to sample distance will lead to an increase in impedance. If, however, a conductive sample is measured in a low conductivity electrolyte, or with a high ac frequency, decreasing the probe to sample distance will result in a lower measured impedance.
SECM imaging
Changes in current as a function of distance between electrode tip and substrate surface allow imaging of insulating and conducting surfaces for topology and reactivity information by moving the tip across surfaces and measuring tip current.The most common scanning mode is constant-height mode, where the tip height is unchanging and is scanned across the surface in the x-y plane. Alternatively, constant distance measurements are possible, which change the z position to maintain the probe to sample distance as the probe is scanned across the surface in the x-y plane. The constant distance measurement can be based on an electrical signal as is the case in the constant-current mode, where the device attempts to maintain a constant current by changing the substrate to tip distance, d, and recording the change in d. A mechanical signal can also be used to control the probe to sample distance. Examples of this are the intermittent contact -SECM and shear force techniques which use changes in probe vibration to maintain the probe to sample distance.
Spatial resolution is dependent on the tip radius, the substrate to tip distance, the precision of the electronics, and other considerations.
Instrumentation
Early SECMs were constructed solely by individual lab groups from a set of common components including potentiostat and potential programmer, current amplifier, piezoelectric positioner and controller, computer, and UME. Many SECM experiments are highly specific in nature, and in-house assembly of SECMs remains common. The development of new techniques toward the reliable nanofabrication of electrodes has been a primary focus in the literature due to several distinct advantages including high mass-transfer rates and low levels of reactant adsorption in kinetic experiments. Additionally, enhanced spatial resolution afforded by reduced tip size expands the scope of SECM studies to smaller and faster phenomena. The following methods encompass an abbreviated summary of fabrication techniques in a rapidly developing field.Preparation of electrodes
SECM probes use platinum as the active core material, however carbon, gold, mercury, and silver have all been used. Typical preparation of a microscale electrode is performed by heat sealing a microwire or carbon fiber in a glass capillary under vacuum. This tip can be connected to a larger copper electrode through the use of silver epoxy then polished to yield a sharpened tip. Nanofabrication of electrodes can be performed by etching a metal wire with sodium cyanide and sodium hydroxide. Etched metal wires can then be coated with wax, varnish, molten paraffin or glass, poly, polyimide,electropolymerized phenol, and electrophoretic paint. Nanotips produced by these methods are conical, however disc-shaped tips can be obtained by micropipette pulling of glass sealed electrodes. Nanoscale electrodes allow for high resolution experiments of biological features of sub micron scale or single molecule analysis. "Penetration" experiments, where the tip is inserted into a microstructure to probe kinetic and concentration parameters, also require the use of nanoscale electrodes. However, microelectrodes remain ideal for quantitative kinetic and feedback mode experiments due to their increased surface area.
Modification of electrodes has developed beyond the size parameter. SECM-AFM probes can act as both a force sensor and electrode through the utilization of a flattened, etched metal wire coated by electrophoretic paint. In this system, the flattened wire acts as a flexible cantilever to measure the force against a sample as the wire electrode measures the current. Similarly, SECM functionality can be imparted into standard AFM probes by sputtering the surface with a conductive metal or by milling an insulated tip with a focused ion beam. Electron-beam lithography has also been demonstrated to reproducibly generate SECM-AFM probes using silicon wafers. AFM probe manufacturers, such as Scuba Probe Technologies fabricate SECM-AFM probes with reliable electrical contacts for operation in liquids.
Images of the chemical environment that is decoupled from localized topographies are also desirable to study larger or uneven surfaces. "Soft stylus probes" were recently developed by filling a microfabricated track on a polyethylene terephthalate sheet with a conductive carbon ink. Lamination with a polymer film produced v-shaped stylus that was cut to expose the carbon tip. The flexibility inherent in the probe design allows for constant contact with the substrate that bends the probe. When dragged across a sample, probe bending accommodates for topographical differences in the substrate and provides a quasi-constant tip-to-substrate distance, d.
Micro-ITIES probes represent another type of specialty probe that utilizes the Interface between Two Immiscible Electrolyte Solutions. These tips feature a tapered pipette containing a solution containing a metal counter electrode, and are used to measure electron and ion transfer events when immersed in a second, immiscible liquid phase containing a counter-reference electrode.
Often the probing of liquid/liquid and air/liquid interfaces via SECM require the use of a submarine electrode. In this configuration, the electrode is fashioned into a hook shape where the electrode can be inverted and submerged within the liquid layer. The UME tip points upwards and can be positioned directly beneath the liquid/liquid or air/liquid interface. The portion of the electrode passing through the interface region is electrically insulated to prevent indirect interfacial perturbations.
Increases in the complexity of electrodes along with decreases in size have prompted the need for high resolution characterization techniques. Scanning electron microscopy, cyclic voltammetry, and SECM approach curve measurements are frequently applied to identify the dimension and geometry of fabricated probes.