Electrochemical Mechanism of Ion–Ionophore Recognition at Plasticized Polymer Membrane/Water Interfaces

Title:  Electrochemical Mechanism of Ion–Ionophore Recognition at Plasticized Polymer Membrane/Water Interfaces

Author:  Ryoichi Ishimatsu, Anahita Izadyar, Benjamin Kabagambe, Yushin Kim, Jiyeon Kim, and Shigeru Amemiya

Journal:  Journal of the American Chemical Society

Affiliation:  Department of Chemistry, University of Pittsburgh, 219 Parkman Avenue, Pittsburgh, Pennsylvania, 15260

Take Home Importance:  First electrochemical study that reveals kinetics and molecular level mechanism of facilitated IT at plasticized PVC membrane/water interfaces. Elucidation of one-step electron transfer (E) mechanism rather than two-steps process of electron transfer followed by a chemical change (EC) mechanism for ion-ionophore association.

Element Specific Ionophores

Summary:  After electropolymerizing a polymer thin film on the surface of the working electrode, a thin piece of tubing, which holds target metal ions in aqueous solution, is directly attached to the electrode surface to create kinetically-limited conditions. By using cyclic voltammetry, the intrinsic kinetic mechanisms can be elucidated by analyzing voltammogram behaviors. Additional parameters can be extracted via modeling, with numerical fit on α, and k0. α is the transfer coefficient that describes the symmetry of energy barrier surrounding one electrochemical reaction, while k0 describes the rate constant of the electrochemical reaction. The novelty of this study is that the kinetics have been slowed down enough to clearly identify the binding mechanism is E and not EC.

A polished electrode (gold or glassy carbon) was modified with a polymer film and then a PVC membrane by drop-cast. The ionophores are cast in the PVC membrane. The thickness of the film is about 14 μm. A piece of Teflon tubing was put on the membrane to create a water/PVC membrane interface. The PVC membrane immobilizes ionophores for the study.

First, the author showed that the addition of polymer thin film does not interfere the electrochemical process by imposing a large Ohmic potential drop (often due to solution or interfacial resistance in electron transfer). As seen by the cyclic voltammograms, Ag+, K+, Ba2+, and Pb2+ are fast quasi-reversible processes that peak potential does not drift given different scan rate. The shift in the case of Ca2+ showed that the kinetics are slow enough and quasi-reversibility is fairly clear (reaction rate of a reversible reaction is proportional to current or nerstian. In cyclic voltammetry, it results in nicely symmetric “duck” shape or the ratio between reductive and oxidative current to be 1).

In Figure 2c, the shift of peak potential with respect to scan rate shows the electron transfer on calcium ions is clearly not reversible; however, the current is still diffusion limited both in oxidation and reduction, which indicates its quasi-reversibility.

In addition, by modeling the system as first-order process with Bulter-Volmer relation, the transfer coefficient $alpha; is found to be close to 0.5, another indication of one-step electron transfer (energy barrier for oxidation and reduction at the equilibrium is about the same). If EC mechanism is used to analyze the cyclic voltammograms, the ion-ionophore interactions have to be labile enough to exceed diffusion limitation (i.e. weak chemical binding), which is not the case where the model presented an association rate constant several magnitude larger than highest reported value for ion transfer (So in this case, either dissociation rate constant is also several magnitude larger than any reported value or the model does not accurate describe the transfer mechanism. The fitted kinetic constant is smaller than the diffusion-limited value thus causes the first hypothesis to be rejected).

With a micropipet on modified electrode, DCE (1,2-dichloroethane)/water interface was examined, which showed similar ion transfer behavior. Additional evidence supporting E mechanism was showed in this system (check out the rest of the paper for yourself :)). So in conclusion, E mechanism describes the data set much better than the EC mechanism. And further application of surface modified electrode may increase the current limitations on ion-specific sensing by improving both sensitivity and selectivity. Why would that be useful? Many biological processes are controlled by specific ions, such as your muscle contractions (sodium and potassium) or neuronal transmission (copper and zinc). Better ion sensing techniques give us more insight into the exact biochemical pathways and contribute toward medicinal applications to treat disorders in cellular signaling.

It may take a while to understand the results and the implications in this paper, but once it clicks, it is pretty cool.

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