Site-Directed Immobilization of Bilirubin Oxidase for Electrocatalytic Oxygen Reduction

Conductance Channels in a Single-Entity Enzyme

For a long time, the prevailing view in the scientific community was that proteins, being complex macromolecules composed of amino acid chains linked by peptide bonds, adopt folded structure with insulating or semiconducting properties, with high bandgaps. However, recent discoveries of unexpectedly high conductance levels, reaching values in the range of dozens of nanosiemens (nS) in proteins, have challenged this conventional understanding. In this study, we used scanning tunneling microscopy (STM) to explore the single-entity conductance properties of enzymatic channels, focusing on bilirubin oxidase (BOD) as a model metalloprotein. By immobilizing BOD on a conductive carbon surface, we discern its preferred orientation, facilitating the formation of electronic and ionic channels. These channels show efficient electron transport (ETp), with apparent conductance up to the 15 nS range. Notably, these conductance pathways are localized, minimizing electron transport barriers due to solvents and ions, underscoring BOD's redox versatility. Furthermore, electron transfer (ET) within the BOD occurs via preferential pathways. The alignment of the conductance channels with hydrophilicity maps, molecular vacancies, and regions accessible to electrolytes explains the observed conductance values. Additionally, BOD exhibits redox activity, with its active center playing a critical role in the ETp process. These findings significantly advance our understanding of the intricate mechanisms that govern ETp processes in proteins, offering new insights into the conductance of metalloproteins.

Enzyme-Modified Microelectrode for Simultaneous Local Measurements of O2 and pH

The use of miniaturized probes opens a new dimension in the analysis of (bio)chemical processes, enabling the possibility to perform measurements with local resolution. In addition, multiparametric measurements are highly valuable for a holistic understanding of the investigated process. Therefore, different strategies have been suggested for simultaneous local measurements of various parameters. Electroanalytical methods are a powerful strategy in this direction. However, they have been mainly restricted to coupling concurrent independent measurements with different miniaturized probes. Here, we present an enzymatic microbiosensor for the simultaneous detection of O2 and pH. The sensing strategy is based on the pH-dependent bioelectrocatalytic process associated with O2 reduction at a gold microelectrode modified with a multicopper oxidase. After initial investigations of the bioelectrocatalytic reaction over gold macroelectrodes, the fabrication and characterization of micrometer-sized probes are presented. The microbioelectrode exhibits a linear current increase with O2 concentration extending to 17.2 mg L-1, with a sensitivity of (5.56 ± 0.13) nA L mg-1 and a limit of detection of (0.5 ± 0.3) mg L-1. Moreover, a linear response allowing pH detection is obtained between pH 5.2 and 7.5 with a slope of -(47 ± 8) mV per pH unit. In addition, two proof-of-concept analytical examples are shown, demonstrating the capability of the developed sensing system for simultaneous local measurements of O2 and pH. Compared with other miniaturized probes reported before for simultaneous detection, our strategy stands out as the two investigated parameters are acquired from the very same measurement. This strategy greatly simplifies the analytical setup and for the first time provides truly simultaneous local detection in the micrometer scale.

Advances in aldo-keto reductases immobilization for biocatalytic synthesis of chiral alcohols

Chiral alcohols are essential building blocks of numerous pharmaceuticals and fine chemicals. Aldo-keto reductases (AKRs) constitute a superfamily of oxidoreductases that catalyze the reduction of aldehydes and ketones to their corresponding alcohols using NAD(P)H as a coenzyme. Knowledge about the crucial roles of AKRs immobilization in the biocatalytic synthesis of chiral alcohols is expanding. Herein, we reviewed the characteristics of various AKRs immobilization approaches, the applications of different immobilization materials, and the prospects of continuous flow bioreactor construction by employing these immobilized biocatalysts for synthesizing chiral alcohols. Finally, the opportunities and ongoing challenges for AKR immobilization are discussed and the outlook for this emerging area is analyzed.

Site-directed immobilization of enzymes on nanoparticles using self-assembly systems

Enzyme immobilization is an effective method for improving the stability and reusability. However, linking at random sites on the enzyme results in low catalytic efficiency due to blockage of the active site or conformational changes. Therefore, controlling the orientation of enzymes on the carrier has been developed. Here, the site-specific mutation and the SpyTag/SpyCatcher systems were used to prepare a site-directed immobilized enzyme. The thermal stability of the immobilized enzyme was better than that of the free enzyme, and ≥80 % of the catalytic activity was retained after 30 days of storage. Furthermore, the Michaelis constant (Km) and the turnover number (kcat) of the immobilized enzyme were 5.23-fold lower and 6.11-fold higher than those of the free enzyme, respectively, which appeared to be related to changes in secondary structure after immobilization. These findings provide a new and effective option for enzyme-directed immobilization.

Emerging Trends of Bilirubin Oxidases at the Bioelectrochemical Interface: Paving the Way for Self-Powered Electrochemical Devices and Biosensors

Bilirubin oxidases (BODs) [EC 1.3.3.5 - bilirubin: oxygen oxido-reductase] are enzymes that belong to the multicopper oxidase family and can oxidize bilirubin, diphenols, and aryl amines and reduce the oxygen by direct four-electron transfer from the electrode with almost no electrochemical overpotential. Therefore, BOD is a promising bioelectrocatalyst for (self-powered) biosensors and/or enzymatic fuel cells. The advantages of electrochemically active BOD enzymes include selective biosensing, biocatalysis for efficient energy conversion, and electrosynthesis. Owing to the rise in publications and patents, as well as the expanding interest in BODs for a range of physiological conditions, this Review analyzes scientific literature reports on BOD enzymes and current hypotheses on their bioelectrocatalysis. This Review evaluates the specific research outcomes of the BOD in enzyme (protein) engineering, immobilization strategies, and challenges along with their bioelectrochemical properties, limitations, and applications in the fields of (i) biosensors, (ii) self-powered biosensors, and (iii) biofuel cells for powering bioelectronics.

Oxidoreductase enzymes: Characteristics, applications, and challenges as a biocatalyst

Oxidoreductases are enzymes with distinctive characteristics that favor their use in different areas, such as agriculture, environmental management, medicine, and analytical chemistry. Among these enzymes, oxidases, dehydrogenases, peroxidases, and oxygenases are very interesting. Because their substrate diversity, they can be used in different biocatalytic processes by homogeneous and heterogeneous catalysis. Immobilization of these enzymes has favored their use in the solution of different biotechnological problems, with a notable increase in the study and optimization of this technology in the last years. In this review, the main structural and catalytical features of oxidoreductases, their substrate specificity, immobilization, and usage in biocatalytic processes, such as bioconversion, bioremediation, and biosensors obtainment, are presented.

Optimising Electrical Interfacing between the Trimeric Copper Nitrite Reductase and Carbon Nanotubes

The immobilization of copper-containing nitrite reductase (NiR) from Alcaligenes faecalis on functionalised multi-walled carbon nanotube (MWCNT) electrodes is reported. It is demonstrated that this immobilization is mainly driven by hydrophobic interactions, promoted by the modification of MWCNTs with adamantyl groups. Direct electrochemistry shows high bioelectrochemical reduction of nitrite at the redox potential of NiR with high current density of 1.41 mA cm-2 . Furthermore, the desymmetrization of the trimer upon immobilization induces an independent electrocatalytic behavior for each of the three enzyme subunits, corroborated by an electron-tunneling distance dependence.

High electrolyte concentration effect on enzymatic oxygen reduction

The nature, the composition and the concentration of electrolytes is essential for electrocatalysis involving redox enzymes. Here, we discuss the effect of various electrolyte compositions with increasing ionic strengths on the stability and activity towards O₂ reduction of the bilirubin oxidase from Myrothecium verrucaria (Mv BOD). Different salts, Na₂SO₄, (NH₄)₂SO₄, NaCl, NaClO₄, added to a phosphate buffer (PB) were evaluated with concentrations ranging from 100 mM up to 1.7 M. On functionalized carbon nanotube-modified electrodes, it was shown that the catalytic current progressively decreased with increasing salt concentrations. The process was reversible suggesting it was not related to enzyme leakage. The enzyme was then immobilized on gold electrodes modified by self-assembling of thiols. When the enzyme was simply adsorbed, the catalytic current decreased in a reversible way, thus behaving similarly as on carbon nanotubes. Enzyme mobility at the interface induced by a modification in the interactions between the protein and the electrode upon salt addition may account for this behavior. When the enzyme was covalently attached, the catalytic current increased. Enzyme compaction is proposed to be at the origin of such catalytic current increase because of shorter distances between the first copper site electron acceptor and the electrode.

Grafting of proteins onto polymeric surfaces: A synthesis and characterization challenge

This review aims at answering the following question: how can a researcher be sure to succeed in grafting a protein onto a polymer surface? Even if protein immobilization on solid supports has been used industrially for a long time, hence enabling natural enzymes to serve as a powerful tool, emergence of new supports such as polymeric surfaces for the development of so-called intelligent materials requires new approaches. In this review, we introduce the challenges in grafting protein on synthetic polymers, mainly because compared to hard surfaces, polymers may be sensitive to various aqueous media, depending on the pH or reductive molecules, or may exhibit state transitions with temperature. Then, the specificity of grafting on synthetic polymers due to difference of chemical functions availability or difference of physical properties are summarized. We present next the various available routes to covalently bond the protein onto the polymeric substrates considering the functional groups coming from the monomers used during polymerization reaction or post-modification of the surfaces. We also focus our review on a major concern of grafting protein, which is avoiding the potential loss of function of the immobilized protein. Meanwhile, this review considers the different methods of characterization used to determine the grafting efficiency but also the behavior of enzymes once grafted. We finally dedicate the last part of this review to industrial application and future prospective, considering the sustainable processes based on green chemistry.

Biotic-abiotic hybrids for bioanalytics and biocatalysis

The advances in biotic-abiotic interfaced systems open new directions toward bioanalytics and biocatalysis applications. Conjugating the unique electronic and optic properties of nanoelements with the high selectivity and extraordinary catalytic abilities of biotic materials holds great promise to gain superior new features. Herein, we present a wide scope of biotic-abiotic research, with key examples for its utilization in bioanalytics applications as well as in biocatalysis. The described configurations feature methodologies that enable extending the known scientific toolbox to gain synergy. These new nanobiohybrids may contribute to major global challenges, for example, developing alternative energy utilization or new affordable biodiagnostics and therapeutics tools.

Influence of distal glycan mimics on direct electron transfer performance for bilirubin oxidase bioelectrocatalysts

Bilirubin oxidase (BOD) is a bioelectrocatalyst that reduces dioxygen (O₂) to water and is capable of direct electron transfer (DET)-type bioelectrocatalysis via its electrode-active site (T1 Cu). BOD from Myrothecium verrucaria (mBOD) has been widely studied and has strong DET activity. mBOD contains two N-linked glycans (N-glycans) with N472 and N482 binding sites distal to T1 Cu. We previously reported that different N-glycan compositions affect the enzymatic orientation on the electrode by using recombinant BOD expressed in Pichia pastoris and the deglycosylation method. However, the individual function of the two N-glycans and the effects of N-glycan composition (size, structure, and non-reducing termini) on DET-type reactions are still unclear. In this study, we utilize maleimide-functionalized polyethylene glycol (MAL-PEG) as an N-glycan mimic to evaluate the aforementioned effects. Site-specific enzyme-PEG crosslinking was carried out by specific binding of maleimide to Cys residues. Recombinant BOD expressed in Escherichia coli (eBOD), which does not have a glycosylation system, was used as a benchmark to evaluate the effect. Site-directed mutagenesis of Asn residue (N472 or N482) into Cys residue is utilized to realize site-specific glycan mimic modification to the original binding site.

A disposable enzymatic biofuel cell for glucose sensing via short-circuit current

It is essential to construct a biofuel cell-based sensor and develop an effective strategy to detect glucose without any potentiostat circuitry in order to create a simple and miniaturized device. In this report, an enzymatic biofuel cell (EBFC) is fabricated by the facile design of an anode and cathode on a screen-printed carbon electrode (SPCE). To construct the anode, thionine and flavin adenine dinucleotide-dependent glucose dehydrogenase (FAD-GDH) are covalently immobilized via a crosslinker to make a cross-linked redox network. As a cathode, the Pt-free oxygen reduction carbon catalyst is employed alternative to the commonly used bilirubin oxidase. We proposed the importance of EBFC-based sensors through the connection of anode and cathode; they can identify a short-circuit current by means of applied zero external voltage, thereby capable of glucose detection without under the operation of the potentiostat. The result shows that the EBFC-based sensor could be able to detect based on a short-circuit current with a wide range of glucose concentrations from 0.28 to 30 mM. Further, an EBFC is employed as a one-compartment model energy harvester with a maximum power density of (36 ± 3) μW cm^{- 2} in sample volume 5 μL. In addition, the constructed EBFC-based sensor demonstrates that the physiological range of ascorbic acid and uric acid shows no significant effect on the short-circuit current generation. Moreover, this EBFC can be used as a sensor in artificial plasma without losing its performance and thereby used as a disposable test strip in real blood sample analysis.

Enzymatic and Microbial Electrochemistry: Approaches and Methods

The coupling of enzymes and/or intact bacteria with electrodes has been vastly investigated due to the wide range of existing applications. These span from biomedical and biosensing to energy production purposes and bioelectrosynthesis, whether for theoretical research or pure applied industrial processes. Both enzymes and bacteria offer a potential biotechnological alternative to noble/rare metal-dependent catalytic processes. However, when developing these biohybrid electrochemical systems, it is of the utmost importance to investigate how the approaches utilized to couple biocatalysts and electrodes influence the resulting bioelectrocatalytic response. Accordingly, this tutorial review starts by recalling some basic principles and applications of bioelectrochemistry, presenting the electrode and/or biocatalyst modifications that facilitate the interaction between the biotic and abiotic components of bioelectrochemical systems. Focus is then directed toward the methods used to evaluate the effectiveness of enzyme/bacteria-electrode interaction and the insights that they provide. The basic concepts of electrochemical methods widely employed in enzymatic and microbial electrochemistry, such as amperometry and voltammetry, are initially presented to later focus on various complementary methods such as spectroelectrochemistry, fluorescence spectroscopy and microscopy, and surface analytical/characterization techniques such as quartz crystal microbalance and atomic force microscopy. The tutorial review is thus aimed at students and graduate students approaching the field of enzymatic and microbial electrochemistry, while also providing a critical and up-to-date reference for senior researchers working in the field.

High-performance paper-based biocathode fabricated by screen-printing an improved mesoporous carbon ink and by oriented immobilization of bilirubin oxidase

In this study, the performance of a paper-based, screen-printed biofuel cell with mesoporous MgO-templated carbon (MgOC) electrodes was improved in two steps. First, a small amount of carboxymethyl cellulose (CMC) was added to the MgOC ink. Next, the cathode was modified with bilirubin prior to immobilizing the bilirubin oxidase (BOD). The CMC increased the accessibility of the mesopores of the MgOC, and subsequently, the performance of both the bioanode and biocathode. CMC also likely increased the stability of the electrodes. The pre-modification with bilirubin improved the orientation of the BOD, which facilitated direct electron transfer. With these two steps, an open circuit potential of 0.65 V, a maximal current density of 1.94 mA cm⁻², and a maximal power density of 465 μW cm⁻² was achieved with lactate oxidase as bioanode enzyme and lactate as fuel. This is one of the highest reported performances for a biofuel cell.

Engineering Electro- and Photocatalytic Carbon Materials for CO2 Reduction by Formate Dehydrogenase

Semiartificial approaches to renewable fuel synthesis exploit the integration of enzymes with synthetic materials for kinetically efficient fuel production. Here, a CO₂ reductase, formate dehydrogenase (FDH) from Desulfovibrio vulgaris Hildenborough, is interfaced with carbon nanotubes (CNTs) and amorphous carbon dots (a-CDs). Each carbon substrate, tailored for electro- and photocatalysis, is functionalized with positive (-NHMe₂⁺) and negative (-COO^-) chemical surface groups to understand and optimize the electrostatic effect of protein association and orientation on CO₂ reduction. Immobilization of FDH on positively charged CNT electrodes results in efficient and reversible electrochemical CO₂ reduction via direct electron transfer with >90% Faradaic efficiency and -250 μA cm^-2 at -0.6 V vs SHE (pH 6.7 and 25 °C) for formate production. In contrast, negatively charged CNTs only result in marginal currents with immobilized FDH. Quartz crystal microbalance analysis and attenuated total reflection infrared spectroscopy confirm the high binding affinity of active FDH to CNTs. FDH has subsequently been coupled to a-CDs, where the benefits of the positive charge (-NHMe₂⁺-terminated a-CDs) were translated to a functional CD-FDH hybrid photocatalyst. High rates of photocatalytic CO₂ reduction (turnover frequency: 3.5 × 10³ h^-1; AM 1.5G) with dl-dithiothreitol as the sacrificial electron donor were obtained after 6 h, providing benchmark rates for homogeneous photocatalytic CO₂ reduction with metal-free light absorbers. This work provides a rational basis to understand interfacial surface/enzyme interactions at electrodes and photosensitizers to guide improvements with catalytic biohybrid materials.

Control of carbon monoxide dehydrogenase orientation by site-specific immobilization enables direct electrical contact between enzyme cofactor and solid surface

Controlling the orientation of redox enzymes on electrode surfaces is essential in the development of direct electron transfer (DET)-based bioelectrocatalytic systems. The electron transfer (ET) distance varies according to the enzyme orientation when immobilized on an electrode surface, which influences the interfacial ET rate. We report control of the orientation of carbon monoxide dehydrogenase (CODH) as a model enzyme through the fusion of gold-binding peptide (gbp) at either the N- or the C-terminus, and at both termini to strengthen the binding interactions between the fusion enzyme and the gold surface. Key factors influenced by the gbp fusion site are described. Collectively, our data show that control of the CODH orientation on an electrode surface is achieved through the presence of dual tethering sites, which maintains the enzyme cofactor within a DET-available distance (<14 Å), thereby promoting DET at the enzyme-electrode interface.

Use of Protein Engineering to Elucidate Electron Transfer Pathways between Proteins and Electrodes

Herein, we review protein engineering tools for electron transfer enhancement and investigation in bioelectrochemical systems. We present recent studies in the field while focusing on how electron transfer investigation and measurements were performed and discuss the use of protein engineering to interpret electron transfer mechanisms.

The Structure of Bilirubin Oxidase from Bacillus pumilus Reveals a Unique Disulfide Bond for Site-Specific Direct Electron Transfer

Efficient oxygen-reducing biocatalysts are essential for the development of biofuel cells or photo-bioelectrochemical applications. Bilirubin oxidase (BOD) is a promising biocatalyst for oxygen reduction processes at neutral pH and low overpotentials. BOD has been extensively investigated over the last few decades. While the enzyme’s internal electron transfer process and methods to establish electrical communication with electrodes have been elucidated, a crystal structure of BOD from bacterial origin has never been determined. Here we present the first crystal structure of BOD from Bacillus pumilus (BpBOD) at 3.5 Å resolution. Overall, BpBOD shows high homology with the fungal enzymes; however, it holds a unique surface-exposed disulfide bond between Cys229 and Cys322 residues. We present methodologies to orient the T1 site towards the electrode by coupling the reduced disulfide bond with maleimide moiety on the electrodes. The developed configurations were further investigated and revealed improved direct electron transfer rates with the electrodes. The work presented here may contribute to the construction of rationally designed bioanodes or biocathode configurations that are based on redox-active enzymes.

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.

Electron Transfer to the Trinuclear Copper Cluster in Electrocatalysis by the Multicopper Oxidases

High-potential multicopper oxidases (MCOs) are excellent catalysts able to perform the oxygen reduction reaction (ORR) at remarkably low overpotentials. Moreover, MCOs are able to interact directly with the electrode surfaces via direct electron transfer (DET), that makes them the most commonly used electrocatalysts for oxygen reduction in biofuel cells. The central question in MCO electrocatalysis is whether the type 1 (T1) Cu is the primary electron acceptor site from the electrode, or whether electrons can be transferred directly to the trinuclear copper cluster (TNC), bypassing the rate-limiting intramolecular electron transfer step from the T1 site. Here, using site-directed mutagenesis and electrochemical methods combined with data modeling of electrode kinetics, we have found that there is no preferential superexchange pathway for DET to the T1 site. However, due to the high reorganization energy of the fully oxidized TNC, electron transfer from the electrode to the TNC does occur primarily through the T1 site. We have further demonstrated that the lower reorganization energy of the TNC in its two-electron reduced, alternative resting, form enables DET to the TNC, but this only occurs in the first turnover. This study provides insight into the factors that control the kinetics of electrocatalysis by the MCOs and a guide for the design of more efficient biocathodes for the ORR.

Robust, site-specifically immobilized phenylalanine ammonia-lyases for the enantioselective ammonia addition of cinnamic acids

Phenylalanine ammonia-lyases (PALs) catalyse the non-oxidative deamination of l-phenylalanine to trans-cinnamic acid, while in the presence of high ammonia concentration, the synthetically attractive reverse reaction occurs. Although they have been intensively studied, the wider application of PALs for the large scale synthesis of non-natural amino acids is still rather limited, mainly due to the decreased operational stability of PALs under the high ammonia concentration conditions of ammonia addition. Herein, we describe the development of a highly stable and active immobilized PAL-biocatalyst obtained through site-specific covalent immobilization onto single-walled carbon nanotubes (SWCNTs), employing maleimide/thiol coupling of engineered enzymes containing surficial Cys residues. The immobilization method afforded robust biocatalysts (by strong covalent attachment to the support) and allowed modulation of enzymatic activity (by proper selection of binding site, controlling the orientation of the enzyme attached to the support). The novel biocatalysts were investigated in PAL-catalyzed reactions, focusing on the synthetically challenging ammonia addition reaction. The optimization of the immobilization (enzyme load) and reaction conditions (substrate : biocatalyst ratio, ammonia source, reaction temperature) involving the best performing biocatalyst SWCNTNH2 -SS-PcPAL was performed. The biocatalyst, under the optimal reaction conditions, showed high catalytic efficiency, providing excellent conversion (c ∼90% in 10 h) of cinnamic acid into l-Phe, and more importantly, possesses high operational stability, maintaining its high efficiency over >7 reaction cycles. Moreover, the site-specifically immobilized PcPAL L134A/S614C and PcPAL I460V/S614C variants were successfully applied in the synthesis of several l-phenylalanine analogues of high synthetic value, providing perspectives for the efficient replacement of classical synthetic methods for l-phenylalanines with a mild, selective and eco-friendly enzymatic alternative.

Self-Powered Detection of Glucose by Enzymatic Glucose/Oxygen Fuel Cells on Printed Circuit Boards

Monitoring glucose levels in physiological fluids can help prevent severe complications associated with hypo- and hyper-glycemic events. Current glucose-monitoring systems require a three-electrode setup and a power source to function, which can hamper the system miniaturization to the patient discomfort. Enzymatic fuel cells (EFCs) offer the opportunity to develop self-powered and minimally invasive glucose sensors by eliminating the need for an external power source. Nevertheless, practical applications demand for cost-effective and mass-manufacturable EFCs compatible with integration strategies. In this study, we explore for the first time the use of gold electrodes on a printed circuit board (PCB) for the development of an EFC and demonstrate its application in saliva. To increase the specific surface area, the PCB gold-plated electrodes were modified with porous gold films. At the anode, glucose oxidase is immobilized with an osmium redox polymer that serves as an electron-transfer mediator. At the cathode, bilirubin oxidase is adsorbed onto the porous gold surface with a blocking agent that prevents parasitic reactions while maintaining the enzyme catalytic activity. The resulting EFC showed a linear response to glucose in phosphate buffer within the range 50 μM to 1 mM, with a sensitivity of 14.13 μA cm-2 mM-1. The sensor was further characterized in saliva, showing the linear range of detection of 0.75 to 2 mM, which is within the physiological range, and sensitivity of 21.5 μA cm-2 mM-1. Overall, this work demonstrates that PCBs are suitable platforms for EFCs, paving the way for the development of fully integrated systems in a seamless and miniaturized device.

From Enzyme Stability to Enzymatic Bioelectrode Stabilization Processes

Bioelectrocatalysis using redox enzymes appears as a sustainable way for biosensing, electricity production, or biosynthesis of fine products. Despite advances in the knowledge of parameters that drive the efficiency of enzymatic electrocatalysis, the weak stability of bioelectrodes prevents large scale development of bioelectrocatalysis. In this review, starting from the understanding of the parameters that drive protein instability, we will discuss the main strategies available to improve all enzyme stability, including use of chemicals, protein engineering and immobilization. Considering in a second step the additional requirements for use of redox enzymes, we will evaluate how far these general strategies can be applied to bioelectrocatalysis.

Direct Electrochemical Enzyme Electron Transfer on Electrodes Modified by Self-Assembled Molecular Monolayers

Self-assembled molecular monolayers (SAMs) have long been recognized as crucial “bridges” between redox enzymes and solid electrode surfaces, on which the enzymes undergo direct electron transfer (DET)—for example, in enzymatic biofuel cells (EBFCs) and biosensors. SAMs possess a wide range of terminal groups that enable productive enzyme adsorption and fine-tuning in favorable orientations on the electrode. The tunneling distance and SAM chain length, and the contacting terminal SAM groups, are the most significant controlling factors in DET-type bioelectrocatalysis. In particular, SAM-modified nanostructured electrode materials have recently been extensively explored to improve the catalytic activity and stability of redox proteins immobilized on electrochemical surfaces. In this report, we present an overview of recent investigations of electrochemical enzyme DET processes on SAMs with a focus on single-crystal and nanoporous gold electrodes. Specifically, we consider the preparation and characterization methods of SAMs, as well as SAM applications in promoting interfacial electrochemical electron transfer of redox proteins and enzymes. The strategic selection of SAMs to accord with the properties of the core redox protein/enzymes is also highlighted.

Rational Surface Modification of Carbon Nanomaterials for Improved Direct Electron Transfer-Type Bioelectrocatalysis of Redox Enzymes

Interfacial electron transfer between redox enzymes and electrodes is a key step for enzymatic bioelectrocatalysis in various bioelectrochemical devices. Although the use of carbon nanomaterials enables an increasing number of redox enzymes to carry out bioelectrocatalysis involving direct electron transfer (DET), the role of carbon nanomaterials in interfacial electron transfer remains unclear. Based on the recent progress reported in the literature, in this mini review, the significance of carbon nanomaterials on DET-type bioelectrocatalysis is discussed. Strategies for the oriented immobilization of redox enzymes in rationally modified carbon nanomaterials are also summarized and discussed. Furthermore, techniques to probe redox enzymes in carbon nanomaterials are introduced.

Bilirubin oxidase oriented on novel type three-dimensional biocathodes with reduced graphene aggregation for biocathode

Aggregation of reduced graphene oxide (RGO) due to π-π stacking is a recurrent problem in graphene-based electrochemistry, decreasing the effective working area and therefore the performance of the RGO electrodes. Dispersing RGO on three-dimensional (3D) carbon paper electrodes is one strategy towards overcoming this challenge, with partial relief aggregation. In this report, we describe the grafting of negatively charged 4-aminobenzoic acid (4-ABA) onto a graphene functionalized carbon paper electrode surface. 4-ABA functionalization induces separation of the RGO layers, at the same time leading to favorable orientation of the blue multi-copper enzyme Myrothecium verrucaria bilirubin oxidase (MvBOD) for direct electron transfer (DET) in the dioxygen reduction reaction (ORR) at neutral pH. Simultaneous electroreduction of graphene oxide to RGO and covalent attachment of 4-ABA are achieved by applying alternating cathodic and anodic electrochemical potential pulses, leading to a high catalytic current density (Δj_cat:193 ± 4 μA cm^-2) under static conditions. Electrochemically grafted 4-ABA not only leads to a favorable orientation of BOD as validated by fitting a kinetic model to the electrocatalytic data, but also acts to alleviate RGO aggregation as disclosed by scanning electron microscopy, most likely due to the electrostatic repulsion between 4-ABA-grafted graphene layers. With a half-lifetime of 55 h, the bioelectrode also shows the highest operational stability for DET-type MvBOD-based bioelectrodes reported to date. The bioelectrode was finally shown to work well as a biocathode of a membrane-less glucose/O₂ enzymatic biofuel cell with a maximum power density of 22 μW cm^-2 and an open circuit voltage of 0.51 V.

Nanostructured pencil graphite electrodes for application as high power biocathodes in miniaturized biofuel cells and bio-batteries

This work describes a simple method for the fabrication of an enzymatic electrode with high sensitivity to oxygen and good performance when applied as biocathode. Pencil graphite electrodes (PGE) were chosen as disposable transducers given their availability and good electrochemical response. After electrochemical characterization regarding hardness and surface pre-treatment suited modification with carbon-based nanostructures, namely with reduced graphene, MWCNT and carbon black for optimal performance was proceeded. The bioelectrode was finally assembled through immobilization of bilirubin oxidase (BOx) lashed on the modified surface of MWCNT via π-π stacking and amide bond functionalization. The high sensitivity towards dissolved oxygen of 648 ± 51 µA mM-1 cm-2, and a LOD of 1.7 µM, was achieved for the PGE with surface previously modified with reduced graphene (rGO), almost the double registered for direct anchorage on the bare PGE surface. Polarization curves resulted in an open circuit potential (OCP) of 1.68 V (vs Zn electrode) and generated a maximum current density of about 650 μA cm-2 in O2 saturated solution.

Photoelectrochemically-assisted biofuel cell constructed by redox complex and g-C3N4 coated MWCNT bioanode

Herein, we report a membraneless glucose and air photoelectrochemical biofuel cell (PBFC) with a visible light assisted photobioanode. Flavin adenine dinucleotide dependent glucose dehydrogenase (FADGDH) was immobilized on the combined photobioanode for the visible light assisted glucose oxidation (GCE|MWCNT|g-C₃N₄|Ru-complex|FADGDH) with a quinone mediated electron transfer. Bilirubine oxidase (BOx) immobilized on MWCNT coated GCE (GCE|BOx) was used as the cathode with direct electron transfer (DET). An improvement of biocatalytic oxidation current was observed by 6.2% due in part to the light-driven electron-transfer. The large oxidation currents are probably owing to the good contacting of the immobilized enzymes with the electrode material and the utilization of light assisted process. Under the visible light, the photobioanode shows an anodic photocurrent of 1.95 μA cm² at attractively low potentials viz. -0.4 vs Ag/AgCl. The lower-lying conduction band of g-C₃N₄ as compared to Ru-complexes decreases the rate of hole and electron recombination and enhances the charge transportation. The bioanode shows maximum current density for glucose oxidation up to 6.78 μA cm^-2 at 0.2 V vs Ag/AgCl at pH:7. The performance of three promising Ru-complexes differing in chemical and redox properties were compared as electron mediators for FADGDH. Upon illumination, the PBFC delivered a maximum power density of 28.5 ± 0.10 μW cm^-2 at a cell voltage of +0.4 V with an open circuit voltage of 0.64 V.

Insight into multicopper oxidase laccase from Myrothecium verrucaria ITCC-8447: a case study using in silico and experimental analysis

The oxidation activity of multicopper-oxidases overlaps with different substrates of laccases and bilirubin oxidases, thus in the present study an integrated approach of bioinformatics using homology modeling, docking, and experimental validation was used to confirm the type of multicopper-oxidase in Myrothecium verrucaria ITCC-8447. The result of peptide sequence of M. verrucaria ITCC-8447 enabled to predict the 3 D-structure of multicopper-oxidase. It was overlapped with the structure of laccase and root mean square deviation (RMSD) was 1.53 Å for 533 and, 171 residues. The low binding energy with azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) (-5.64) as compared to bilirubin (-4.39) suggested that M. verrucaria ITCC-8447 have laccase-like activity. The experimental analysis confirmed high activity with laccase specific substrates, phenol (18.3 U/L), ampyrone (172.4 U/L) and, ampyrone phenol coupling (50 U/L) as compared to bilirubin oxidase substrate bilirubin (16.6 U/L). In addition, lowest binding energy with ABTS (-5.64), syringaldazine SYZ (-4.83), guaiacol GCL (-4.42), and 2,6-dimethoxyphenol DMP (-4.41) confirmed the presence of laccase. Further, complete remediation of two hazardous model pollutants i.e., phenol and resorcinol (1.5 mM) after 12 h of incubation and low binding energy of -4.32 and, -4.85 respectively confirmed its removal by laccase. The results confirmed the presence of laccase in M. verrucaria ITCC-8447 and its effective bioremediation potential.

Enzyme-Based Biosensors: Tackling Electron Transfer Issues

This review summarizes the fundamentals of the phenomenon of electron transfer (ET) reactions occurring in redox enzymes that were widely employed for the development of electroanalytical devices, like biosensors, and enzymatic fuel cells (EFCs). A brief introduction on the ET observed in proteins/enzymes and its paradigms (e.g., classification of ET mechanisms, maximal distance at which is observed direct electron transfer, etc.) are given. Moreover, the theoretical aspects related to direct electron transfer (DET) are resumed as a guideline for newcomers to the field. Snapshots on the ET theory formulated by Rudolph A. Marcus and on the mathematical model used to calculate the ET rate constant formulated by Laviron are provided. Particular attention is devoted to the case of glucose oxidase (GOx) that has been erroneously classified as an enzyme able to transfer electrons directly. Thereafter, all tools available to investigate ET issues are reported addressing the discussions toward the development of new methodology to tackle ET issues. In conclusion, the trends toward upcoming practical applications are suggested as well as some directions in fundamental studies of bioelectrochemistry.

Proteins-Based Nanocatalysts for Energy Conversion Reactions

In recent years, the incorporation of molecular enzymes into nanostructured frameworks to create efficient energy conversion biomaterials has gained increasing interest as a promising strategy owing to both the dynamic behavior of proteins for their electrocatalytic function and the unique properties of the synergistic interactions between proteins and nanosized materials. Herein, we review the impact of proteins on energy conversion fields and the contribution of proteins to the improved activity of the resulting nanocomposites. We address different strategies to fabricate protein-based nanocatalysts as well as current knowledge on the structure-function relationships of enzymes during the catalytic processes. Additionally, a comprehensive review of state-of-the-art bioelectrocatalytic materials for water-splitting reactions such as hydrogen evolution reaction (HER) and oxygen evolution reactions (OER) is afforded. Finally, we briefly envision opportunities to develop a new generation of electrocatalysts towards the electrochemical reduction of N₂ to NH₃ using theoretical tools to built nature-inspired nitrogen reduction reaction catalysts.

Bismuth Vanadate/Bilirubin Oxidase Photo(bio)electrochemical Cells for Unbiased, Light-Triggered Electrical Power Generation

The construction of bias- and donor-free photobioelectrochemical cells for the generation of light-triggered electrical power is presented. The developed oxygen reduction biocathodes are based on bilirubin oxidase (BOD) that originates from Myrothecium verrucaria (MvBOD) and a thermophilic Bacillus pumilus (BpBOD). Methods to entrap the BOD with 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) redox molecules in a polydopamine layer are presented. A pH-independent, positively charged pyrenebetaine linker was synthesized, utilized, and led to a threefold improvement to the bioelectrocatalytic current. Both the developed polydopamine/ABTS/MvBOD and the pyrenebetaine/BpBOD biocathodes were further coupled with BiVO₄ /cobalt phosphate water-oxidation photoanodes to construct biotic/abiotic photobioelectrochemical cells, which generated power outputs of 0.74 and 0.85 mW cm^-2 , respectively. The presented methods are versatile, show the strength of biotic/abiotic hybrids, and can be further used to couple different redox enzymes with electrodes.

Development of graphene-based enzymatic biofuel cells: A minireview

Enzymatic biofuel cells (EBFCs) have attracted increasing attention due to their potential to harvest energy from a wide range of fuels under mild conditions. Fabrication of effective bioelectrodes is essential for the practical application of EBFCs. Graphene possesses unique physiochemical properties making it an attractive material for the construction of EBFCs. Despite these promising properties, graphene has not been used for EBFCs as frequently as carbon nanotubes, another nanoscale carbon allotrope. This review focuses on current research progress in graphene-based electrodes, including electrodes modified with graphene derivatives and graphene composites, as well as free-standing graphene electrodes. Particular features of graphene-based electrodes such as high conductivity, mechanical flexibility and high porosity for bioelectrochemical applications are highlighted. Reports on graphene-based EBFCs from the last five years are summarized, and perspectives for graphene-based EBFCs are offered.

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.

Inhibition in multicopper oxidases: a critical review

This review critiques the literature on inhibition of O₂-reduction catalysis in multicopper oxidases like laccase and bilirubin oxidase and provide recommendations for best practice when carrying out experiments and interpreting published data.

DNA Release from a Modified Electrode Triggered by a Bioelectrocatalytic Process

DNA release from an electrode surface was stimulated by application of a mild electrical potential (0 V vs Ag/AgCl). The release process was activated by interfacial pH increase originating from H⁺ consumption during O₂ reduction bio-electrocatalyzed by bilirubin oxidase immobilized at the electrode surface. The pH increase resulted in a change of the electrical charge from positive to negative at the surface of SiO₂ nanoparticles (200 nm) associated with the electrode surface and functionalized with trigonelline and boronic acid. While the negatively charged DNA molecules were electrostatically bound to the positively charged surface, the negative charge produced upon O₂ reduction resulted in the DNA repulsion and release from the modified interface. The small electrical potential for O₂ reduction resulting in the interface recharge was allowed due to the bio-electrocatalysis using bilirubin oxidase enzyme. While, in the first set of experiments, the potential was applied on the modified electrode from an electrochemical instrument, later it was generated in situ by biocatalytic or photo-biocatalytic processes at a connected electrode. A multistep biocatalytic cascade generating NADH or photosynthetic process in thylakoid membranes was used to produce in situ a small potential to stimulate the DNA release catalyzed by bilirubin oxidase. The designed system can be used for different release processes triggered by various signals (electrical, biomolecular, and light signals, etc.), thus representing a general interfacial platform for the controlled release of different biomolecules and nanosize species.

Tackling the Challenges of Enzymatic (Bio)Fuel Cells

The ever-increasing demands for clean and sustainable energy sources combined with rapid advances in biointegrated portable or implantable electronic devices have stimulated intensive research activities in enzymatic (bio)fuel cells (EFCs). The use of renewable biocatalysts, the utilization of abundant green, safe, and high energy density fuels, together with the capability of working at modest and biocompatible conditions make EFCs promising as next generation alternative power sources. However, the main challenges (low energy density, relatively low power density, poor operational stability, and limited voltage output) hinder future applications of EFCs. This review aims at exploring the underlying mechanism of EFCs and providing possible practical strategies, methodologies and insights to tackle these issues. First, this review summarizes approaches in achieving high energy densities in EFCs, particularly, employing enzyme cascades for the deep/complete oxidation of fuels. Second, strategies for increasing power densities in EFCs, including increasing enzyme activities, facilitating electron transfers, employing nanomaterials, and designing more efficient enzyme-electrode interfaces, are described. The potential of EFCs/(super)capacitor combination is discussed. Third, the review evaluates a range of strategies for improving the stability of EFCs, including the use of different enzyme immobilization approaches, tuning enzyme properties, designing protective matrixes, and using microbial surface displaying enzymes. Fourth, approaches for the improvement of the cell voltage of EFCs are highlighted. Finally, future developments and a prospective on EFCs are envisioned.