Tackling the Challenges of Enzymatic (Bio)Fuel Cells

Implementation of π-π interaction in AuNPs@GDY to boost the bioelectrocatalysis in enzymatic biofuel cells

The main challenges (sluggish electron transfer, low energy density) hinder the future application of enzymatic biofuel cells (EBFCs), which urgent to take effective measures to solve these issues. In this work, a composite of Au nanoparticles decorated graphdiyne (AuNPs@GDY) is fabricated and employed as the carrier of enzyme (G6PDH), and a mechanism based on π-π interaction of electron transfer is proposed to understand bioelectrocatalysis processes. The results show that the AuNPs@GDY composite exhibits the highest current density among the three materials (GDY, AuNPs, and AuNPs@GDY), which is 3.4 times higher than that of GDY and 2.5 times higher than that of AuNPs. Furthermore, the results reveal that the AuNPs could increase the loading of enzymes and provide more active site for reaction, while GDY provides highly π-conjugated structure and unique sp/sp2-hybridized linkages interface. This work provides new insights to explore a theoretical basis for the development of more efficient bioelectrocatalytic systems.

Development of the BioBattery: A novel enzyme fuel cell using a multicopper oxidase as an anodic enzyme

This work presents the development of an enzyme fuel cell, termed "BioBattery", that utilizes multicopper oxidases as the anodic enzyme in a non-diffusion limited system. We evaluated various enzyme variants as the anode, including multicopper oxidase from Pyrobaculum aerophilum, laccase from Trametes versicolor, and bilirubin oxidase from Myrothecium verrucaria. Several combinations of cathodes were also examined, focusing on the reduction of oxygen as the primary electron acceptor. The optimal pairing used multicopper oxidase from Pyrobaculum aerophilum as the anode and amine reactive phenazine ethosulfate modified bovine serum albumin as the cathode. BioBattery was integrated with our previously developed BioCapacitor, proving capable of consistently powering a 470 μF capacitor, positioning it as a modular power source for wearable and implantable systems. This research work addresses and overcomes some of the fundamental limitations seen in enzyme fuel cells, where power and current are often limited by substrate accessibility to the active electrode surface. (152 words).

Microbial electrosynthesis of valuable chemicals from the reduction of CO2: a review

The present energy demand of the world is increasing but the fossil fuels are gradually depleting. As a result, the need for alternative fuels and energy sources is growing. Fuel cells could be one alternative to address the challenge. The fuel cell can convert CO2 to value-added chemicals. The potential of bio-fuel cells, specifically enzymatic fuel cells and microbial fuel cells, and the importance of immobilization technology in bio-fuel cells are highlighted. The review paper also includes a detailed explanation of the microbial electrosynthesis system to reduce CO2 and the value-added products during microbial electrosynthesis. Future research in bio-electrochemical synthesis for CO2 conversion is expected to prioritize enhancing biocatalyst efficiency, refining reactor design, exploring novel electrode materials, understanding microbial interactions, integrating renewable energy sources, and investigating electrochemical processes for carbon capture and selective CO2 reduction. The challenges and perspectives of bio-electrochemical systems in the application of CO2 conversion are also discussed. Overall, this review paper provides valuable insights into the latest developments and criteria for effective research and implementation in bio-fuel cells, immobilization technology, and microbial electro-synthesis systems.

DNA-induced assembly of biocatalytic nanocompartments for sensitive and selective aptasensing of aflatoxin B1

Enzyme cascade with high specificity and catalytic efficiency has significant applications for developing efficient bioanalysis methods. In this work, a sensitive and selective aptasensor was constructed based on the DNA-induced assembly of biocatalytic nanocompartments. Different from the conventional co-immobilization in one pot, the cascade enzymes of glucose oxidase (GOX) and horseradish peroxidase (HRP) were separately encapsulated in ZIF-90 nanoparticles. After conjugating complementary DNA or aptermer on enzyme@ZIF-90, DNA hybridization drove enzyme@ZIF-90 connected into clusters or linked on other DNA modified biocatalytic nanocompartment (such as invertase loaded Fe3O4@SiO2). Owing to the shortened distance between enzymes, the catalytic efficiency of connected clusters was significantly enhanced. However, the specifically interaction between the substrate molecule and aptermer sequence would lead to the disassembly of DNA duplexes, resulting in the gradual "switching-off" of cascade reactions. With aflatoxin B1 (AFB1) as the model substrate, the compartmentalized three-enzyme nanoreactors showed good analytical performance in the linear range from 0.01 ng mL-1 to 50 ng mL-1 with a low detection limit (3.3 pg mL-1). In addition, the proposed aptasensor was applied to detect AFB1 in corn oil and wheat powder samples with total recoveries ranging from 94 % to 109 %. As a result, this DNA-induced strategy for enzyme cascade nanoreactors opens new avenues for stimuli-responsive applications in biosensing.

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.

Enzymatic biofuel cell: A potential power source for self-sustained smart textiles

Self-sustained smart textiles require a miniaturized and flexible power source, while the state-of-the-art lithium-ion battery cannot be seamlessly integrated into smart textiles. Enzymatic biofuel cells (EBFC), utilizing physiological glucose or lactate as fuels to convert chemical energy into electricity, are a potential alternative power source. In comparison to other proposed energy harvesters relying on solar and biomechanical energy, EBFCs feature several key properties, including continuous power generation, biocompatible interfaces without using toxic elements, simple configuration without extra packaging, and biodegradability. There is an urgent need to introduce EBFCs to the researchers working on smart textiles, who typically are not expert on bioelectrochemistry. This minireview first introduces the working principle of EBFC and then summarizes its recent progress on fibers, yarns, and textiles. It's expected that this review can help to bridge the knowledge gap and provide the community of smart textiles with information on both the strengths and limitations of EBFCs.

Electrochemical sensing fibers for wearable health monitoring devices

Real-time monitoring of health conditions is an emerging strong issue in health care, internet information, and other strongly evolving areas. Wearable electronics are versatile platforms for non-invasive sensing. Among a variety of wearable device principles, fiber electronics represent cutting-edge development of flexible electronics. Enabled by electrochemical sensing, fiber electronics have found a wide range of applications, providing new opportunities for real-time monitoring of health conditions by daily wearing, and electrochemical fiber sensors as explored in the present report are a promising emerging field. In consideration of the key challenges and corresponding solutions for electrochemical sensing fibers, we offer here a timely and comprehensive review. We discuss the principles and advantages of electrochemical sensing fibers and fabrics. Our review also highlights the importance of electrochemical sensing fibers in the fabrication of "smart" fabric designs, focusing on strategies to address key issues in fiber-based electrochemical sensors, and we provide an overview of smart clothing systems and their cutting-edge applications in therapeutic care. Our report offers a comprehensive overview of current developments in electrochemical sensing fibers to researchers in the fields of wearables, flexible electronics, and electrochemical sensing, stimulating forthcoming development of next-generation "smart" fabrics-based electrochemical sensing.

Copper Nanoclusters as Multienzymes Mimic Activities of Oxidase and Ascorbic Acid Oxidase in the Presence of Imidazole

Artificial nanoenzymes based on metal nanoclusters have received great attention for multienzyme activities nowadays. In this work, pepsin-capped copper NCs (Cu-Pep NCs) are used as oxidase, ascorbic acid oxidase (AAO), and peroxidase mimics, and their activities are enhanced by the introduction of imidazole. The oxidase activity increased almost 7.5-fold, while 5-fold and 2-fold increases were observed for the peroxidase and AAO-like activity, respectively. The enhanced radical formation in the presence of imidazole moieties facilitates the enzymatic activity of the Cu-Pep-NCs/Imid system. This work describes the different enzymatic activities of the NCs, paving a new way for artificial nanoenzymes having enhanced activities.

Research Progress in Enzyme Biofuel Cells Modified Using Nanomaterials and Their Implementation as Self-Powered Sensors

Enzyme biofuel cells (EBFCs) can convert chemical or biochemical energy in fuel into electrical energy, and therefore have received widespread attention. EBFCs have advantages that traditional fuel cells cannot match, such as a wide range of fuel sources, environmental friendliness, and mild reaction conditions. At present, research on EBFCs mainly focuses on two aspects: one is the use of nanomaterials with excellent properties to construct high-performance EBFCs, and the other is self-powered sensors based on EBFCs. This article reviews the applied nanomaterials based on the working principle of EBFCs, analyzes the design ideas of self-powered sensors based on enzyme biofuel cells, and looks forward to their future research directions and application prospects. This article also points out the key properties of nanomaterials in EBFCs, such as electronic conductivity, biocompatibility, and catalytic activity. And the research on EBFCs is classified according to different research goals, such as improving battery efficiency, expanding the fuel range, and achieving self-powered sensors.

Virtual Model Compound Approach for Calculating Redox Potentials of [Fe2S2]-Cys4 Centers in Proteins - Structure Quality Matters

The midpoint potential of the [Fe2S2]-Cys4-cluster in proteins is known to vary between -200 and -450 mV. This variation is caused by the different electrostatic environment of the cluster in the respective proteins. Continuum electrostatics can quantify the impact of the protein environment on the redox potential. Thus, if the redox potential of a [Fe2S2]-Cys4-cluster model compound in aqueous solution would be known, then redox potentials in various protein complexes could be calculated. However, [Fe2S2]-Cys4-cluster models are not water-soluble, and thus, their redox potential can not be measured in aqueous solution. To overcome this problem, we introduce a method that we call Virtual Model Compound Approach (VMCA) to extrapolate the model redox potential from known redox potentials of proteins. We carefully selected high-resolution structures for our analysis and divide them into a fit set, for fitting the model redox potential, and an independent test set, to check the validity of the model redox potential. However, from our analysis, we realized that the some structures can not be used as downloaded from the PDB but had to be re-refined in order to calculate reliable redox potentials. Because of the re-refinement, we were able to significantly reduce the standard deviation of our derived model redox potential for the [Fe2S2]-Cys4-cluster from 31 mV to 10 mV. As the model redox potential, we obtained -184 mV. This model redox potential can be used to analyze the redox behavior of [Fe2S2]-Cys4-clusters in larger protein complexes.

Enzyme Cascade Electrode Reactions with Nanomaterials and Their Applicability towards Biosensor and Biofuel Cells

Nanomaterials, including carbon nanotubes, graphene oxide, metal-organic frameworks, metal nanoparticles, and porous carbon, play a crucial role as efficient carriers to enhance enzyme activity through substrate channeling while improving enzyme stability and reusability. However, there are significant debates surrounding aspects such as enzyme orientation, enzyme loading, retention of enzyme activity, and immobilization techniques. Consequently, these subjects have become the focus of intensive research in the realm of multi-enzyme cascade reactions. Researchers have undertaken the challenge of creating functional in vitro multi-enzyme systems, drawing inspiration from natural multi-enzyme processes within living organisms. Substantial progress has been achieved in designing multi-step reactions that harness the synthetic capabilities of various enzymes, particularly in applications such as biomarker detection (e.g., biosensors) and the development of biofuel cells. This review provides an overview of recent developments in concurrent and sequential approaches involving two or more enzymes in sequence. It delves into the intricacies of multi-enzyme cascade reactions conducted on nanostructured electrodes, addressing both the challenges encountered and the innovative solutions devised in this field.

Biofluid-Activated Biofuel Cells, Batteries, and Supercapacitors: A Comprehensive Review

Recent developments in wearable and implanted devices have resulted in numerous, unprecedented capabilities that generate increasingly detailed information about a user's health or provide targeted therapy. However, options for powering such systems remain limited to conventional batteries which are large and have toxic components and as such are not suitable for close integration with the human body. This work provides an in-depth overview of biofluid-activated electrochemical energy devices, an emerging class of energy sources judiciously designed for biomedical applications. These unconventional energy devices are composed of biocompatible materials that harness the inherent chemistries of various biofluids to produce useable electrical energy. This work covers examples of such biofluid-activated energy devices in the form of biofuel cells, batteries, and supercapacitors. Advances in materials, design engineering, and biotechnology that form the basis for high-performance, biofluid-activated energy devices are discussed. Innovations in hybrid manufacturing and heterogeneous integration of device components to maximize power output are also included. Finally, key challenges and future scopes of this nascent field are provided.

A Mediator-Free Multi-Ply Biofuel Cell Using an Interfacial Assembly between Hydrophilic Enzymes and Hydrophobic Conductive Oxide Nanoparticles with Pointed Apexes

Biofuel cells (BFCs) based on enzymatic electrodes hold great promise as power sources for biomedical devices. However, their practical use is hindered by low electron transfer efficiency and poor operational stability of enzymatic electrodes. Here, a novel mediator-free multi-ply BFC that overcomes these limitations and exhibits both substantially high-power output and long-term operational stability is presented. The approach involves the utilization of interfacial interaction-induced assembly between hydrophilic glucose oxidase (GOx) and hydrophobic conductive indium tin oxide nanoparticles (ITO NPs) with distinctive shapes, along with a multi-ply electrode system. For the preparation of the anode, GOx and oleylamine-stabilized ITO NPs with bipod/tripod type are covalently assembled onto the host fiber electrode composed of multi-walled carbon nanotubes and gold (Au) NPs. Remarkably, despite the contrasting hydrophilic and hydrophobic properties, this interfacial assembly approach allows for the formation of nanoblended GOx/ITO NP film, enabling efficient electron transfer within the anode. Additionally, the cathode is prepared by sputtering Pt onto the host electrode. Furthermore, the multi-ply fiber electrode system exhibits unprecedented high-power output (≈10.4 mW cm-2 ) and excellent operational stability (2.1 mW cm-2 , ≈49% after 60 days of continuous operation). The approach can provide a basis for the development of high-performance BFCs.

Recent advances of biosensors on microneedles

Biosensors have attracted a considerable attention in recent years due to their enormous potential to provide insights into the physical condition of individuals. However, the widespread use of biosensors has experienced difficulties regarding the stability of the biological response and the poor miniaturization and portability of biosensors. Hence, there is an urgent need for more reliable biosensor devices. Microneedle (MN) technology has become a revolutionary approach to biosensing strategies, setting new horizons for improving existing biosensors. MN-based biosensors allow for painless injection, and in situ extraction or monitoring. However, the accuracy and practicality of detection need to be improved. This review begins by discussing the classification of MNs, manufacturing methods and other design parameters to develop a more accurate MN-based detection sensing system. Herein, we categorize and analyze the energy supply of wearable biosensors. Specifically, we describe the detection methods of MN biosensors, such as electrochemical, optical, nucleic acid recognition and immunoassays, and how MNs can be combined with these methods to detect biomarkers. Furthermore, we provide a detailed overview of the latest applications (drug release, drug detection, etc.). The MN-based biosensors are followed by a summary of key challenges and opportunities in the field.

Developing high-affinity, oxygen-insensitive [NiFe]-hydrogenases as biocatalysts for energy conversion

The splitting of hydrogen (H2) is an energy-yielding process, which is important for both biological systems and as a means of providing green energy. In biology, this reaction is mediated by enzymes called hydrogenases, which utilise complex nickel and iron cofactors to split H2 and transfer the resulting electrons to an electron-acceptor. These [NiFe]-hydrogenases have received considerable attention as catalysts in fuel cells, which utilise H2 to produce electrical current. [NiFe]-hydrogenases are a promising alternative to the platinum-based catalysts that currently predominate in fuel cells due to the abundance of nickel and iron, and the resistance of some family members to inhibition by gases, including carbon monoxide, which rapidly poison platinum-based catalysts. However, the majority of characterised [NiFe]-hydrogenases are inhibited by oxygen (O2), limiting their activity and stability. We recently reported the isolation and characterisation of the [NiFe]-hydrogenase Huc from Mycobacterium smegmatis, which is insensitive to inhibition by O2 and has an extremely high affinity, making it capable of oxidising H2 in air to below atmospheric concentrations. These properties make Huc a promising candidate for the development of enzyme-based fuel cells (EBFCs), which utilise H2 at low concentrations and in impure gas mixtures. In this review, we aim to provide context for the use of Huc for this purpose by discussing the advantages of [NiFe]-hydrogenases as catalysts and their deployment in fuel cells. We also address the challenges associated with using [NiFe]-hydrogenases for this purpose, and how these might be overcome to develop EBFCs that can be deployed at scale.

Surfactant-Free Formate/O2 Biofuel Cell with Electropolymerized Phenothiazine Derivative-Modified Enzymatic Bioanode

A formate (HCOO-) bioanode was developed by utilizing a phenothiazine-based electropolymerized layer deposited on sucrose-derived carbon. The electrode modified with NAD-dependent formate dehydrogenase and the electropolymerized layer synergistically catalyzed the oxidation of the coenzyme (NADH) and fuel (HCOO-) to achieve efficient electron transfer. Further, the replacement of carbon nanotubes with water-dispersible sucrose-derived carbon used as the electrode base allowed the fabrication of a surfactant-free bioanode delivering a maximum current density of 1.96 mA cm-2 in the fuel solution. Finally, a separator- and surfactant-free HCOO-/O2 biofuel cell featuring the above bioanode and a gas-diffusion biocathode modified with bilirubin oxidase and 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) was fabricated, delivering a maximum power density of 70 μW cm-2 (at 0.24 V) and an open-circuit voltage of 0.59 V. Thus, this study demonstrates the potential of formic acid as a fuel and possibilities for the application of carbon materials in bioanodes.

Conductive Polymers and Their Nanocomposites: Application Features in Biosensors and Biofuel Cells

Conductive polymers and their composites are excellent materials for coupling biological materials and electrodes in bioelectrochemical systems. It is assumed that their relevance and introduction to the field of bioelectrochemical devices will only grow due to their tunable conductivity, easy modification, and biocompatibility. This review analyzes the main trends and trends in the development of the methodology for the application of conductive polymers and their use in biosensors and biofuel elements, as well as describes their future prospects. Approaches to the synthesis of such materials and the peculiarities of obtaining their nanocomposites are presented. Special emphasis is placed on the features of the interfaces of such materials with biological objects.

Nanozymatic Biofuel Cell-Enabled Self-Powered Sensing System for a Sensitive Immunoassay

Self-powered sensing system (SPSS) integrating the enzymatic biofuel cell and biosensing platform has attracted tremendous interest. However, natural enzymes suffer from the intrinsic drawbacks of enzymes and enzymatic proteins. Nanozymes with enzyme-like activities are the ideal alternatives to enzymes, and it is greatly challenging to explore high-performance nanozymatic biofuel cell for SPSS. Herein, the advanced nanozymatic biofuel cell-enabled SPSS is developed for the sensitive detection of the prostate-specific antigen (PSA), where Ir single atoms supported by nitrogen-doped carbon and Au nanozymes serve as the cathode and anode, respectively. Based on the excellent electrochemical activity and stability, the resultant nanozymatic biofuel cell exhibits a higher power output and open-circuit potential than the Pt/C-based counterpart, which is beneficial for the application of SPSS. As a proof of concept, the nanozymatic biofuel cell-enabled SPSS shows a wide detection range of 0.2-500 ng mL-1 with a detection limit of 62 pg mL-1 for PSA, which provides new insight into broadening the application scenarios of nanozymes.

Stimuli-Responsive DNA-Based Hydrogels on Surfaces for Switchable Bioelectrocatalysis and Controlled Release of Loads

The assembly of enzyme [glucose oxidase (GOx)]-loaded stimuli-responsive DNA-based hydrogels on electrode surfaces, and the triggered control over the stiffness of the hydrogels, provides a means to switch the bioelectrocatalytic functions of the hydrogels. One system includes the assembly of GOx-loaded, pH-responsive, hydrogel matrices cross-linked by two cooperative nucleic acid motives comprising permanent duplex nucleic acids and "caged" i-motif pH-responsive duplexes. Bioelectrocatalyzed oxidation of glucose leads to the formation of gluconic acid that acidifies the hydrogel resulting in the separation of the i-motif constituents and lowering the hydrogel stiffness. Loading of the hydrogel matrices with insulin results in the potential-triggered, glucose concentration-controlled, switchable release of insulin from the hydrogel-modified electrodes. The switchable bioelectrocatalyzed release of insulin is demonstrated in the presence of ferrocenemethanol as a diffusional electron mediator or by applying an electrically wired integrated matrix that includes ferrocenyl-modified GOx embedded in the hydrogel. The second GOx-loaded, stimuli-responsive, DNA-based hydrogel matrix associated with the electrode includes a polyacrylamide hydrogel cooperatively cross-linked by duplex nucleic acids and "caged" G-quadruplex-responsive duplexes. The hydrogel matrix undergoes K+-ions/crown ether-triggered stiffness changes by the cyclic K+-ion-stimulated formation of G-quadruplexes (lower stiffness) and the crown ether-induced separation of the G-quadruplexes (higher stiffness). The hydrogel matrices demonstrate switchable bioelectrocatalytic functions guided by the stiffness properties of the hydrogels.

Hierarchical Porous Metal-Organic Framework as Biocatalytic Microreactor for Enzymatic Biofuel Cell-Based Self-Powered Biosensing of MicroRNA Integrated with Cascade Signal Amplification

Enzymatic biofuel cells have become powerful tools in biosensing, which however generally suffer from the limited loading efficiency as well as low catalytic activity and poor stability of bioenzymes. Herein, the hierarchical porous metal-organic frameworks (MOFs) are synthesized using tannic acid (TA) for structural etching, which realizes co-encapsulation of glucose dehydrogenase (GDH) and nicotinamide adenine dinucleotide (NAD+ ) cofactor in zeolitic imidazolate framework (ZIF-L) and are further used as the biocatalytic microreactors to modify bioanode. In this work, the TA-controlled etching can not only expand the pore size of microreactors, but also achieve the reorientation of enzymes in their lower surface energy form, therefore enhancing the biocatalysis of cofactor-dependent enzyme. Meanwhile, the topological DNA tetrahedron is assembled on the microreactors, which acts as the microRNA-responsive "lock" to perform the cascade signal amplification of exonuclease III-assisted target recycling on bioanode and hybridization chain reaction (HCR) on biocathode. The proposed self-powered biosensor has achieved a detection limit as low as 2 aM (6 copies miRNA-21 in a 5 µL of sample), which is further successfully applied to identify cancer cells and clinical serums of breast cancer patients based on the different levels of miRNA-21, holding great potential in accurate disease identification and clinical diagnosis.

Recent advances in enzymatic biofuel cells enabled by innovative materials and techniques

The past few decades have seen increasingly rapid advances in the field of sustainable energy technologies. As a new bio- and eco-friendly energy source, enzymatic biofuel cells (EBFCs) have garnered significant research interest due to their capacity to power implantable bioelectronics, portable devices, and biosensors by utilizing biomass as fuel under mild circumstances. Nonetheless, numerous obstacles impeded the commercialization of EBFCs, including their relatively modest power output and poor long-term stability of enzymes. To depict the current progress of EBFC and address the challenges it faces, this review traces back the evolution of EBFC and focuses on contemporary advances such as newly emerged multi or single enzyme systems, various porous framework-enzyme composites techniques, and innovative applications. Besides emphasizing current achievements in this field, from our perspective part we also introduced novel electrode and cell design for highly effective EBFC fabrication. We believe this review will assist readers in comprehending the basic research and applications of EBFCs as well as potentially spark interdisciplinary collaboration for addressing the pressing issues in this field.

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.

Molecular Insights of Cellobiose Dehydrogenase Adsorption on Self-Assembled Monolayers

Cellobiose dehydrogenase (CDH) is capable of direct electron transfer (DET) on electrodes and is a promising redox enzyme for bioelectrochemical applications. Its unique two-domain structure makes the function of CDH adsorbed on the surface of the electrode deeply affected by the external environment, such as ion species, strength, pH, and surface charge density. To date, however, the exact mechanism of how the external environment tailors the structure and dynamics of CDH adsorbed on the electrode surface still remains poorly understood. Here, multiscale simulations were performed to look for insight into the effect of Na⁺ and Ca²⁺ ions on the activation of CDH on oppositely charged self-assembled monolayer (NH₂-SAM and COOH-SAM) surfaces with different surface charge densities (SCDs). Both Na⁺ and Ca²⁺ can promote CDH conformation switch from the open state to the closed state, while the promotion effect of Ca²⁺ is stronger than that of Na⁺ at the same conditions. However, the high ionic strength (IS) of Ca²⁺ renders the cytochrome (CYT) domain of CDH away from the NH₂-SAM with low SCD. In contrast, whatever the IS, the NH₂-SAM surface with high SCD can not only enhance the CYT-surface interaction but also achieve a closed-state conformation due to a similar role of Ca²⁺. Overall, this study gains molecular-level insights into the role of ion species and surface charge in modulating the structure and conformation of CDH on the SAM surface, thereby tailoring its activity.

Putting the electro-bugs to work: A systematic review of 22 years of advances in bio-electrochemical systems and the parameters governing their performance

Wastewater treatment using bioelectrochemical systems (BESs) can be considered as a technology finding application in versatile areas such as for renewable energy production and simultaneous reducing environmental problems, biosensors, and bioelectrosynthesis. This review paper reports and critically discusses the challenges, and advances in bio-electrochemical studies in the 21st century. To sum and critically analyze the strides of the last 20+ years on the topic, this study first provides a comprehensive analysis on the structure, performance, and application of BESs, which include Microbial Fuel Cells (MFCs), Microbial Electrolysis Cells (MECs) and Microbial Desalination Cells (MDCs). We focus on the effect of various parameters, such as electroactive microbial community structure, electrode material, configuration of bioreactors, anode unit volume, membrane type, initial COD, co-substrates and the nature of the input wastewater in treatment process and the amount of energy and fuel production, with the purpose of showcasing the modes of operation as a guide for future studies. The results of this review show that the BES have great potential in reducing environmental pollution, purifying saltwater, and producing energy and fuel. At a larger scale, it aspires to facilitate the path of achieving sustainable development and practical application of BES in real-world scenarios.

Shield, Anchor, and Adhesive Roles of Methylene Blue in Tyrosinase Adsorbed on Carbon Felt for a Flow Injection Amperometric Enzyme Biosensor for Phenolic Substrates and Inhibitors

Methylene blue (MB) acted as a stabilizer for preventing surface-induced denaturation of tyrosinase (TYR) adsorbed on a carbon felt (CF) surface, which is based on shield and anchor roles preventing the unfavorable conformational change of TYR on the hydrophobic CF surface. Furthermore, MB acted as an effective adhesive for TYR immobilization on CF. The resulting TYR and MB coadsorbed CF (TYR/MB-CF) worked as an excellent working electrode unit in an electrochemical detector in a flow injection amperometric biosensor, which allowed highly sensitive consecutive determination of not only TYR substrates but also competitive inhibitors. Simultaneous adsorption of TYR and MB from their mixed solution was much useful as compared with step-wise separated adsorption of TYR on the MB-adsorbed CF, which suggests that the binding interaction of MB with TYR in the solution phase is important for this phenomenon. Fluorescence and UV-vis spectroscopy revealed that not only electrostatic forces between the cationic MB and anionic amino acid residues of TYR but also hydrophobic interactions via the phenothiazine ring of MB play a principal binding driving force of MB with TYR at the surface of the TYR molecules. Synchronous fluorescence, three-dimensional fluorescence, and circular dichroism (CD) spectroscopy clarified that the conformation and the secondary structure of TYR slightly changed upon the MB binding, implying that MB binding leads to the modification of the original intramolecular bonding in part.

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.

Nanostructured electrodes based on multiwalled carbon nanotube/glyconanoparticles for the specific immobilization of bilirubin oxidase: Application to the electrocatalytic O2 reduction

Here we describe the design and the characterization of novel electrode materials consisting of multi-walled carbon nanotubes coated with glyconanoparticles (GNPs) functionalized with anthraquinone sulfonate. The resulting modified electrodes were characterized by scanning electron microscopy and cyclic voltammetry. Their electrochemical behavior reveals a stable pH-dependent redox signal characteristic of anthraquinone sulfonate. Immobilization of bilirubin oxidase on these three-dimensional electrodes leads to the electroenzymatic reduction of O₂ to water with an onset potential of 0.5 V/SCE (saturated calomel electrode). A catalytic cathodic current of 174 µA (0.88 mA cm^-2) at 0.1 V/SCE, demonstrates that glyconanoparticles modified by anthraquinone sulfonate were able to interact and orientate bilirubin oxidase by electrostatic interactions.

Availability of Biomass and Potential of Nanotechnologies for Bioenergy Production in Jordan

Jordan’s energy situation is in a critical state of dependency, with the country relying heavily on imports to satisfy its ever-increasing energy requirements. Renewable energy is a more competitive and consistent source of energy that can supply a large proportion of a country’s energy demand. It is environmentally friendly and minimizes atmospheric pollutant emissions. Thus, bioenergy has the potential to be a crucial alternative energy source in Jordan. Biomass is the principal source of bioenergy; it accounts for approximately 13% of the primary energy demand and is anticipated to supply half of the total primary energy demand by 2050. Nanotechnology has emerged as an important scientific research area with numerous applications, including biofuels. This review summarizes the application of nanoparticles to improve the properties and processes of biofuels. It presents the availability and viability of nanotechnology-supported bioenergy production in Jordan. Jordan generates up to 5.8 million tons of biomass each year and has access to abundant nonedible plant resources (such as Jojoba, Handal, and Jatropha). The theoretical energy potential of waste and residue available in Jordan was also assessed; it was discovered that the 1.28 million tons of dry crop residues (vegetables, fruits, and farming crops) could generate 6.8 PJ of energy per year and that biogas could be generated at a rate of 817 MCM/year

Bioinspired and Bioderived Aqueous Electrocatalysis

The development of efficient and sustainable electrochemical systems able to provide clean-energy fuels and chemicals is one of the main current challenges of materials science and engineering. Over the last decades, significant advances have been made in the development of robust electrocatalysts for different reactions, with fundamental insights from both computational and experimental work. Some of the most promising systems in the literature are based on expensive and scarce platinum-group metals; however, natural enzymes show the highest per-site catalytic activities, while their active sites are based exclusively on earth-abundant metals. Additionally, natural biomass provides a valuable feedstock for producing advanced carbonaceous materials with porous hierarchical structures. Utilizing resources and design inspiration from nature can help create more sustainable and cost-effective strategies for manufacturing cost-effective, sustainable, and robust electrochemical materials and devices. This review spans from materials to device engineering; we initially discuss the design of carbon-based materials with bioinspired features (such as enzyme active sites), the utilization of biomass resources to construct tailored carbon materials, and their activity in aqueous electrocatalysis for water splitting, oxygen reduction, and CO₂ reduction. We then delve in the applicability of bioinspired features in electrochemical devices, such as the engineering of bioinspired mass transport and electrode interfaces. Finally, we address remaining challenges, such as the stability of bioinspired active sites or the activity of metal-free carbon materials, and discuss new potential research directions that can open the gates to the implementation of bioinspired sustainable materials in electrochemical devices.

Enzymatic biofuel cell-powered iontophoretic facial mask for enhanced transdermal drug delivery

Recent advances in enzymatic biofuel cells (EBFCs) have resulted in great progress in health monitoring and supplying power to medical applications, such as drug delivery. On the other hand, to enhance the electric field-assisted transdermal permeation for facial mask application, an external power source is usually required. Herein, we attempted to combine an EBFC with a facial mask so that the microcurrent generated can boost the transdermal permeability of target molecules in the facial mask essence. When screen-printed onto a polypropylene-based non-woven fabric, the three-layered flexible EBFC could produce a voltage of ∼0.4 V and a maximum power density of 23.3 μW cm^-2, leading to an approximately 2-3-fold increase in permeated nicotinamide, arbutin, and aspirin levels within 15 min compared to non-iontophoretic transdermal drug delivery. Both cell viability and animal experiments further demonstrated that the EBFC-powered iontophoresis worked well in living animals with good biocompatibility. These results suggest that the EBFC-powered iontophoretic facial mask can effectively improve the permeation of drugs and holds a promise for the possible cosmetic application.

Functional Fiber Materials to Smart Fiber Devices

The development of fiber materials has accompanied the evolution of human civilization for centuries. Recent advances in materials science and chemistry offered fibers new applications with various functions, including energy harvesting, energy storing, displaying, health monitoring and treating, and computing. The unique one-dimensional shape of fiber devices endows them advantages to work as human-interfaced electronics due to the small size, lightweight, flexibility, and feasibility for integration into large-scale textile systems. In this review, we first present a discussion of the basics of fiber materials and the design principles of fiber devices, followed by a comprehensive analysis on recently developed fiber devices. Finally, we provide the current challenges facing this field and give an outlook on future research directions. With novel fiber devices and new applications continuing to be discovered after two decades of research, we envision that new fiber devices could have an important impact on our life in the near future.

Facile Functionalization of Carbon Electrodes for Efficient Electroenzymatic Hydrogen Production

Enzymatic electrocatalysis holds promise for new biotechnological approaches to produce chemical commodities such as molecular hydrogen (H₂). However, typical inhibitory limitations include low stability and/or low electrocatalytic currents (low product yields). Here we report a facile single-step electrode preparation procedure using indium-tin oxide nanoparticles on carbon electrodes. The subsequent immobilization of a model [FeFe]-hydrogenase from Clostridium pasteurianum ("CpI") on the functionalized carbon electrode permits comparatively large quantities of H₂ to be produced in a stable manner. Specifically, we observe current densities of >8 mA/cm² at -0.8 V vs the standard hydrogen electrode (SHE) by direct electron transfer (DET) from cyclic voltammetry, with an onset potential for H₂ production close to its standard potential at pH 7 (approximately -0.4 V vs. SHE). Importantly, hydrogenase-modified electrodes show high stability retaining ∼92% of their electrocatalytic current after 120 h of continuous potentiostatic H₂ production at -0.6 V vs. SHE; gas chromatography confirmed ∼100% Faradaic efficiency. As the bioelectrode preparation method balances simplicity, performance, and stability, it paves the way for DET on other electroenzymatic reactions as well as semiartificial photosynthesis.

Tailoring enzymatic loading capacity on 3D macroporous gold by catalytic hairpin assembly and hybridization chain reaction: Application for ultrasensitive self-powered microRNA detection

It is important to develop effective strategies to construct enzymatic biofuel cell based self-powered biosensors. We report here the facile regulation of enzymatic loading capacity on the bioanode by utilizing a concatenated catalytic hairpin assembly (CHA)/hybridization chain reaction (HCR) and its application for self-powered microRNA-141 (miRNA-141) detection. To construct the bioanode, a concatenated CHA/HCR process triggered by miRNA-141 was conducted on the three-dimensional macroporous gold (3DMG) electrode to generate long double-stranded DNA nanowires for glucose oxidase immobilization. Quartz crystal microbalance study reveals that the enzymatic loading capacity on the bioanode increases at an increasing miRNA-141 concentration, leading to enhanced catalytic performance for glucose oxidation. The short-circuit currents of the assembled glucose/O2 biofuel cells increase at increasing miRNA-141 concentrations, enabling ultrasensitive detection of miRNA-141. The self-powered biosensor features a wide dynamic range for detecting miRNA-141 from 10-17 to 10-11 M, with an ultralow detection limit of 1.3 aM. This work provides a highly sensitive self-powered biosensing platform for miRNA detection.

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.

Optimizing Covalent Immobilization of Glucose Oxidase and Laccase on PV15 Fluoropolymer-Based Bioelectrodes

Enzymatic biofuel cells (EBCs) represent a promising technology for biosensors, biodevices, and sustainable green energy applications, thanks to enzymes' high specificity and catalytic efficiency. Nevertheless, drawbacks such as limited output power and short lifetime have to be solved. Nowadays, research is addressed to the use of 3D electrode structures, but the high cost and the industrialization difficulties of such electrodes represent a key issue. The purpose of the paper is thus to describe the use of a low-cost commercial conductive polymer (Sigracell^® PV15) as support for the covalent immobilization of glucose oxidase and laccase, for bioanode and biocathode fabrication, respectively. Efficient immobilization protocols were determined for the immobilized enzymes in terms of employed linkers and enzyme concentrations, resulting in significant enzymatic activities for units of area. The analysis focuses specifically on the optimization of the challenging immobilization of laccase and assessing its stability over time. In particular, an optimum activity of 23 mU/cm² was found by immobilizing 0.18 mg/cm² of laccase, allowing better performances, as for voltage output and electrochemical stability, and a direct electron transfer mechanism to be revealed for the fabricated biocathode. This study thus poses the basis for the viable development of low-cost functional EBC devices for biomedical applications.

Structural design of anthraquinone bridges in direct electron transfer of fructose dehydrogenase

Multi-functional small molecules attached to an electrode surface can bind non-covalently to the redox enzyme fructose dehydrogenase (FDH) to ensure efficient electrochemical electron transfer (ET) and electrocatalysis of the enzyme in both mediated (MET) and direct (DET) ET modes. The present work investigates the potential of exploiting secondary, electrostatic and hydrophobic interactions between substituents on a small molecular bridge and the local FDH surfaces. Such interactions ensure alignment of the enzyme in an orientation favourable for both MET and DET. We have used a group of novel synthesized anthraquinones as the small molecule bridge, functionalised with electrostatically neutral, anionic, or cationic substituents. Particularly, we investigated the immobilisation of FDH on a nanoporous gold (NPG) electrode decorated with the novel synthesised anthraquinones using electrochemical methods. The best DET-capable fraction out of four anthraquinone derivatives tested is achieved for an anthraquinone functionalised with an anionic sulphonate group. Our study demonstrates, how the combination of chemical design and bioelectrochemistry can be brought to control alignment of enzymes in productive orientations on electrodes, a paradigm for thiol modified surfaces in biosensors and bioelectronics.

Modeling of the Electrostatic Interaction and Catalytic Activity of [NiFe] Hydrogenases on a Planar Electrode

Hydrogenases are a group of enzymes that have caught the interest of researchers in renewable energies, due to their ability to catalyze the redox reaction of hydrogen. The exploitation of hydrogenases in electrochemical devices requires their immobilization on the surface of suitable electrodes, such as graphite. The orientation of the enzyme on the electrode is important to ensure a good flux of electrons to the catalytic center, through an array of iron-sulfur clusters. Here we present a computational approach to determine the possible orientations of a [NiFe] hydrogenase (PDB 1e3d) on a planar electrode, as a function of pH, salinity, and electrode potential. The calculations are based on the solution of the linearized Poisson-Boltzmann equation, using the PyGBe software. The results reveal that electrostatic interactions do not truly immobilize the enzyme on the surface of the electrode, but there is instead a dynamic equilibrium between different orientations. Nonetheless, after averaging over all thermally accessible orientations, we find significant differences related to the solution's salinity and pH, while the effect of the electrode potential is relatively weak. We also combine models for the protein adsoption-desorption equilibria and for the electron transfer between the proteins and the electrode to arrive at a prediction of the electrode's activity as a function of the enzyme concentration.

Development of an Integrated Salt Cartridge-Reverse Electrodialysis (Red) Device to Increase Electrolyte Concentrations to Biomedical Devices

Emerging technologies in nanotechnology and biomedical engineering have led to an increase in the use of implantable biomedical devices. These devices are currently battery powered which often means they must be surgically replaced during a patient's lifetime. Therefore, there is an important need for a power source that could provide continuous, stable power over a prolonged time. Reverse electrodialysis (RED) based biopower cells have been previously used to generate continuous power from physiologically relevant fluids; however, the low salinity gradient that exists within the body limited the performance of the biopower cell. In this study, a miniaturized RED biopower cell design coupled with a salt cartridge was evaluated for boosting the salt concentration gradient supplied to RED in situ. For the salt cartridge, polysulfone (PSf) hollow fibers were prepared in-house and saturated with NaCl solutions to deliver salt and thereby enhance the concentration gradient. The effect of operational parameters including solution flow rate and cartridge salt concentration on salt transport performance was evaluated. The results demonstrated that the use of the salt cartridge was able to increase the salt concentration of the RED inlet stream by 74% which in turn generated a 3-fold increase in the open circuit voltage (OCV) of the biopower cell. This innovative adaptation of the membrane-based approach into portable power generation could help open new pathways in various biomedical applications.

Engineering Self-Powered Electrochemical Sensors Using Analyzed Liquid Sample as the Sole Energy Source

Many healthcare and environmental monitoring devices use electrochemical techniques to detect and quantify analytes. With sensors progressively becoming smaller-particularly in point-of-care (POC) devices and wearable platforms-it creates the opportunity to operate them using less energy than their predecessors. In fact, they may require so little power that can be extracted from the analyzed fluids themselves, for example, blood or sweat in case of physiological sensors and sources like river water in the case of environmental monitoring. Self-powered electrochemical sensors (SPES) can generate a response by utilizing the available chemical species in the analyzed liquid sample. Though SPESs generate relatively low power, capable devices can be engineered by combining suitable reactions, miniaturized cell designs, and effective sensing approaches for deciphering analyte information. This review details various such sensing and engineering approaches adopted in different categories of SPES systems that solely use the power available in liquid sample for their operation. Specifically, the categories discussed in this review cover enzyme-based systems, battery-based systems, and ion-selective electrode-based systems. The review details the benefits and drawbacks with these approaches, as well as prospects of and challenges to accomplishing them.

A novel membraneless β-glucan/O2 enzymatic fuel cell based on β-glucosidase (RmBgl3B)/pyranose dehydrogenase (AmPDH) co-immobilized onto buckypaper electrode

A novel membraneless β-glucan/O₂ enzymatic fuel cell was developed by combining a bioanode based on buckypaper modified with co-immobilized Agaricus meleagris pyranose dehydrogenase (AmPDH) and Rhodothermus marinus β-glucosidase (RmBgl3B) (RmBgl3B-AmPDH/buckypaper) with a biocathode based on solid graphite modified with Myrothecium verrucaria bilirubin oxidase (MvBOx/graphite). AmPDH was connected electrochemically with the buckypaper using an osmium redox polymer in a mediated reaction, whereas MvBOx was connected with graphite in a direct electron transfer reaction. The fuel for the bioanode was produced by enzymatic hydrolysis of β-glucan by the exoglucanase RmBgl3B into d-glucose, which in turn was enzymatically oxidised by AmPDH to generate a current response. This design allows to obtain an efficient enzymatic fuel cell, where the chemical energy converted into electrical energy is higher than the chemical energy stored in complex carbohydrate based fuel. The maximum power density of the assembled β-glucan/O₂ biofuel cell reached 26.3 ± 4.6 μWcm^-2 at 0.36 V in phosphate buffer containing 0.5 % (w/v) β-glucan at 40 °C with excellent stability retaining 68.6 % of its initial performance after 5 days. The result confirms that β-glucan can be employed as fuel in an enzymatic biofuel cell.

Modulating the adsorption orientation of methionine-rich laccase by tailoring the surface chemistry of single-walled carbon nanotubes

Achieving fast electron transfer process between oxidoreductase and electrodes is pivotal for the biocathode of enzymatic biofuel cells (EBFCs). However, in-depth understanding of the interplay mechanism between enzymes and electrode materials remains challenging when designing and constructing EBFCs. Herein, atomic-scale insight into the direct electron transfer (DET) behavior of Thermus thermophilus laccase (TtLac) with a special methionine-rich β-hairpin motif adsorbed on the carboxyl-functionalized carbon nanotube (COOH-CNT) and amino-functionalized carbon nanotube (NH₂-CNT) surfaces were disclosed by multi-scale molecular simulations. Simulation results reveal that electrostatic modification is an effective way to tune the DET behavior for TtLac on the modified-CNTs electrode surface. Surprisingly, the positively charged TtLac can be attracted by both negatively charged COOH-CNT and positively charged NH₂-CNT surfaces, yet only the latter is capable to trigger the DET process due to the 'lying-on' adsorption orientation. Specifically, the T1 copper site is near the methionine-rich β-hairpin motif, which is the key binding site for TtLac binding onto the NH₂-CNT surface via electrostatic interaction, π-π stacking and cation-π interaction. Moreover, TtLac on the NH₂-CNT surface undergoes less conformational changes than those on the COOH-CNT surface, which allows the laccase stability and catalytic efficiency to be well preserved. These findings provide a fundamental guidance for future design and fabrication of methionine-rich laccase-based EBFCs with high power output and long lifespan.

Engineering bio-interfaces for the direct electron transfer of Myriococcum thermophilum cellobiose dehydrogenase: Towards a mediator-less biosupercapacitor/biofuel cell hybrid

Direct electron transfer (DET) of enzymes on electrode surfaces is highly desirable both for fundamental mechanistic studies and to achieve membrane- and mediator-less bioenergy harvesting. In this report, we describe the preparation and comprehensive structural and electrochemical characterization of a three-dimensional (3D) graphene-based carbon electrode, onto which the two-domain redox enzyme Myriococcum thermophilum cellobiose dehydrogenase (MtCDH) is immobilized. The electrode is prepared by an entirely novel method, which combines in a single step electrochemical reduction of graphene oxide (GO) and simultaneous electrodeposition of positively charged polyethylenimine (PEI), resulting in a well dispersed MtCDH surface. The resulting MtCDH bio-interface was characterized structurally in detail, optimized, and found to exhibit a DET maximum current density of 7.7 ± 0.9 μA cm^-2 and a half-lifetime of 48 h for glucose oxidation, attributed to favorable MtCDH surface orientation. A dual, entirely DET-based enzymatic biofuel cell (EBFC) was constructed with a MtCDH bioanode and a Myrothecium verrucaria bilirubin oxidase (MvBOD) biocathode. The EBFC delivers a maximum power density (P_max) of 7.6 ± 1.3 μW cm^-2, an open-circuit voltage (OCV) of 0.60 V, and an operational lifetime over seven days, which exceeds most reported CDH based DET-type EBFCs. A biosupercapacitor/EBFC hybrid was also constructed and found to register maximum power densities 62 and 43 times higher than single glucose/air and lactose/air EBFCs, respectively. This hybrid also shows excellent operational stability with self-charging/discharging over at least 500 cycles.

Hollow Bioelectrodes Based on Buckypaper Assembly. Application to the Electroenzymatic Reduction of O2

A new concept of hollow electrode based on the assembly of two buckypapers creating a microcavity which contains a biocatalyst is described. To illustrate this innovative concept, hollow bioelectrodes containing 0.16–4 mg bilirubin oxidase in a microcavity were fabricated and applied to electroenzymatic reduction of O2 in aqueous solution. For hemin-modified buckypaper, the bioelectrode shows a direct electron transfer between multi-walled carbon nanotubes and bilirubin oxidase with an onset potential of 0.77 V vs. RHE. The hollow bioelectrodes showed good storage stability in solution with an electroenzymatic activity of 30 and 11% of its initial activity after 3 and 6 months, respectively. The co-entrapment of bilirubin oxidase and 2,2-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) in the microcavity leads to a bioelectrode exhibiting mediated electron transfer. After 23 h of intermittent operation, 5.66 × 10−4 mol of O2 were electroreduced (turnover number of 19,245), the loss of catalytic current being only 54% after 7 days.

Electrochemical immobilization of glucose oxidase for the controlled production of H2O2 in a biocatalytic flow reactor

Electrochemical methods can be used to selectively modify the surfaces of electrodes, enabling the immobilisation of enzymes on defined areas on the surfaces of electrodes. Such selective immobilisation methods can be used to pattern catalysts on surfaces in a controlled manner. Using this approach, the selective patterning of the enzyme glucose oxidase on the electrodes was used to develop a flow reactor for the controlled delivery of the oxidant H₂O₂. GOx was immobilised on a glassy carbon electrode using polypyrrole, silica films, and diazonium linkers. The rate of production of H₂O₂ and the stability of the response was dependent on the immobilisation method. GOx encapsulated in polypyrrole was selected as the optimal method of immobilisation, with a rate of production of 91±11 μM h⁻¹ for 4 hours of continuous operation. The enzyme was subsequently immobilised on carbon rod electrodes (surface area of 5.76 cm²) using a polypyrrole/Nafion® film and incorporated into a flow reactor. The rate of production of H₂O₂ was 602±57 μM h⁻¹, with 100 % retention of activity after 7 h of continuous operation, demonstrating that such a system can be used to prepare H₂O₂ at continuous and stable rate for use in downstream oxidation reactions.