Electrochemistry.

F or this special issue of PNAS, it seems appropriate to begin with a brief commentary on the path(s) leading to where the discipline of electrochemistry is today. The papers in this issue serve as a sampling of the current themes and directions; we thank the authors for their contributions. We should also point to a nice collection of historical perspectives gathered together by Leddy et al. (1). The development of the electrochemistry discipline up to the 1960s was substantially that of the physical principles of describing electrochemical reactions and electrode reactions in which current or working electrode potential was controlled in some way. Those were aptly summarized in the classic 1954 text “New Instrumental Methods in Electrochemistry” (2). According to his introductory chapter, Delahay (2) wrote this influential text at the urging of the legendary I. M. Kolthoff, who prepared a preface to the book that stated “This book deserves a wide circle of friends.” It was indeed very influential and added impetus for (then young) researchers like Bard and Murray to enter the discipline. The introduction of flexible electrochemical equipment in the late 1950s, particularly including instruments based on the now-ubiquitous operational amplifier, unleashed an accelerated evolution of different ways to manipulate applied currents and potentials in electrochemical experiments. This evolution included the development by Nicholson and Shain of a technique first labeled as “stationary electrode polarography” but shortly afterward as “cyclic voltammetry” (CV) (3, 4). This was a very important development, because observation of the current pattern evoked by application of a single triangular wave potential scan, repeated at varied potential scan rates, allowed the informed researcher to deduce whether the current was controlled by mass transport, affected by a chemical reaction involving either the initial reactant or the product of the forward potential sweep, and involved reaction(s) of adsorbed species, as well as whether or not the reaction was electrochemically reversible. Such an enticing bonanza of information provided by the CV experiment provoked the present and continuing use of electrochemistry by a wide circle of researchers having a multitude of objectives. For example, CV has become an important technique that complements spectroscopic techniques in studies of organic and inorganic species. Analyses of CV responses and further probing of ever more complex and/or multiple chemical reactions accompanying electrochemical reactions encounter mathematical obstacles, centered on an inability to obtain explicit equation solutions of the mass transport processes coupled to the chemical processes. The necessity of using numerical solutions was a serious impediment until the effective aid of digital simulation was demonstrated (5) and commercial programs using implicit finite difference algorithms by Rudolph (6) in collaboration with Feldberg (7) became available. The variety of electrochemical approaches to study chemical processes was boosted starting in the 1960s by combinations with spectroscopic measurements in which the working electrode is either optically “transparent” or highly reflecting, such that an incident light beam traverses the volume of reactant and product solution near the electrode surface (8, 9). This informs the electrochemical response with chemical structural information available from the spectra. “Spectroelectrochemistry” now encompasses a very wide range of spectral approaches that include reflectance, ellipsometry, internal reflection, surface plasmon resonance, second harmonic spectroscopy, and infrared and Raman spectroscopies, as well as X-rays. Spectroelectrochemical methods can also be devised in schemes in which the electrode reaction is monitored by the mass of its product (electrochemical quartz crystal microbalance), its magnetic properties (as in electron spin and NMR spectrometries), properties observed in high-vacuum experiments like electron spectrometry, and scanning probe methods. The book written by Bard and Faulkner (10) contains a summary of these developments. A further development was small electrodes (i.e., microelectrodes), which were introduced by Adams in studying neurotransmitters in vivo and extended to even smaller sizes, sometimes called ultramicroelectrodes (UMEs), by Wightman and Fleischmann around 1980 (ref. 10, p. 169). The development of applications of UMEs was carried forward by Wightman and colleagues (11) and others to show that the combination of small currents and the associated loosening of tolerable solution resistance effects on electrochemical cell time constants, on the one hand lowered the accessible time domains of electrochemical experiments and, on the other hand broadened the scope of usable low conductivity media. It is rare today to find electrochemical reports that do not take advantage of the virtues of UMEs. Current research leads onward into the land of nanoelectrodes, where ambitions of molecule-sized electrodes can be found. In summary, the modern world of electrochemistry has been built on a range of experimental modalities and, in regard to its methods, may be said to be approaching “maturity.” This maturity is not an end point, however, but one that enhances the ways in which new chemistry can be measured and understood. The span of chemical topics in the papers in this issue represents a sampling of that enhancement.