A Potent Auto-Umpolung Ligand for Conjugative Radical Stabilization.

Carbenes with conjugatively connected redox system act as "auto-umpolung" ligands. Due to their electronic flexibility, they should also be particularly suitable to stabilize open-shell species. Herein, the first neutral radical of such sort is described in form of a dialkylamino-substituted bis(dicyanomethylene)cyclopropanide. Despite the absence of steric shielding, the radical is stable for an extended amount of time and was consequently characterized in solution via EPR measurements. These data and accompanying X-ray structural analyses indicate that the radical species is in equilibrium with aggregates (formed via π-stacking) and dimers (obtained via σ-bond formation between methylene carbons).

Real-Time Measurement of Cellobiose and Glucose Formation during Enzymatic Biomass Hydrolysis.

Enzymatic hydrolysis of lignocellulosic biomass for biofuel production relies on complex multi-enzyme ensembles. Continuous and accurate measurement of the released key products is crucial in optimizing the industrial degradation process and also investigating the activity and interaction between the involved enzymes and the insoluble substrate. Amperometric biosensors have been applied to perform continuous cellobiose measurements during the enzymatic hydrolysis of pure cellulose powders. The oxygen-sensitive mediators used in these biosensors restricted their function under physiological or industrial conditions. Also, the combined measurements of the hydrolysis products cellobiose and glucose require a high selectivity of the biorecognition elements. We employed an [Os(2,2'-bipyridine)2Cl]Cl-modified polymer and cellobiose dehydrogenase to fabricate a cellobiose biosensor, which can accurately and specifically detect cellobiose even in the presence of oxygen and the other main product glucose. Additionally, a glucose biosensor was fabricated to simultaneously measure glucose produced from cellobiose by ß-glucosidases. The cellobiose and glucose biosensors work at applied potentials of +0.25 and +0.45 V versus Ag|AgCl (3 M KCl), respectively, and can selectively detect their substrate. Both biosensors were used in combination to monitor the hydrolysis of pure cellulose of low crystallinity or industrial corncob samples. The obtained results correlate with the high-performance liquid chromatography pulsed amperometric detection analysis and demonstrate that neither oxygen nor the presence of redox-active compounds from the lignin fraction of the corncob interferes with the measurements.

A Tandem Solar Biofuel Cell: Harnessing Energy from Light and Biofuels.

We report on a photobioelectrochemical fuel cell consisting of a glucose-oxidase-modified BiFeO3 photobiocathode and a quantum-dot-sensitized inverse opal TiO2 photobioanode linked to FAD glucose dehydrogenase via a redox polymer. Both photobioelectrodes are driven by enzymatic glucose conversion. Whereas the photobioanode can collect electrons from sugar oxidation at rather low potential, the photobiocathode shows reduction currents at rather high potential. The electrodes can be arranged in a sandwich-like manner due to the semi-transparent nature of BiFeO3 , which also guarantees a simultaneous excitation of the photobioanode when illuminated via the cathode side. This tandem cell can generate electricity under illumination and in the presence of glucose and provides an exceptionally high OCV of about 1 V. The developed semi-artificial system has significant implications for the integration of biocatalysts in photoactive entities for bioenergetic purposes, and it opens up a new path toward generation of electricity from sunlight and (bio)fuels.

Closing the Gap for Electronic Short-Circuiting: Photosystem†I Mixed Monolayers Enable Improved Anisotropic Electron Flow in Biophotovoltaic Devices.

Well-defined assemblies of photosynthetic protein complexes are required for an optimal performance of semi-artificial energy conversion devices, capable of providing unidirectional electron flow when light-harvesting proteins are interfaced with electrode surfaces. We present mixed photosystem I (PSI) monolayers constituted of native cyanobacterial PSI trimers in combination with isolated PSI monomers from the same organism. The resulting compact arrangement ensures a high density of photoactive protein complexes per unit area, providing the basis to effectively minimize short-circuiting processes that typically limit the performance of PSI-based bioelectrodes. The PSI film is further interfaced with redox polymers for optimal electron transfer, enabling highly efficient light-induced photocurrent generation. Coupling of the photocathode with a [NiFeSe]-hydrogenase confirms the possibility to realize light-induced H2 evolution.

Redox-Polymer-Based High-Current-Density Gas-Diffusion H2 -Oxidation Bioanode Using [FeFe] Hydrogenase from Desulfovibrio desulfuricans in a Membrane-free Biofuel Cell.

The incorporation of highly active but also highly sensitive catalysts (e.g. the [FeFe] hydrogenase from Desulfovibrio desulfuricans) in biofuel cells is still one of the major challenges in sustainable energy conversion. We report the fabrication of a dual-gas diffusion electrode H2 /O2 biofuel cell equipped with a [FeFe] hydrogenase/redox polymer-based high-current-density H2 -oxidation bioanode. The bioanodes show benchmark current densities of around 14 mA cm-2 and the corresponding fuel cell tests exhibit a benchmark for a hydrogenase/redox polymer-based biofuel cell with outstanding power densities of 5.4 mW cm-2 at 0.7 V cell voltage. Furthermore, the highly sensitive [FeFe] hydrogenase is protected against oxygen damage by the redox polymer and can function under 5 % O2 .

Electroenzymatic Nitrogen Fixation Using a MoFe Protein System Immobilized in an Organic Redox Polymer.

We report an organic redox-polymer-based electroenzymatic nitrogen fixation system using a metal-free redox polymer, namely neutral-red-modified poly(glycidyl methacrylate-co-methylmethacrylate-co-poly(ethyleneglycol)methacrylate) with a low redox potential of -0.58 V vs. SCE. The stable and efficient electric wiring of nitrogenase within the redox polymer matrix enables mediated bioelectrocatalysis of N3- , NO2- and N2 to NH3 catalyzed by the MoFe protein via the polymer-bound redox moieties distributed in the polymer matrix in the absence of the Fe protein. Bulk bioelectrosynthetic experiments produced 209±30 nmol NH3 nmol MoFe-1 h-1 from N2 reduction. 15 N2 labeling experiments and NMR analysis were performed to confirm biosynthetic N2 reduction to NH3 .

Extended Operational Lifetime of a Photosystem-Based Bioelectrode.

The development of bioelectrochemical assemblies for sustainable energy transformation constitutes an increasingly important field of research. Significant progress has been made in the development of semiartificial devices for conversion of light into electrical energy by integration of photosynthetic biomolecules on electrodes. However, sufficient long-term stability of such biophotoelectrodes has been compromised by reactive species generated under aerobic operation. Therefore, meeting the requirements of practical applications still remains unsolved. We present the operation of a photosystem I-based photocathode using an electron acceptor that enables photocurrent generation under anaerobic conditions as the basis for a biodevice with substantially improved stability. A continuous operation lifetime considerably superior to previous reports and at higher light intensities is paving the way toward the potential application of semiartificial energy conversion devices.

A Z-Scheme-Inspired Photobioelectrochemical H2 O/O2 Cell with a 1†V Open-Circuit Voltage Combining Photosystem†II and PbS Quantum Dots.

A biohybrid photobioanode mimicking the Z-scheme has been developed by functional integration of photosystem II (PSII) and PbS quantum dots (QDs) within an inverse opal TiO2 architecture giving rise to a rather negative water oxidation potential of about -0.55 V vs. Ag/AgCl, 1 m KCl at neutral pH. The electrical linkage between both light-sensitive entities has been established through an Os-complex-modified redox polymer (POs ), which allows the formation of a multi-step electron-transfer chain under illumination starting with the photo-activated water oxidation at PSII followed by an electron transfer from PSII through POs to the photo-excited QDs and finally to the TiO2 electrode. The photobioanode was coupled to a novel, transparent, inverse-opal ATO cathode modified with an O2 -reducing bilirubin oxidase for the construction of a H2 O/O2 photobioelectrochemical cell reaching a high open-circuit voltage of about 1 V under illumination.

Photoreduction of CO2 with a Formate Dehydrogenase Driven by Photosystem II Using a Semi-artificial Z-Scheme Architecture.

Solar-driven coupling of water oxidation with CO2 reduction sustains life on our planet and is of high priority in contemporary energy research. Here, we report a photoelectrochemical tandem device that performs photocatalytic reduction of CO2 to formate. We employ a semi-artificial design, which wires a W-dependent formate dehydrogenase (FDH) cathode to a photoanode containing the photosynthetic water oxidation enzyme, Photosystem II, via a synthetic dye with complementary light absorption. From a biological perspective, the system achieves a metabolically inaccessible pathway of light-driven CO2 fixation to formate. From a synthetic point of view, it represents a proof-of-principle system utilizing precious-metal-free catalysts for selective CO2-to-formate conversion using water as an electron donor. This hybrid platform demonstrates the translatability and versatility of coupling abiotic and biotic components to create challenging models for solar fuel and chemical synthesis.

Tuned Amperometric Detection of Reduced ß-Nicotinamide Adenine Dinucleotide by Allosteric Modulation of the Reductase Component of the p-Hydroxyphenylacetate Hydroxylase Immobilized within a Redox Polymer.

We report the fabrication of an amperometric NADH biosensor system that employs an allosterically modulated bacterial reductase in an adapted osmium(III)-complex-modified redox polymer film for analyte quantification. Chains of complexed Os(III) centers along matrix polymer strings make electrical connection between the immobilized redox protein and a graphite electrode disc, transducing enzymatic oxidation of NADH into a biosensor current. Sustainable anodic signaling required (1) a redox polymer with a formal potential that matched the redox switch of the embedded reductase and avoided interfering redox interactions and (2) formation of a cross-linked enzyme/polymer film for stable biocatalyst entrapment. The activity of the chosen reductase is enhanced upon binding of an effector, i.e. p-hydroxy-phenylacetic acid ( p-HPA), allowing the acceleration of the substrate conversion rate on the sensor surface by in situ addition or preincubation with p-HPA. Acceleration of NADH oxidation amplified the response of the biosensor, with a 1.5-fold increase in the sensitivity of analyte detection, compared to operation without the allosteric modulator. Repetitive quantitative testing of solutions of known NADH concentration verified the performance in terms of reliability and analyte recovery. We herewith established the use of allosteric enzyme modulation and redox polymer-based enzyme electrode wiring for substrate biosensing, a concept that may be applicable to other allosteric enzymes.

The Open Circuit Voltage in Biofuel Cells: Nernstian Shift in Pseudocapacitive Electrodes.

In the development of biofuel cells great effort is dedicated to achieving outstanding figures of merit, such as high stability, maximum power output, and a large open circuit voltage. Biofuel cells with immobilized redox mediators, such as redox polymers with integrated enzymes, show experimentally a substantially higher open circuit voltage than the thermodynamically expected value. Although this phenomenon is widely reported in the literature, there is no comprehensive understanding of the potential shift, the high open circuit voltages have not been discussed in detail, and hence they are only accepted as an inherent property of the investigated systems. We demonstrate that this effect is the result of a Nernstian shift of the electrode potential when catalytic conversion takes place in the absence or at very low current flow. Experimental evidence confirms that the immobilization of redox centers on the electrode surface results in the assembled biofuel cell delivering a higher power output because of charge storage upon catalytic conversion. Our findings have direct implications for the design and evaluation of (bio)fuel cells with pseudocapacitive elements.

Interrogation of a PS1-Based Photocathode by Means of Scanning Photoelectrochemical Microscopy.

In the development of photosystem-based energy conversion devices, the in-depth understanding of electron transfer processes involved in photocurrent generation and possible charge recombination is essential as a basis for the development of photo-bioelectrochemical architectures with increased efficiency. The evaluation of a bio-photocathode based on photosystem 1 (PS1) integrated within a redox hydrogel by means of scanning photoelectrochemical microscopy (SPECM) is reported. The redox polymer acts as a conducting matrix for the transfer of electrons from the electrode surface to the photo-oxidized P700 centers within PS1, while methyl viologen is used as charge carrier for the collection of electrons at the reduced FB site of PS1. The analysis of the modified surfaces by SPECM enables the evaluation of electron-transfer processes by simultaneously monitoring photocurrent generation at the bio-photoelectrode and the associated generation of reduced charge carriers. The possibility to visualize charge recombination processes is illustrated by using two different electrode materials, namely Au and p-doped Si, exhibiting substantially different electron transfer kinetics for the reoxidation of the methyl viologen radical cation used as freely diffusing charge carrier. In the case of p-doped Si, a slower recombination kinetics allows visualization of methyl viologen radical cation concentration profiles from SPECM approach curves.

Design of an Os Complex-Modified Hydrogel with Optimized Redox Potential for Biosensors and Biofuel Cells.

Multistep synthesis and electrochemical characterization of an Os complex-modified redox hydrogel exhibiting a redox potential ≈+30 mV (vs. Ag/AgCl 3 M KCl) is demonstrated. The careful selection of bipyridine-based ligands bearing N,N-dimethylamino moieties and an amino-linker for the covalent attachment to the polymer backbone ensures the formation of a stable redox polymer with an envisaged redox potential close to 0 V. Most importantly, the formation of an octahedral N6-coordination sphere around the Os central atoms provides improved stability concomitantly with the low formal potential, a low reorganization energy during the Os(3+/2+) redox conversion and a negligible impact on oxygen reduction. By wiring a variety of enzymes such as pyrroloquinoline quinone (PQQ)-dependent glucose dehydrogenase, flavin adenine dinucleotide (FAD)-dependent glucose dehydrogenase and the FAD-dependent dehydrogenase domain of cellobiose dehydrogenase, low-potential glucose biosensors could be obtained with negligible co-oxidation of common interfering compounds such as uric acid or ascorbic acid. In combination with a bilirubin oxidase-based biocathode, enzymatic biofuel cells with open-circuit voltages of up to 0.54 V were obtained.

Functionalized branched EDOT-terthiophene copolymer films by electropolymerization and post-polymerization "click"-reactions.

The electrocopolymerization of 3,4-ethylenedioxythiophene (EDOT) with the branched thiophene building block 2,2':3',2″-terthiophene (3T) is presented as a versatile route to functional polymer films. Comparisons to blend systems of the respective homopolymers PEDOT and P3T by in situ spectroelectrochemistry and Raman spectroscopy prove the successful copolymer formation and the access to tailored redox properties and energy levels. The use of EDOT-N3 as co-monomer furthermore allows modifications of the films by polymer analogous reactions. Here, we exemplarily describe the post-functionalization with ionic moieties by 1,3-dipolar cycloaddition ("click"-chemistry) which allows to tune the surface polarity of the copolymer films from water contact angles of 140° down to 40°.

[6,6]-Open and [6,6]-closed isomers of C70(CF2): synthesis, electrochemical and quantum chemical investigation.

Novel difluoromethylenated [70]fullerene derivatives, C70(CF2 )n (n=1-3), were obtained by the reaction of C70 with sodium difluorochloroacetate. Two major products, isomeric C70(CF2 ) mono-adducts with [6,6]-open and [6,6]-closed configurations, were isolated and their homofullerene and methanofullerene structures were reliably determined by a variety of methods that included X-ray analysis and high-level spectroscopic techniques. The [6,6]-open isomer of C70(CF2 ) constitutes the first homofullerene example of a non-hetero [70]fullerene derivative in which functionalisation involves the most reactive bond in the polar region of the cage. Voltammetric estimation of the electron affinity of the C70(CF2 ) isomers showed that it is substantially higher for the [6,6]-open isomer (the 70-electron π-conjugated system is retained) than the [6,6]-closed form, the latter being similar to the electron affinity of pristine C70. In situ ESR spectroelectrochemical investigation of the C70(CF2 ) radical anions and DFT calculations of the hyperfine coupling constants provide evidence for the first example of an inter-conversion between the [6,6]-closed and [6,6]-open forms of a cage-modified fullerene driven by an electrochemical one-electron transfer. Thus, [6,6]-closed C70(CF2 ) constitutes an interesting example of a redox-switchable fullerene derivative.

Electropolymerized three-dimensional randomly branched EDOT-containing copolymers.

The potential of 2,2';3,2″-terthiophene (3T) as branching units in 3D copolymers is presented with EDOT as an example comonomer. Branched EDOT/3T polythiophenes were prepared by electropolymerization, and their electrochemical and optical properties are discussed. Two different approaches were employed: (i) the direct electropolymerization of a novel branched thiophene monomer (3TE3) consisting of a 3T core that contains three outer EDOT end groups and (ii) the electrochemical copolymerization of a EDOT/3T mixture in different ratios from [1:1] to [1:10]. Cyclic voltammetric and vis spectrometric experiments show that the EDOT content within the polymer has a strong influence on the electronic properties of the material: with increasing EDOT content, the HOMO-LUMO gap is decreased. To prove copolymer formation of EDOT and 3T, chemically synthesized reference copolymers of EDOT and 3T were prepared by oxidative coupling using FeCl3, and their optical and electronic properties were compared to those of the electrodeposited films. In addition, the copolymer formation is indicated by the comparison of the electrochemical and spectroscopic results with those of the homopolymers P3T and PEDOT.

Anti-[2.2](1,4)pentacenophane: a covalently coupled pentacene dimer.

Two in a row: A pentacene dimer in which both units are covalently linked through a [2.2]paracyclophane bridge, has been synthesized. The electronic properties of the molecule were elucidated by a combination of experimental and computational methods. Such molecules could lead to materials with improved charge-transport properties.

Light-controlled imaging of biocatalytic reactions via scanning photoelectrochemical microscopy for multiplexed sensing.

A light-controlled multiplexing platform has been developed on the basis of a quantum dot-sensitized inverse opal TiO2 electrode with integrated biocatalytic reactions. Spatially resolved illumination enables multiplexed sensing and imaging of enzymatic oxidation reactions at relatively negative applied potentials.

Drug Release from Polymer Thin Films and Gel Pellets: Insights from Programmed Microplate Electroanalysis.

Robotic electroanalysis in 24-well microplates was used to determine Paracetamol (PCT) release from thin films of chitosan and two pH-sensitive synthetic polymers as well as blends of the polymers with each other and with agarose. Square-wave voltammograms were recorded automatically in a potential window of 0.35 V-0.85 V vs. Ag/AgCl/0.1 M KCl and their evaluation revealed time-dependent PCT release into acidic and basic media. Comparison of the release profiles showed that pure chitosan layers released PCT quickly in a single-phase process while liberation from synthetic polymer thin films was slower with a sigmoidal shape at pH 1.2 and pH 8.0 with a maximum release of PCT after approximately 150 and 140 min, respectively. The release profile from thicker agarose films was between those of the thin films. Agarose blended with chitosan or synthetic polymers formed films with biphasic release behavior. Chitosan linearized the initial section of the release profile in chitosan/polymer blends. The automated procedure for release testing offers the advantage of low-cost, labor-effective and error-free data acquisition. The procedure has been validated as a useful microplate assay option for release profile testing.

Polymer/enzyme-modified HF-etched carbon nanoelectrodes for single-cell analysis.

Carbon-based nanoelectrodes fabricated by means of pyrolysis of an alkane precursor gas purged through a glass capillary and subsequently etched with HF were modified with redox polymer/enzyme films for the detection of glucose at the single-cell level. Glucose oxidase (GOx) was immobilized and electrically wired by means of an Os-complex-modified redox polymer in a sequential dip coating process. For the synthesis of the redox polymer matrix, a poly(1-vinylimidazole-co-acrylamide)-based backbone was used that was first modified with the electron transfer mediator [Os(bpy)2Cl]+ (bpy = 2,2'-bipyridine) followed by the conversion of the amide groups within the acrylamide monomer into hydrazide groups in a polymer-analogue reaction. The hydrazide groups react readily with bifunctional epoxide-based crosslinkers ensuring high film stability. Insertion of the nanometre-sized polymer/enzyme modified electrodes into adherently growing single NG108-15 cells resulted in a positive current response correlating with the intracellular glucose concentration. Moreover, the nanosensors showed a stable current output without significant loss in performance after intracellular measurements.