Electron Transfer in Binary Hemin-Modified Alkanethiol Self-Assembled Monolayers on Gold: Hemin's Lateral and Interfacial Interactions

Orientated coupling of redox enzymes to electrodes by their reconstitution onto redox cofactors, such as hemin conjugated to self-assembled monolayers (SAMs) formed on the electrodes, poses the requirements for a SAM design enabling reconstitution. We show that the kinetics of electron transfer (ET) in binary SAMs of alkanethiols on gold composed of in situ hemin-conjugated 11-amino-1-undecanethiol (AUT) and diluting OH-terminated alkanethiols with 11, 6, and 2 methylene groups (MC₁₁OH, MC₆OH, and MC₂OH) depends on both the SAM composition and surface density of hemin, Γ_heme. In AUT/MC₁₁OH SAMs composed of equal linker/diluent lengths, the heterogeneous ET rate constant k_s decreased with the Γ_heme and varied between 70 and 500 s^-1. For shorter diluents, the k_s of 245-330 s^-1 (C₆) and 300-340 s^-1 (C₂) showed a little (if any) Γ_heme dependence. In AUT/MC₁₁OH SAMs, the increasing Γ_heme resulted in the steric crowding of hemin species and their neighboring lateral interactions in the plane of hemin localization, affecting the potential distribution at the SAM/electrode interface and inducing local electrostatic effects interfering with hemin oxidation. In AUT/MC₆OH and AUT/MC₂OH SAMs, hemin discharged at the plane of the closest approach to the gold surface, equal to the diluent length and permeable to electrolyte ions, which lessened those effects. All studied binary SAMs provided steric hindrance for protein reconstitution on the hemin cofactor conjugated to the extended AUT linker. Further use of SAM-modified electrodes with the covalently attached hemin as interfaces for heme proteins' reconstitution should consider SAMs with loosely dispersed redox centers terminating more rigid molecular wires. Such wires place hemin at fixed distances from the electrode surface and thus ensure the interfacial properties required for the effective on-surface reconstitution of proteins and enzymes.

The laccase mediator system at carbon nanotubes for anthracene oxidation and femtomolar electrochemical biosensing

We investigated the use of POXA1b laccase from Pleurotus ostreatus for the oxidation of anthracene into anthraquinone. We show that different pathways can occur depending on the nature of the redox mediator combined to laccase, leading to different structural isomers. The laccase combined with 2,2'-azine-bis(3-ethylbenzothiazoline-6-sulfonic acid (ABTS) leads to the formation of 1,4-anthraquinone and/or 1,2-anthraquinone. The unprecedented role of carbon nanotubes (CNTs) as redox mediators for oxidation of anthracene into 9,10-anthraquinone is shown and corroborated by density-functional theory (DFT) calculations. Owing to the efficient adsorption of anthraquinones at CNT electrodes, anthracene can be detected with low limit-of-detection using either laccase in solution, CNT-supported laccase or laccase immobilized at magnetic beads exploiting the adhesive property of a chimeric hydrophobin-laccase.

Fundamental insight into redox enzyme-based bioelectrocatalysis

Redox enzymes can work as efficient electrocatalysts. The coupling of redox enzymatic reactions with electrode reactions is called enzymatic bioelectrocatalysis, which imparts high reaction specificity to electrode reactions with nonspecific characteristics. The key factors required for bioelectrocatalysis are hydride ion/electron transfer characteristics and low specificity for either substrate in redox enzymes. Several theoretical features of steady-state responses are introduced to understand bioelectrocatalysis and to extend the performance of bioelectrocatalytic systems. Applications of the coupling concept to bioelectrochemical devices are also summarized with emphasis on the achievements recorded in the research group of the author.

A review: Evolution of enzymatic biofuel cells

Ever-growing demands for energy, the unsustainability of fossil fuel due to its scarcity and massive impact on global economies and the environment, have encouraged the research on alternative power sources to work upon for the governments, companies, and scientists across the world. Enzymatic biofuel cells (eBFCs) is one category of fuel cell that can harvest energy from biological moieties and has the future to be used as an alternative source of energy. The aim of this review is to summarize the background and state-of-the-art in the field of eBFCs. This review article will be very beneficial for a wide audience including students and new researchers in the field. A part of the paper summarized the challenges in the preparation of anode and cathode and the involvement of nanomaterials and conducting polymers to construct the effective bioelectrodes. It will provide an insight for the researchers working in this challenging field. Furthermore, various applications of eBFCs in implantable power devices, tiny electronic gadgets, and self powered biosensors are reported. This review article explains the development in the area of eBFCs for several years from its origin to growth systematically. It reveals the strategies that have been taken for the improvements required for the better electrochemical performance and operational stability of eBFCs. It also mentions the challenges in this field that will require proper attention so that the eBFCs can be utilized commercially in the future. The review article is written and structurized in a way so that it can provide a decent background of eBFCs to its reader. It will definitely help in enhancing the interest of reader in eBFCs.

Single Particle Nanoelectrochemistry Reveals the Catalytic Oxygen Evolution Reaction Activity of Co3 O4 Nanocubes

Co₃O₄ nanocubes are evaluated concerning their intrinsic electrocatalytic activity towards the oxygen evolution reaction (OER) by means of single‐entity electrochemistry. Scanning electrochemical cell microscopy (SECCM) provides data on the electrocatalytic OER activity from several individual measurement areas covering one Co₃O₄ nanocube of a comparatively high number of individual particles with sufficient statistical reproducibility. Single‐particle‐on‐nanoelectrode measurements of Co₃O₄ nanocubes provide an accelerated stress test at highly alkaline conditions with current densities of up to 5.5 A cm⁻², and allows to derive TOF values of up to 2.8×10⁴ s⁻¹ at 1.92 V vs. RHE for surface Co atoms of a single cubic nanoparticle. Obtaining such high current densities combined with identical‐location transmission electron microscopy allows monitoring the formation of an oxy(hydroxide) surface layer during electrocatalysis. Combining two independent single‐entity electrochemistry techniques provides the basis for elucidating structure–activity relations of single electrocatalyst nanoparticles with well‐defined surface structure.

Efficiency of Site-Specific Clicked Laccase-Carbon Nanotubes Biocathodes towards O2 Reduction

A maximization of a direct electron transfer (DET) between redox enzymes and electrodes can be obtained through the oriented immobilization of enzymes onto an electroactive surface. Here, a strategy for obtaining carbon nanotube (CNTs) based electrodes covalently modified with perfectly control-oriented fungal laccases is presented. Modelizations of the laccase-CNT interaction and of electron conduction pathways serve as a guide in choosing grafting positions. Homogeneous populations of alkyne-modified laccases are obtained through the reductive amination of a unique surface-accessible lysine residue selectively engineered near either one or the other of the two copper centers in enzyme variants. Immobilization of the site-specific alkynated enzymes is achieved by copper-catalyzed click reaction on azido-modified CNTs. A highly efficient reduction of O₂ at low overpotential and catalytic current densities over -3 mA cm^-2 are obtained by minimizing the distance from the electrode surface to the trinuclear cluster.

Wearable biofuel cells based on the classification of enzyme for high power outputs and lifetimes

Wearable enzymatic biofuel cells would be the most prospective fuel cells for wearable devices because of their low cost, compactness and flexibility. As the high specificity and catalytic properties of enzymes, enzymatic biofuel cells (EBFCs) catalyze the fuel associated with the redox reaction and get electrical energy. Available biofuels such as glucose, lactate and pyruvate can be harvested from biofluids of sweat, tears and blood, which afford cells a favorable use in implantable and wearable devices. However, the development of wearable enzymatic biofuel cells requires significant improvements on the power density and enzymes lifetime. In this paper, some new advances in improving the performance of wearable enzymatic biofuel cells are reviewed based on the bioanode and biocathode by classifying single-enzyme and multi-enzyme catalysis system. Thereinto, the bioanode usually contains oxidases and dehydrogenases as catalyst, and the biocathode utilizes the catalysis of multi-copper oxidases (MCOs) in the single system. For further enhancing the power density, efforts to develop multi-enzyme catalysis strategies are discussed in bioanode and biocathode respectively. Moreover, some potential technologies in recent years, such as carbon nanodots, CNT sponges and mixed operational/storage electrode are summarized owing to notable efficiency and the capability of enhancing electron transfer on the electrode. Finally, major challenges and future prospects are discussed for the high power output, stable and practical wearable enzymatic biofuel cells.

Orientation Control of Trametes Laccases on a Carbon Electrode Surface to Understand the Orientation Effect on the Electrocatalytic Activity

By using a carbon-coated anodic aluminum oxide (CAAO) film as a monolithic porous electrode for the immobilization of Trametes laccases (LACs), an attempt is made to control the orientation of LAC molecules toward the electrode surface simply by applying an electric potential to the CAAO film. Because the resulting film is characterized by a myriad of open, simple, and straight nanochannels with diameters as large as 40 nm, the O₂ diffusion problem in pores is minimized, thereby making it possible to highlight the effect of such orientation on the electrocatalytic activity as a biocathode. It has been evidenced that LAC molecules are favorably oriented for a smooth electron transfer from the electrode when the LACs are immobilized with applying a positive voltage to the electrode, and such favorable orientation exhibits 3.7-times higher electrocatalytic activity than unfavorable orientation. Furthermore, the orientation mechanism has been rationally explained in terms of local surface chemistry on a LAC molecule.

A self-powered biosensing device with an integrated hybrid biofuel cell for intermittent monitoring of analytes

In this work, we propose an integrated self-powered sensing system, driven by a hybrid biofuel cell (HBFC) with carbon paper discs coated with multiwalled carbon nanotubes. The sensing system has a biocathode made from laccase or bilirubin oxidase, and the anode is made from a zinc plate. The system includes a dedicated custom-built electronic control unit for the detection of oxygen and catechol analytes, which are central to medical and environmental applications. Both the HBFC and sensors, operate in a mediatorless direct electron transfer mode. The measured characteristics of the HBFC with externally applied resistance included the power-time dependencies under flow cell conditions, the sensors performance (evaluated by cyclic voltammetry), and chronoamperometry. The HBFC is integrated with analytical devices and operating in a pulse mode form long-run monitoring experiments. The HBFC generated sufficient power for wireless data transmission to a local computer.

Structure and Modification of Electrode Materials for Protein Electrochemistry

The interactions between proteins and electrode surfaces are of fundamental importance in bioelectrochemistry, including photobioelectrochemistry. In order to optimise the interaction between electrode and redox protein, either the electrode or the protein can be engineered, with the former being the most adopted approach. This tutorial review provides a basic description of the most commonly used electrode materials in bioelectrochemistry and discusses approaches to modify these surfaces. Carbon, gold and transparent electrodes (e.g. indium tin oxide) are covered, while approaches to form meso- and macroporous structured electrodes are also described. Electrode modifications include the chemical modification with (self-assembled) monolayers and the use of conducting polymers in which the protein is imbedded. The proteins themselves can either be in solution, electrostatically adsorbed on the surface or covalently bound to the electrode. Drawbacks and benefits of each material and its modifications are discussed. Where examples exist of applications in photobioelectrochemistry, these are highlighted.

XPS analysis of enzyme and mediator at the surface of a layer-by-layer self-assembled wired enzyme electrode

High potential purified Trametes trogii laccase has been deposited in mono- and multilayer thin films on gold surfaces by layer-by-layer electrostatic adsorption self-assembly. The osmium bipyridil redox relay sites on polycation poly(allylamine) backbone efficiently work as a molecular "wire" in oxygen cathodes for biofuel cells. X-ray photoelectron spectroscopy of Cu 2p3/2 and Os 4f signals provided chemical information on the enzyme and redox mediator surface concentrations after different adsorption steps. The electrical charge involved in oxidation-reduction cycles of the osmium sites, the ellipsometric enzyme film thickness, and the mass uptake from quartz crystal microbalance experiments, correlate with the XPS surface concentration, which provides unique evidence on the chemical identity of the composition in the topmost layers. XPS is shown to be an important analytical tool to investigate stratified copper and osmium distribution in LbL thin films relevant to biosensors and biofuel cells.

Effect of enzymatic orientation through the use of syringaldazine molecules on multiple multi-copper oxidase enzymes

The effect of proper enzyme orientation at the electrode surface was explored for two multi-copper oxygen reducing enzymes: Bilirubin Oxidase (BOx) and Laccase (Lac).

A 96-well electrochemical method for the screening of enzymatic activities

The rapid electrochemical screening of enzyme activities in bioelectronics is still a challenging issue. In order to solve this problem, we propose to use a 96-well electrochemical assay. This system is composed of 96 screen-printed electrodes on a printed circuit board adapted from a commercial system (carbon is used as the working electrode and silver chloride as the counter/reference electrode). The associated device allows for the measurements on the 96 electrodes to be performed within a few seconds. In this work, we demonstrate the validity of the screening method with the commercial laccase from the fungus Trametes versicolor. The signal-to-noise ratio (S/N) is found to be the best way to analyze the electrochemical signals. The S/N follows a saturation-like mechanism with a dynamic linear range of two decades ranging from 0.5 to 75 ng of laccase (corresponding to enzymatic activities from 62 × 10(-6) to 9.37 × 10(-3) μmol min(-1)) and a sensitivity of 3027 μg(-1) at +100 mV versus Ag/AgCl. Laccase inhibitors (azide and fluoride anions), pH optima, and interfering molecules could also be identified within a few minutes.

Integration of photoswitchable proteins, photosynthetic reaction centers and semiconductor/biomolecule hybrids with electrode supports for optobioelectronic applications

Light-triggered biological processes provide the principles for the development of man-made optobioelectronic systems. This Review addresses three recently developed topics in the area of optobioelectronics, while addressing the potential applications of these systems. The topics discussed include: (i) the reversible photoswitching of the bioelectrocatalytic functions of redox proteins by the modification of proteins with photoisomerizable units or by the integration of proteins with photoisomerizable environments; (ii) the integration of natural photosynthetic reaction centers with electrodes and the construction of photobioelectrochemical cells and photobiofuel cells; and (iii) the synthesis of biomolecule/semiconductor quantum dots hybrid systems and their immobilization on electrodes to yield photobioelectrochemical and photobiofuel cell elements. The fundamental challenge in the tailoring of optobioelectronic systems is the development of means to electrically contact photoactive biomolecular assemblies with the electrode supports. Different methods to establish electrical communication between the photoactive biomolecular assemblies and electrodes are discussed. These include the nanoscale engineering of the biomolecular nanostructures on surfaces, the development of photoactive molecular wires and the coupling of photoinduced electron transfer reactions with the redox functions of proteins. The different possible applications of optobioelectronic systems are discussed, including their use as photosensors, the design of biosensors, and the construction of solar energy conversion and storage systems.

High-throughput synthesis and electrochemical screening of a library of modified electrodes for NADH oxidation

We report the combinatorial preparation and high-throughput screening of a library of modified electrodes designed to catalyze the oxidation of NADH. Sixty glassy carbon electrodes were covalently modified with ruthenium(II) or zinc(II) complexes bearing the redox active 1,10-phenanthroline-5,6-dione (phendione) ligand by electrochemical functionalization using one of four different linkers, followed by attachment of one of five different phendione metal complexes using combinatorial solid-phase synthesis methodology. This gave a library with three replicates of each of 20 different electrode modifications. This library was electrochemically screened in high-throughput (HTP) mode using cyclic voltammetry. The members of the library were evaluated with regard to the surface coverage, midpeak potential, and voltammetric peak separation for the phendione ligand, and their catalytic activity toward NADH oxidation. The surface coverage was found to depend on the length and flexibility of the linker and the geometry of the metal complex. The choices of linker and metal complex were also found to have significant impact on the kinetics of the reaction between the 1,10-phenanthroline-5,6-dione ligand and NADH. The rate constants for the reaction were obtained by analyzing the catalytic currents as a function of NADH concentration and scan rate, and the influence of the surface molecular architecture on the kinetics was evaluated.

Covalent attachment of FeFe hydrogenases to carbon electrodes for direct electron transfer

Direct electron transfer between enzymes and electrodes is now commonly achieved, but obtaining protein films that are very stable may be challenging. This is particularly crucial in the case of hydrogenases, the enzymes that catalyze the biological conversion between dihydrogen and protons, because the instability of the hydrogenase films may prevent the use of these enzymes as electrocatalysts of H(2) oxidation and production in biofuel cells and photoelectrochemical cells. Here we show that two different FeFe hydrogenases (from Chamydomonas reinhardtii and Clostridium acetobutylicum) can be covalently attached to functionalized pyrolytic graphite electrodes using peptidic coupling. In both cases, a surface patch of lysine residues makes it possible to favor an orientation that is efficient for fast, direct electron transfer. High hydrogen-oxidation current densities are maintained for up to one week, the only limitation being the intrinsic stability of the enzyme. We also show that covalent attachment has no effect on the catalytic properties of the enzyme, which means that this strategy can also used be for electrochemical studies of the catalytic mechanism.