Invited: Microbiology Leads the Way: From Protein Nanowires to Hybrid Devices
Authors: Reguera Gemma 
Published: February 2020

In Nature, highly efficient and diverse consortia of microbes cycle carbon and other elements in electrochemical reactions that discharge metabolic electrons onto soluble and insoluble metals. One group in particular, Geobacter bacteria, uses hair-like protein filaments or pili to bind and transfer electrons to iron (Fe[III]) oxides, a reaction that solubilizes part of the iron as Fe(II) and also generates the semi-conducting mineral, magnetite. Geobacter cells also use the conductive pili to bind and reduce toxic metals such as uranium (present as the uranyl divalent cation). The reaction reduces the U(VI) in the uranyl cation to a mononuclear mineral phase of U(IV), effectively precipitating the radionuclide extracellularly and preventing its permeation inside the cells. Interestingly, the Geobacter pili are composed of a single peptide subunit, which polymerizes in a helical fashion such that contacts are formed between the side chains of aromatic residues to promote fast rates of charge hopping. Structural studies also identified ligands on the pilus surface that could function as metal traps for the uranyl cation and other cationic metals. Once bound, the metals are positioned in close proximity to a terminal tyrosine residue of the pilus multistep hopping path to promote their reductive precipitation. The ability of a peptide assembly such as Geobacter pili to transport electrons at micrometer distances is especially significant for applications in nanotechnology. We show that recombinant techniques can be applied to develop sustainable manufacturing protocols for the mass-production of conductive peptides inspired in the pilus subunit or pilin. Information about the amino acids residues and structural features of the pilin that contribute to its self-assembly and conductivity was used to design protocols for their in vitro assembly in a cell-free environment, effectively mass-producing protein nanowires. Further, genetic engineering was used to functionalize the recombinant peptides with chemical tags for their selective and specific integration into nanoelectronic devices. This approach allowed us to manufacture conductive Peptide Self-Assembled Monolayers (PSAMs) on electrodes that concentrate the metal traps on the monolayer surface and maximize metal binding and reduction. The ability to custom-design and mass-produce protein nanowires and PSAMs via recombinant and genetic engineering approaches contrasts with fabrication methods for inorganic nanowires or hybrid devices with synthetic peptides, which can involve high temperatures, toxic solvents, vacuums, and/or specialized equipment, and the limited methods available for their functionalization. Protein nanowires and PSAMs also circumvent major concerns regarding cyto- and genotoxicity that limit commercial applications of carbon, metal, and metal-oxide based nanomaterials, making them suitable for the development of biodegradable and biocompatible nanoelectronic devices. Of special significance is the ability of the recombinant protein nanowires and PSAMs to retain the ability to bind and reductively precipitate cationic metal contaminants such as uranium, a process that could be harnessed to develop sensors and deployable devices for bioremediation.