Designing Across the Micro-Nano Boundary: Hierarchically Organized Nafion-Nanopore Structures Enable Electrochemical Diodes

A wide variety of notable technological advances have been achieved through the process of mimicry. Nature has inspired scientists into developing ingenious, yet very essential, techniques of resolving problems. As such, two classical but yet important phenomena that support the functioning of higher organisms have inspired research in areas related to the nanofabrication of complex structures. Existing literature has shown that a number of innovative current measurement/control schemes are based on non-Faradic techniques- as they involve no direct electron transfer since the ions used are typically inert salts. In a majority of these systems, nonlinear current responses arise either because asymmetric nanopore geometries break the symmetry of the ion distribution, creating unequal surface charge across the nanopore, or by coupling unidirectional electron transfer within a nanopore electrode.

Previous studies have also demonstrated the development and characterization of nanopore-based redox-cycling. Nonetheless, the crucial step of combining these ideas was missing – a step which is needed to enable wide scale application of ion-based measurements and signal processing.

Recently, a team of researchers at the University of Notre Dame led by Professor Paul Bohn from the Department of Chemical and Biomolecular Engineering and the Department of Chemistry and Biochemistry successfully fabricated a redox-cycling-based electrochemical diode in a hierarchically organized structure by coating a Nafion membrane on top of nanopore electrode arrays (Nafion@NEA), thereby integrating the asymmetric ion-transport characteristics of Nafion into the redox-cycling system. Their work is currently published in the research journal, ACS Nano.

The novelty of this research arises from the integration of the permselective Nafion membrane with their confined nanopore system, which was previously demonstrated to be an efficient electrochemical vessel to amplify redox signals, as well a powerful nanoparticle sorter to gate particle transport to/from the confined space. In the current work, the research team exploited the ability of Nafion membranes to function as efficient molecular sieves, allowing only cations to be transported into the interior volume of the NEAs, while repelling anions. Additionally, the top ring electrode of dual-electrode NEAs was used to control ion transport for subsequent redox-cycling reaction inside the Nafion@NEA structure, that is, switching between “gate open” and “gate closed” conditions. Overall, this hierarchically-organized system accesses new chemical and signal processing capabilities, because it integrates structures with micrometer and nanometer-scale features to effect molecular transport control at the molecular level.

In summary, Professor Bohn and his research team presented in the report the development of a hierarchically organized redox-cycling-based electrochemical diode by coating defect-free Nafion membranes on the top of a nanopore electrode arrays. Their study enabled the simplification of the electrical connection by using the top electrode and bottom electrode of NEAs in a two-terminal configuration, in which the structure exhibited strong rectification and functioned as an electrochemical diode. By doing so, the Notre Dame researchers managed to develop a high performance, redox-cycling-based electrochemical diode with low operation voltage, fast response, and good stability.