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Thin really does matter after all
I’ve never seen Bill Heineman in a boastful mood. Early this summer I was asked to contribute to a symposium honoring Distinguished Professor William R. Heineman of the University of Cincinnati, who this year was awarded the top analytical chemistry recognition from the American Chemical Society. Bill and I first met as graduate students in Chapel Hill North Carolina in the summer of 1966. Just a few years earlier it was observed by chance that productive things happen if electrochemical experiments were confined to a thin layer of solution. Here diffusion was restricted to a film with a thickness on the order of 0.01-0.10 mm.
For reference, a piece of copy paper is typically about 0.1 mm. Fifty years ago, this was a very novel idea. Once the news broke several years earlier, groups at Cal Tech and UNC were energized to think of how to do it and how to solve the associated diffusion equations in these liquid films. While it was long known that electrochemical reactions were heterogeneous and occurred only at atomic dimensions at surfaces, electroanalytical chemistry traditions of that day involved 50 mL to 100 mL cells operating such that reactions at electrode surfaces of a few square millimeters did not significantly alter the composition of the solution under study. As is often the case, this established dogma was very limiting.
Confining the solution in a film afforded some unique advantages. For one, the solution could be exhaustively reacted in a few seconds. For another, steady-state cycling could be performed with opposing electrodes. Total solution volumes of as little as 0.010 mL were very feasible.
Bill, working with Prof. Royce Murray, pursued the idea of optically transparent thin-layer electrochemistry, which enabled optical interrogation of the conversion of reactant to product without interference from a large path length of non-participating solution. Electrodes could be very fine transparent grids. Ultimately transparent metal thin and semiconducting films were also explored along with both transmission and reflectance spectroscopy (specular and internal reflectance). This was all good fun. Technology developed for other purposes got put to use for entirely new and unexpected benefits.
In the hands of Bill’s group, thin-layer spectroelectrochemistry became a means to determine the redox potentials for proteins with metal centers inaccessible to an electrode surface. It became helpful in studying bone imaging agents based on technetium. Working with radioactive materials suggested possible applications of electroanalytical chemistry in tracking Superfund sites. Electrochemistry in small volumes suggested an alternative to radioimmunoassays based on electrochemistry to supply the signal.
From there, enzyme-linked electrochemical immunoassays were born in the Heineman lab and today are used in important point-of-care diagnostic measurements. Then others added electrochemiluminescent immunoassays to the armamentarium and they, too, are a commercial success today. Thin-layer electrochemical cells are sold in millions a week for home glucose monitoring. They are also used in automatically diming electrochromic rearview mirrors on millions of cars and trucks.
Thin matters in many places, such as liquid crystal displays, interstitial distances in chromatography columns, small diameters in capillary electrophoresis, microdialysis probes in tissue, detectors for chromatography and channels in microfluidic devices. Fick’s Laws of diffusion still work fine. Combining them with Nernst’s equation (1888) and Faraday’s Law of electrolysis (1834) has brought endless pleasure to scientists and productive results for millions of diabetics and truck drivers once blinded by the sun in their rearview mirrors.
In isocratic chromatography we say that the young peaks must be thin, while the more strongly retained older peaks can be a little chubby. I am happy to report that Bill Heineman is still thin, five decades after graduate school. Although he’s taken on his last graduate student, the new ideas are still there and he’s achieved everlasting life as his academic grandchildren contribute to advancing science with impact.
We bond to people we served with on sports teams, in war and in graduate school. Summer is a wonderful time to reconnect with those you served with and share perspective on how much has happened since then. Bill and I thank the USSR for stimulating science and engineering funding in the 1960s. Thin-layer electrochemistry owes a lot to the United States Air Force Office of Scientific Research, and so does the computer screen where you may be reading this column.
Thank you, Khrushchev.