Anne de Poulpiquet

O2 Reduction in Enzymatic Biofuel Cells

Catalytic four-electron reduction of O2 to water is one of the most extensively studied electrochemical reactions due to O2 exceptional availability and high O2/H2O redox potential, which may in particular allow highly energetic reactions in fuel cells. To circumvent the use of expensive and inefficient Pt catalysts, multicopper oxidases (MCOs) have been envisioned because they provide efficient O2 reduction with almost no overpotential. MCOs have been used to elaborate enzymatic biofuel cells (EBFCs), a subclass of fuel cells in which enzymes replace the conventional catalysts. A glucose/O2 EBFC, with a glucose oxidizing anode and a O2 reducing MCO cathode, could become the in vivo source of electricity that would power sometimes in the future integrated medical devices. This review covers the challenges and advances in the electrochemistry of MCOs and their use in EBFCs with a particular emphasis on the last 6 years. First basic features of MCOs and EBFCs are presented. Clues provided by electrochemistry to understand these enzymes and how they behave once connected at electrodes are described. Progresses realized in the development of efficient biocathodes for O2 reduction relying both on direct and mediated electron transfer mechanism are then discussed. Some implementations in EBFCs are finally presented.

Mechanism of Chloride Inhibition of Bilirubin Oxidases and Its Dependence on Potential and pH

Bilirubin oxidases (BODs) belong to the multi-copper oxidase (MCO) family and efficiently reduce O2 at neutral pH and in physiological conditions where chloride concentrations are over 100 mM. BODs were consequently considered to be Cl- resistant contrary to laccases. However, there has not been a detailed study on the related effect of chloride and pH on the redox state of immobilized BODs. Here, we investigate by electrochemistry the catalytic mechanism of O2 reduction by the thermostable Bacillus pumilus BOD immobilized on carbon nanofibers in the presence of NaCl. The addition of chloride results in the formation of a redox state of the enzyme, previously observed for different BODs and laccases, which is only active after a reductive step. This behavior has not been previously investigated. We show for the first time that the kinetics of formation of this state is strongly dependent on pH, temperature, Cl- concentration and on the applied redox potential. UV-visible spectroscopy allows us to correlate the inhibition process by chloride with the formation of the alternative resting form of the enzyme. We demonstrate that O2 is not required for its formation and show that the application of an oxidative potential is sufficient. In addition, our results suggest that the reactivation may proceed thought the T3 β.

Dual Enzymatic Detection by Bulk Electrogenerated Chemiluminescence.

The combination of enzymes, as recognition elements for specific analytes, and of electrogenerated chemiluminescence (ECL) as a readout method has proven to be a valuable strategy for sensitive and specific analytical detection. However, ECL is intrinsically a 2D process which could potentially limit the analysis of inhomogeneous samples. Here, we show how a bulk ECL signal, generated by thousands of carbon microbeads remotely addressed via bipolar electrochemistry, are implemented as a powerful tool for the concomitant ECL sensing and imaging of two enzymatic substrates. We selected two enzymes (glucose dehydrogenase and choline oxidase) that react with their respective model substrates and produce in situ chemical species (β-nicotinamide adenine dinucleotide (NADH) and H2O2) acting as coreactants for the ECL emission of different luminophores ([Ru(bpy)3](2+) at λ = 620 nm and luminol at λ = 425 nm, respectively). Both enzymes are spatially separated in the same capillary. We demonstrate thus the simultaneous quantitative determination of both glucose and choline over a wide concentration range. The originality of this remote approach is to provide a global chemical view through one single ECL image of inhomogeneous samples such as a biochemical concentration gradient in a capillary configuration. Finally, we report the first proof-of-concept of dual biosensing based on this bulk ECL method for the simultaneous imaging of both enzymatic analytes at distinct wavelengths.

Carbon Nanotube-Enzyme Biohybrids in a Green Hydrogen Economy

Alternative energy pathways to replace depleting oil reserves and to limit the effects of glob‐ al warming by reducing the atmospheric emissions of carbon dioxide are nowadays re‐ quired. Dihydrogen appears as an attractive candidate because it represents the highest energy output relative to the molecular weight (120 MJ kg-1 against 50 MJ kg-1 for natural gas), and because its combustion delivers only water and heat. Whereas the main renewable sources of energy available in nature (solar, wind, geothermal...) need to be transformed, di‐ hydrogen is able to transport and store energy. Dihydrogen can be produced from renewa‐ ble energies, indirectly from photosynthesis via biomass transformation, or directly by bacteria. It can be converted into electricity using fuel cell technology. From all these proper‐ ties and because it does not compete with food and water resources, dihydrogen has been defined as third generation biofuel. It thus emerges as a new fully friendly environmental energy vector. The use of dihydrogen as an energy carrier is not a new idea. Let us simply remember that Jules Verne, a famous French visionary novelist, wrote early in 1874: “I be‐ lieve that O2 and H2 will be in the future our energy and heat sources” [1]. His prediction simply relied on the discovery a few years before of the fuel cell concept by C. Schonbein, then W. Groove, who demonstrated that when stopping water electrolysis, a current flow occurred in the reverse way [2]. However in order to implement the dihydrogen economy and replace fossil fuels, there are significant technical challenges that need to be overcome in each of the following domains: