Electrocatalytic Oxidation of Glucose by Rhodium Porphyrin-Functionalized MWCNT Electrodes: Application to a Fully Molecular Catalyst-Based Glucose/O₂ Fuel Cell

This paper details the electrochemical investigation of a deuteroporphyrin dimethylester (DPDE) rhodium­(III) (<b>(DPDE)­Rh</b>^<b>III</b>) complex, immobilized within a MWCNT/Nafion electrode, and its integration into a molecular catalysis-based glucose fuel cell. The domains of present <b>(DPDE)­Rh</b>^<b>I</b>, <b>(DPDE)­Rh–H</b>, <b>(DPDE)­Rh</b>^<b>II</b>, and <b>(DPDE)­Rh</b>^<b>III</b> were characterized by surface electrochemistry performed at a broad pH range. The Pourbaix diagrams (plots of <i>E</i>_1/2 vs pH) support the stability of <b>(DPDE)­Rh</b>^<b>II</b> at intermediate pH and the predominance of the two-electron redox system <b>(DPDE)­Rh</b>^<b>I</b>/<b>(DPDE)­Rh</b>^<b>III</b> at both low and high pH. This two-electron system is especially involved in the electrocatalytic oxidation of alcohols and was applied to the glucose oxidation. The catalytic oxidation mechanism exhibits an oxidative deactivation coupled with a reductive reactivation mechanism, which has previously been observed for redox enzymes but not yet for a metal-based molecular catalyst. The MWCNT/<b>(DPDE)­Rh</b>^<b>III</b> electrode was finally integrated in a novel design of an alkaline glucose/O₂ fuel cell with a MWCNT/phthalocyanin cobalt­(II) (<b>CoPc</b>) electrode for the oxygen reduction reaction. This nonenzymatic molecular catalysis-based glucose fuel cell exhibits a power density of <i>P</i>_max = 0.182 mW cm^–2 at 0.22 V and an open circuit voltage (OCV) of 0.64 V

Osmium(II) Complexes Bearing Chelating N‑Heterocyclic Carbene and Pyrene-Modified Ligands: Surface Electrochemistry and Electron Transfer Mediation of Oxygen Reduction by Multicopper Enzymes

We report the synthesis of original osmium­(II) complexes bearing chelating N-heterocyclic (NHC) and bipyridine ligands. The pincer ligand 1,1′-dimethyl-3,3′-methylenediimidazole-2,2′-diylidene was used to tune the redox properties of osmium complexes. Bipyridine ligands modified with pyrene groups were chosen to study the electrosynthesis of Os^II-NHC-based metallopolymers as well as the noncovalent immobilization of these complexes on carbon-nanotube (CNT) electrodes. Poly-[Os^II-NHC] polypyrene polymer was electrogenerated on a GC electrode, whereas the pyrene-modified [Os^II-NHC] could interact with the CNTs’ sidewalls through π–π interactions, allowing the immobilization of the NHC complexes at the surface of π-extended nanostructured electrodes. Furthermore, an Os^II-NHC complex was studied in water, showing electron transfer mediation with multicopper enzymes. UV–visible and electrochemical experiments demonstrate that redox properties of the Os^II-NHC complex provide sufficient driving force for electron transfer with bilirubin oxidase from <i>Myrothecium verrucaria</i> while achieving high potential electroenzymatic oxygen reduction at <i>E</i> = +0.45 V vs Ag/AgCl at pH 6.5

Osmium(II) Complexes Bearing Chelating N‑Heterocyclic Carbene and Pyrene-Modified Ligands: Surface Electrochemistry and Electron Transfer Mediation of Oxygen Reduction by Multicopper Enzymes

We report the synthesis of original osmium­(II) complexes bearing chelating N-heterocyclic (NHC) and bipyridine ligands. The pincer ligand 1,1′-dimethyl-3,3′-methylenediimidazole-2,2′-diylidene was used to tune the redox properties of osmium complexes. Bipyridine ligands modified with pyrene groups were chosen to study the electrosynthesis of Os^II-NHC-based metallopolymers as well as the noncovalent immobilization of these complexes on carbon-nanotube (CNT) electrodes. Poly-[Os^II-NHC] polypyrene polymer was electrogenerated on a GC electrode, whereas the pyrene-modified [Os^II-NHC] could interact with the CNTs’ sidewalls through π–π interactions, allowing the immobilization of the NHC complexes at the surface of π-extended nanostructured electrodes. Furthermore, an Os^II-NHC complex was studied in water, showing electron transfer mediation with multicopper enzymes. UV–visible and electrochemical experiments demonstrate that redox properties of the Os^II-NHC complex provide sufficient driving force for electron transfer with bilirubin oxidase from <i>Myrothecium verrucaria</i> while achieving high potential electroenzymatic oxygen reduction at <i>E</i> = +0.45 V vs Ag/AgCl at pH 6.5

Hosting Adamantane in the Substrate Pocket of Laccase: Direct Bioelectrocatalytic Reduction of O₂ on Functionalized Carbon Nanotubes

We report the efficient immobilization and orientation of laccase from <i>Trametes versicolor</i> on MWCNT electrodes using 1-pyrenebutyric acid adamantyl amide as a supramolecular linker. We demonstrate the ability of adamantane to specifically interact with the hydrophobic cavity of laccase, while pyrene interacts with MWCNT sidewalls by π–π interactions. Adamantane allows the oriented immobilization of laccases on MWCNT electrodes. Using an anthraquinone-modified pyrene derivative for comparison, adamantane-modified MWCNTs achieve the stable immobilization and orientation of a higher number of enzymes per surface units, as confirmed by electrochemistry, theoretical calculations, and quartz crystal microbalance experiments. Furthermore, the efficient direct electron transfer ensures bioelectrocatalytic oxygen reduction at high half-wave potential of 0.55 V vs SCE accompanied by no kinetic limitation by the heterogeneous electron transfer and maximum current densities of 2.4 mA cm^–2

Electroanalytical Sensing Properties of Pristine and Functionalized Multilayer Graphene

This paper describes the heterogeneous electron transfer (ET) properties of high-quality multilayer graphene (MLG) films grown using chemical vapor deposition (CVD) on nickel and transferred to insulating poly­(ethylene terephthalate) (PET) sheets. An oxygen plasma treatment is used to enhance the ET properties of the films by generating oxygenated functionalities and edge-plane sites and defects. Scanning electron microscopy (SEM), Raman, and X-ray photoelectron spectroscopy (XPS) along with voltammetry of the standard redox probes [Ru­(NH₃)₆]^3+/2+, [Fe­(CN)₆]^3–/4–, and Fe^3+/2+ are used to demonstrate this effect. The biologically relevant molecules dopamine, NADH, ascorbic acid, and uric acid are employed to show the improved sensing characteristics of the treated films. Control experiments involving commercially available edge-plane and basal-plane pyrolytic graphite (EPPG and BPPG) electrodes help to explain the different responses observed for each probe, and it is shown that, in certain cases, treated MLG provides a viable alternative to EPPG, hitherto considered to be the “best-case scenario” in carbon electrochemistry. This is the first comprehensive study of the electroanalytical properties of pristine and functionalized CVD-grown MLG, and it will serve as an important benchmark in the clarification of ET behavior at graphene surfaces, with a view to the development of novel electrochemical sensors

Assembly and Stacking of Flow-through Enzymatic Bioelectrodes for High Power Glucose Fuel Cells

Bioelectrocatalytic carbon nanotube based pellets comprising redox enzymes were directly integrated in a newly conceived flow-through fuel cell. Porous electrodes and a separating cellulose membrane were housed in a glucose/oxygen biofuel cell design with inlets and outlets allowing the flow of electrolyte through the entire fuel cell. Different flow setups were tested and the optimized single cell setup, exploiting only 5 mmol L^–1 glucose, showed an open circuit voltage (OCV) of 0.663 V and provided 1.03 ± 0.05 mW at 0.34 V. Furthermore, different charge/discharge cycles at 500 Ω and 3 kΩ were applied to optimize long-term stability leading to 3.6 J (1 mW h) of produced electrical energy after 48 h. Under continuous discharge at 6 kΩ, about 0.7 mW h could be produced after a 24 h period. The biofuel cell design further allows a convenient assembly of several glucose biofuel cells in reduced volumes and their connection in parallel or in series. The configuration of two biofuel cells connected in series showed an OCV of 1.35 V and provided 1.82 ± 0.09 mW at 0.675 V, and when connected in parallel, showed an OCV of 0.669 V and provided 1.75 ± 0.09 mW at 0.381 V. The presented design is conceived to stack an unlimited amount of biofuel cells to reach the necessary voltage and power for portable electronic devices without the need for step-up converters or energy managing systems