Reports: UNI550258-UNI5: Studies of Charge Transfer Across the Interface Between a Small Aqueous Phase (<1femto liter) and a Bulk Organic Phase

Peng Sun, Ph.D , East Tennessee State University

There are wide applications of the system in which tiny heterogeneous droplets (Vdroplet <10-15 Liter) dispersed in another bulk phase. Such systems include cells and microemulsion etc. The study of charge transfer across the interface between a small aqueous (or organic) phase and a bulk organic (or aqueous) phase are of fundamental importance for life science and enhanced oil recovery. The phenomenon could be studied by using a working electrode whose entire surface is covered by a small volume aqueous (or organic) phase. Since the size of the small phase is directly proportional to the size of the working electrode, it is necessary to use a nanometer-sized electrode so as to study the reaction in a small phase whose volume is less than 1 femto-liter. In the last one year, we concentrated on the study of the electrochemical properties and the surface modification of very small electrode, because it is fundamentally and experimentally important to understand the stability of the very small droplet on the electrode. We found annealing is a very important step in preparing a well polished puller-made nanometer-sized electrode with a radius larger than 10nm. For a well polished nanometer-sized electrode with an effective radius larger than 20nm, the effective radius is almost equal to its geometric radius. A relatively big difference between the effective and the geometric electrode radii can be observed when the effective electrode radius is smaller than 20nm. Cyclic voltammograms show that there is apparent current fluctuation on a 1.6nm radius electrode at slow scan rate. The current fluctuation may result from the potential fluctuation at the electrode/solution interface or the electric double layer effect. We also found that it is possible to attach a single gold nanoparticle on a nanometer-sized Pt electrode with a radius comparable to the size of a gold nanoparticle. Our preliminary results show that the gold oxide stripping peak position becomes more negative and the gold oxidation peak position becomes more positive when its size is decreasing. We also found that nanometer-sized gold electrodes with radii down to 60nm could be chemically modified. The electrochemical behavior at a modified nanometer sized electrode is similar to that at a macro electrode. For example, the cyclic voltammograms obtained on a 4-aminothiophenol (4-ATP) self-assembled monolayers modified electrode show peaks and the peak height is proportional to the scan rate, which is similar to that on an electroactive SAMs modified macro electrode. The electrochemical behavior and mechanism of outer-sphere electron transfer reaction on the 4-ATP SAMs modified nanometer-sized electrode has also been studied. A nanometer-sized gold electrode modified with a monolayer of alkanethiols has also been studied. There are pin holes in the monolayer, and the pinholes can be used as very small electrode. Our evaluation shows that it is possible to have only one pinhole on the monolayer covered electrode. The single pinhole electrode has been used to study the electrochemical behaviors of fast and slow electrochemical reactions. Right now, we are studying the stability of very small (aqueous or organic) droplet supported on the small electrode.
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