A Role for Microbial Palladium Nanoparticles in Extracellular Electron Transfer

Abstract

Supplementary Fig. 1. Summary of previously reported elemental transformation into lower valence species via microbial reduction reactions 1-7. Products of elemental metals (valence = 0), elemental non-metals (valence = 0), metallic oxides/ions (valence ≠ 0) and metalloid (valence = 0) indicated by red, yellow, green and brown respectively, and the methylation of Hg, As, Se, Sn and Te etc are not considered in this diagram. The elements (i.e. O, S, Se and Te), which are in the same group, can be reduced to the products of valence = 0. However, no bio-produced Fe, Ni or Cu (valence = 0) particles, which elements are in the same group with the noble metals, were reported. Through microbial evolution of billions of years, there is potentially a wide diversity of electron transport pathways/mechanisms that are yet to be fully elucidated and understood e.g. what features make the microbial nano-pili electronically conductive? 1 Are there any effects of the membrane bounded metallic-nanoparticles of high electronic conductivity on microbial growth or metabolism? S3 Supplementary Scheme 1 hydrogenase and metal nanoparticles as catalysts for hydrogen oxidation and production. 1) [NiFe]-hydrogenase; 2) the structure of active site; 3) membrane bound bio-nanoparticles. The following mechanism of the nanoparticle production is proposed: Pd(II) is absorbed into the cell and then reduced by hydrogenases and/or cytochrome to elemental Pd(0); periplasmic [FeFe]-hydrogenases serve as the nucleation sites for the initial formation of the metallic nanoparticles, which are then secreted through the cell membrane and undergo size growth. The bio-nanoparticles (e.g. Pd, Pt, Rh and Au) have outstanding catalytic properties in various redox reactions. Fe or Ni (valence = 0) particles generally have little catalytic activity, but the [Fe-Ni] unit (i.e. active site) are key part of hydrogenase, which has been known as catalyst for hydrogen reactions of energy chain in membrane.

Enrichment of Microbial Electrolysis Cell Biocathodes from Sediment Microbial Fuel Cell Bioanodes

“…Desulfovibrio species are rarely autotrophic but typically possess hydrogenases and are common electrode biofilm constituents in BESs (18,20). In Desulfovibrio desulfuricans, membrane enzymes link oxidative metabolism to extracellular electron transfer (42). Desulfovibrio biofilms on graphite electrodes have been shown to accept electrons and reduce the overpotential for hydrogen formation by about 200 mV (12,44).…”

Modification of Abiotic–Biotic Interfaces with Small Molecules and Nanomaterials for Improved Bioelectronics

“…101 Desulfovibrio desulfuricans was cultured in the presence of 0.07 and 1.0 mM ammonium tetrachloropalladate(II), leading to different cellassociated Pd(0) NP loadings. In the case of lower Pd NP loading, current generation linked to lactate oxidation (as measured by cyclic voltammetry) was increased 4-fold.…”

Hollow Palladium Nanoparticles Facilitated Biodegradation of an Azo Dye by Electrically Active Biofilms

“…The CV analysis confirmed the outstanding redox behavior of the hollow Pd nanoparticles over the solid Pd nanoparticles (Figure 3). Because of these properties, hollow Pd nanoparticles significantly enhanced the bacterial EET process by binding with outer membrane proteins [35] and the enhanced bacterial EET accelerated the MO degradation. …”