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The heterotrimeric flavin adenine dinucleotide (FAD) dependent glucose dehydrogenase derived from Burkholderia cepacia (BcGDH) has many exceptional features for its use in glucose sensing—including that this enzyme is capable of direct electron transfer with an electrode in its heterotrimeric configuration. However, this enzyme's high catalytic activity towards not only glucose but also galactose presents an engineering challenge. To increase the substrate specificity of this enzyme, it must be engineered to reduce its activity towards galactose while maintaining its activity towards glucose. To aid in these mutagenesis studies, the crystal structure composed of BcGDH's small subunit and catalytic subunit (BcGDHγα), in complex with D -glucono-1,5-lactone was elucidated and used to construct the three-dimensional model for targeted, site-directed mutagenesis. BcGDHγα was then mutated at three different residues, glycine 322, asparagine 474 and asparagine 475. The single mutations that showed the greatest glucose selectivity were combined to create the resulting mutant, α-G322Q-N474S-N475S. The α-G322Q-N474S-N475S mutant and BcGDHγα wild type were then characterized with dye-mediated dehydrogenase activity assays to determine their kinetic parameters. The α-G322Q-N474S-N475S mutant showed more than a 2-fold increase in V max towards glucose and this mutant showed a lower activity towards galactose in the physiological range (5 mM) of 4.19 U mg−1, as compared to the wild type, 86.6 U mg−1. This resulting increase in specificity lead to an 81.7 gal/glc % activity for the wild type while the α-G322Q-N474S-N475S mutant had just 10.9 gal/glc % activity at 5 mM. While the BcGDHγα wild type has high specificity towards galactose, our engineering α-G322Q-N474S-N475S mutant showed concentration dependent response to glucose and was not affected by galactose. • X-ray crystal structure of catalytic subunit of DET-type GDH in complex with 1,5-Glucono-δ-lactone was elucidated. • Three residues, G322, N474 and N475, were solicited to be responsible for enzyme substrate specificity. • G322Q/N474S/N475S mutant improved specificity toward glucose by decreasing activity toward galactose. • Structural model provided the impact of mutations, confirming the narrowed substrate binding pocket in the mutant. [ABSTRACT FROM AUTHOR]

The metalloenzyme [FeFe]‐hydrogenase is of interest to future biotechnologies targeting the production of “green” hydrogen (H2). We recently developed a simple two‐step functionalized procedure to immobilize the [FeFe]‐hydrogenase from Clostridium pasteurianum (“CpI”) on mesoporous indium tin oxide (ITO) electrodes to achieve elevated H2 production with high operational stability and current densities of 8 mA cm−2. Here, we use a combination of atomic force microscopy (AFM), scanning electron microscopy (SEM) and electrochemical quartz crystal microbalance (EQCM) to understand how mesoporous ITO stabilizes and activates CpI for electroenzymatic H2 production. Examination of the topography and morphology of the mesoporous ITO surface revealed a hierarchical morphology containing cavities and well‐defined nanoparticle agglomerates. Any potential effect of mesoporosity was investigated by comparing the stability and electroenzymatic activity of CpI on mesoporous ‘nanoITO’ and planar ITO, where we determined that CpI has a higher turnover frequency and adsorbs with greater stability (with respect to electroenzymatic activity over time) to nanoITO surfaces. [ABSTRACT FROM AUTHOR]

Electron donor and acceptor interactions are the fundamentals for the emergence of optoelectronic functions in most organic devices. In particular, exciplexes, which hold the possibility to utilize dark triplets, are widely investigated in organic light‐emitting diodes. However, the relatively low radiative decay rate with a low photoluminescence quantum yield poses further advanced challenges. Traditionally, it is well recognized that exciplexes are formed through a charge transfer process, i.e., direct electron transfer, by photoexciting either a donor or an acceptor component. In this study, the focus is on a long‐range exciplex system where tris(4‐carbazoyl‐9‐ylphenyl)amine (TCTA) and 2,4,6‐tris[3‐(diphenylphosphinyl)phenyl]−1,3,5‐triazine (PO‐T2T) serve as the donor and acceptor, respectively. Moreover, 1,2‐bis(triphenylsilyl) benzene (UGH2) and bis[2‐(diphenylphosphino)phenyl]ether oxide (DPEPO), having a wide energy gap and a high triplet energy level, are employed as the double insulating layer to suppressing direct electron transfer between the donor and acceptor. The indirect through‐space exciplex formation is demonstrated by the simultaneous excitation of both donor and acceptor layers optically and electrically, providing a new route for exciplex formation. [ABSTRACT FROM AUTHOR]

Graphene and its variants exhibit excellent electrical properties for the construction of enzymatic interfaces. In particular, the direct electron transfer of glucose oxidase on the electrode surface is a very important issue in the development of enzyme-based bioelectrodes. However, the number of studies conducted to assess how pristine graphene forms different interfaces with other carbon materials is insufficient. Enzyme-based electrodes (formed using carbon materials) have been extensively applied because of their low manufacturing costs and easy production techniques. In this study, the characteristics of a single-walled carbon nanotube/graphene-combined enzyme interface are analyzed at the atomic level using molecular dynamics simulations. The morphology of the enzyme was visualized using an elastic network model by performing normal-mode analysis based on electrochemical and microscopic experiments. Single-carbon electrodes exhibited poorer electrical characteristics than those prepared as composites with enzymes. Furthermore, the composite interface exhibited 4.61- and 2.45-fold higher direct electron efficiencies than GOx synthesized with single-carbon nanotubes and graphene, respectively. Based on this study, we propose that pristine graphene has the potential to develop glucose oxidase interfaces and carbon-nanotube–graphene composites for easy fabrication, low cost, and efficient electrode structures for enzyme-based biofuel cells. [ABSTRACT FROM AUTHOR]

The iron transport system plays a crucial role in the extracellular electron transfer process of Shewanella sp. In this study, we fabricated a vertically oriented α-Fe2O3 nanoarray on carbon cloth to enhance interfacial electron transfer in Shewanella putrefaciens CN32 microbial fuel cells. The incorporation of the α-Fe2O3 nanoarray not only resulted in a slight increase in flavin content but also significantly enhanced biofilm loading, leading to an eight-fold higher maximum power density compared to plain carbon cloth. Through expression level analyses of electron transfer-related genes in the outer membrane and core genes in the iron transport system, we propose that the α-Fe2O3 nanoarray can serve as an electron mediator, facilitating direct electron transfer between the bacteria and electrodes. This finding provides important insights into the potential application of iron-containing oxide electrodes in the design of microbial fuel cells and other bioelectrochemical systems, highlighting the role of α-Fe2O3 in promoting direct electron transfer. [ABSTRACT FROM AUTHOR]

The integrated system of anaerobic digestion and microbial electrolysis cells (AD-MEC) was a novel approach to enhance the degradation of food waste anaerobic digestate and recover methane. Through long-term operation, the start-up method, organic loading, and methane production mechanism of the digestate have been investigated. At an organic loading rate of 4000 mg/L, AD-MEC increased methane production by 3–4 times and soluble chemical oxygen demand (SCOD) removal by 20.3% compared with anaerobic digestion (AD). The abundance of bacteria Fastidiosipila and Geobacter, which participated in the acid degradation and direct electron transfer in the AD-MEC, increased dramatically compared to that in the AD. The dominant methanogenic archaea in the AD-MEC and AD were Methanobacterium (44.4–56.3%) and Methanocalculus (70.05%), respectively. Geobacter and Methanobacterium were dominant in the AD-MEC by direct electron transfer of organic matter into synthetic methane intermediates. AD-MEC showed a perfect SCOD removal efficiency of the digestate, while methane as clean energy was obtained. Therefore, AD-MEC was a promising technology for deep energy transformation from digestate. [ABSTRACT FROM AUTHOR]

A direct electron tunneling of myoglobin (cardiac biomarker) onto a magnesium oxide templated carbon electrode surface (MgOC) was observed. The porous carbon material employed in this work possesses a 3D macropores structure. Herein, the presence of a 3D curvature‐like structure provides a favorable environment for anchoring the protein myoglobin, which facilitates direct electron transfer by the myoglobin encapsulated within electroactive carbon and improves the mass transfer inside the carbon. The direct electron transfer of myoglobin at MgOC demonstrated the usefulness of this carbon material as an effective platform for studying the redox properties of various proteins and biomolecules that are of clinical relevance. The proposed strategy is distinct from the other known electrochemical strategies that involve a denaturant to partially open up myoglobin's redox‐active center, and thereby myoglobin is not detected in its native state. Our results also indicated a concentration‐dependent irreversible electron transfer from the heme center to the electrode surface. Further, the detection was rapid, and the analysis can be completed within a few minutes than other existing approaches that may take a few days up to a week for the results. The MgOC electrode also exhibited excellent stability towards the detection of myoglobin. [ABSTRACT FROM AUTHOR]

Among the various types of enzyme-based biosensors, sensors utilizing enzymes capable of direct electron transfer (DET) are recognized as the most ideal. However, only a limited number of redox enzymes are capable of DET with electrodes, that is, dehydrogenases harboring a subunit or domain that functions specifically to accept electrons from the redox cofactor of the catalytic site and transfer the electrons to the external electron acceptor. Such subunits or domains act as built-in mediators for electron transfer between enzymes and electrodes; consequently, such enzymes enable direct electron transfer to electrodes and are designated as DET-type enzymes. DET-type enzymes fall into several categories, including redox cofactors of catalytic reactions, built-in mediators for DET with electrodes and by their protein hierarchic structures, DET-type oxidoreductases with oligomeric structures harboring electron transfer subunits, and monomeric DET-type oxidoreductases harboring electron transfer domains. In this review, we cover the science of DET-type oxidoreductases and their biomedical applications. First, we introduce the structural biology and current understanding of DET-type enzyme reactions. Next, we describe recent technological developments based on DET-type enzymes for biomedical applications, such as biosensors and biochemical energy harvesting for self-powered medical devices. Finally, after discussing how to further engineer and create DET-type enzymes, we address the future prospects for DET-type enzymes in biomedical engineering. [ABSTRACT FROM AUTHOR]

In this study, Ti-Sn-Sb/γ-Al2O3 particle electrodes were prepared by sol–gel method and a three-dimensional electrocatalytic system was constructed to treat methylene blue (MB) dye wastewater. The experimental results demonstrate that the degradation of methylene blue conforms to the pseudo-first order reaction kinetics and had a low activation energy (Ea = 36.97 kJ/mol). Through quenching experiments, linear voltammetry tests, and radical trapping experiments utilizing benzoic acid as a probe, we have determined that the oxidation of MB by the particle electrode is primarily induced through direct surface electron transfer (86 ~ 88%) and surface-bound radicals (12 ~ 14%). Methylene blue undergoes degradation through the synergistic interplay of direct electron transfer (86% ~ 88%) and surface-bound radicals ·OHads (12% ~ 14%) on the particle electrode's surface. [ABSTRACT FROM AUTHOR]

This study focuses on micro-electrochemical screening to select microbial strains capable of directly transfer electrons to the working electrode dependent on specific enzymatic machinery. The main objective of this work is to select and identify promising strains for allow the bioelectrolysis production of hydrogen. To achieve this goal, microbial composites (artificial biofilms), have been developed using Escherichia coli CGE1 from LCPME. CNRS Fransh, Pseudomonas putrifisciens (CIP 69.13) (CIP, Collection Institut Pasteur, Fransh). Shewanella oneidensis MR-1 (ATCC 700550), and Thiobacillus denitrificans, from (ATCC, American Type Culture Collection), each one enclosed in a matrix carbon nanotubes and protamine matrix, forming an artificial biofilm on buckypaper. Cyclic Voltammetry (CV) measurements were performed over a potential range of +0.4 to -0.7V at 5mV/s under 30°C, using a saturated KCl Ag/AgCl reference electrode and a stainless-steel grid counter electrode. For E. coli and P. putrifisciens, the measurement focused on the oxidation of 20mM glucose, while the former bacteria were growth with and without O2. For S. oneidensis and T. denitrificans the focus was on the reduction of fumarate and 20 mM of NaNOH3+, respectively. As results, E. coli and P. putrifisciens species show no notable electrochemical activity, with no signal of glucose oxidation, due to the absence of type C cytochromes in the cytoplasmic membrane, unlike S. oneidensis and T. denitrificans, that demonstrate a direct electron transfer.

Abstract In the present study, three process parameters optimization were assessed as controlling factors for the biogas and biomethane generation from brown algae Cystoceira myrica as the substrate using RSM for the first time. The biomass amount, Co3O4NPs dosage, and digestion time were assessed and optimized by RSM using Box-Behnken design (BBD) to determine their optimum level. BET, FTIR, TGA, XRD, SEM, XPS, and TEM were applied to illustrate the Co3O4NPs. FTIR and XRD analysis established the formation of Co3O4NPs. The kinetic investigation confirmed that the modified model of Gompertz fit the research results satisfactorily, with R 2 ranging between 0.989–0.998 and 0.879–0.979 for biogas and biomethane production, respectively. The results recommended that adding Co3O4NPs at doses of 5 mg/L to C. myrica (1.5 g) significantly increases biogas yield (462 mL/g VS) compared to all other treatments. The maximum biomethane generation (96.85 mL/g VS) was obtained with C. myrica at (0 mg/L) of Co3O4NPs. The impacts of Co3O4NPs dosages on biomethane production, direct electron transfer (DIET) and reactive oxygen species (ROS) were also investigated in detail. The techno-economic study results demonstrate the financial benefits of this strategy for the biogas with the greatest net energy content, which was 2.82 kWh with a net profit of 0.60 USD/m3 of the substrate and was produced using Co3O4NPs (5 mg/L).

In the present study, three process parameters optimization were assessed as controlling factors for the biogas and biomethane generation from brown algae Cystoceira myrica as the substrate using RSM for the first time. The biomass amount, Co3O4NPs dosage, and digestion time were assessed and optimized by RSM using Box-Behnken design (BBD) to determine their optimum level. BET, FTIR, TGA, XRD, SEM, XPS, and TEM were applied to illustrate the Co3O4NPs. FTIR and XRD analysis established the formation of Co3O4NPs. The kinetic investigation confirmed that the modified model of Gompertz fit the research results satisfactorily, with R2 ranging between 0.989–0.998 and 0.879–0.979 for biogas and biomethane production, respectively. The results recommended that adding Co3O4NPs at doses of 5 mg/L to C. myrica (1.5 g) significantly increases biogas yield (462 mL/g VS) compared to all other treatments. The maximum biomethane generation (96.85 mL/g VS) was obtained with C. myrica at (0 mg/L) of Co3O4NPs. The impacts of Co3O4NPs dosages on biomethane production, direct electron transfer (DIET) and reactive oxygen species (ROS) were also investigated in detail. The techno-economic study results demonstrate the financial benefits of this strategy for the biogas with the greatest net energy content, which was 2.82 kWh with a net profit of 0.60 USD/m3 of the substrate and was produced using Co3O4NPs (5 mg/L). [ABSTRACT FROM AUTHOR]

The iron transport system plays a crucial role in the extracellular electron transfer process of Shewanella sp. In this study, we fabricated a vertically oriented α-Fe2O3 nanoarray on carbon cloth to enhance interfacial electron transfer in Shewanella putrefaciens CN32 microbial fuel cells. The incorporation of the α-Fe2O3 nanoarray not only resulted in a slight increase in flavin content but also significantly enhanced biofilm loading, leading to an eight-fold higher maximum power density compared to plain carbon cloth. Through expression level analyses of electron transfer-related genes in the outer membrane and core genes in the iron transport system, we propose that the α-Fe2O3 nanoarray can serve as an electron mediator, facilitating direct electron transfer between the bacteria and electrodes. This finding provides important insights into the potential application of iron-containing oxide electrodes in the design of microbial fuel cells and other bioelectrochemical systems, highlighting the role of α-Fe2O3 in promoting direct electron transfer.

Electrochemical manipulation of enzymes is expected to enable the rapid detection of marker molecules and the efficient production of valuable materials. We have fabricated an Au nanoparticle (AuNP) thin film electrode in our previous study, which enables more enhanced heterogeneous electron transfer reactions than a conventional planner Au electrode. In this study, electrochemical evaluation of cytochrome c was performed using the AuNP thin film electrodes with and without self-assembled monolayer (SAM) modification to assess its practicality for protein electrochemistry. The 4-pyridinethiol- and 7-carboxy-1-heptanethiol-modified AuNP thin film electrodes showed much larger faradaic current values attributed to redox reactions compared to those of the Au electrodes. The results indicate that the nanostructural effects of the AuNP thin film electrode are beneficial. Importantly, even when the non-modified (bare) AuNP thin film electrode was applied, it exhibited the clear electrochemical response of cyt c, while the bare Au electrode showed no such a response. This study provides evidence that the AuNP thin film electrode functions as a potent bioelectrocatalysts due to the nanostructure-specific properties.

Massive efforts are invested in developing innovative CO2‐sequestration strategies to counter climate change and transform CO2 into higher‐value products. CO2‐capture by reduction is a chemical challenge, and attention is turned toward biological systems that selectively and efficiently catalyse this reaction under mild conditions and in aqueous solvents. While a few reports have evaluated the effectiveness of isolated bacterial formate dehydrogenases as catalysts for the reversible electrochemical reduction of CO2, it is imperative to explore other enzymes among the natural reservoir of potential models that might exhibit higher turnover rates or preferential directionality for the reductive reaction. Here, we present electroenzymatic catalysis of formylmethanofuran dehydrogenase, a CO2‐reducing‐and‐fixing biomachinery isolated from a thermophilic methanogen, which was deposited on a graphite rod electrode to enable direct electron transfer for electroenzymatic CO2 reduction. The gas is reduced with a high Faradaic efficiency (109±1 %), where a low affinity for formate prevents its electrochemical reoxidation and favours formate accumulation. These properties make the enzyme an excellent tool for electroenzymatic CO2‐fixation and inspiration for protein engineering that would be beneficial for biotechnological purposes to convert the greenhouse gas into stable formate that can subsequently be safely stored, transported, and used for power generation without energy loss. [ABSTRACT FROM AUTHOR]

Graphene and its variants exhibit excellent electrical properties for the construction of enzymatic interfaces. In particular, the direct electron transfer of glucose oxidase on the electrode surface is a very important issue in the development of enzyme-based bioelectrodes. However, the number of studies conducted to assess how pristine graphene forms different interfaces with other carbon materials is insufficient. Enzyme-based electrodes (formed using carbon materials) have been extensively applied because of their low manufacturing costs and easy production techniques. In this study, the characteristics of a single-walled carbon nanotube/graphene-combined enzyme interface are analyzed at the atomic level using molecular dynamics simulations. The morphology of the enzyme was visualized using an elastic network model by performing normal-mode analysis based on electrochemical and microscopic experiments. Single-carbon electrodes exhibited poorer electrical characteristics than those prepared as composites with enzymes. Furthermore, the composite interface exhibited 4.61- and 2.45-fold higher direct electron efficiencies than GOx synthesized with single-carbon nanotubes and graphene, respectively. Based on this study, we propose that pristine graphene has the potential to develop glucose oxidase interfaces and carbon-nanotube–graphene composites for easy fabrication, low cost, and efficient electrode structures for enzyme-based biofuel cells.

Massive efforts are invested in developing innovative CO2‐sequestration strategies to counter climate change and transform CO2 into higher‐value products. CO2‐capture by reduction is a chemical challenge, and attention is turned toward biological systems that selectively and efficiently catalyse this reaction under mild conditions and in aqueous solvents. While a few reports have evaluated the effectiveness of isolated bacterial formate dehydrogenases as catalysts for the reversible electrochemical reduction of CO2, it is imperative to explore other enzymes among the natural reservoir of potential models that might exhibit higher turnover rates or preferential directionality for the reductive reaction. Here, we present electroenzymatic catalysis of formylmethanofuran dehydrogenase, a CO2‐reducing‐and‐fixing biomachinery isolated from a thermophilic methanogen, which was deposited on a graphite rod electrode to enable direct electron transfer for electroenzymatic CO2 reduction. The gas is reduced with a high Faradaic efficiency (109±1 %), where a low affinity for formate prevents its electrochemical reoxidation and favours formate accumulation. These properties make the enzyme an excellent tool for electroenzymatic CO2‐fixation and inspiration for protein engineering that would be beneficial for biotechnological purposes to convert the greenhouse gas into stable formate that can subsequently be safely stored, transported, and used for power generation without energy loss. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A disposable paper-based glucose biosensor with direct electron transfer (DET) of glucose oxidase (GOX) was developed through simple covalent immobilization of GOX on a carbon electrode surface using zero-length cross-linkers. This glucose biosensor exhibited a high electron transfer rate (ks, 3.363 s−1) as well as good affinity (km, 0.03 mM) for GOX while keeping innate enzymatic activities. Furthermore, the DET-based glucose detection was accomplished by employing both square wave voltammetry and chronoamperometric techniques, and it achieved a glucose detection range from 5.4 mg/dL to 900 mg/dL, which is wider than most commercially available glucometers. This low-cost DET glucose biosensor showed remarkable selectivity, and the use of the negative operating potential avoided interference from other common electroactive compounds. It has great potential to monitor different stages of diabetes from hypoglycemic to hyperglycemic states, especially for self-monitoring of blood glucose. [ABSTRACT FROM AUTHOR]

A glucose biosensor was layer-by-layer assembled on a modified glassy carbon electrode (GCE) from a nanocomposite of NAD(P)+-dependent glucose dehydrogenase, aminated polyethylene glycol (mPEG), carboxylic acid-functionalized multi-wall carbon nanotubes (fMWCNTs), and ionic liquid (IL) composite functional polymers. The electrochemical electrode was denoted as NF/IL/GDH/mPEG-fMWCNTs/GCE. The composite polymer membranes were characterized by cyclic voltammetry, ultraviolet-visible spectrophotometry, electrochemical impedance spectroscopy, scanning electron microscopy, and transmission electron microscopy. The cyclic voltammogram of the modified electrode had a pair of well-defined quasi-reversible redox peaks with a formal potential of −61 mV (vs. Ag/AgCl) at a scan rate of 0.05 V s−1. The heterogeneous electron transfer constant (ks) of GDH on the composite functional polymer-modified GCE was 6.5 s−1. The biosensor could sensitively recognize and detect glucose linearly from 0.8 to 100 µM with a detection limit down to 0.46 μM (S/N = 3) and a sensitivity of 29.1 nA μM−1. The apparent Michaelis–Menten constant ( K m app ) of the modified electrode was 0.21 mM. The constructed electrochemical sensor was compared with the high-performance liquid chromatography method for the determination of glucose in commercially available glucose injections. The results demonstrated that the sensor was highly accurate and could be used for the rapid and quantitative determination of glucose concentration. [ABSTRACT FROM AUTHOR]

Direct electron transfer (DET)-enabled oxidoreductase has been utilized to develop mediator-free bioelectronic devices. Fungal pyrroloquinoline quinone-dependent dehydrogenase from Coprinopsis cinerea is an attractive quinohemoprotein in which the catalytic PQQ domain and cytochrome domain perform DET. Here, we examined the difference in the distance from the active center to the protein surface between PQQ and heme. Then, we studied the direct bioelectrocatalysis of the full-length enzyme by regulating the distance between the enzyme and electrode with various alkyl chains of self-assembled monolayers (SAMs). The catalytic currents by the full-length enzyme were obtained on SAM of chain alkyl lengths of C6 or more, while no catalytic currents have been obtained by the isolated PQQ domain. After approximately 15 Å from PQQ in the active site to the electrode surface, the onset potential of the current shifted from the redox potential of PQQred/PQQsemi to Hemered/Hemeox. The results indicated that the chain length-dependent electron transfer pathway from the PQQ domain directly to the electrode changed via the cytochrome domain. The amount of electroactive cytochrome domain that had immobilized decreased with increasing pH (pH 6.0 to pH 9.5). In direct bioelectrocatalysis through interdomain electron transfer of the cytochrome domain, kcat was found to be pH dependent with an optimal pH of 8.5; therefore, the rate-limiting step that governs pH dependence is likely the IET process.

The electrochemical enzyme sensors based on direct electron transfer (DET)-type oxidoreductase-based enzymes are ideal for continuous and in vivo monitoring. However, the number and types of DET-type oxidoreductases are limited. The aim of this research is the development of a versatile method to create a DET-type oxidoreductase complex based on the SpyCatcher/SpyTag technique by preparing SpyCatcher-fused heme c and SpyTag-fused non-DET-type oxidoreductases, and by the in vitro formation of DET-type oxidoreductase complexes. A heme c containing an electron transfer protein derived from Rhizobium radiobacter (CYTc) was selected to prepare SpyCatcher-fused heme c. Three non-DET-type oxidoreductases were selected as candidates for the SpyTag-fused enzyme: fungi-derived flavin adenine dinucleotide (FAD)-dependent glucose dehydrogenase (GDH), an engineered FAD-dependent d-amino acid oxidase (DAAOx), and an engineered FMN-dependent l-lactate oxidase (LOx). CYTc-SpyCatcher (CYTc-SC) and SpyTag-Enzymes (ST-GDH, ST-DAAOx, ST-LOx) were prepared as soluble molecules while maintaining their redox properties and catalytic activities, respectively. CYTc-SC/ST-Enzyme complexes were formed by mixing CYTc-SpyCatcher and SpyTag-Enzymes, and the complexes retained their original enzymatic activity. Remarkably, the heme domain served as an electron acceptor from complexed enzymes by intramolecular electron transfer; consequently, all constructed CYTc-SC/ST-Enzyme complexes showed DET ability to the electrode, demonstrating the versatility of this method. [ABSTRACT FROM AUTHOR]

This review focuses on the development of microbial biofuel cells to demonstrate how similar principles apply to the development of bioelectronic devices. The low specificity of microorganism-based amperometric biosensors can be exploited in designing microbial biofuel cells, enabling them to consume a broader range of chemical fuels. Charge transfer efficiency is among the most challenging and critical issues while developing biofuel cells. Nanomaterials and particular redox mediators are exploited to facilitate charge transfer between biomaterials and biofuel cell electrodes. The application of conductive polymers (CPs) can improve the efficiency of biofuel cells while CPs are well-suitable for the immobilization of enzymes, and in some specific circumstances, CPs can facilitate charge transfer. Moreover, biocompatibility is an important issue during the development of implantable biofuel cells. Therefore, biocompatibility-related aspects of conducting polymers with microorganisms are discussed in this review. Ways to modify cell-wall/membrane and to improve charge transfer efficiency and suitability for biofuel cell design are outlined. [ABSTRACT FROM AUTHOR]

Nitrate (NO3−-N) and nitrites (NO2−-N) are common pollutants in various water bodies causing serious threats not only to aquatic, but also to animals and human beings. In this study, we developed a strategy for efficiently reducing nitrates in microbial fuel cells (MFCs) powered by a granular activated carbon (GAC)-biocathode. GAC was developed by acclimatizing and enriching denitrifying bacteria under a redox potential (0.3 V) generated from MFCs. Thus, using the formed GAC-biocathode we continued to study their effect on denitrification with different cathode materials and circulation speeds in MFCs. The GAC-biocathode with its excellent capacitive property can actively reduce nitrate for over thirty days irrespective of the cathode material used. The stirring speed of GAC in the cathode showed a steady growth in potential generation from 0.25 V to 0.33 V. A rapid lag phase was observed when a new carbon cathode was used with enriched GAC. While a slow lag phase was seen when a stainless-steel cathode was replaced. These observations showed that effective storage and supply of electrons to the GAC plays a crucial role in the reduction process in MFCs. Electrochemical analysis of the GAC properties studied using electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and zeta potential showed distinct properties with different abiotic and biocathode conditions. We found that the enrichment of electrotrophic bacteria on GAC facilitates the direct electron transfer in the cathode chamber for reducing NO3−-N in MFCs as observed by scanning electron microscopy. [ABSTRACT FROM AUTHOR]

Untreated and treated (with chemical electron acceptors like Fe (III) oxide and Fumarate) sludge samples were subjected to microbial fuel cell (MFC) studies. The study focused on both anode biofilm as well as the anolyte consortia developed in the MFC reactor after treatment. Through impedance and voltammogram data, it was seen that untreated inoculum, when used in MFC followed a synergistic electron transfer mechanism (EET), dual EET (3.726 ± 0.130 mWm−3). The Fe (III) oxide treatment promoted the development of electrogenic biofilm that followed direct electron transfer (DET) mechanism (5.439 ± 0.009 mWm−3), whereas Fumarate treatment promoted the growth of electrogenic microbes in anolyte and followed mediator-based electron transfer (MET) mechanism (4.500 ± 0.0009 mWm−3). The microbial cultures like Alcaligenes sp. and Pseudomonas sp. were isolated from MFC reactors having Fe (III) oxide treated biofilm and Fumarate treated anolyte respectively. The occurrence of these microbes indicates their role in the EET mechanism adopted by various MFC reactors. [ABSTRACT FROM AUTHOR]

• CNT/3DG hybrid is a suitable supporting material for enzyme immobilization. • CNT/3DG/Gox/GCE fabricate a modified bioanode in a glucose/O 2 EBFC. • maximum power density of 253.36 µWcm-2 at 0.65 V is harvested. • The immobilized GOx lifetime was increased to 210 days. Long-lasting, high-performance enzymatic biofuel cells (EBFCs) are among the most promising candidates for renewable and green power generation. There are two main drawbacks to EBFCs: 1) short enzyme lifetime due to the enzyme's (protein) denaturation; and 2) low electron transfer efficiency as the enzyme's active site is deeply buried inside the non-conductive protein structure of the enzyme. In this study, a hybrid of carbon nanotubes (CNT) and three-dimensional graphene (3DG) was used to immobilize the glucose oxidase (GOx) on the surface of a glassy carbon electrode (GCE) to fabricate a modified bioanode in a glucose/O 2 EBFC. This facile and low-cost CNT/3DG hybrid is a solution to the problems of instability, short lifespan, and poor electron transport in EBFCs. The exceptional electrocatalytic activity of the porous CNT/3DG hybrid is demonstrated by its effective immobilization of GOx and ability to collect energy from glucose oxidation by direct electron transfer (DET) while preserving the structure of the enzyme. The CNT acts like a tiny electrical wire that reaches and harvests the electron directly from the flavin adenine dinucleotide (FAD) (the redox center of GOx) and transfers it to the electrode surface, thus enhancing the performance of the EBFC. The immobilized GOx lifetime was increased to 210 days with a maximum power density of 253.36 µWcm-2 at 0.65 V. These results demonstrate the attractive capability of the CNT/3DG hybrid in the development of efficient biofuel cells and biosensors that employ enzymes in their structures. [Display omitted] [ABSTRACT FROM AUTHOR]

The incorporation of conductive materials to enhance electron transfer in bioelectrochemical systems (BES) is considered a promising approach. However, the specific effects and mechanisms of these materials on trichloroethylene (TCE) reductive dechlorination in BES remains are not fully understood. This study investigated the use of magnetite nanoparticles (MNP) and biochars (BC) as coatings on biocathodes for TCE reduction. Results demonstrated that the average dechlorination rates of MNP-Biocathode (122.89 μM Cl·d−1) and BC-Biocathode (102.88 μM Cl·d−1) were greatly higher than that of Biocathode (78.17 μM Cl·d−1). Based on MATLAB calculation, the dechlorination rate exhibited a more significantly increase in TCE-to-DCE step than the other dechlorination steps. Microbial community analyses revealed an increase in the relative abundance of electroactive and dechlorinating populations (e.g., Pseudomonas , Geobacter , and Desulfovibrio) in MNP-Biocathode and BC-Biocathode. Functional gene analysis via RT-qPCR showed the expression of dehalogenase (RDase) and direct electron transfer (DET) related genes was upregulated with the addition of MNP and BC. These findings suggest that conductive materials might accelerate reductive dechlorination by enhancing DET. The difference of physicochemical characteristics (e.g. particle size and specific surface area), electron transfer enhancement mechanism between MNP and BC as well as the reduction of Fe(III) by hydrogen may explain the superior dechlorination rate observed with MNP-Biocathode. [Display omitted] • Conductive materials accelerate reductive dechlorination rate by enhancing direct electron transfer. • Conductive materials promote the proportion of electroactive and dechlorinating microorganisms. • The manifold differences between magnetite nanoparticles and biochars can explain the higher dechlorination rate in magnetite nanoparticles-biocathode. [ABSTRACT FROM AUTHOR]

Anaerobic microbiologically influenced corrosion (MIC) of Fe (0) metals causes great harm to the environment and economy, which depends on the key electron transfer process between anaerobic microorganisms and Fe (0) metals. However, the key electron transfer process in microbiota dominating MIC remains unclear, especially for methanogenic microbiota wildly distributed in the environment. Herein, three different methanogenic microbiota (Methanothrix , Methanospirillum , and Methanobacterium) were acclimated to systematically investigate electron transfer pathways on corroding Q235A steel coupons. Results indicated that microbiota dominated by Methanothrix , Methanospirillum , or Methanobacterium accelerated the steel corrosion mainly through direct electron transfer (DET) pathway, H 2 mediated electron transfer (HMET) pathway, and combined DET and HMET pathways, respectively. Compared with Methanospirillum dominant microbiota, Methanothrix or Methanobacterium dominant microbiota caused more methane production, higher weight loss, corrosion pits with larger areas, higher corrosion depth, and smaller corrosion pits density. Such results reflected that the DET process between microbiota and Fe (0) metals decided the biocorrosion degree and behavior of Fe (0) metals. This study insightfully elucidates the mechanisms of methanogenic microbiota on corroding steels, in turn providing new insights for anti-corrosion motives. • Methanobacterium or Methanothrix dominant microbiota promoted surface corrosion. • Methanospirillum dominant microbiota promoted pit corrosion. • Methanothrix dominant microbiota caused the most serious corrosion of steel. • Direct electron transfer process of microbiota decided the biocorrosion of steel. [ABSTRACT FROM AUTHOR]

Periprosthetic infection is one of the most devastating complications following orthopaedic surgery. Rapid detection of an infection can change the treatment pathway and improve outcomes for the patient. In here, we propose a miniaturized lactate biosensor developed on a flexible substrate and integrated on a small-form bone implant to detect infection. The methods for lactate biosensor fabrication and integration on a bone implant are fully described within this study. The system performance was comprehensively electrochemically characterised, including with L-lactate solutions prepared in phosphate-buffered saline and culture medium, and interferents such as acetaminophen and ascorbic acid. A proof-of-concept demonstration was then conducted with ex vivo ovine femoral heads incubated with and without exposure to Staphylococcus epidermidis. The sensitivity, current density and limit-of-detection levels achieved by the biosensor were 1.25 μA mM−1, 1.51 μA.M−1.mm−2 and 66 μM, respectively. The system was insensitive to acetaminophen, while sensitivity to ascorbic acid was half that of the sensitivity to L-lactate. In the ex vivo bone model, S. epidermidis infection was detected within 5 h of implantation, while the control sample led to no change in the sensor readings. This pioneering work demonstrates a pathway to improving orthopaedic outcomes by enabling early infection diagnosis. • Flexible lactate sensor with direct electron transfer using a 3rd-generation enzyme. • Biosensor testing against lactate solutions in PBS/culture medium and interferents. • Biosensor integration in a bone implant for detection of infection in orthopaedics. • Low-powered implant bioelectronics with wireless data readings of lactate levels. • E x vivo detection in ovine femoral heads infected by Staphylococcus epidermidis. [ABSTRACT FROM AUTHOR]

• Electrochemical property of FDH lacking a cytochrome subunit (ΔC FDH) was studied. • DET of ΔC FDH was observed at porous Au electrodes modified with thiols. • Electrode-active site of ΔC FDH was estimated by kinetic analysis of DET signals. • Uncharged aromatic thiols on porous Au electrodes enhance DET of ΔC FDH. • An enhancement mechanism for DET by aromatic thiols was proposed. A heterotrimeric membrane-bound d -fructose dehydrogenase (FDH) from Gluconobacter japonicus , composed of subunits L, C, and S, shows strong direct electron transfer (DET)-type bioelectrocatalytic performance in the two-electron oxidation of d -fructose. The electrode-active site of the enzyme is the heme c moiety in the subunit C. A previous study reported the construction of the subunit L/S subcomplex (ΔC FDH) lacking the subunit C. In this work, we attempted to realize the DET-type reaction of ΔC FDH using porous gold electrodes functionalized with several thiols, and investigated the influence of the thiols on DET. Kinetic analysis of the steady-state catalytic waves revealed that the electrode-active site of ΔC FDH is the [3Fe-4S] iron-sulfur cluster in the L subunit. The experimental results demonstrated that uncharged aromatic thiols on the electrode enhance the DET-type reaction of ΔC FDH. Based on the pH-dependent profile of the DET-type activity and the surface conditions of the modified thiols evaluated by electrochemical reductive desorption, we suggest an enhancement mechanism for DET by aromatic thiols. [ABSTRACT FROM AUTHOR]

Developing non radical systems for antibiotic degradation is crucial for addressing the inefficiency of conventional radical systems. In this study, novel magnetic-modified sludge biochar (MASBC) was synthesized to significantly enhance the oxidative degradation of sulfamethoxazole (SMX) by ferrate (Fe (VI)). In the Fe (VI)/MASBC system, 90.46% of SMX at a concentration of 10 μM and 49.34% of the total organic carbon (TOC) could be removed under optimal conditions of 100 μM of Fe (VI) and 0.40 g/L of MASBC within 10 min. Furthermore, the Fe (VI)/MASBC system was demonstrated with broad-spectrum removal capability towards sulfonamides in single or mixture. Quenching experiments, EPR analyses, and electrochemical experiments revealed that direct electron transfer (DET) and •O 2 − were mainly responsible for the removal of SMX, with functional groups (e.g., -OH, C=O) and Fe-O (redox of Fe (III)/Fe (II)) acting as the active sites, while the probe experiments showed that Fe (IV)/Fe (V) made a minor contribution to the degradation of SMX. Benefiting from the DET, the Fe (VI)/MASBC system exhibited a wide pH adaptation range (e.g., from 5.0 to 10.0) and strong anti-interference ability. The N atoms and their neighboring atoms in SMX were the prior degradation sites, with the cleavage of bond and ring opening. The degradation products showed low or non-toxicity according to ECOSAR program assessment. The removal of SMX remained within a reasonable range of 71.33%–90.46% over five consecutive cycles. Also, the Fe (VI)/MASBC system was demonstrated to be effectively applied for successful SMX removal in various water matrices, including ultrapure water, tap water, lake water, Yangtze River water, and wastewater. Therefore, this study offered new insights into the mechanism of Fe (VI) oxidation and would contribute to the efficient treatment of organic pollutants. [Display omitted] • Magnetized sludge-based biochar enhanced the oxidation properties of Fe (VI). • Direct electron transfer and.•O 2 − contributed to SMX removal in Fe (VI)/MASBC system. • N atoms and their neighboring atoms in SMX were the primary degradation sites. • Oxygen-containing functional groups and Fe-O served as the main active sites. • Fe (VI)/MASBC system had strong adaptability to various real water environments. [ABSTRACT FROM AUTHOR]

Electrodes composed of nanostructured surfaces have been shown to be promising electrocatalysts, offering enhanced catalytic activity. Gold nanoparticles (AuNPs) are often used to produce such a nanostructured surface. However, these are commonly prepared using a reductant and surfactant, often hindering the electrocatalytic application of the surface. In the present study, we fabricated a nanostructured gold thin film electrode derived from a solution of AuNPs containing only Au, H, and O, which was prepared by solution plasma processing in the presence of hydrogen peroxide. The concentrated AuNP dispersed aqueous solution was dropped onto the glassy carbon substrate and then dried to form a AuNP thin film at room temperature. The resultant gold film was nanostructured owing to the presence of the AuNPs, could be used directly in electrochemical reactions without pre-treatment (such as surfactant/reductant removal), and gave stable voltammetric responses, indicating its suitability as an electrode material. In addition, the AuNP thin film electrode exhibited enhanced electrocatalytic activity compared with conventional planar gold electrodes, which can be attributed to its surface nanostructure. In particular, the electrocatalytic current in the hydrogen evolution reaction was more than 10 times larger at the nanostructured AuNP thin film electrode than at a planar gold electrode. Further, the AuNP thin film electrode enabled direct electron transfer (DET) between a redox enzyme, neuroglobin, and the electrode surface, while the planar electrode showed no DET response. The results indicate the usefulness of the present AuNP thin film electrode, prepared by a reagent-free process, for various electrocatalytic applications.

Inorganic iron-sulfur clusters (Fe–S) have emerged as potential electrode anchors and electron shuttles for enzymatic biosensor systems involving nicotinamide adenine dinucleotide (NAD) cofactor. In this work, we report how other cofactors that include nicotinamide adenine dinucleotide phosphate (NADP), flavin adenine dinucleotide (FAD), pyrroloquinoline quinone (PQQ), and coenzyme-Q10 (CoQ10), electrochemically respond to iron-sulfur mediated gold electrodes. Here, select redox cofactors were tethered onto gold electrode supports via inorganic Fe–S (FeS, FeS2, Fe2S3, Fe3S4) and organic Fe–S (Ferredoxin), using the layer-by-layer self-assembly. A combination of docking simulations, secondary ion mass spectrometry, and Fourier-transform infrared spectroscopy revealed the formation of iron carbonyl bonds between FeS and NAD, leading to electrochemical interactions between Fe–S and the redox cofactors. Electrochemical studies confirmed both inorganic and organic iron-sulfur molecules can anchor cofactors onto the gold electrode supports. Up to a 300% increase in anodic peak currents was observed when the inorganic Fe–S replaced the organic counterpart, Ferredoxin, to wire the cofactors onto the gold electrodes. Electrode kinetic measurements revealed the reversible and quasi-reversible behavior of the inorganic and organic Fe–S electrodes, respectively. Reductive desorption studies indicated Fe3S4 closely followed by FeS, forming robust wiring associations with gold by displaying the highest surface coverage, desorption power, and the lowest resistance. Consequently, the inorganic iron-sulfur clusters facilitate an effective method for fabricating enzymatic biosensors for many analytes based on the enzyme-cofactor combination.

2-Methyl imidazole and Zn(NO3)2 were proposed as raw materials for the hemoglobin (Hb) accommodation matrix ZIF-8, and the encapsulation of Hb molecules onto ZIF-8 was achieved by hydrogen bond interaction and electrostatic attraction. The electrochemical sensing performance of H2O2 for Hb integration into metal–organic framework (MOF) material was analyzed systematically and evaluated quantitatively to identify the key factor in limiting the sensing efficiency. The influence of mutual interactions between Hb and ZIF-8 on the spectroscopic features and sensing efficiency for complexes with Hb incorporation was estimated by multiple techniques to explore the impact of those interactions on the relevant steps involved in the catalysis. The results from experiments revealed that the hydrogen bond between Hb and ZIF-8 led to non-oriented aggregation of enzyme molecules onto the MOF with reduced active surface area and inhibited catalytic activity. The relatively weak interaction between heme protein and MOF material did not result in serious distortion in the structure or chemical nature of Hb entrapment into ZIF-8. The presence of a mutual interaction between Hb and ZIF-8 did not impose a serious negative effect on heterogeneous electron shuttle (28.2–91.2 s−1) for the heme site in Hb entrapment into ZIF-8, which was classified into a typical quasi-reversible and thin-film confining mode with the involvement of four electrons and two H+ ions. The results showed that hb-induced electrocatalysis was limited by the step of substrate diffusion (1.3 × 10−3 s−1). The Hb-based electrode demonstrated excellent affinity to dissolved H2O2 (Michaelis constant: 28.0 μM) and desirable sensing performance (low detection limitation: 3.7 μM). Illustration of the procedure for preparing the electrode based on MOF material (ZIF-8) with hemoglobin accommodation via physical adsorption and electrostatic attraction, which gives rise to non-oriented distribution of hemoglobin attachment onto the surface and inner cavity of ZIF-8. [ABSTRACT FROM AUTHOR]

Cellobiose dehydrogenase (CDH) is capable of oxidizing cellobiose and related carbohydrates and generating electrical current at carbon‐based electrodes through direct electron transfer (DET) or mediated electron transfer (MET) mechanisms. As a result, CDHs have been utilized as biocatalysts in biosensors and biofuel cell anodes. A novel engineered ascomycetous Class II CDH with enhanced glucose activity was tested as a bioelectrocatalyst for application to DET or MET‐based glucose biosensors with the electrode component amount selection optimized for maximum current in 5 mM glucose solutions. The optimised DET biosensor showed a similar sensitivity and 3‐fold lower KM,app when compared to non‐optimised DET sensor based on the same engineered CDH. The optimized MET biosensor had a similar KM,app to non‐optimized MET biosensor. However, it showed 15‐fold improvement in jmax and 17‐fold improvement in sensitivity over the DET biosensor. The sensor signals are not affected by the presence of oxygen, although operation in artificial serum results in 43 % and 28 % lower sensitivity for the DET and MET sensors, respectively. While no individually tested potential interferent breaches a mean absolute relative difference of 20 % of the current, the cumulative co‐operative effect in complex media, such as artificial serum, decreases the glucose oxidation current signal. [ABSTRACT FROM AUTHOR]

[Display omitted] • A novel Bi-Ti 4 O 7 anode was successfully fabricated for efficient RAN electro-oxidation. • Bi coating elevated the oxygen vacancy concentration and charge transfer on Ti 4 O 7 surface. • Direct electron transfer played an important role in RAN electro-oxidation by Bi-Ti 4 O 7. • Bi-Ti 4 O 7 showcased a wide applicability under complex and realistic conditions. • RAN toxicity could be significantly reduced after electro-oxidation by Bi-Ti 4 O 7. Ranitidine (RAN) is a commonly prescribed medication that poses ecological risks due to its potential to transform into the carcinogenic byproduct N-nitrosodimethylamine (NDMA) in the environment. Herein, we synthesized a novel bismuth-coated titanium oxide (Bi-Ti 4 O 7) electrode using a straightforward coating method and evaluated its effectiveness in the electrochemical oxidation of RAN. The Bi-Ti 4 O 7 electrode achieved a degradation efficiency of 90.41 % for an initial RAN concentration of 10 mg/L within 120 min, greatly outperforming the pristine Ti 4 O 7 electrode. The incorporation of Bi enhances the oxygen vacancies and charge transfer characteristics, facilitating direct electron transfer between RAN and the Bi-Ti 4 O 7 electrode. The potential degradation pathways of RAN by the Bi-Ti 4 O 7 electrode included C–S bond cleavage, C–N bond cleavage, and electrophilic oxidation. No NDMA was generated during the oxidation process, and the toxicity of RAN oxidation intermediates was greatly reduced. The Bi-Ti 4 O 7 electrode demonstrated excellent stability, maintaining over 90 % degradation efficiency across 30 cycles, and showed wide applicability across different pH levels and wastewater types with negligible Bi ion release (0.148 µg/L). These results underscore the potential of Bi-Ti 4 O 7 as an effective and sustainable solution for RAN-polluted wastewater treatment. [ABSTRACT FROM AUTHOR]

Transition metal nitrides have demonstrated potential as activators for peroxymonosulfate (PMS) in the degradation of contaminants because of their intrinsic properties. However, there is a pressing need to develop effective strategies to enhance catalyst stability and reusability across consecutive cycles. Herein, a peanut-shaped nitrogen-doped carbon framework-encapsulated cobalt nitride material (Co 4 N/NC) was successfully synthesized by directly pyrolyzing commercial ethylenediaminetetraacetic acid disodium cobalt salt in air, followed by a nitridation process. The peanut-like Co 4 N/NC structure, which features encapsulated Co 4 N functional centers and heterojunction interfaces between Co 4 N and NC, were advantageous for enhancing active centers and facilitating electron transfer, resulting in improved efficiency in degrading tetracycline hydrochloride (TCH). The Co 4 N/NC-500/PMS system exhibited an impressive TCH removal rate of 95.48 % within 20 min. Furthermore, the Co 4 N/NC-500/PMS system maintained consistent activity across varying solution pH values, in the presence of different anions and organic matter, and demonstrated high degradation efficiency after five cycles. Quenching experiments and electron paramagnetic resonance tests validated the presence of radicals, non-radicals, and direct electron transfer pathways for TCH degradation. Additionally, the degradation intermediates and pathways were elucidated, along with an assessment of the toxicity of these intermediate products. This study provides an efficient and simple strategy that leverages spatial confinement and heterojunction effects to enhance pollutant degradation through PMS activation. [ABSTRACT FROM AUTHOR]

Schematic representing the influence of non-native graphite-specific peptide (Gr; IMVTESSDYSSY) as molecular binder on NAD+/NADH-dependent CO 2 reductase (CR) immobilization to assess the influence of site-specific fusion on graphite electrode towards electrocatalyzed CO 2 reduction to formate. [Display omitted] • NAD+/NADH-dependent CO 2 reductase for catalytic CO 2 to formate inter-conversion. • Solid-binding peptide enabled direct electron transfer towards effective CO 2 reduction. • Graphite surface characteristics benefitted in capacitive enzyme-electrode interfacing. • Amino-acids position in peptide inserts impact CO 2 -specific binding for redox catalysis. • Graphite in enzyme-electrode system include low-costs for bioelectronics upscaling. NAD+/NADH-dependent CO 2 reductase (CR) adapted from Candida methylica (E.C. 1.17.1.9) was introduced with a non-native graphite-specific peptide (Gr; IMVTESSDYSSY) as molecular binder to modify the native enzyme (CR-WT) with peptide insertion at N, C and NC terminus (CR-GrN, CR-GrC and CR-GrNC) to assess the influence of site-specific fusion on electrode binding. Graphite surface-binding activity relative to the electrode topography was evaluated for both native and synthetic CRs to establish the enzyme-electrode interfacing potentiality for efficient electron channelling. Impact of site-specific peptide fusion and amino-acids positioning was assessed for the active site binding availability and adsorption/desorption capability towards competent CO 2 -based redox catalysis. Solid-binding peptide and graphite surface interactive ability on direct electron transfer was studied with structural, enzymatic and electrochemical characterizations for efficient CO 2 electrosynthesis. Overall, enzymatic CO 2 reduction to formate based on interactive potentiality of enzyme-electrode complex with peptide modifications and graphite surface towards possibility of bioelectronics upscaling was depicted. [ABSTRACT FROM AUTHOR]

[Display omitted] • Carboxyl plays a key role in enhancing CO 2 -reduction methanogensis. • CH 4 yield increased by up to 2–3 times by loading the carboxyl group. • H 2 evolution was enhanced, thus enhancing H 2 -mediated electron transfer. • Protein in EPS, genes related to cytochrome c and Methanosaeta were increased. • The direct electron transfer capacity was enhanced. Microbial electrosynthesis is a promising technology that recovers energy from wastewater while converting CO 2 into CH 4. Constructing a biocathode with both strong H 2 -mediated and direct electron transfer capacities is crucial for efficient startup and long-term stable CH 4 production. This study found that introducing carboxyl groups onto the cathode effectively enhanced both electron transfer pathways, improving the reduction rate and coulombic efficiency of CH 4 production and increasing the CH 4 yield by 2–3 times. Carboxyl groups decreased the overpotential for H 2 evolution and increased current density, thereby enhancing H 2 -mediated electron transfer. Additionally, carboxyl groups increased the relative abundance of Methanosaeta by 3%-10%, doubled the protein content in extracellular polymeric substances, and boosted the expression of cytochrome c -related genes, thereby enhancing direct electron transfer capacity. These findings present a novel and efficient approach for constructing a stable, high-performance biocathode, contributing to energy recovery and CO 2 fixation. [ABSTRACT FROM AUTHOR]

Heptafluoropropylene oxide dimer acid (HFPO-DA), as an alternative to perfluorooctanoic acid (PFOA), has been shown to pose similar environmental and health risks as other perfluorinated compounds. The electrochemical-based advanced oxidation processes are promising techniques for the treatment of perfluorinated compounds, and the boron-doped diamond (BDD) anode could degrade HFPO-DA under mild conditions. However, the roles of radicals in the degradation and how to overcome the steric hindrance of the –CF 3 branch on the carboxyl group were not yet clear. In this study, we investigated the degradation mechanism of HFPO-DA on the BDD anode. Instead of other non-active anodes (PbO 2 and SnO 2 electrodes), HFPO-DA can be degradable on the BDD electrode with a rate constant logarithmic correlation to the applied current density. The hydroxyl radical (•OH) was one of the key factors in the degradation of HFPO-DA, accounting for almost 89% of the significant effect, and the direct electron transfer was the rate-limiting step in the degradation reaction. Physicochemical characterization including field emission scanning electron microscope (FE-SEM), X-ray photo-electron spectroscopy (XPS), water contact angle, and electrochemical property indicated that the BDD electrode was fluorinated after electrolysis, the electrode surface became more hydrophobic due to the bonding of –C x F y , leading to a decrease in the electrochemically active area. Moreover, degradation products (pentafluoropropionic acid, trifluoroacetic acid, and fluorine ion) were detected and the mass balance of carbon and fluorine was calculated during the degradation. Therefore, a degradation mechanism for HFPO-DA was proposed, which involved direct electron transfer, decarboxylation, radical reaction, decarboxylation, and decarboxylation. The de–CF 3 step initiated the fluorination of the BDD electrode, which was initiated by the defluorination process. This study contributes to the understanding of the electro-oxidative degradation of perfluoroalkyl ether carboxylic acids and provides guidance for the application of electrochemical advanced oxidation processes. [Display omitted] • BDD anode could effectively degrade HFPO-DA. • DET was the rate-limiting step of the degradation. • •OH was an indispensable but not the only factor in degradation. • Surface fluorination may promote the DET on BDD anode. [ABSTRACT FROM AUTHOR]

[Display omitted] • Evidence for direct electron transfer denitrification (DETD) was observed. • The nitrogen removal rate reached 60.17 mmol NO 3 −-N L−1 day−1 m−2. • Thiobacillus highly expressed extracellular multiheme c -type cytochrome genes. Cathodes can play a pivotal role as electron donors for denitrification in wastewater treatment plants, facilitating the removal of nitrogen-containing compounds such as nitrate. Denitrifying bacteria can either accept electrons directly from the electrode or indirectly via electron shuttling intermediaries generated at the cathode. In this study, comparative analyses were conducted between two denitrifying electrochemical reactors with biocathodes poised at different potentials, one that generated H 2 through water electrolysis and one that did not (−500 mV and −800 mV vs. SHE). Nitrogen removal efficiencies were much higher in the −500 mV (vs. SHE) system, >93 % compared to only 20 %. Notably, a biofilm formed exclusively on the −500 mV (vs. SHE) cathode, predominantly composed of Thiobacillus species, with the majority most similar to Thiobacillus strain 65-29. Metatranscriptomics showed that genes from Thiobacillus strain 65-29 encoding outer surface c -type cytochrome and porin proteins that could facilitate direct electron uptake, were markedly up-regulated in the −500 mV (vs. SHE) reactor. Reduced sulfur species generated by the cathode and sulfate reducing bacteria might have also provided a source of electrons for nitrate reduction. In fact, Thiobacillus in the −500 mV (vs. SHE) system exhibited elevated expression of genes involved in thiosulfate (soxABXYZ), sulfite (soeABC), and sulfide (fccAB) oxidation. These findings offer valuable insights into the mechanisms of autotrophic denitrification by Thiobacillus in mixed culture systems and provide practical guidance for engineering applications. [ABSTRACT FROM AUTHOR]

A disposable paper-based glucose biosensor with direct electron transfer (DET) of glucose oxidase (GOX) was developed through simple covalent immobilization of GOX on a carbon electrode surface using zero-length cross-linkers. This glucose biosensor exhibited a high electron transfer rate (ks, 3.363 s−1) as well as good affinity (km, 0.03 mM) for GOX while keeping innate enzymatic activities. Furthermore, the DET-based glucose detection was accomplished by employing both square wave voltammetry and chronoamperometric techniques, and it achieved a glucose detection range from 5.4 mg/dL to 900 mg/dL, which is wider than most commercially available glucometers. This low-cost DET glucose biosensor showed remarkable selectivity, and the use of the negative operating potential avoided interference from other common electroactive compounds. It has great potential to monitor different stages of diabetes from hypoglycemic to hyperglycemic states, especially for self-monitoring of blood glucose.

While flow-through anodic oxidation (FTAO) technique has demonstrated high efficiency to treat various refractory waste streams, there is an increasing concern on the secondary hazard generation thereby. In this study, we developed an integrated system that couples FTAO and cathodic reduction processes (termed FTAO-CR) for sustainable treatment of chlorine-laden industrial wastewater. Among four common electrode materials (i.e. , Ti 4 O 7 , β -PbO 2 , RuO 2 , and SnO 2 -Sb), RuO 2 flow-through anode exhibited the best pollutant removal performance and relatively low ClO 3 and ClO 4 yields. Because of the significant scavenging effect of Cl− in real wastewater treatment, the direct electron transfer process played a dominant role in contaminant degradation for both active and nonactive anodes though active species (i.e. , active chlorine) were involved in the subsequent transformation of the organic matter. A continuous FTAO-CR system was then constructed for simultaneous COD removal and organic and inorganic chlorinated byproduct control. The quality of the treated effluent could meet the national discharge permit limit at low energy cost (∼4.52 kWh m 3 or ∼0.035 kWh g 1-COD). Results from our study pave the way for developing novel electrochemical platforms for the purification of refractory waste streams whilst minimizing the secondary pollution. [Display omitted] • Flow-through anodic oxidation removes refractory organics from industrial effluent. • ClO 3 − forms during the efficient direct electron transfer process of oxidation. • Flow-through anodic oxidation is coupled with cathodic reduction (FTAO-CR). • FTAO-CR achieves simultaneous COD removal and chlorinated byproducts control. • Energy cost is ∼0.035 kWh g 1-COD to achieve discharge permit limit. [ABSTRACT FROM AUTHOR]

Enzymatic biosensors based on glucose oxidase has been proven to be one of the effective strategies for the detection of glucose and contributed to health improvements. Therefore, research and debates to date are ongoing in an attempt to find the most effective way to detect this analyte using this enzyme as the recognition center. The 3rd generation biosensors using direct electron transfer (DET) type enzymes are a great way towards practical devices. In this work, we developed a simple method for the functionalization of glucose oxidase with redoxable ferrocene groups in chloroform. The enzyme retained its activity after storage in this organic solvent and after the functionalization procedures. This enzyme functionalization strategy was employed to develop the biosensing monolayer-based platforms for the detection of glucose utilizing the quasi-DET mechanism. As a result of an electrochemical regeneration of the catalytic center, the formation of harmful H2O2 is minimized during enzymatic electrocatalysis. [ABSTRACT FROM AUTHOR]

The simulation model of the direct electron transfer (DET)-type bioelectrocatalysis at a porous electrode indicates that the catalytic current is proportional to the real surface area (A), when A is small. When A is large, the catalytic current is saturated at a limited value controlled by the mass transfer in the porous structure. In this study, the bioelectrocatalytic currents at porous electrodes are analyzed based on their charging currents. Comparisons among the carbon composites constructed with Vulcan and exfoliated graphite (JSP) show that the interface between the Vulcan and JSP particles is more suitable for the DET-type bioelectrocatalysis of bilirubin oxidase (BOD) than that between the particles of the same type. In particular, the most suitable properties were achieved at a Vulcan : JSP ratio of 1 : 1 in the composite electrode. In these composites, the multipoint contact between the BOD molecule and electrode seems to result in a higher DET-type bioelectrocatalytic activity.

Enzymatic Biofuel Cell (EBC) represents a promising green source since it is capable of harvesting electricity from renewable and abundantly available biofuels using enzymes as catalysts. Nevertheless, nowadays long-term stability and low power output are currently the main concerns. To this end, several research studies focus on using complex tridimensional and highly expensive nanostructures as electrode support for enzymes. This increases cell performance whilst drastically reducing the economic feasibility needed for industrial viability. Thus, this paper analyzes a novel flow-based EBC consisting of covalent immobilized GOx (bioanode) and Laccase (biocathode) on a commercial flat conductive polymer. A suitable immobilization technique based on covalent ligands is carried out to enhance EBC durability. The experimental characterization demonstrates that the cell generates power over three weeks, reaching 590 mV and 2.41 µW cm−2 as maximum open circuit voltage and power density, respectively. The most significant contributions of this configuration are definitely ease of implementation, low cost, high scalability, and reproducibility. Therefore, such a design can be considered a step forward in the viable EBC industrialization process for a wide range of applications. [ABSTRACT FROM AUTHOR]

Glucose oxidase (GOx) was immobilized between aligned carbon nanotube films (ACNTFs) using a cross-stacking method without any purification and functionalization processes. Direct electron transfer of GOx was achieved on the ACNTFs with reversible redox peaks observed at the formal potential of –0.448 V. When applied to the determination of glucose by cyclic voltammetry, the prepared enzymatic electrode exhibited a linear range from 1 to 5 mM under mediator-free physiological conditions, which makes it a promising candidate for the development of third-generation glucose biosensors. A new strategy was developed to immobilize glucose oxidase (GOx) between aligned carbon nanotube films, which achieved direct electron transfer of GOx for the electrocatalytic determination of glucose under mediator-free physiological conditions. [ABSTRACT FROM AUTHOR]