The electrode potential determines the yield coefficients of early-stage Geobacter sulfurreducens biofilm anodes

Geobacter sulfurreducens is the model for electroactive microorganisms (EAM). EAM can use solid state terminal electron acceptors (TEA) including anodes via extracellular electron transfer (EET). Yield coefficients relate the produced cell number or biomass to the oxidized substrate or the reduced TEA. These data are not yet sufficiently available for EAM growing at anodes. Thus, this study provides information about kinetics as well as yield coefficients of early-stage G. sulfurreducens biofilms using anodes as TEA at the potentials of -200 mV, 0 mV and +200 mV (vs. Ag/AgCl sat. KCl). The selected microorganism was therefore cultivated in single and double chamber batch reactors on graphite or AuPd anodes. Interestingly, whereas the lag time and maximum current density within 12 days of growth differed, the anode potential does not influence the coulombic efficiency and the formal potential of the EET, which remains constant for all the experiments at ~ -300 to -350 mV. We demonstrated for the first time that the anode potential has a strong influence on single cell yield coefficients which ranged from 2.69 × 10¹² cells mol_e-^-1 at -200 mV and 1.48 × 10¹² cells mol_e-^-1 at 0 mV to 2.58 × 10¹¹ cells mol_e-^-1 at +200 mV in single chamber reactors and from 1.15 × 10¹² cells mol_e-^-1 at -200 mV to 8.98× 10¹¹ cells mol_e-^-1 at 0 mV in double chamber reactors. This data can be useful for optimization and scaling-up of primary microbial electrochemical technologies.

A platinum-nickel bimetallic nanocluster ensemble-on-polyaniline nanofilm for enhanced electrocatalytic oxidation of dopamine

We report a new approach to design flexible functional material platforms based on electropolymerized polyaniline (PANI) polymer nanofilms modified with bimetallic nanoclusters (NCs) for efficient electro-oxidation of small organic molecules. Composition defined ligand free Pt_0.75Ni_0.25 NCs were synthesized in the gas phase using the Cluster Beam Deposition (CBD) technology and characterized using RToF, HAADF-STEM, XAFS and XPS. NCs were then directly deposited on PANI coated templates to construct electrodes. Dopamine (DP) molecules were used as a representative organic analyte and the influence of the NC-PANI hybrid atomistic structure on the electrochemical and electrocatalytic performance was investigated. The as prepared, nearly monodispersed, Pt_0.75Ni_0.25 NCs of ca. 2 nm diameter featuring a PtOx surface combined with a shallow platelet-like Ni-O(OH) phase formed a densely packed active surface on PANI at ultralow metal coverages. Electrochemical measurements (EIS and CV) show a 2.5 times decrease in charge transfer resistance and a remarkable 6-fold increase at lower potential in the mass activity for Pt_0.75Ni_0.25 NCs in comparison with their pure Pt counterparts. The enhanced electrochemical performance of the Pt_0.75Ni_0.25 NC hybrid's interface is ascribed to the formation of mixed Pt metal and Ni-O(OH) phases at the surface of the alloyed PtNi cores of the bimetallic NCs under electrochemical conditions combined with an efficient charge conduction pathway between NCs.

Interfacial electron transfer between Geobacter sulfurreducens and gold electrodes via carboxylate-alkanethiol linkers: Effects of the linker length

Geobacter sulfurreducens (Gs) attachment and biofilm formation on self-assembled monolayers (SAMs) of carboxyl-terminated alkanethiol linkers with varied chain length on gold (Au) was investigated by electrochemical and microscopic methods to elucidate the effect of the surface modification on the current production efficiency of Gs cells and biofilms. At the initial stage of the cell attachment, the electrochemical activity of Gs cells at a submonolayer coverage on the SAM-Au surface was independent of the linker length. Subsequently, multiple potential cyclings indicated that longer linkers provided more biocompatible conditions for Gs cells than shorter ones. For Gs biofilms, on the other hand, the turnover current decreased exponentially with the linker length. During the biofilm formation, bacteria need to adjust from the initial planktonic state to an electrode-respiring state, which was triggered by a strong electrochemical stress found for shorter linkers, resulting in the formation of mature biofilms. Our results suggest that the initial cell attachment and the biofilm formation are two inherently different processes. Therefore, the effects of linker molecules, electron transfer efficiency and biocompatibility, must be explored simultaneously to understand both processes to increase the current production of electrogenic microorganisms in microbial fuel cells.

Spectroscopic elucidation of energy transfer in hybrid inorganic-biological organisms for solar-to-chemical production

The rise of inorganic-biological hybrid organisms for solar-to-chemical production has spurred mechanistic investigations into the dynamics of the biotic-abiotic interface to drive the development of next-generation systems. The model system, Moorella thermoacetica-cadmium sulfide (CdS), combines an inorganic semiconductor nanoparticle light harvester with an acetogenic bacterium to drive the photosynthetic reduction of CO₂ to acetic acid with high efficiency. In this work, we report insights into this unique electrotrophic behavior and propose a charge-transfer mechanism from CdS to M. thermoacetica Transient absorption (TA) spectroscopy revealed that photoexcited electron transfer rates increase with increasing hydrogenase (H₂ase) enzyme activity. On the same time scale as the TA spectroscopy, time-resolved infrared (TRIR) spectroscopy showed spectral changes in the 1,700-1,900-cm^-1 spectral region. The quantum efficiency of this system for photosynthetic acetic acid generation also increased with increasing H₂ase activity and shorter carrier lifetimes when averaged over the first 24 h of photosynthesis. However, within the initial 3 h of photosynthesis, the rate followed an opposite trend: The bacteria with the lowest H₂ase activity photosynthesized acetic acid the fastest. These results suggest a two-pathway mechanism: a high quantum efficiency charge-transfer pathway to H₂ase generating H₂ as a molecular intermediate that dominates at long time scales (24 h), and a direct energy-transducing enzymatic pathway responsible for acetic acid production at short time scales (3 h). This work represents a promising platform to utilize conventional spectroscopic methodology to extract insights from more complex biotic-abiotic hybrid systems.

Analysis of electron transfer dynamics in mixed community electroactive microbial biofilms

Electrochemically active microbial biofilms are capable to produce electric current when grown onto electrodes. This work investigates the dynamics of electron transfer inside the biofilm as well as at the biofilm/electrode interface.

Electronic communication in phosphine substituted bridged dirhenium complexes – clarifying ambiguities raised by the redox non-innocence of the C4H2- and C4-bridges

Dinuclear rhenium complexes with C₄H₂- or C₄- bridges can show open or closed shell structures.