“Natural” Organic Metals: Biologically Produced Nanomaterials for Bioenergy, Biofuels, and Bioelectronic Devices

Natural organic nanostructures are potentially attractive over their synthetic, inorganic counterparts due to relatively low cost, facile synthesis and absence of toxicity. The discovery of metallic conductivity in polyacetylene started the field of plastic electronics and synthetic organic metals with widespread applications such as solar cells, transistors and light-emitting diodes. In this talk, I will present our discovery of first "natural organic metal" using nanowires synthesized biologically from natural amino acids [1].

Our in situ measurements revealed metallic-like conductivity in protein nanofilaments, pili, of Geobacter sulfurreducens. Pili show conductivities comparable to synthetic organic metallic nanostructures. Moreover, the conductivity can be tuned by gene expression and also by gate voltage. Temperature, magnetic field and gate-voltage dependence of conductivity is akin to those of quasi-one-dimensional disordered metals and the metallic nature can be improved further via processing. Magnetoresistance measurements of pili filaments show the evidence for quantum interference and metal-insulator transition. Furthermore, pili can be doped with protons in a manner similar to synthetic organic metals. Structural and mutagenesis studies indicate the possibility of molecular pi stacking in pili that can confer metallic-like conductivity. Using a novel scanning probe microscopy-based nanoscopic approach, we have found out that native pili filaments propagate charges in a manner similar to metallic carbon nanotubes [2]. We have also discovered a new mode of energy transfer in which microorganisms directly exchange electrons via networks of conductive pili instead of relying on diffusion of molecular intermediates [3].

The discovery of metallic-like conductivity in natural proteins represents a paradigm shift in our understanding of electron transfer in biology and electronic properties of biomaterials. It provides a strong foundation for an emerging field of biologically-produced electronic materials. These pili are a new class of electronically functional proteins that can be used to generate future generation of nanomaterials and nanoelectronic devices. We have developed high-performance microbial fuel cells [4], electrolyte-gated field-effect transistors, [1] and supercapacitors [5] using these living, self-renewing, nanostructured electronic biomaterials.

[1] Malvankar et al. Nature Nanotechnology, 6, 573-579 (2011)
[2] Malvankar et al. Nature Nanotechnology, manuscript under 2nd stage of review
[ [3] Summers, Fogarty, Leang, Franks, Malvankar and Lovley. Science, 330, 1413-1415 (2010)
[ [4] Malvankar et al., Energy and Environmental Science, 5, 5790–5797 (2012).
[ [5] Malvankar et al., ChemPhysChem 13, 463–468 (2012).