Scanning electrochemical microscopy (SECM) is a technique within the broader class of scanning probe microscopy (SPM) 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 (UME) 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 (SPR), electrochemical scanning tunneling microscopy (ESTM), and atomic force microscopy (AFM) 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 (UMEs) 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 (ESTM) 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 (e.g. Fe2+/Fe3+). When a sufficiently negative potential is applied, (Fe3+) is reduced to (Fe2+) 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:
i T , ∞ = 4 n F C D a {\displaystyle i_{T,\infty }=4nFCDa}
where iT,∞ is the diffusion-limited current, n is the number of electrons transferred at the electrode tip (O + ne− → R), 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 (TG/SC). 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 (SG/TC), 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.
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