A Pyrene-Substituted Tris(bipyridine)osmium(II) Complex as a Versatile Redox Probe for Characterizing and Functionalizing Carbon Nanotube- and Graphene-Based Electrodes

We report the functionalization of nanostructured graphene-based electrode with an original (bis­(2,2′-bipyridine)­(4,4′-bis­(4-pyrenyl-1-ylbutyloxy)-2,2′-bipyridine]­osmium­(II) hexafluorophosphate complex bearing pyrene groups. Graphene oxide (GO) and chemically reduced graphene oxide (c-RGO) paper electrodes were prepared by the flow-directed filtration method. After film transfer via the soluble membrane technique, the homogeneous and stable GO electrode was electrochemically reduced in water to achieve electrochemically reduced graphene oxide (e-RGO) film on the electrode. The electrochemical properties of GO, c-RGO, and e-RGO electrodes were characterized by scanning electron microscopy and electrochemistry. Cyclic voltammetry of the Ru­(NH₃)₆^2+/3+ redox probe underlines the important influence of the RGO preparation method on electrochemical properties. We finally achieved the flexible functionalization of graphene-based electrodes using either supramolecular binding of the Os­(II) complex bearing pyrene groups or its electropolymerization via the irreversible oxidation of pyrene. The properties of these functionalized graphene paper electrodes were compared to glassy carbon (GC) and multiwalled carbon nanotube (MWCNT) electrodes. Thanks to its divalent binding sites, the Os­(II) complex constitutes a useful tool to probe the π-extended graphitic surface of RGO and MWCNT films. The Os­(II) complex interacts strongly via noncovalent π–π interactions, with π-extended graphene planes, thus acting as a marker to quantify the electroactive surface of both MWCNT and RGO electrodes and to illustrate their ease of functionalization

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

Micelle-Assisted Confined Coordination Spaces for Benzimidazole: Enhanced Electrochemiluminescence for Nitrite Determination

Selective and sensitive detection of nitrite has important medical and biological implications. In the present work, to obtain an enhanced electrochemiluminescence (ECL) determination of nitrite, a novel nano-ECL emitter CoBIM/cetyltrimethylammonium bromide (CTAB) was prepared via a micelle–assisted, energy-saving, and ecofriendly method based on benzimidazole (BIM) and CTAB. Unlike conventional micelle assistance, the deprotonated BIM (BIM–) preferential placement was in the palisade layer of cationic CTAB-based micelles. Enriching the original CTAB micelle with BIM– disrupted its stability and resulted in the formation of considerably smaller BIM/CTAB-based micelles, providing a confined coordination environment for BIM– and Co2+. As a result, the growth of CoBIM/CTAB was also limited. Owing to the unusual nitration reaction between BIM and nitrite, the prepared CoBIM/CTAB was successfully applied as a novel ECL probe for the detection of nitrite with a wide linear range of 1–1500 μM and a low detection limit of 0.67 μM. This work also provides a promising ECL platform for ultrasensitive monitoring of nitrite and it was applied with sausages and pickled vegetables

Oriented Immobilization of [NiFeSe] Hydrogenases on Covalently and Noncovalently Functionalized Carbon Nanotubes for H₂/Air Enzymatic Fuel Cells

We report the oriented immobilization of [NiFeSe] hydrogenases on both covalently and noncovalently modified carbon nanotubes (CNTs) electrodes. A specific interaction of the [NiFeSe] hydrogenase from <i>Desulfomicrobium baculatum</i> with hydrophobic organic molecules was probed by electrochemistry, quartz crystal microbalance with dissipation monitoring (QCM-D), and theoretical calculations. Taking advantage of these hydrophobic interactions, the enzyme was efficiently wired on anthraquinone and adamantane-modified CNTs. Because of rational immobilization onto functionalized CNTs, the O₂-tolerant [NiFeSe]-hydrogenase is able to efficiently operate in a H₂/air gas-diffusion enzymatic fuel cell

Redox-Active Carbohydrate-Coated Nanoparticles: Self-Assembly of a Cyclodextrin–Polystyrene Glycopolymer with Tetrazine–Naphthalimide

The controlled self-assembly of precise and well-defined photochemically and electrochemically active carbohydrate-coated nanoparticles offers the exciting prospect of biocompatible catalysts for energy storage/conversion and biolabeling applications. Here an aqueous nanoparticle system has been developed with a versatile outer layer for host–guest molecule encapsulation via β-cyclodextrin inclusion complexes. A β-cyclodextrin-modified polystyrene polymer was first obtained by copper nanopowder click chemistry. The glycopolymer enables self-assembly and controlled encapsulation of tetrazine-naphthalimide, as a model redox-active agent, into nanoparticles via nanoprecipitation. Cyclodextrin host–guest interactions permit encapsulation and internanoparticle cross-linking for the formation of fluorescent compound and clustered self-assemblies with chemically reversible electroactivity in aqueous solution. Light scattering experiments revealed stable particles with hydrodynamic diameters of 138 and 654 nm for nanoparticles prepared with tetrazine, of which 95% of the nanoparticles represent the smaller objects by number. Dynamic light scattering revealed differences as a function of preparation method in terms of size, 3-month stability, polydispersity, radius of gyration, and shape factor. Individual self-assemblies were visualized by atomic force microscopy and fluorescence microscopy and monitored in real-time by nanoparticle tracking analysis. UV–vis and fluorescence spectra provided insight into the optical properties and critical evidence for host–guest encapsulation as evidenced by solvachromatism and enhanced tetrazine uptake. Cyclic voltammetry was used to investigate the electrochemical properties and provided further support for encapsulation and an estimate of the tetrazine loading capacity in tandem with light scattering data

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