1. Electrical Characterization of Reflectin Isoforms
Amanda 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 Accepted
9. 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.