Mediated Electron Transfer

Microbial fuel cells (MFCs) hold great promise for the simultaneous treatment of wastewater and electricity production. However, the electricity recovery is currently poor, typically <10% of what is theoretically possible, and the extracellular electron transfer mechanisms are poorly understood. The influence of using cocultures as a way of improving substrate turnover rate and hence electricity produced was investigated using synthetic wastewater as a substrate. Cocultures used were (i) Shewanella oneidensis and Clostridium beijerinckii; (ii) combinations of Geobacter sulphurreducens, Clostridium beijerinckii and Saccharomyces cerevisiae. The relative abundances test showed mutualistic relationship within the cocultures and was determined using RT-PCR at the end of the investigation. The coculture of S.oneidensis and C.beijerinkii gave a maximum power density of 87mWm-2 compared to 60 mWm-2 for C.beijerinckii alone and 48 mWm-2 for S.oneidensis alone. In the second study the bes...

Dissimilatory metal reducing bacteria can exchange electrons extracellularly and hold great promise for their use in simultaneous wastewater treatment and electricity production. This study investigated the role of riboflavin, an electron carrier, in the decolourisation of Congo red in microbial fuel cells (MFCs) using Shewanella oneidensis MR-1 as a model organism. The contribution of the membrane-bound protein MtrC to the decolourisation process was also investigated. Within the range of riboflavin concentrations tested, 20 µM was found to be the best with >95% of the dye (initial concentration 200 mg/L) decolourised in MFCs within 50 h compared to 90% in the case where no riboflavin was added. The corresponding maximum power density was 45 mW/m(2). There was no significant difference in the overall decolourisation efficiencies of Shewanela oneidensis MR-1 ΔMtrC mutants compared to the wild type. However, in terms of power production the mutant produced more power (Pmax 76 mW/m...

In the present study three different strains of Escherichia coli (JM109-a native "wild type" strain, JM109/ pBSD 1300-a strain overproducing the membrane anchor domain of Bacillus subtilis succinate-quinone reductase, SQR, a protein that contains two transmembraneously arranged heme groups and JM109/ pLUV 1900-a strain overproducing cytochrome c 550 from B. subtilis, a protein where the cytochrome domain is anchored to the membrane with a transmembrane helix) were immobilised on the surface of a spectrographic graphite electrode and tested for electrical communication using mediators. Such compounds as ferricyanide, 2,6-dichlorophenolindophenol (DCPIP) and ubiquinone (Q 0) were used as soluble mediators and two flexible osmium redox polymers; poly(1-vinylimidazole) 12-[Os-(4,4-dimethyl-2,2-di'pyridyl) 2 Cl 2 ] 2+/+ (osmium redox polymer I) and poly(vinylpyridine)-[Os-(N,Nmethylated-2,2-biimidazole) 3 ] 2+/3+ (osmium redox polymer II) were co-immobilised with the bacterial cells onto the electrode surface. The effects of applied potential, buffer pH and different substrates were compared for the different combinations bacterial strains-mediators. Through the introduction of the cytochromes in the bacterial membrane it was established that it had great effect on the ability of the bacterial cells to effectively communicate with artificial mediators. The introduction of the transmembraneously arranged heme groups of B. subtilis made it possible for this strain to communicate with the Os-polymers, whereas the introduction of the cytochrome c 550 had an effect especially increasing ability of Q 0 to act as an efficient e − acceptor.

We have constructed and characterised a glucose sensor using glucose oxidase (GOD) covalently attached to carboxylic acid polyethyleneglycol (PEG), called (PEG-GOD). This modified enzyme was entrapped afterwards within poly(3,4-ethylenedioxythiophene) (PEDT) films electrogenerated on glassy carbon (GC) electrodes. The composite (PEG-GOD/PEDT) film is more porous than the film without enzyme (PEDT+PEG). Data from electrochemical quartz microbalance (ECQM) and pH-stat experiments indicate a good relative activity of the modified enzyme, ca. 12-15%. Amperometric measurements, using ferrocenemethanol as the redox mediator, confirms that the modified enzyme is catalytically active. The effect of film thickness was also investigated. The sensitivities were quite similar for modified-GOD electrodes (ca. 3 mA cm −2 M −1) and unmodified-GOD electrodes (ca. 2.7 mA cm −2 M −1) but a better stability was obtained with modified PEG-GOD electrodes.

This work describes a simple methodology to bio-functionalize flat Au(1 1 1) electrodes through the onestep reaction between gold nanoparticles (AuNPs), carbon disulfide and a secondary amine (epinephrine) and an aminoacid (tryptophan). The process relies on the in situ dithiocarbamate formation between carbon disulphide and amine groups and also on the strong linkage between sulfur and gold. The redox behavior of modified gold with epinephrine or tryptophan, prepared from both ethanolic and aqueous solutions confirms their covalent immobilization and reveals a significant increase of their amount on the electrode due to the presence of AuNPs. Electrochemical reductive desorption in basic solution provided qualitative information on the amount of sulfur linked the gold surface and complements the redox studies. The co-immobilization of an enzyme (glucose oxidase, GOx) and gold nanoparticles, using carbon disulfide has been also tested. The presence of GOx on modified Au(1 1 1) electrodes has been confirmed by electrochemical detection of the catalytic oxidation of glucose in the presence of a redox mediator and through the evaluation of H 2 O 2 reduction, formed during the catalytic reaction. XPS analysis, topographic and phase imaging by atomic force microscopy (AFM) further confirmed surface modification by AuNPs and also their functionalization. The successful one-step amine adsorption, in the presence of AuNPs, from aqueous solutions reveals the potential of this method in the construction of nanostructured biosensing interfaces.

One of the processes most studied in bioenergetic systems in recent years is the oxygen reduction reaction (ORR). An important challenge in bioelectrochemistry is to achieve this reaction under physiological conditions. In this study, we used bilirubin oxidase (BOD) from Myrothecium verrucaria, a subclass of multicopper oxidases (MCOs), to catalyse the ORR to water via four electrons in physiological conditions. The active site of BOD, the T2/T3 cluster, contains three Cu atoms classified as T2, T3α, and T3β depending on their spectroscopic characteristics. A fourth Cu atom; the T1 cluster acts as a relay of electrons to the T2/T3 cluster. Graphite electrodes were modified with BOD and the direct electron transfer (DET) to the enzyme, and the mediated electron transfer (MET) using an osmium polymer (OsP) as a redox mediator, were compared. As a result, an alternative resting (AR) form was observed in the catalytic cycle of BOD. In the absence and presence of the redox mediator, the AR direct reduction occurs through the trinuclear site (TNC) via T1, specifically activated at low potentials in which T2 and T3α of the TNC are reduced and T3β is oxidized. A comparative study between the DET and MET was conducted at various pH and temperatures, considering the influence of inhibitors like H 2 O 2 , F − , and Cl −. In the presence of H 2 O 2 and F − , these bind to the TNC in a non-competitive reversible inhibition of O 2. Instead; Cl − acts as a competitive inhibitor for the electron donor substrate and binds to the T1 site.

Prior studies indicated that –OH and/or –NH 2 substituent containing auxochrome compounds (e.g., 2-aminophenol and 1-amino-2-naphthol) could act as electron shuttles (ESs) to stimulate wastewater decolorization and bioelectricity generation in microbial fuel cells (MFCs). This study provided first-attempt to disclose how and why thionine-associated textile dyes (i.e., azure A and azure C) could also own such redox-mediating capabilities in MFCs. Due to the presence of iminium part as mediating group, –N(CH 3 ) 2 or –N(CH 3 )H substituent could effectively mediate electron transport compared to –NH 2 substituent for bioelectricity generation in MFCs. For dye-bearing wastewater treatment, the presence of electron-mediating textile dyes (e.g., thionine, azure A and azure C) in MFCs is promising to stimulate biodegradation of organics and bioelectricity generation. With such ESs as stimulants, using MFC as operation strategy would be cost-effective for wastewater treatment as oxidation ...

Through equivalent concepts of electrochemical resistances, this first-attempt study disclosed optimal operation mode(s) of microbial fuel cells (MFCs)-assisted dye decontamination. With supplementation of energy substrate(s) and textile dye(s), internal resistances could be significantly reduced for effective electron transfer (ET) to dye decolorization (DD) and bioelectricity generation (BG). The findings indicated that increases in nutrient substrates and azo dye-reactive blue 160 would favor DD in MFC systems. According to analyses for the best contacting patterns of reacting systems, a batch or plug-flow MFC system is the most appropriate mode of operation for DD. The also suggested that continuous flow-modes of operation seem to be relatively favorable for BG. Apparently, whether DD or BG is electron-transfer dominant in MFCs directly depended on reaction order(s) of nutrient substrate and dye concentrations.

Optimization of mass transfer within a porous material is a highly promising strategy to improve the efficiency of electrode reactions. Herein, nanoporous gold (NPG) modified gold electrode is investigated to study the mass transport of ascorbic acid (AA), which oxidation process is characterized by an electrochemical coupled 1 st order chemical reaction, commonly termed as EC1 reaction. The template-assisted synthesis of NPG results into the formation of a highly pure and porous film of gold. However, the surface porosity of NPG depends on the choice of electrodeposition parameters, such as deposition time (td) and potential (Ed) and the size of the substrate. Such porosity variation of NPG strongly influences the voltammetric profile of AA anodic reaction, displaying sigmoidal, non-symmetric and symmetric peak features. The analysis of mass transport behaviour of AA reveals a combination of diffusion and thin layer EC1 mechanism, predominance of which is determined by the Ed and td selected for NPG synthesis, as well as the size of the Au substrate. The mass transport of AA on NPG prepared on Au microelectrodes experienced a significant diffusion from bulk solution, owing to the larger pores, which permits the easier exchange of redox species between the NPG volume and the bulk solution. On the contrary, mass transport of AA on NPG deposited on big Au electrode has a significant contribution of thin layer diffusion, attributed to the smaller surface pores of NPG, which limits the exchange of AA and its oxidized form from the bulk solution.

We have constructed and characterised a glucose sensor using glucose oxidase (GOD) covalently attached to carboxylic acid polyethyleneglycol (PEG), called (PEG-GOD). This modified enzyme was entrapped afterwards within poly(3,4-ethylenedioxythiophene) (PEDT) films electrogenerated on glassy carbon (GC) electrodes. The composite (PEG-GOD/PEDT) film is more porous than the film without enzyme (PEDT+PEG). Data from electrochemical quartz microbalance (ECQM) and pH-stat experiments indicate a good relative activity of the modified enzyme, ca. 12-15%. Amperometric measurements, using ferrocenemethanol as the redox mediator, confirms that the modified enzyme is catalytically active. The effect of film thickness was also investigated. The sensitivities were quite similar for modified-GOD electrodes (ca. 3 mA cm −2 M −1) and unmodified-GOD electrodes (ca. 2.7 mA cm −2 M −1) but a better stability was obtained with modified PEG-GOD electrodes.

This review tended to decipher the expression of electron transfer capability (e.g., biofilm formation, electron shuttles, swarming motility, dye decolorization, bioelectricity generation) to microbial fuel cells (MFCs). As mixed culture were known to perform better than pure microbial cultures for optimal expression of electrochemically stable activities to pollutant degradation and bioenergy recycling, Proteus hauseri isolated as a "keystone species" to maintain such ecologically stable potential for power generation in MFCs was characterized. P. hauseri expressed outstanding performance of electron transfer (ET)-associated characteristics [e.g., reductive decolorization (RD) and bioelectricity generation (BG)] for electrochemically steered bioremediation even though it is not a nanowire-generating bacterium. This review tended to uncover taxonomic classification, genetic or genomic characteristics, enzymatic functions, and bioelectricity-generating capabilities of Proteus spp. with perspectives for electrochemical practicability. As a matter of fact, using MFCs as a tool to evaluate ET capabilities, dye decolorizer(s) could clearly express excellent performance of simultaneous bioelectricity generation and reductive decolorization (SBG and RD) due to feedback catalysis of residual decolorized metabolites (DMs) as electron shuttles (ESs). Moreover, the presence of reduced intermediates of nitroaromatics or DMs as ESs could synergistically augment efficiency of reductive decolorization and power generation. With swarming mobility, P. hauseri could own significant biofilm-forming capability to sustain ecologically stable consortia for RD and BG. This mini-review evidently provided lost episodes of great significance about bioenergysteered applications in myriads of fields (e.g., biodegradation, biorefinery, and electro-fermentation).

Glucose sensors are essential tools for diabetes patients to use in monitoring their blood glucose levels. However, to be able to detect glucose in non-invasively collected physiological fluids, such as tears and urine, the sensitivity of these glucose sensors must be significantly higher than sensors that are currently used to detect glucose concentrations in blood. Increasing the specific surface area of enzyme-based glucose sensors through the use of ordered porous gold electrodes has been shown to enhance the sensitivity of these sensors. The enzyme-based ordered porous gold glucose sensor was demonstrated to be suitable in detecting glucose concentrations ranges that are similar to those occurring in tears. Although sensitivity of the glucose sensor is enhanced, the saturation threshold of the sensor is lowered. Further optimizations of the porous gold electrodes are required to eliminate signal saturation of these improved sensors.

Dissimilatory metal reducing bacteria can exchange electrons extracellularly and hold great promise for their use in simultaneous wastewater treatment and electricity production. This study investigated the role of riboflavin, an electron carrier, in the decolourisation of Congo red in microbial fuel cells (MFCs) using Shewanella oneidensis MR-1 as a model organism. The contribution of the membrane-bound protein MtrC to the decolourisation process was also investigated. Within the range of riboflavin concentrations tested, 20 µM was found to be the best with >95% of the dye (initial concentration 200 mg/L) decolourised in MFCs within 50 h compared to 90% in the case where no riboflavin was added. The corresponding maximum power density was 45 mW/m(2). There was no significant difference in the overall decolourisation efficiencies of Shewanela oneidensis MR-1 ΔMtrC mutants compared to the wild type. However, in terms of power production the mutant produced more power (Pmax 76 mW/m...

In the present study three different strains of Escherichia coli (JM109-a native "wild type" strain, JM109/ pBSD 1300-a strain overproducing the membrane anchor domain of Bacillus subtilis succinate-quinone reductase, SQR, a protein that contains two transmembraneously arranged heme groups and JM109/ pLUV 1900-a strain overproducing cytochrome c 550 from B. subtilis, a protein where the cytochrome domain is anchored to the membrane with a transmembrane helix) were immobilised on the surface of a spectrographic graphite electrode and tested for electrical communication using mediators. Such compounds as ferricyanide, 2,6-dichlorophenolindophenol (DCPIP) and ubiquinone (Q 0) were used as soluble mediators and two flexible osmium redox polymers; poly(1-vinylimidazole) 12-[Os-(4,4-dimethyl-2,2-di'pyridyl) 2 Cl 2 ] 2+/+ (osmium redox polymer I) and poly(vinylpyridine)-[Os-(N,Nmethylated-2,2-biimidazole) 3 ] 2+/3+ (osmium redox polymer II) were co-immobilised with the bacterial cells onto the electrode surface. The effects of applied potential, buffer pH and different substrates were compared for the different combinations bacterial strains-mediators. Through the introduction of the cytochromes in the bacterial membrane it was established that it had great effect on the ability of the bacterial cells to effectively communicate with artificial mediators. The introduction of the transmembraneously arranged heme groups of B. subtilis made it possible for this strain to communicate with the Os-polymers, whereas the introduction of the cytochrome c 550 had an effect especially increasing ability of Q 0 to act as an efficient e − acceptor.

Due to the recent interest in enzymatic biofuel cells (BFCs), sugar oxidizing enzymes other than the commonly used glucose oxidase are gaining more importance as possible bioelements of implantable microscale-devices, which can, for example, be used in biosensors and pacemakers. In this study we used rational and semi-rational protein design to improve the catalytic activity of the enzyme pyranose 2-oxidase (P2Ox) with its alternative soluble electron acceptors 1,4-benzoquinone and ferricenium ion, which can serve as electron mediators, to possibly boost the power output of enzymatic BFCs. Using a screening assay based on 96-well plates, we identified the variant H450G, which showed lower K M and higher k cat values for both 1,4-benzoquinone and ferricenium ion compared to the wild-type enzyme, when either d-glucose or d-galactose were used as saturating electron donors. Besides this variant, we analyzed the variants V546C and T169G/V546C for their possible application in enzymatic BFCs. The results obtained in homogeneous solution were compared with those obtained when P2Ox was immobilized on the surface of graphite electrodes and either "wired" to an osmium redox polymer or using soluble 1,4-benzoquinone as mediator. According to the spectrophotometrically determined kinetic constants, the possible energy output, measured in flow injection analysis experiments with these variants, increased up to 4-fold compared to systems employing the wild-type enzyme. Our results show that by increasing the catalytic activity of the redox enzyme P2Ox with its alternative electron acceptors 1,4-benzoquinone and ferricenium ion, it is possible to achieve a higher energy output of an enzymatic BFC when using the same concentration of sugar substrate.

We demonstrate self-assembly, characterization and bioelectrocatalysis of redox-active cyclodextrin-coated nanoparticles. The nanoparticles with host-guest functionality are easy to assemble and permit entrapment of hydrophobic redox molecules in aqueous solution. Bis-pyrene-ABTS encapsulated nanoparticles were investigated electrochemically and spectroscopically. Their use as electron shuttles is demonstrated via an intraelectron transfer chain between neighboring redox units of clustered particles (Dh,DLS = 195 nm) and the mono- and trinuclear Cu sites of bilirubin oxidases. Enhanced current densities for mediated O2 reduction are observed with the redox nanoparticle system compared to equivalent bioelectrode cells with dissolved mediator. Improved catalytic stability over 2 days was also observed with the redox nanoparticles, highlighting a stabilizing effect of the polymeric architecture. Bioinspired nanoparticles as mediators for bioelectrocatalysis promises to be valuable for ...

This paper reports cyclic voltammograms of cytochrome c on recessed nanodisk-array electrodes (RNEs) based on nanoporous films (11, 14 or 24 nm in average pore diameter; 30 nm thick) derived from polystyrene-poly(methylmethacrylate) diblock copolymers. The faradic current of cytochrome c was observed on RNEs, indicating the penetration of cytochrome c (hydrodynamic diameter ≈ 4 nm) through the nanopores to the underlying electrodes. The faradic current on RNEs with 11-and 14-nm nanopores mainly originated from cytochrome c adsorbed on the underlying electrodes, whereas the current on RNEs with 24-nm pores was diffusion-controlled. Interestingly, the diffusion-controlled current of cytochrome c was significantly smaller than that estimated from the faradic current of 1,1-ferrocenedimethanol on the RNEs. The smaller faradic current suggested more effective decrease in the diffusion coefficient of cytochrome c as compared to that of 1,1-ferrocenedimethanol, which probably reflected enhanced steric and chemical interactions within the nanopores. Comparison between experimental data and results of finite-element computer simulations made it possible to assess the structure of the nanoporous films and the diffusion coefficients of redox species within the nanopores.

Dissimilatory metal reducing bacteria can exchange electrons extracellularly and hold great promise for their use in simultaneous wastewater treatment and electricity production. This study investigated the role of riboflavin, an electron carrier, in the decolourisation of Congo red in microbial fuel cells (MFCs) using Shewanella oneidensis MR-1 as a model organism. The contribution of the membrane-bound protein MtrC to the decolourisation process was also investigated. Within the range of riboflavin concentrations tested, 20 µM was found to be the best with >95% of the dye (initial concentration 200 mg/L) decolourised in MFCs within 50 h compared to 90% in the case where no riboflavin was added. The corresponding maximum power density was 45 mW/m(2). There was no significant difference in the overall decolourisation efficiencies of Shewanela oneidensis MR-1 ΔMtrC mutants compared to the wild type. However, in terms of power production the mutant produced more power (Pmax 76 mW/m...

Wiring glucose oxidase in the membrane with an immobilized mediator is possible due to the diffusion ability of the latter, if the enzyme containing membrane is formed according to the proposed protocol, including exposing proteins to water-organic mixtures with the high content of organic solvent. In the course of the study, the new glucose oxidase mediator, unsubstituted phenothiazine, was discovered. The diffusion coefficient of the mediator in the resulting membrane is independent of the presence of enzyme. The cyclic voltammograms of the enzyme electrode after appearance of the only glucose in solution obtain a well-defined catalytic shape, which is normally observed for both the enzyme and the mediator in solution. Analytical performances of the resulting biosensor are comparable to the advanced second generation ones, which, however, require covalent linking of the mediator either to the membrane forming polymer or to the enzyme. Even without such covalent linking, the reported biosensor is characterized by an appropriate long-term operational stability allowing reagentless sensing.

This article describes the controllable construction of a layered superstructure of gold nanooctahedra via a molecularly mediated assembly and their electrochemical application as a host matrix toward the oxidation of glucose. The layered superstructures of the Au nanooctahedra were confirmed with atomic force microscopy, X-ray electron spectroscopy, and cyclic voltammograms. The effects of the interface of gold spherical nanoparticles and the different layered interface of gold nanooctahedra on the electrochemical responses to Fe(CN) 6 3-and glucose were investigated in detail. The glucose biosensor, which uses the Au nanooctahedra layer structure as a matrix, shows excellent electrocatalytic activities, such as a high level of sensitivity, a fast response, and a wide response range.

Graphene (GR) and Carbon nanotubes (CNTs) are highperformance carbon non-materials. Their excellent electron transfer and electrocatalytic properties have attracted much interest in the field of electrochemistry. In this work graphene as a nanohybrid composites based on noble metal/metal alloy were applied to study the electrochemical sensing of hydrogen peroxide and glucose. The main contents were described as follows. We have developed the design and synthesis of a new type of high-performance nanocatalysts, i.e. deposit tiny palladium and platinum alloy nanoparticles on the surface of freestanding ultrathin nanoporous gold film (Pd&Pt@NPG), and transfer it to the surface of graphene paper for the fabricate free-standing and flexible Pd&Pt@NPG/graphene paper, which enable to be used for real-time tracking the secretion of H2O2 in different types of living human cells.

Dissimilatory metal reducing bacteria can exchange electrons extracellularly and hold great promise for their use in simultaneous wastewater treatment and electricity production. This study investigated the role of riboflavin, an electron carrier, in the decolourisation of Congo red in microbial fuel cells (MFCs) using Shewanella oneidensis MR-1 as a model organism. The contribution of the membrane-bound protein MtrC to the decolourisation process was also investigated. Within the range of riboflavin concentrations tested, 20 µM was found to be the best with >95% of the dye (initial concentration 200 mg/L) decolourised in MFCs within 50 h compared to 90% in the case where no riboflavin was added. The corresponding maximum power density was 45 mW/m(2). There was no significant difference in the overall decolourisation efficiencies of Shewanela oneidensis MR-1 ΔMtrC mutants compared to the wild type. However, in terms of power production the mutant produced more power (Pmax 76 mW/m...

The electrochemical characterization of a class II cellobiose dehydrogenase (CDH), from the ascomycete fungus Neurospora crassa, adsorbed on graphite (G), was performed in regard to direct (DET) and mediated electron transfer (MET). The effects of the applied potential, mediator (1,4 benzoquinone) concentration and flow carrier pH on the amperometric response of the G/CDH modified electrodes were investigated under flow conditions. From the calibration curves, recorded at two pH values (5.2 and 7.0) for nine different sugars, the kinetic and the analytical parameters were evaluated under DET and MET operation modes. These results together with those obtained from long term operational stability measurements showed that: (i) for all nine investigated sugars the sensitivity was higher for MET than for DET and for pH 5.2 compared to pH 7.0; (ii) irrespective of DET or MET operation mode, the sensitivity of the new enzyme towards the investigated sugars decreased in the following sequen...

Nanoporous gold (np-Au) represents a novel nanostructured bulk material with very interesting 10 perspectives in heterogeneous catalysis. Its monolithic porous structure and the absence of a support or other stabilizing agents opens up unprecedented possibilities to tune structure and surface chemistry in order to adapt the material to specific catalytic applications. We investigated three of these tuning options in more detail: change of the porosity by annealing, increase of activity by the deposition of oxides and change of activity and selectivity by bimetallic effects. As an example for the latter case, the effect of Ag impurities will be discussed. The presence and concentration of Ag can be correlated to the availability of active oxygen. While for the oxidation of CO the activity of the catalyst can be significantly enhanced when increasing the content of Ag, we show for the oxidation of methanol that the selectivity is shifted from partial to total oxidation. In a second set of experiments, two different metal-oxides were deposited on np-Au, praseodymia and titania. In both 20 cases, the surface chemistry changed significantly. The activity of the catalyst for oxidation of CO was increased by up to one order of magnitude after modification. Finally, we used adsorbate controlled coarsening to tune the structure of np-Au. In this way, even gradients in the pore-and ligament size could be induced, taking advantage of mass transport phenomena.

The electrochemical characterization of a class II cellobiose dehydrogenase (CDH), from the ascomycete fungus Neurospora crassa, adsorbed on graphite (G), was performed in regard to direct (DET) and mediated electron transfer (MET). The effects of the applied potential, mediator (1,4 benzoquinone) concentration and flow carrier pH on the amperometric response of the G/CDH modified electrodes were investigated under flow conditions. From the calibration curves, recorded at two pH values (5.2 and 7.0) for nine different sugars, the kinetic and the analytical parameters were evaluated under DET and MET operation modes. These results together with those obtained from long term operational stability measurements showed that: (i) for all nine investigated sugars the sensitivity was higher for MET than for DET and for pH 5.2 compared to pH 7.0; (ii) irrespective of DET or MET operation mode, the sensitivity of the new enzyme towards the investigated sugars decreased in the following sequence: cellobiose > lactose > (cellotriose ≈ cellopentaose) > > (maltotriose ≈ maltotetraose≈ maltopentaose) > (maltose ≈ glucose); (iii) for all tested substrates, the apparent CDH affinity was roughly higher in DET than in MET operation mode.

The electrochemical characterization of a class II cellobiose dehydrogenase (CDH), from the ascomycete fungus Neurospora crassa, adsorbed on graphite (G), was performed in regard to direct (DET) and mediated electron transfer (MET). The effects of the applied potential, mediator (1,4 benzoquinone) concentration and flow carrier pH on the amperometric response of the G/CDH modified electrodes were investigated under flow conditions. From the calibration curves, recorded at two pH values (5.2 and 7.0) for nine different sugars, the kinetic and the analytical parameters were evaluated under DET and MET operation modes. These results together with those obtained from long term operational stability measurements showed that: (i) for all nine investigated sugars the sensitivity was higher for MET than for DET and for pH 5.2 compared to pH 7.0; (ii) irrespective of DET or MET operation mode, the sensitivity of the new enzyme towards the investigated sugars decreased in the following sequence: cellobiose > lactose > (cellotriose ≈ cellopentaose) > > (maltotriose ≈ maltotetraose≈ maltopentaose) > (maltose ≈ glucose); (iii) for all tested substrates, the apparent CDH affinity was roughly higher in DET than in MET operation mode.

Progress in the development of commercially available non-enzymatic glucose sensors continues to be problematic due to issues regarding selectivity, reproducibility and stability. Overcoming these issues is a research challenge of significant importance. This study reports a novel fabrication process using a double-layer self-assembly of (3 mercaptopropyl)trimethoxysilane (MPTS) on a gold substrate and co-deposition of a platinum–copper alloy. The subsequent electrochemical dealloying of the less noble copper resulted in a nanoporous platinum structure on the uppermost exposed thiol groups. Amperometric responses at 0.4 V vs. Ag/AgCl found the modification to be highly selective towards glucose in the presence of known interferants. The sensor propagated a rapid response time <5 s and exhibited a wide linear range from 1 mM to 18 mM. Additionally, extremely robust stability was attributed to enhanced attachment due to the strong chemisorption between the gold substrate and the ex...

Glucose sensors are essential tools for diabetes patients to use in monitoring their blood glucose levels. However, to be able to detect glucose in non-invasively collected physiological fluids, such as tears and urine, the sensitivity of these glucose sensors must be significantly higher than sensors that are currently used to detect glucose concentrations in blood. Increasing the specific surface area of enzyme-based glucose sensors through the use of ordered porous gold electrodes has been shown to enhance the sensitivity of these sensors. The enzyme-based ordered porous gold glucose sensor was demonstrated to be suitable in detecting glucose concentrations ranges that are similar to those occurring in tears. Although sensitivity of the glucose sensor is enhanced, the saturation threshold of the sensor is lowered. Further optimizations of the porous gold electrodes are required to eliminate signal saturation of these improved sensors.

An effect of permeabilisation and lyophilisation of the yeast cells Hansenula polymorpha on their electrochemical behaviour in the presence of mediators, substrates (formaldehyde, glucose, methanol, ethanol), and cofactors (NAD + , NADP + , NADH, NADPH, glutathione) has been studied. Two amperometric techniques differing in the cell immobilisation methods were applied. The cells of a wild strain (356) and mutant strains (C-105 and KCA 33) of the yeast, grown in the presence of glucose or methanol, were used in the experiments. The intact cells revealed the highest reduction rates of mediators, 2,6-dichlorphenolindophenol (DCIP) and 2,4-benzoquinone (BQ), as measured by amperometry. The addition of formaldehyde significantly enhanced the response, if the cells were grown in the presence of glucose. The permeabilised cells showed the lowest current level in the presence of DCIP and BQ and no response to the addition of formaldehyde and NAD +. However, the addition of NADH gave significant current surge. All these phenomena imply that the permeabilised cells lost cofactors and the activity of dehydrogenases producing NADH, but they remained the activity of NADHubiquinone oxidoreductase and of some components of the electron transport chain. The electrochemical behaviour of the lyophilised cells shows they are heterogeneous. The partial degradation of the outer membrane of the cells after their lyophilisation was electrochemically confirmed.

Graphene (GR) and Carbon nanotubes (CNTs) are high performance
carbon non-materials. Their excellent electron transfer and
electrocatalytic properties have attracted much interest in the field of
electrochemistry. In this work graphene as a nanohybrid composites
based on noble metal/metal alloy were applied to study the
electrochemical sensing of hydrogen peroxide and glucose. The main
contents were described as follows. We have developed the design and
synthesis of a new type of high-performance nanocatalysts, i.e. deposit
tiny palladium and platinum alloy nanoparticles on the surface of freestanding
ultrathin nanoporous gold film (Pd&Pt@NPG), and transfer it to
the surface of graphene paper for the fabricate free-standing and flexible
Pd&Pt@NPG/graphene paper, which enables to be used for real-time
tracking the secretion of H2O2 in different types of living human cells.

In this study, nanostructured porous gold electrodes were prepared by the anodization of gold in the presence of oxalic acid or glucose as a reductant, and applied as scaffolds for direct electron transfer (DET)-type bioelectrocatalysis. Gold cations generated in the anodization seem to be reduced by the reductant to construct a porous gold structure. The DET-type performance of the electrode was examined using two DET-type model enzymes, bilirubin oxidase (BOD) and peroxidase (POD), for the four-electron reduction of dioxygen and the two-electron reduction of peroxide, respectively. BOD and POD on the anodized porous gold electrodes exhibited well-defined sigmoidal steady-state waves corresponding to DET-type bioelectrocatalysis. Scanning electron microscopy images revealed sponge-like pores on the electrodes. The anodized porous gold electrodes demonstrate promise as scaffolds for DET-type bioelectrocatalysis.

We performed numerical simulations on an extremely fast, mediated, electron transfer-type bioelectrocatalytic reaction using a microband electrode. The simulations under fast-enzyme-kinetics conditions predicted that the decrement of the current density by increaseing the microband thickness would effectively improve the upper limit of detection. These predictions were accurate for an ultrathin-ring with thickness of 100 nm and gold leaf with thickness of 10 μm electrodes, acting as novel amperometric glucose sensors with FAD-dependent glucose dehydrogenase. The gold leaf electrode provided pseudo-steady-state currents which were proportional to the glucose concentration up to a concentration of 20 times higher than the mediator concentration.

The electrocatalytic evolution of oxygen gas is investigated at manganese oxide nanorods (nano-MnO x) modified Au, Pt and GC electrodes in a wide range of pH values, ranging from highly acidic to highly basic. Morphological investigation has been carried out by a scanning electron microscopy (SEM), which revealed the deposition of nano-MnO x in a nanorod morphology. A significant enhancement of the electrocatalytic activity of the Au, Pt and GC electrodes towards the oxygen evolution reaction (OER) was observed upon the electrodeposition of nano-MnO x onto the aforementioned electrodes. The effect of the surface coverage of the manganese oxide and the pH of the electrolyte was investigated to seek an optimization. The highest cathodic shift in the onset potential of the OER was obtained in 0.5 M KOH irrespective of the substrate whereas the optimum loading (surface coverage) was about ca. 52%. The origin of the enhancement of the OER is addressed with the assistance of an X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) techniques. The preferential electrodeposition of crystallographically oriented nano-MnOx (in the manganite phase, ␥-MnOOH) is thought to play the primary role in the observed enhancement.

Trametes trogii laccase has been studied as biocatalyst for the oxygen electro-reduction in three different systems: (i) soluble laccase was studied in solution; (ii) an enzyme monolayer was tethered to a gold surface by dithiobis N-succinimidyl propionate (DTSP), with a soluble osmium pyridine-bipyridine redox mediator in both cases. The third case (iii) consisted in the sequential immobilization of laccase and the osmium complex derivatized poly(allylamine) self-assembled layer-by-layer (LbL) on mercaptopropane sulfonate modified gold to produce an all integrated and wired enzymatic oxygen cathode. The polycation was the same osmium complex covalently bound to poly-(ally-lamine) backbone (PAH-Os), the polyanion was the enzyme adsorbed from a solution of a suitable pH so that the protein carries a net negative charge. The adsorption of laccase was studied by monitoring the mass uptake with a quartz crystal microbalance and the oxygen reduction electrocatalysis was studied by linear scan voltammetry. While for the three cases, oxygen electrocatalysis mediated by the osmium complex was observed, for tethered laccase direct electron transfer in the absence of redox mediator was also apparent but no electrocatalysis for the oxygen reduction was recorded in the absence of mediator in solution. For the fully integrated LbL self-assembled laccase and redox mediator (case iii) a catalytic reduction of oxygen could be recorded at different oxygen partial pressures and different electrolyte pH. The tolerance of the reaction to methanol and chloride was also investigated.

Schematic shows the morphology of the adsorbed BSA layer on nanoporous gold. Initial response of the electrode from biofouling resulted in faradaic current decay followed by its regeneration due to slow diffusion of analytes through the fouled layer.

A nanostructured gold surface consisting of closely packed outwardly growing spikes is investigated for the electrochemical detection of dopamine and cytochrome c. A significant electrocatalytic effect for the electrooxidation of both dopamine and ascorbic acid at the nanostructured electrode was found due to the presence of surface active sites which allowed the detection of dopamine in the presence of excess ascorbic acid to be achieved by differential pulse voltammetry. By simple modification with a layer of Nafion, the enhanced electrocatalytic properties of the nanostructured surface was maintained while increasing the selectivity of dopamine detection in the presence of interfering species such as excess ascorbic and uric acids. Also, upon modification of the nanostructured surface with a monolayer of cysteine, the electrochemical response of immobilised cytochrome c in two distinct conformations was observed. This opens up the possibility of using such a nanostructured surface for the characterisation of other biomolecules and in bio-electroanalytical applications.

The discovery by M.C. Potter in 1911 that some bacteria can generate electricity in devices called microbial fuel cells (MFCs) opened up a new opportunity in exploitation of microbes' potential; but limited interest was shown for some time. However, since 1980's research in this area has intensified. MFCs work on the principle that electricigens can oxidise substrates in an anode chamber releasing electrons and protons. The electrons go through an external circuit to a cathode chamber, while protons travel from the anode to the cathode through a membrane that separates the two chambers. Recombination of electrons and protons in the cathodic chamber completes the circuit in presence of an oxidant, typically oxygen. MFCs have promise in a number of areas including bioremediation, electricity production, biosensing and water desalination. To enhance feasibility of MFC technology in biotechnology sectors, a number of challenges need to be overcome. These include selection/design of efficient microbes, electrodes, membranes and chambers; better understanding of the mechanism and improving the process of electron transfer from the microorganisms to the electrodes; integration of MFCs in the wastewater treatment train; extending potential of MFCs from applications in bioremediation to bioproduction; and cost-effective scale-up of the reactors. This 'In-focus' section of the Journal of Chemical Technology and Biotechnology (JCTB) covers a total of six manuscripts (two review papers 1,6 and four original research articles 2-4) in microbial fuel cells reporting recent developments in MFC technology. Alleviating the accumulation of xenobiotics in the environment, has been subject to extensive research. However, the use of bioelectrochemical systems (BES) in remediation is a relatively new endeavour. Fernando et al. 1 report in a comprehensive review, the history of electromicrobiology, contaminants treated by MFC, and types of BES used, addressing BES advantages. The review concludes that BES is promising for both in situ and ex situ environmental remediation applications in a sustainable manner. Gomaa et al. 2 address the mechanism of concomitant degradation of the dye Congo red and bioelectricity generation using a recombinant strain of E. coli. Their work shows that although there seems to exist a link between dye decolourisation and COD values in their reactor, the efficiency of the system for generation of electricity is low. This highlights the importance of appropriately engineered efficient strains for multiple desired outputs. In another study investigating multifunctional

The interaction between platinum nanoparticles and polypyrrole or poly(3,4-ethylenedioxythiophene) soluble in apolar solvents has been investigated. Regular and full monolayers and multilayers are produced via solution processing of platinum nanoparticles and poly(3,4-ethylenedioxythiophene). The new layered materials, obtained with the layer-by-layer technique and characterized by cyclic voltammetry, ultraviolet-visible spectroscopy, atomic force microscopy, and conductivity, are uniform, stable, and mechanically robust. Catalytic oxidation of methanol on such layers occurs at levels comparable with that of a bare platinum surface.

This report describes the applications of cobalt tetracarboxylic acid phthalocyanine (CoTCAPc) self-assembled monolayer (SAM) immobilized onto a preformed 2-mercaptoethanol (Au-ME) SAM on gold surface (Au-ME-CoTCAPc SAM) as a potential amperometric sensor for the detection of hydrogen peroxide (H 2 O 2 ) at neutral pH conditions. The Au-ME-CoTCAPc SAM sensor showed a very fast amperometric response time of approximately 1 s, good linearity at the studied concentration range of up to 5 M with a coefficient R 2 = 0.993 and a detection limit of 0.4 M oxidatively. Also reductively, the sensor exhibited a very fast amperometric response time (∼1 s), linearity up to 5 M with a coefficient R 2 = 0.986 and a detection limit of 0.2 M. The cobalt tetracarboxylic acid phthalocyanine self-assembled monolayer was then evaluated as a mediator for glucose oxidase (GOx)-based biosensor. The GOx (enzyme) was immobilized covalently onto Au-ME-CoTCAPc SAM using coupling agents: Nethyl-N(3-dimethylaminopropyl) carbodiimide (EDC) and N-hydroxy succinimide (NHS), and the results demonstrated a good catalytic behavior. Kinetic parameters associated with the enzymatic and mediator reactions were estimated using electrochemical versions of Lineweaver-Burk and Hanes equation, and the stability of the sensor was tested. The biosensor (Au-ME-CoTCAPc-GOx SAM) electrode showed good sensitivity (7.5 nA/mM) with a good detection limit of 8.4 M at 3σ, smaller Michaelis-Menten constant (4.8 mM from Hanes plot) and very fast response time of approximately 5 s.

Dissimilatory metal reducing bacteria can exchange electrons extracellularly and hold great promise for their use in simultaneous wastewater treatment and electricity production. This study investigated the role of riboflavin, an electron carrier, in the decolourisation of Congo red in microbial fuel cells (MFCs) using Shewanella oneidensis MR-1 as a model organism. The contribution of the membrane-bound protein MtrC to the decolourisation process was also investigated. Within the range of riboflavin concentrations tested, 20 µM was found to be the best with >95% of the dye (initial concentration 200 mg/L) decolourised in MFCs within 50 h compared to 90% in the case where no riboflavin was added. The corresponding maximum power density was 45 mW/m2. There was no significant difference in the overall decolourisation efficiencies of Shewanela oneidensis MR-1 ΔMtrC mutants compared to the wild type. However, in terms of power production the mutant produced more power  (Pmax 76 mW/m2) compared to the wild type  (Pmax 46 mW/m2) which was attributed to higher levels of riboflavin secreted in solution. Decolourisation efficiencies in non-MFC systems (anaerobic bottles) were similar to those under MFC systems indicating that electricity generation in MFCs does not impair dye decolourisation efficiencies. The results suggest that riboflavin enhances both decolourisation of dyes and simultaneous electricity production in MFCs.

In this paper, we demonstrate the fabrication of a highly sensitive, selective, reproducible and reliable amperometric sensor for cyanide ion in aqueous solutions based on PEG modified ZnS nanoparticles (NPs). The fabricated cyanide ion sensor exhibits a very high and reproducible sensitivity of ∼79.7 A cm −2 nM −1 with a response time less than 2 s and a detection limit of 0.177 × 10 −6 mol L −1 , which is ∼16 times lower than maximum value of cyanide (2.7 × 10 −6 mol L −1 ) permitted by the World Health Organization (WHO) in drinking water. Moreover, various amperometric responses of some common interfering ions (Cl − , Br − , NO 3 − , SO 4 2− and CO 3 2− ) have also been recorded which confirm that the fabricated sensor is highly selective. This work demonstrates a simple, easy and promising technique for the detection of highly toxic and deadly cyanide ion in aqueous medium.

Au nanocorals are grown on gold screen-printed electrodes (SPEs) by using a novel and simple one-step electro-deposition process. Scanning electron microscopy was used for the morphological characterization. The devices were assembled on a three-electrode SPE system, which is flexible and mass producible. The electroactive surface area, determined by cyclic voltammetry in sulphuric acid, was found to be 0.07 ± 0.01 cm 2 and 35.3 ± 2.7 cm 2 for bare Au and nanocoral Au, respectively. The nanocoral modified SPEs were used to develop an enzymatic glucose biosensor based on H 2 O 2 detection. Au nanocoral electrodes showed a higher sensitivity of 48.3 ± 0.9 μA/ (mM cm 2) at +0.45 V vs Ag|AgCl compared to a value of 24.6 ± 1.3 μA/(mM cm 2) at +0.70 V vs Ag|AgCl obtained with bare Au electrodes. However, the modified electrodes have indeed proven to be extremely powerful for the direct detection of glucose with a non-enzymatic approach. The results confirmed a clear peak observed by using nanocoral Au electrode even in the presence of chloride ions at physiological concentration. Amperometric study carried out at + 0.15 V vs Ag | AgCl in the presence of 0.12 M NaCl showed a linear range for glucose between 0.1 and 13 mM.

A nanostructured gold surface consisting of closely packed outwardly growing spikes is investigated for the electrochemical detection of dopamine and cytochrome c. A significant electrocatalytic effect for the electrooxidation of both dopamine and ascorbic acid at the nanostructured electrode was found due to the presence of surface active sites which allowed the detection of dopamine in the presence of excess ascorbic acid to be achieved by differential pulse voltammetry. By simple modification with a layer of Nafion, the enhanced electrocatalytic properties of the nanostructured surface was maintained while increasing the selectivity of dopamine detection in the presence of interfering species such as excess ascorbic and uric acids. Also, upon modification of the nanostructured surface with a monolayer of cysteine, the electrochemical response of immobilised cytochrome c in two distinct conformations was observed. This opens up the possibility of using such a nanostructured surface for the characterisation of other biomolecules and in bio-electroanalytical applications.

In this paper, we report the results of the surface oxidation of platinum (Pt) microwires in aqueous sulfuric acid (H 2 SO 4 ) solutions by using a cyclic voltammetry technique. The Pt microwire chips were scanned and applied with voltage potentials ranging from 0 to 1.4 V in the H 2 SO 4 solution with concentrations from 0.0003 to 0.0018 M to find out the optimized concentration of sulfuric acid for the oxidation process. The cyclic voltammetry (CV) measurements show the oxidation peak at a potential range from 1.1 to 1.2 V. This is the peak of the interfacial place exchange of chemisorbed O(O chem ) and surface Pt atoms, resulting in the formation of a quasi-3D surface PtO lattice comprising Pt 2+ and O 2− . The oxidized surface Pt microwires were then functionalized with a 3-aminopropyl triethoxy silane (APTES) and glucose oxidase (GOD) was immobilized onto the functionalized chips for further application in glucose detection. By using this process, Pt microwires have been used for the successful detection of glucose in solution with concentrations in the range of 4-20 mM.