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

Significance 

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 ([email protected]), 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 [email protected] 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.

Designing Across the Micro-Nano Boundary: Hierarchically Organized Nafion-Nanopore Structures Enable Electrochemical Diodes - Advances in Engineering
Schemes and SEM images of asymmetric Nafion-coated nanopore electrode arrays, [email protected] (Top, left). (a) Schematic illustration of [email protected], where Nafion (light blue) acts as a cation exchange membrane to allow cations (red spheres) to pass through, while rejecting anions (green spheres), for subsequent redox cycling inside the NEA. (b) The [email protected] acts as a redox cycling-based diode when using the top and bottom electrodes are operated in a two terminal configuration. (c) Tilted SEM image near the edge of the Nafion film, indicating that the Nafion conformally coats the NEA (scale bar is 2 mm), and cross-sectional SEM image showing the stacked metal (disk)-insulator-metal (ring) (MIM) structure in the vertical direction, as well the well-sealed Nafion at the top (scale bar is 400 nm).

About the author

Dr. Kaiyu Fu is a postdoctoral associate in the Department of Electrical Engineering and Radiology at Stanford University. He got his bachelor and master’s degree both in polymer science and engineering from Sichuan University and Fudan University, respectively. Then he received his Ph.D. in chemistry from the University of Notre Dame. He is a co-author of more than 20 papers and one book chapter. For the period 2018/2019, he is one of recipients of ACS Division of Analytical Chemistry Graduate Fellowship. His research is focused on the advancement of electroanalytical methods, including the fabrication of micro/nanoscale electrodes for lab-on-a-chip and the application of electrochemical biosensors for point-of-care diagnostics.

About the author

Dr. Donghoon Han is an assistant professor in the Department of Chemistry at The Catholic University of Korea. He received his Ph.D. in chemistry from Seoul National University, Republic of Korea in 2014 and worked as postdoctoral research associate at the University of Notre Dame, US (2014-2018). His research interests lie the intersection of electrochemistry, nanoscience, and the biological interface. One of his main objective is to develop new (bio)analytical methods to probe chemical and biological systems with unprecedented spatial and temporal resolutions afforded by working at the micro/nanoscale.

About the author

Dr. Seung-Ryong Kwon earned his M.S. degree in the department of Chemistry from Incheon National University, South Korea in 2011 and his Ph.D. degree from Seoul National University in August 2016, South Korea. Since November 2016, he joined professor Dr. Paul Bohn’s group as a postdoctoral researcher in the department of Chemical and Biomolecular Engineering at the University of Notre Dame. Currently, his research focuses on studying ion transport behaviors within electric fields in attoliter-volume nanopore electrode arrays.

About the author

Dr. Paul W. Bohn received the B.S. in Chemistry from the University of Notre Dame in 1977 and the Ph.D. in Chemistry from the University of Wisconsin-Madison in 1981. Since 2006, he is the Arthur J. Schmitt Professor of Chemical and Biomolecular Engineering and Professor of Chemistry and Biochemistry of the University of Notre Dame. Dr. Bohn’s research interests include: (a) molecular approaches to, and uses of, nanotechnology, (b) integrated nanofluidic and microfluidic chemical measurement strategies for personal monitoring, and (c) correlated chemical imaging, especially of microbial communities.

Reference

Kaiyu Fu, Donghoon Han, Seung-Ryong Kwon, Paul W. Bohn. Asymmetric Nafion-Coated Nanopore Electrode Arrays as Redox-Cycling-Based Electrochemical Diodes. ACS Nano 2018, volume 12, page 9177−9185.

Go To ACS Nano

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