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From theory to practice: How did Allen J. Bard lead the revolution in scanning electrochemical microscopy?
Scanning electrochemical microscopy (SECM) is a technique used to measure the local electrochemical behavior at liquid/solid, liquid/gas, and liquid/liquid interfaces. The technique was first characterized in 1989 by Allen J. Bard, an electrochemist at the University of Texas. Since then, the theoretical foundation has been gradually improved, making the technique widely used in fields such as chemistry, biology, and materials science.
SECM acquires spatially resolved electrochemical signals by precisely moving the ultramicroelectrode (UME) tip over the substrate region of interest. The interpretation of SECM signals is based on the concept of diffusion-limited current. Users can aggregate information from two-dimensional raster scans to generate images of surface reactivity and chemical kinetics. This technique complements other characterization methods such as surface plasmon resonance (SPR), electrochemical scanning tunneling microscopy (ESTM), and atomic force microscopy (AFM) to provide in-depth insights into different interfacial phenomena.
SECM not only provides topographic information, but is also frequently used to probe the surface reactivity of solid-state materials, electrocatalytic materials, enzymes, and other biophysical systems.
With the advent of electrochemical nanoelectrodes (UMEs) in the 1980s, the sensitive electrochemical analysis technique of SECM was developed. In 1986, Engstrom performed the first SECM-like experiments, which enabled direct observation of reaction profiles and short-lived intermediates. Soon after, Bard performed experiments using electrochemical scanning tunneling microscopy (ESTM), which showed that currents could still be detected at large tip-sample distances, which was inconsistent with electron tunneling. This phenomenon was related to Faraday currents and prompted a deeper analysis of electrochemical microscopy.
Bard's theoretical foundations in 1989 were also refreshing, and he first coined the term "scanning electrochemical microscopy". By showing the application of various feedback modes, Bard explained the wide applicability of SECM. As the theoretical foundations developed, the number of SECM-related publications increased year by year, from about 80 in 1999. The popularity of SECM is not only due to theoretical innovations, but also driven by technological advances, which further expanded the experimental mode, a wide range of substrates and improved sensitivity.
Principle of Operation
SECM studies redox pairs by manipulating the potential at the tip of an ultramicroelectrode in an electrolyte. By applying a sufficiently negative potential, (Fe3+) ions are reduced to (Fe2+) at the tip of the ultramicroelectrode, resulting in a diffusion-limited current.
The current variation in this process is related to multiple factors, including the concentration of the oxidizing species, the diffusion coefficient, and the radius of the ultramicroelectrode tip.
SECM has two main operating modes: feedback mode and collection-generation mode. In feedback mode, the ultramicroelectrode approaches a conductive substrate and the current increases. In contrast, when the probe contacts an insulating surface, the current decreases because the oxidizing species cannot be regenerated.
Applications
SECM has been used to probe the morphology and reactivity of solid-state surfaces, track the dissolution kinetics of ionic crystals in aqueous environments, screen electrocatalytic materials, elucidate the activity of enzymes, and study biophysical systems such as dynamic transport in synthetic and natural membranes. Early experiments focused on solid-liquid interfaces and provided higher spatial resolution and sensitivity than traditional electrochemical experiments.
In recent years, SECM technology has been improved to explore the dynamics of chemical transfer at liquid-liquid and gas-liquid interfaces.
In terms of microstructuring, SECM is also used for surface fabrication, patterning, and microstructuring. The SECM configuration allows for operations such as scanning probe lithography (SPL), which helps study metal deposition, surface etching, and surface patterning reactions by enzymes. Combined with electrochemical properties, SECM overcomes the size limitations of conventional microfabrication processes.
Summary
Allen J. Bard's contribution to the development of the scanning electrochemical microscope is undoubtedly extremely important. His research provides an irreplaceable platform for subsequent scientific exploration. With the continuous advancement of technology and theory, can SECM lead to new scientific discoveries in the future? What do you think?
