Electrical Characterization of Reflectin IsoformsAmanda G. Smith, Laliphat (Mai) Kositchaimongkol, Michael N. Nguyen, Nikka Mofid

Department of Chemical Engineering and Materials Science, University of California, Irvine

1. Colomban, P. Proton Conductors: Solids, Membrains and Gels—Materials and Devices (Cambridge Univ. Press, 1992).2. Fabbri, E. Pergolesi, D. & Traversa, E. Materials challenges toward proton-conducting oxide fuel cells: a critical review. Chem. Soc. Rev. 39, 4355- 4369 (2010).3. Kruerer, K., Paddison, S. J., Spohr, R. & Schuster, M. Transport in proton conductors for fuel-cell applications: stimulations, elementary reactions, and phenomenology. Chem. Rev. 104, 4637-4678 (2004).4. Kreuer, K. Proton conductivity: materials and applications. Chem. Mater. 8, 610-641 (1996)5. I. Kymissis, 2009, Organic Field Transistors Theory, Fabrication, and Chacterization, Springer, New York, 136 p.6. Mauritz, K. A. & Moore, R. B. State of understanding of Nafion. Chem. Rev. 4535-4385 (2004).7. Norby, T. Proton conduction in solids: bulk and interfaces. MRS Bull. 923- 928 (2009).8. Ordinario, D. D., Phan, L., Walkup IV, W. G., Jocson, J-M., Karshalev, E., Husken, N., Gorodetsky, A. A. Bulk protonic conductivity in a cephalopod structural protein. Nat. Chem. 2014 Accepted9. Yoon, M., Suh, K., Natarajan, S. & Kim, K. Proton conduction in metal- organic frameworks and related modularly built porous solids. Angew. Chem. Int. Ed. 52. 2688-2700 (2013).

We would like to acknowledge the University of California, Irvine,the Gorodetsky Group, the Undergraduate Research Oppurtunities Program, the Air Force Office of Scientific Research,the National Science Foundation, the Delta Hardware & Industry Co. For their financial support.

References Acknowledgement

Ionic transistors from organic and biological materials represent an emerging class of devices for bioelectronics applications. Within this context, protonic transistors represent exciting targets for further research and development despite the fact that they have received relatively little attention. Given the ubiquity of proton transport and transfer phenomena, protonic devices represent a natural choice for interfacing rugged traditional electronics and biological systems, facilitating the sensitive transduction of biochemical events into electrical signals. In previous research, we fabricated and characterized protonic transistors from the cephalopod structural protein reflectin isoform A1 and recently fabricated devices from reflectin isoforms A2 and B1. We have investigated these devices with standard electrical and electrochemical techniques, and our findings indicate performance comparable to the state-of-the-art for protonic transistors. Overall, our findings may hold significance for a broad range of biomedical and bioelectrochemical devices.

Abstract

Reflectins possess a number of interesting yet unique properties which set them apart from most other proteins. They contain a large number of charged amino acid residues, consist of one to six highly conserved repeating subdomains separated by variable linker regions and possess little-to-no organized secondary structure.

Background: Properties of Reflectin

All previous characterization of the ability for reflectin to act as the active element of a protonic transistor has been performed on the A1 isoform. However, in addition to A1 there are many other isoforms of reflectin that have been discovered. Isoforms that are currently of interest to this project are A2, B1, and 1b. A family tree showing the known isoforms of reflectin is shown to the right.

Materials and Methods Results

Finally, devices were fabricated into 3-terminal field effect transistors, which allows for greater electrostatic control over conduction.  If these devices utilized protons as the charge carrier in conduction, a negative applied gate bias  should induce more injection of protons into the channel and increase the observed current ; conversely, a positive applied gate bias would deplete the channel of protons and a decreasing current would be observed.

When conducting standard electrical benchmark tests, similar trends were observed across all reflectin isoforms, thereby indicating a huge similarity in their behavior and conduction mechanisms.

* Isoform devices were tested with Pd versus PdHx in the procedure described above.  There was a notable increase in current when Palladium Hydride electrodes were used across all reflectin isoforms, A1, A2, and B1.  Palladium Hydride is a proton injecting material, indicating that protons are the charge carrier used in all the reflectin isoforms.

* The currents for all reflectin isoforms were collected at varying humidities, from 70% RH to 90% RH.  There is an increase in current across all isoforms with increasing humidity which shows water absorbed in the films plays in integral role in proton conduction.  The Grotthus mechanism was proposed in the A1 isoform and therefore is likely used by isoforms A2 and B1.

* Protein isoforms were fabricated into field effect transistors.  All isoforms exhibit current increases with more negative gate biases and current decreases with more positive gate biases.  Standard electron-conducting transistors behavior would do the opposite, thereby suggesting isoforms A2 and B1 are proton conductors like A1.

* Integrating these transistors on living organic material. * Characterization of other isoforms to see if any isoform is more ideally suited* Selectively breeding the protein that produce reflectin to create an isoform with maximized electrical properties.* Selective breeding of reflectin to engineer an isoform with optimized electrical properties

Future Work

For electrical measurements, we fabricated two- and three-terminal bottom contact devices, where reflectin served as the active material. Shadow mask lithography was used to electron-beam evaporate arrays of paired palladium (Pd) electrodes onto silicon dioxide/silicon (SiO2/Si) substrates. Subsequently, we drop cast smooth and featureless thin films of reflectin directly onto these electrodes from aqueous solution and mechanically scribed away excess material, taking great care to avoid damaging the electrodes. The completed devices were then subjected to systematic electrical interrogation.

To test whether isoforms implemented proton-conduction like reflectin RfA1, completed devices were exposed to hydrogen (H2) gas, transforming the Pd electrodes into proton-transparent palladium hydride (PdHx) electrodes in situ. If reflectin protonic conducting material, the devices with proton-injecting electrodes should show much higher currents that those which do not.