From squid protein to bioelectronic applications

(Nanowerk Spotlight) Electrical devices, from light bulbs to iPods, send information using electrons. All living things, on the other hand, send signals and perform work using ions or protons. Protons activate "on" and "off" switches and are key players in biological energy transfer. Proton-conducting materials have become important for a wide range of technologies, such as fuel cells, batteries, and biosensors. A great deal of research has been devoted to developing improved and application-specific proton conducting materials. Researchers even developed a proton-based transistor that could let machines communicate with living things.
Scientists now have discovered and characterized novel electrical properties for the cephalopod structural protein reflectin.
"The figures of merit for reflectin's protonic conductivity compare favorably to those found for many artificial proton-conducting materials, such as ceramic oxides, solid acids, polymers and metal–organic frameworks" Alon A. Gorodetsky, Assistant Professor at the Henry Samueli School of Engineering at the University of California, Irvine, tells Nanowerk. "We believe that a better understanding of reflectin’s proton conducting properties could inform the design and engineering of artificial proton-conducting materials."
Gorodetsky and his team report their latest findings in the June 1, 2014 online edition of Nature Chemistry ("Bulk protonic conductivity in a cephalopod structural protein").
Squid. (Photo by Tom Kleindinst, ©Woods Hole Oceanographic Institution)
The protein reflectin has been previously explored for its optical properties. For instance, in a previous Nanowerk Spotlight we reported on how squid and octopus might point the way to nanotechnology-based stealth coatings.
However, reflectin's electrical properties had never been previously studied.
"We were inspired to pursue our studies in part by the Hanlon Group's work on electrical triggering of iridophores in squid skin," says Gorodetsky. "We postulated that reflectins might have unique electrical properties."
In their paper, first-authored by David Ordinario, the team describes how they interrogated the protein by humidity-dependent direct current electrical measurements with both proton-blocking and proton-injecting contacts, alternating current electrical measurements in the presence of water and deuterium oxide, rationally guided mutagenesis experiments and temperature-dependent electrochemical impedance spectroscopy. Together, these experiments indicate that reflectin functions as an efficient proton-conduction medium.
"Based on our measurements, we infer that reflectin exhibits the characteristics of a dilute acid, with an average proton conductivity of 1 × 10-4 S cm-1, a proton transport activation energy of ∼0.2 eV and a proton mobility of ∼7 x 10-3 cm2 V-1 s-1," explains Ordinario. "Bulk reflectin is quite unique in this regard; as far as we are aware, no other protein has been shown to mimic a dilute acidic solution so closely. Moreover, reflectin's maximum conductivity of 2.6 × 10-3 S cm-1 at 65 °C is among the largest values found for any naturally occurring protein. Within the context of other biological (and even artificial) proton-conducting materials, reflectin's figures of merit are impressive and may represent new benchmarks for proteins in the solid state."
Gorodetsky notes that the findings indicate that reflectins could represent a promising new class of modular proton-conducting materials for bioelectronics and other applications.
"We believe that reflectin-based transistors and devices might be especially useful for bioelectronic applications that require an inherently biocompatible conductive material," he says. "For example, we envision using reflectin-based devices to interface with neural cells and to read out ionic and protonic fluxes from these cells."
The team's future research directions will revolve around engineering and evolving reflectin for not only improved properties but also for specific applications outside of bioelectronics.
"One of the key challenges facing our work is that we do not know reflectin's precise structure, either in thin films or in solution," Gorodetsky points out. "In our future work, by gaining insight into reflectin's structure, we hope to engineer the protein for optimum functionality in different types of electrical devices."
Michael Berger By – Michael is author of three books by the Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology,
Nanotechnology: The Future is Tiny, and
Nanoengineering: The Skills and Tools Making Technology Invisible
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