Transmembrane transport characterization across ionic redox transistors using surface-tracked scanning ion conductance microscopy

This study is led by Dr. Vijay Venkatesh (Department of Mechanical and Aerospace Engineering, The Ohio State University). Characterization of transmembrane ion transport through porous substrates at the nanoscale is critical to integrating membrane separators in drug delivery, gating circuits, and batteries. The suite of existing scanning probe microscopy techniques are limited to investigating ion transport as a surface phenomena and do not provide an insight into the fundamental mechanisms of ion transport within the material under investigation. In a recent article, they introduced ‘surface-tracked scanning ion conductance microscopy’ as a high-resolution microscopy technique to quantitatively characterize transmembrane ion transport across porous substrates. The objective of this paper is to extend the aforementioned technique to enhance their understanding of ion transport across an electrochemical arrangement referred to as an ‘ionic redox transistor’. An ionic redox transistor is a device that was developed previously to regulate transmembrane transport and mitigate thermal runaway in commercial Li-ion batteries.

Conducting polymers such as polypyrrole (PPy) doped with anions such as dodecylbenzenesulfonate (DBS^－), polystyrenesulfonate (PSS^－) and dodecylsulfate (DS^－) are attractive candidates for regulating ion transport due to insertion/expulsion of cations into/out of the polymer backbone during the reduction/oxidation. The concept of using conducting polymers as an ‘ionic gate’ was first proposed by Murray and Burgmayer. It was noted that the membrane switches between an ON-OFF state during reduction/oxidation of the polymer, thus resulting in controlled ion transport. Price and coworkers extended this principle to develop ion-transport systems for separating various metal ions from aqueous solutions. They showed that the application of a pulsed potential waveform to the conducting polymer results in higher ionic flux. Misoska and coworkers investigated permeability characteristics of polypyrrole doped with bathocuproinedisulfonic acid (BCS^－) to transition metal ions such as Co²⁺, Ni²⁺, Zn²⁺. To achieve pulsatile drug delivery, Santini and coworkers fabricated a porous microchip on which a thin film of Au was sputtered. The application of an electric field resulted in expulsion of chemicals across the porous channels in the device. Abidian and coworkers synthesized poly(3,4-ethylene dioxythiophene) nanotubes for transporting dexamethasone as a function of electrochemical state of the polymer. Jeon and coworkers fabricated a nanoporous membrane consisting of polypyrrole deposited on anodized aluminum oxide for transporting isothiocyanate-labeled bovine serum albumin as a function of redox state of the polymer. It should be mentioned that the conducting polymer was deposited such that the equivalent pore size of the membrane increased/decreased when PPy was oxidized/reduced, thus resulting in greater/lower flux of ions through the nanoporous columns in the substrate. Recently, Hery and Sundaresan fabricated an ionic redox transistor which consists of polypyrrole spanning across the pores of a polycarbonate track-etch membrane. The application of an electric potential to the polymer resulted in bi-directional transport of Li⁺ ions across the porous membrane due to hopping pathways created in the polymer backbone. The ionic redox transistor was used as a smart membrane separator in a Li⁺ battery to regulate ion transport at elevated temperatures and prevent thermal runaway.

Despite aforementioned advances in using conducting polymers for various technologies such as gas filtration, gating channels and membrane separators, ion transport at the nanoscale in conducting polymers is poorly understood. Although there are a plethora of research articles which have quantified transmembrane properties of synthetic membranes using ion conductance microscopy, literature on in-situ imaging of transmembrane ion transport across a membrane separator which can regulate ionic current as a function of its electrochemical signature is limited to a report which investigates increase in equivalent pore size of nanoporous channels deposited with PPy using atomic force microscopy (AFM). While AFM images of the underlying substrate depicted a change in equivalent pore size as a function of redox state of the polymer, no information was revealed on the variations in ionic flux across the polymer membrane. Further, Laslau and coworkers reported the need for advances in ion conductance microscopy, primarily for the purpose of distinguishing variations in ionic flux of conducting polymers from volumetric expansion due to ion ingress.

To address the need for an imaging technique that can quantitatively image ion transport across conducting polymers, this article uses surface-tracked scanning ion conductance microscopy using shear-force (SF) imaging as a technique to investigate kinetics of ion transport in ionic redox transistors. It was shown that an application of a reduction potential to PPy(DBS) (V_m) under a constant transmembrane potential (VAC) facilitates ion ingress into redox sites in the polymer and drives transmembrane transport. The transmembrane current increases as the potential applied to PPy(DBS) (V_m) increases, thus switching the transistor from its OFF state to its ON state. An equivalent circuit model of the system was developed and it was shown that the transmembrane current was a result of an increase in conductivity of the polymer under a reduction potential. Finally, surface-tracked scanning ion conductance microscopy was used to map topography and topography-correlated transmembrane transport over an array of pores. The increase in local transmembrane currents was attributed to a higher potential drop between the PE and QRCE, and was quantified using a modified GoldmanHodgkin-Katz (GHK) Equation. It is anticipated that surface-tracked scanning ion conductance microscopy would serve as a tool to characterize transmembrane ion transport across various ionic devices used in chemical separation, gas filtration drug delivery, and desalination.