Frédéric Barrière

Bacteria and yeasts as catalysts in microbial fuel cells: electron transfer from micro-organisms to electrodes for green electricity

This article reviews the use of micro-organisms as catalysts at the electrodes of microbial fuel cells (MFCs). The principle of MFCs and their intended use for water treatment and clean electricity production is discussed. We address the different microbial structure and metabolic pathways found in prokaryote (bacteria) and eukaryote (yeasts) that allow the understanding of why electron transfer is possible between a microbe and an electrode. The different mechanisms of microbe–electrode electron transfer are discussed: direct electron transfer or through natural nanowires (pili), mediated electron transfer by natural or artificial redox mediator and finally direct redox transformation of excreted metabolites at the electrodes. This is followed by a review of the different bacteria that have been found and studied in MFCs mainly in the anodic compartment but also more recently in the cathodic side of the fuel cells. A perspective on the possible advantages and challenges of the use of yeasts in MFCs is provided, as this aspect has not been thoroughly studied so far. The fourth section of the review focuses on how to improve the performance and sustainability of MFCs through the functionalisation of the electrode surface, for instance with the covalent grafting of redox mediators and/or enzymes.

Modeling of the molybdenum center in the nitrogenase FeMo-cofactor

Abstract The functional, structural and theoretical chemical approaches to specifically model the molybdenum center of the nitrogenase enzyme are reviewed. We show how dinitrogen can be reduced at monometallic centers and highlight attempts to develop a nitrogenase-relevant dinitrogen reduction chemistry at molybdenum–sulfur complexes and clusters. The theoretical work addressing the molybdenum issue is also reported together with models for nitrogenase function, some of them directly involving the molybdenum atom.

Tuning the optical properties of aryl-substituted dispirofluorene-indenofluorene isomers through intramolecular excimer formation.

Two families of positional isomers of dispirofluorene-indenofluorene substituted by phenyl groups at the 2,7-positions of the fluorene moieties present drastically different optical properties. The emission wavelengths may be gradually and conveniently modulated for one of the two isomers by the phenyl groups substituent whose bulkiness controls the extent of the excimeric interaction evidenced in this paper.

New Dispiro Compounds: Synthesis and Properties

We report the synthesis and structural characterization of two dispiro compounds. These two positional isomers have been designed and synthesized through an efficient method. Because of the rigidity and orthogonality of the spiro bridge, both molecules exhibit a well-defined architecture which consists of two fluorene rings connecting to an indenofluorenyl unit via two sp3 carbon atoms. The structural, electrochemical, optical, and thermal properties of these dispiro isomers are discussed.

Characterisation of yeast microbial fuel cell with the yeast Arxula adeninivorans as the biocatalyst.

Yeast microbial fuel cells have received little attention to date. Yeast should be ideal MFC catalyst because they are robust, easily handled, mostly non-pathogenic organisms with high catabolic rates and in some cases a broad substrate spectrum. Here we show that the non-conventional yeast Arxula adeninvorans transfers electrons to an electrode through the secretion of a reduced molecule that is not detectable when washed cells are first resuspended but which accumulates rapidly in the extracellular environment. It is a single molecule that accumulates to a significant concentration. The occurrence of mediatorless electron transfer was first established in a conventional microbial fuel cell and that phenomenon was further investigated by a number of techniques. Cyclic voltammetry (CV) on a yeast pellet shows a single peak at 450 mV, a scan rate study showed that the peak was due to a solution species. CVs of the supernatant confirmed a solution species. It appears that, given its other attributes, A. adeninivorans is a good candidate for further investigation as a MFC catalyst.

Encumbered DiSpiro[Fluorene–IndenoFluorene]: Mechanistic Insights

Research and advances in organic electronics have increased the demand for oligomers and polymers of controlled structure needed for applications as diverse as, for example, solar cells, light-emitting diodes, field-effect transistors or field-effect light-emitting transistors. Well characterized smaller molecular units, however, are more suitable than extended p-conjugated polymers for detailed structure–property relationships studies. In such molecular frameworks, the introduction of spiro linkages has interesting structural consequences and have been extensively developed, for instance, in the Tour and Salbeck groups. In our quest for efficient materials for organic light emitting diodes, we designed and studied molecules that combine two fluorene moieties to the indenofluorene core through a spiro-linkage (i.e., dispiro[fluorene-9,6’-indenoACHTUNGTRENNUNG[1,2-b]fluorene-12’,9’’-fluorene], DSF-IF; see R substituted type-2 molecules in Scheme 1). The synthetic approach involves, in the last step, the intramolecular bicyclization of the diol 1 (Scheme 1). Intramolecular electrophilic cyclization in the presence of a Lewis acid is a very common and useful reaction that has been widely used to generate a spiro configuration in mild conditions. For example, Yamagushi and coworkers have reported a double intramolecular cyclization reaction, which should in principle lead to the formation of two regioisomers, however, the authors reported the detection of only one isomer. Similarly, M llen and co-workers have reported the synthesis of poly(p-phenylene-alt-anthrylene) also using an intramolecular cyclization as the last step. Again, the reaction carried out on model compounds indicated that only the anti-ring-closed isomer is formed. We note, however, that indolocarbazole positional isomers with a conceptually related terphenyl backbone have been previously reported by Leclerc and co-workers. While extending the scope of type-2 molecules through the introduction of different R groups on the fluorene moieties (Table 1), we recently managed to separate and fully characterize two positional isomers of DSF-IF derivatives, namely

Enzymatic versus Microbial Bio‐Catalyzed Electrodes in Bio‐Electrochemical Systems

Catalyses of electrode reactions by oxidoreductases or living electroactive bacteria are compared and recent advances reviewed. The relation between the biological and nevertheless inert nature of enzymes and the living machinery of electroactive microbes is discussed. The way these biocatalysts may be electrically contacted to anodes or cathodes is considered with a focus on their immobilization at electrodes and on the issue of time stability of these assemblies. Recent improvements in power output of biofuel cells are reviewed together with applications that have appeared in the literature. This account also reviews new approaches for combining enzymes and living microbes in bioelectrochemical systems such as reproducing microbial metabolisms with enzyme cascades and expressing oxidoreductases on genetically engineered microbes. Finally, the use of surface chemistry for studying the microbe-electrode interface and bioelectrodes with cell organelles, such as mitochondria, or with higher organisms, such as yeasts, are discussed. Some perspectives for future research to extend this field are offered as conclusions.

Polythiophenes Containing In-Chain Cobaltabisdicarbollide Centers

New cobalt(III) bis(dicarbollide) complexes covalently linked to two 2-oligothienyl units have been synthesized and electropolymerized in acetonitrile electrolyte in order to produce the corresponding polythiophene films containing in-chain metallic centers. The polymer films electrogenerated from the bithienyl (4b) and terthienyl (4c) derivatives display redox processes attributed to the Co(III)/Co(II) couple at ca. -1.1 V vs SCE and to the p-doping/undoping of the expected quaterthienyl and sexithienyl segments at ca. 0.8 V vs SCE. In contrast, the anodic oxidation of the thienyl (4a) derivative leads to passivation of the electrode surface. As the length of the oligothiophene substituents increases, the metallic and dicarbollide cage carbon atoms contributions in the HOMO decrease dramatically so that the highest occupied frontier orbitals of 4b and 4c can be considered as almost purely oligothiophene-based. From further UV-vis spectroscopy analysis, it is demonstrated that the polymer incorporating the sexithienyl segments is more conjugated than that with the quaterthienyl segments as the absorption maximum for the interband pi-pi* transition was observed at 410 and 448 nm for poly(4b) and poly(4c) respectively. Furthermore, these polymers display a more extended degree of conjugation than the parent oligothiophenes. Such features indicate a significant electronic delocalization through the cobaltabisdicarbollide moiety. Their conducting probe atomic force microscopy characterization indicates that poly(4b) and poly(4c) behave like heavily doped semiconductors rather than pure semiconductors. Mean conductivity values extracted from the current-voltage profiles are 1.4 x 10(-4) and 7.5 x 10(-4) S cm(-1) for poly(4b) and poly(4c), respectively. Such materials are found to be efficient for the electrocatalytic reduction of protons to dihydrogen, as exemplified for poly(4b). The overpotential for hydrogen evolution is significantly decreased by ca. 230 mV with respect to that obtained with the bare electrode (measured for a current density of 1.4 mA cm(-2) in the presence of 20 mM HBF(4)).

Monophyletic group of unclassified γ-Proteobacteria dominates in mixed culture biofilm of high-performing oxygen reducing biocathode

Several mixed microbial communities have been reported to show robust bioelectrocatalysis of oxygen reduction over time at applicable operation conditions. However, clarification of electron transfer mechanism(s) and identification of essential micro-organisms have not been realised. Therefore, the objective of this study was to shape oxygen reducing biocathodes with different microbial communities by means of surface modification using the electrochemical reduction of two different diazonium salts in order to discuss the relation of microbial composition and performance. The resulting oxygen reducing mixed culture biocathodes had complex bacterial biofilms variable in size and shape as observed by confocal and electron microscopy. Sequence analysis of ribosomal 16S rDNA revealed a putative correlation between the abundance of certain microbiota and biocathode performance. The best performing biocathode developed on the unmodified graphite electrode and reached a high current density for oxygen reducing biocathodes at neutral pH (0.9 A/m(2)). This correlated with the highest domination (60.7%) of a monophyletic group of unclassified γ-Proteobacteria. These results corroborate earlier reports by other groups, however, higher current densities and higher presence of these unclassified bacteria were observed in this work. Therefore, members of this group are likely key-players for highly performing oxygen reducing biocathodes.

Optimized preparation and scanning electrochemical microscopy analysis in feedback mode of glucose oxidase layers grafted onto conducting carbon surfaces.

An optimized immobilization procedure based on the electroreduction of aryldiazonium salt followed by covalent attachment of a cross-linked hydrogel was used to graft glucose oxidase on a carbon surface. Scanning electrochemical microscopy (SECM) and cyclic voltammetry were used to follow the construction steps of the modified electrode. By adjusting the compactness of the layer through the electrografting reaction, the penetration of the mediator through the layer can be controlled to allow the monitoring of the enzymatic activity by both cyclic voltammetry and SECM in feedback mode. The enzymatic activity of the film is finally characterized by SECM.