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Many healthcare and environmental monitoring devices use electrochemical techniques to detect and quantify analytes. With sensors progressively becoming smaller-particularly in point-of-care (POC) devices and wearable platforms-it creates the opportunity to operate them using less energy than their predecessors. In fact, they may require so little power that can be extracted from the analyzed fluids themselves, for example, blood or sweat in case of physiological sensors and sources like river water in the case of environmental monitoring. Self-powered electrochemical sensors (SPES) can generate a response by utilizing the available chemical species in the analyzed liquid sample. Though SPESs generate relatively low power, capable devices can be engineered by combining suitable reactions, miniaturized cell designs, and effective sensing approaches for deciphering analyte information. This review details various such sensing and engineering approaches adopted in different categories of SPES systems that solely use the power available in liquid sample for their operation. Specifically, the categories discussed in this review cover enzyme-based systems, battery-based systems, and ion-selective electrode-based systems. The review details the benefits and drawbacks with these approaches, as well as prospects of and challenges to accomplishing them., (© 2022 The Authors. Advanced Science published by Wiley-VCH GmbH.)

Abstract: Bilirubin oxidases, a sub class of the Multicopper oxidases family, were discovered in 1981 by Tanaka and Murao (Murao and Tanaka, 1981) and first used for the detection of bilirubin. Since 2001 and the pioneering work of Tsujimura, these BODs have attracted a lot of attention for the reduction of O2. Unlike laccases, these BODs are stable in physiological conditions (20mM phosphate buffer, pH 7.4, 0.14M NaCl, 37°C) and more than 120 papers have been published in the last 7 years. Here, we will first briefly describe some general features of BODs and then review the use of BODs for bilirubin biosensors and the recent achievements and progress toward the elaboration of efficient O2 reducing cathodes. [Copyright &y& Elsevier]

A multifunctional mediating system for bioelectrocatalytic oxidation of glucose is described. It comprises a conducting polymer, poly(3,4-ethylenodioxythiophene), carbon nanotubes modified with 4-(pyrrole-1-yl) benzoic acid that provide carboxyl groups that aid immobilization of glucose oxidase in a conductive three-dimensional network, tetrathiafulvalene that mediates electron exchange with the enzyme. This composite produces a system that is capable of effective oxidation of glucose to gluconic acid at pH 7. The current is concentration dependent to at least 60 mM and maintains 77 % of initial response for 35 days. Data supporting the utility of this system for electrochemical sensing and biofuel cell technology are presented. [ABSTRACT FROM AUTHOR]

Glucose oxidase (GOx) from Penicillium funiculosum 46.1 was purified using step-by-step ultrafiltration and it was characterized by spectrophotometric and spectrofluorometric methods. It was shown that spectra of GOx produced by P. funiculosum are typical for flavoproteins. Absorption spectrum has distinct peaks at 380 and 457 nm, excitation spectrum at 373 and 447 nm, and emission spectrum at 530 and 562 nm. The pH correlation of enzyme activity and catalytic characteristics in various buffer systems (phosphate (pH 5.0–9.0), citrate (pH 3.0–5.0), citrate-phosphate (pH 3.0–9.0), and universal (pH 3.0–9.0)) were registered. It was determined that the GOx is the most efficiently interacting with substrate (glucose) in phosphate buffer at pH 7.0 with kcat/ Km = 21,825 M −1 s −1. Interaction of several different redox mediators (9,10-phenantroline-5,6-dione, 9,10-phenanthrenequinone, N-methylphenazonium methyl sulfate, ferrocene, ferrocenecarboxylic acid, α-methylferrocenemethanol, ferrocenecarboxaldehyde) with GOx from P. funiculosum was investigated by evaluation of the difference in fluorescence emission intensity of FAD (oxidized) and FADH 2(reduced) forms. It was found that 9,10-phenantroline-5,6-dione and 9,10-phenanthrenequinone are the best redox mediators for this type of GOx. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

Abstract: Orientation of the enzyme macromolecule on the electrode surface is crucially important for the efficiency of the electron transport between the active site and electrode surface. The orientation can be controlled by affecting the surface charge and the pH of the buffer solution. In this contribution we study laccase physically adsorbed on gold surface modified by mercapto-ethanol, lipid and variously charged diazonium salts. Polarization Modulated Infrared Reflection Absorption Spectroscopy (PMIRRAS) enables the molecular orientation study of the protein molecule by comparison of the amide I to amide II band intensity ratios assuming that the protein secondary structure does not change. We observe significant differences in the intensity ratios depending on the kind of support and the enzyme deposition. The comparison of infrared spectra and cyclic voltammetry responses of variously prepared laccase layers reveals that the parallel orientation of beta-sheet moieties results in high enzyme activity. [Copyright &y& Elsevier]

Biofuel cells are devices for generating electrical energy directly from chemical energy of renewable biomass using biocatalysts such as enzymes. Efficient electrical communication between redox enzymes and electrodes is essential for enzymatic biofuel cells. Carbon nanotubes (CNTs) have been recognized as ideal electrode materials because of their high electrical conductivity, large surface area, and inertness. Electrodes consisting entirely of CNTs, which are known as CNT paper, have high surface areas but are typically weak in mechanical strength. In this study, cellulose (CL)-CNT composite paper was fabricated as electrodes for enzymatic biofuel cells. This composite electrode was prepared by vacuum filtration of CNTs followed by reconstitution of cellulose dissolved in ionic liquid, 1-ethyl-3-methylimidazolium acetate. Glucose oxidase (GOx), which is a redox enzyme capable of oxidizing glucose as a renewable fuel using oxygen, was immobilized on the CL-CNT composite paper. Cyclic voltammograms revealed that the GOx/CL-CNT paper electrode showed a pair of well-defined peaks, which agreed well with that of FAD/FADH the redox center of GOx. This result clearly shows that the direct electron transfer (DET) between the GOx and the composite electrode was achieved. However, this DET was dependent on the type of CNTs. It was also found that the GOx immobilized on the composite electrode retained catalytic activity for the oxidation of glucose. [ABSTRACT FROM AUTHOR]

Enhancement of direct bioelectrocatalysis of dioxygen reduction with laccase has been investigated using π-electron rich compounds, such as anthracene, for docking the laccase active site in close proximity to the electrode. These molecules have been shown to promote direct bioelectrocatalysis by orienting the enzyme to undergo fast, direct electron transfer. In this study, pyrene moieties have been covalently grafted to anthracene groups to be able to functionalize carbon nanotube walls to increase the number of binding sites on nanotubes for laccase. Current densities were compared to unmodified pyrene immobilized by non-covalent interactions on the hydroxylated carbon nanotubes (MWCNT-OH). The presence of pyrene derivatives leads to higher current density than regular MWCNT-OH. In both configurations, the covalent grafting of the anthracene group to pyrene improved electrocatalytic signals. Under normal aerated conditions, 1-pyrenemethanol-modified and 1-aminopyrene-modified electrodes produced 81.4±14.5μA/cm2 and 62.5±4.9μA/cm2, respectively, while for derivatives 1 and 2, current densities produced 186.4±10.4μA/cm2 and 153.0±5.2μA/cm2. Once characterized, the best pyrene-based bioelectrodes were used with laccase as a biocathode and combined with a glucose oxidase (GOx)-based bioanode to form glucose/O2 biofuel cells. [ABSTRACT FROM AUTHOR]

Abstract: We report the first example of a glucose/H2O2 biofuel cell operating at 5mM glucose under air at 37°C in 0.14M NaCl. At a bienzymatic cathode, the direct wiring of horseradish peroxidase (HRP) achieves the electrocatalytic reduction of H2O2 concomitantly produced during the oxidation of glucose by glucose oxidase (GOx) in the presence of O2. At the bienzymatic anode, the indirect wiring of glucose oxidase achieves electrocatalytic glucose oxidation while catalase removes trace of H2O2 and, in combination with GOx, traces of O2. The bienzymatic catalysis involved GOx at both electrodes. The biofuel cell exhibits OCV of 450mV and maximum power output of 30μWcm−2 at 0.3V in 5mM glucose and 0.14M NaCl at 37°C. This represents a novel alternative to the use of copper oxidases in conventional glucose/O2 biofuel cells for implantable applications. [Copyright &y& Elsevier]

No sweat, no gain: Flexible biofuel cells functionalized with lactate oxidase (LOx) and platinum as anode and cathode materials harvested biochemical energy from human perspiration (see picture). Substantial power was generated from human sweat in real‐life scenarios. [ABSTRACT FROM AUTHOR]

Abstract: Graphene is a form of carbon material in which the carbon atoms are arranged in a two-dimensional honeycomb lattice. It is a promising candidate material for future electronic applications, including transistors, gas sensors, electromechanical devices and supercapacitors. We explore the applications of graphene in the field of bioelectroanalysis. This communication reports the influence of graphene surface groups on the bioelectrocatalytic activity of a model enzyme, laccase. A graphite oxide (GO) layer cast onto a glassy carbon electrode is subjected to electrochemical reduction. The surface morphology and composition is monitored with scanning electron microscopy, energy-dispersive X-ray microanalysis and infrared reflectance spectroscopy. The results indicate that carbonyl and epoxide groups are fully removed during electroreduction of GO although some OH groups remain after the electrochemical reduction. The OH groups can serve as redox mediators between laccase and the electrode. The surface composition studies reveal that the functionality and the reactivity of graphene can be optimised by electrochemical treatment. Laccase that is physically adsorbed on the electrochemically reduced GO catalyses oxygen reduction without a mediator at potential values up to +0.6V relative to an Ag/AgCl electrode. [Copyright &y& Elsevier]

Highlights: [•] Flexible biofuel cells and stack have been constructed on filter paper substrates. [•] The maximum power generation of biofuel cell stacks is affected by the connection of series or parallel. [•] The unit with lowest ability of power generation in biofuel cell stack leads to voltage reversal. [ABSTRACT FROM AUTHOR]

We demonstrate the first example of using plant tissues for the operation of biofuel cells (BFC). A banana tissue rich with polyphenol oxidase (PPO), an enzyme able to reduce oxygen to water, was mixed with a carbon paste (banana/CP). The resulting tissue electrode was employed as the biocathode of a glucose biofuel cell based on a glucose dehydrogenase (GDH) bioanode. On the bioanode, multiwall carbon nanotubes (MWCNTs) are used as supports for the glucose-dehydrogenase (GDH) biocatalyst and the Meldola's blue (MDB) mediator, while the phenolic constituents of the banana serve as mediator for PPO. The maximum power output and open circuit voltage (OCV) of the banana-based tissue biofuel cell were 57 µW/cm2 and 0.46 V, respectively. Control experiments using the corresponding pure enzyme tyrosinase (Tyr) for the biocathode resulted in a substantially lower power output. Cost analysis of this tissue biofuel cell system indicates dramatic improvements in terms of the power-output/dollar compared to the common use of isolated enzymes (166 502 µW/$ vs. 17.8 µW/$, i.e., 9354 times improvement). The banana-based biofuel cell displayed an extended lifetime of over 2 weeks with proper storage between experiments. We also demonstrate a fuel-free full plant-tissue based BFC concept, combining the banana biocathode with a bioanode based on germinated tomato-seeds that contain alcohol dehydrogenase (ADH), NAD+ and ethanol. Our findings indicate the potential of using plant tissues for the successful operation of efficient and cost-effective BFC. [ABSTRACT FROM AUTHOR]

Direct electrochemistry of redox proteins provides exciting platforms for performance improvement of biosensors and power enhancement of biofuel cells. Nanomaterials with tailored structures and unique properties are promising building blocks to promote direct electron transfer (DET) between redox-active cofactors of proteins and electrodes. This paper reviews the advances of nanostructured materials for DET of redox proteins in recent years, and is divided into applications in biosensors and biofuel cells. Both of them focus on the performance improvement, together with discussion on major challenges and opportunities for future research. The enhancement mechanism for direct electrochemistry is also reviewed for fundamental insights. [ABSTRACT FROM AUTHOR]

Background: Generation-3 (Gen-3) biosensors and advanced enzyme biofuel cells will benefit from direct electron transfer to oxidoreductases facilitated by single-walled carbon nanotubes (SWNTs). Results: Enzyme activity (KM, kcat, kcat/KM, vmax and n (the Hill parameter)) of pGOx (pristine), sGOx (sonicated) and GOx-SWNT-X-L revealed that sonication of any duration increased both KM and kcat of GOx but did not change kcat/KM. Functionalized tubes had the most dramatic effect, reducing both KM and kcat and reducing kcat/KM. UV-vis spectra over the range of 300 to 550 nm of native enzyme-bound FAD (λmax at 381 and 452 nm) or the blue-shifted solvated FAD of the denatured enzyme (λmax at 377 and 448 nm) revealed that ultrasonication up to 60 minutes had no influence on spectral characteristics of FAD but that the longer SWNTs caused some partial denaturation leading to egress of FAD. Circular dichroism spectral analysis of the 2° structure showed that sonication of any duration caused enrichment in the á-helical content at the sacrifice of the unordered sequences in GOx while the presence of SWNTs, regardless of length and/or functionality, reduced the â-sheet content of pristine GOx. Surface profiling by white light interferometry revealed that ultrasonication produced some aggregation of GOx and that GOx effectively debundled the SWNT. Conclusions: Supramolecular conjugates formed from shorter, -OH functionalized SWNTs using longer sonication times (60 min) gave the most favored combination for forming bioactive conjugates. [ABSTRACT FROM AUTHOR]

Abstract: A new laccase from the filamentous fungus Podospora anserina has been isolated and identified. The 73kDa protein containing 4 coppers, truncated from its first 31 amino acids, was successfully overexpressed in Pichia pastoris and purified in one step with a yield of 48% and a specific activity of 644Umg−1. The kinetic parameters, k cat and K M, determined at 37°C and optimal pH are 1372s−1 and 307μM for ABTS and, 1.29s−1 and 10.9μM, for syringaldazine (SGZ). Unlike other laccases, the new protein displays a better thermostability, with a half life>400min at 37°C, is less sensitive to chloride and more stable at pH 7. Even though, the new 566 amino-acid enzyme displays a large homology with Bilirubin oxidase (BOD) from Myrothecium verrucaria (58%) and exhibits the four histidine rich domains consensus sequences of BODs, the new enzyme is not able to oxidize neither conjugated nor unconjugated bilirubin. [Copyright &y& Elsevier]

Abstract: Linear poly(ethylenimine) (LPEI) was modified by attachment of 3-(tetramethylferrocenyl)propyl groups to ca. 17% of its nitrogen atoms to form a new redox polymer, FcMe4-C3-LPEI, for use as an anodic mediator in glucose/O2 biofuel cells. Electrochemical properties of this polymer were compared to those of 3-(dimethylferrocenyl)propyl-modified LPEI (FcMe2-C3-LPEI). When FcMe4-C3-LPEI was complexed with glucose oxidase (GO x ) and cross-linked with ethylene glycol diglycidyl ether (EGDGE) to form hydrogels on planar, glassy carbon electrodes, limiting catalytic bioanodic current densities of up to ∼1.4mA/cm2 at 37°C were produced. The use of tetramethylferrocene moieties in place of dimethylferrocene moieties lowered the E 1/2 of the films by ca. 85mV. FcMe4-C3-LPEI was shown to be the more effective polymer for use in biofuel cells and, when coupled with a stationary O2 cathode comprised of laccase and cross-linked poly[(vinylpyridine)Os(bipyridyl)2Cl2+/3+] (PVP-Os) as a mediator, produced power densities of up to 57μW/cm2 at 37°C. Power density increased to 126μW/cm2 when a rotating biocathode was used. The power densities of biofuel cells made with either FcMe2-C3-LPEI or FcMe4-C3-LPEI were comparable. The FcMe2-C3-LPEI biofuel cells gave somewhat higher maximum currents, but the operating voltage and the stability of biofuel cells constructed with FcMe4-C3-LPEI was higher than that of cells using FcMe2-C3-LPEI. These polymers have immediate applications as amperometric glucose sensors as well as in biofuel cell materials and have the potential to be used in a wide range of small implantable electronic devices. [Copyright &y& Elsevier]

Enzyme catalysis: Three dehydrogenases have been engineered to self‐assemble into a hydrogel that supports a synthetic metabolic network. The new catalytic biomaterial was used as an anode modification in two enzymatic biobatteries capable of the complete oxidation of methanol to CO2 (see picture). [ABSTRACT FROM AUTHOR]

Abstract: In this communication, we describe a novel and facile method for the immobilization of NAD+/NADH on an electrode surface using a hydrophobic ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C4mim][Tf2N]). By taking advantage of the insolubility of NAD+/NADH in hydrophobic ionic liquids, it is expected that NAD+/NADH can be retained on the electrode''s surface. Alcohol dehydrogenase (ADH) and NAD+/NADH were immobilized with a gelatin hydrogel on an electrode that was modified with an electropolymerized ruthenium complex containing 5-amino-1,10-phenanthroline (pAPRu) as a mediator for NADH oxidation. The (ADH, NAD+)/pAPRu-immobilized electrode exhibited the electrocatalytic oxidation of ethanol in [C4mim][Tf2N]. The obtained catalytic current in [C4mim][Tf2N] was comparable to that in buffer solution containing NAD+. It was confirmed by UV–vis spectroscopy that NAD+ did not dissolve in the [C4mim][Tf2N] and was retained on the electrode''s surface. Furthermore, we succeeded in constructing an ethanol/O2 biofuel cell comprised of an (ADH, NAD+)/pAPRu anode and a bilirubin oxidase cathode using [C4mim][Tf2N] as an electrolyte. [Copyright &y& Elsevier]

Carbon nanotubes (CNTs) grown on carbon cloth substantially increased the surface area of the electrodes. Carbon cloths were pretreated with HNO3 vapor before CNTs growth and electrochemically oxidized afterwards. The CNT-modified carbon cloths were characterized using scanning electron microscopy, Raman spectroscopy, cyclic voltammetry and electrochemical impedance spectroscopy. Biofuel cells based on these CNT-modified electrode materials using Laccase from Trametes hirsuta and cellobiose dehydrogenase from Myriococcum thermophilium entrapped in specifically designed Os-complex modified redox polymers showed a power density of 5.87 µW/cm2 which is 125 fold enhanced as compared with electrodes prepared on untreated carbon cloth. [ABSTRACT FROM AUTHOR]

Molecularly ordered composites of polyvinylimidazole-[Os(bipyridine)2Cl] (PVI-[Os(bpy)2Cl]) and glucose oxidase (GOD) are assembled inside a film of aligned carbon nanotubes. The structure of the prepared GOD/PVI-[Os(bpy)2Cl]/CNT composite film is entirely uniform and stable; more than 90% bioelectrocatalytic activity could be maintained even after storage for 6 d. Owing to the ideal positional relationship achieved between enzyme, mediator, and electrode, the prepared film shows a high bioelectrocatalytic activity for glucose oxidation (ca. 15 mA cm−2 at 25 °C) with an extremely high electron-transfer turnover rate (ca. 650 s−1) comparable to the value for GOD solutions, indicating almost every enzyme molecule entrapped within the ensemble (ca. 3 × 1012 enzymes in a 1 mm × 1 mm film) can work to the fullest extent. This free-standing, flexible composite film can be used by winding on a needle device; as an example, a self-powered sugar monitor is demonstrated. [ABSTRACT FROM AUTHOR]

A novel bilirubin oxidase (BOD), from the rice blast fungus Magnaporthe oryzae, has been identified and isolated. The 64-kDa protein containing four coppers was successfully overexpressed in Pichia pastoris and purified to homogeneity in one step. Protein yield is more than 100 mg for 2 L culture, twice that of Myrothecium verrucaria. The k/ K ratio for conjugated bilirubin (1,513 mM s) is higher than that obtained for the BOD from M. verrucaria expressed in native fungus (980 mM s), with the lowest K measured for any BOD highly desirable for detection of bilirubin in medical samples. In addition, this protein exhibits a half-life for deactivation >300 min at 37 °C, high stability at pH 7, and high tolerance towards urea, making it an ideal candidate for the elaboration of biofuel cells, powering implantable medical devices. Finally, this new BOD is efficient in decolorizing textile dyes such as Remazol brilliant Blue R, making it useful for environmentally friendly industrial applications. [ABSTRACT FROM AUTHOR]

Enzymatic biofuel cells can use a variety of fuels such as glucose and ethanol, and they have the potential to power portable devices. This article summarizes recent advances made in the use of three-dimensional conducting materials as electrode matrices of enzymatic biofuel cells from the point of view of the current density and the power density. [ABSTRACT FROM AUTHOR]

This review details the use of nanomaterials to improve bioelectrocatalysis for biosensor and biological fuel cell applications. Different types of bioelectrocatalysts are described as well as different types of nanomaterials for improving the rate of bioelectrocatalysis, as well as the active surface area of the electrodes. [ABSTRACT FROM AUTHOR]

Abstract: This work presents the results from the development of bio-cathodes for the application on paper-based biofuel cells. Our main goal here is to demonstrate the possibility of using different designs of air-breathing bio-cathodes and ink-based bio-cathodes for this new type of paper based electrochemical cell. The electrochemical performance for the bio-electrocatalytic oxygen reduction reaction was studied by using open circuit voltage and amperometry measurements, as well as polarization curves to probe the four-electron reduction reaction of ambient oxygen catalyzed by bilirubin oxidase (BOx). The electrochemical measurements showed that all procedures allowed the direct electron transfer from the active site of the bilirubin oxidase to the electrode surface with a limiting current density of almost 500μAcm−2 for an air-breathing BOx cathode and 150μAcm−2 for an ink based BOx cathode. Under a load of 300mV a stable current density was obtained for 12h of continuous operation. [Copyright &y& Elsevier]

Abstract: Exploitation of naturally abundant biomass to produce electricity has been a hot topic. This study demonstrates for the first time a new application of enzymatic biofuel cells (BFCs) for direct production of electricity from carboxymethyl cellulose (CMC) biomass. As one of the most important cellulose derivates, CMC can be easily obtained from cellulose and has a good solubility in water. To produce electricity from CMC, cellulase was used as the biocatalyst in solution to catalyze the hydrolysis of CMC into glucose and glucose was then used as the biomass to produce electricity through biofuel cell technology. The bioanode for the oxidation of glucose produced for cellulase-catalyzed CMC hydrolysis was constructed with glucose dehydrogenase (GDH) as the biocatalyst and with methylene green (MG) adsorbed onto single-walled carbon nanotubes (SWNTs) as the electrocatalyst for the oxidation of NADH. The biocathode for the reduction of oxygen was fabricated by crosslinking laccase onto SWNTs. To catalyze CMC hydrolysis into glucose, 0.5mg/mL cellulase was added into 1wt.% CMC solution in 0.10M acetate buffer (pH 5.5) and the mixture was then allowed to stand for 1h to give CMC hydrolyzed solution. Under these conditions, CMC was hydrolyzed into glucose and glucose was oxidized under the biocatalysis of GDH, as characterized with UV–vis spectroscopy. In the CMC hydrolyzed solution containing NAD+ cofactor, both the bioanode and biocathode exhibit a good bioelectrocatalytic activity toward the oxidation of glucose and the reduction of oxygen, respectively. With the presence of 10mM NAD+ into the CMC hydrolyzed solution, the assembled BFC has an open circuit voltage of 0.75V and a maximum power output of 128μWcm−2 at 0.35V under ambient air and room temperature. These values are comparable to or even higher than those with other techniques employed to produce electricity from cellulose or cellulose derivates. This study essentially opens a new application of biofuel cell technology for energy production from naturally abundant biomass. [Copyright &y& Elsevier]

Abstract: A mediated amperometric enzyme electrode, which is constructed by immobilizing an enzyme behind a semipermeable membrane on the surface of a carbon paste electrode containing a mediator compound, has allowed a variety of bioelectrochemical studies in both applied and fundamental aspects such as biosensors, biofuel cells, and enzyme kinetics. This paper describes such bioelectrochemical studies in which we have been involved in these 40 years. [Copyright &y& Elsevier]

Abstract: Carbon nanotubes (CNTs) became a prominent material for its use in bioanalytical devices due to their biocompatibility, their particular structure, and their conductivity. CNTs have shown to be particularly appropriate to establish electronic communication with redox enzymes since the thin diameter can be approached closely to the redox active sites enabling therefore the regeneration of the biocatalysts either by direct electron transfer (DET) or with the help of so-called redox mediators which serve as intermediated for the electron transfer. The possibility to capture the enzymatic redox processes by obtaining catalytic currents, the use of such CNT-enzyme electrode for biological energy conversion represents the logic consequence. The development of CNT based enzyme biofuel cells (BFCs) is a still young but steadily growing research topic where original approaches to construct electron transfer based CNT-bioelectrodes and impressive biofuel cell performances are highlighted in this review. The evolution of reported biofuel cells consisting of CNTs and enzymes at the bioanode and the biocathode are summarized. [Copyright &y& Elsevier]

Discovered in 1981 by Tanaka and Murao ( Agric Biol Chem 45:2383-2384, ), bilirubin oxidase (BOD) is a sub-group of multicopper oxidases (MCOs) also utilizing four Cu ions. It catalyzes the oxidation of bilirubin to biliverdin, hence the classification of bilirubin oxidase, and has been primarily used in the determination of bilirubin in serum and thereby in the diagnostic of jaundice. Unlike laccases, the most studied MCOs, BODs display a high activity and stability at neutral pH, a high tolerance towards chloride anions and other chelators, and for some species, a high thermal tolerance. Therefore, BODs could potentially be an alternative to laccase which are so far mainly restricted to applications in acid media. Because of growing interest in BODs for numerous applications under mild pH conditions, based on the number of patents and publications published in the last 5 years, here I will summarize the available data on the biochemical properties of BODs, their occurrence, and their possible biotechnological use in (1) the field of Healthcare for the elaboration of biofuel cells or bilirubin sensors or (2) the field of environmentally desirable applications such as depollution, decolorization of dyes, and pulp bleaching. [ABSTRACT FROM AUTHOR]

Abstract: We report the fabrication of a tyrosinase bioelectrode by mechanical compression of a MWCNT enzyme mixture. Cyclic voltammetry of the nanostructured bioelectrode demonstrated a Direct Electron Transfer (DET) process between tyrosinase, a copper enzyme, and MWCNT. The latter led to an enzyme redox potential of +0.30V vs SCE, close to the redox potential described for the T3 binuclear copper center. Furthermore, we demonstrate, for the first time, a bioelectrocatalytic reduction of oxygen performed by tyrosinase directly wired within the MWCNT disk. A maximum current density of 0.55mAcm−2 was recorded by chronoamperometric measurements at 0V vs SCE. The bioelectrode exhibits excellent stability over time, conserving more than 50% of its activity after one week. DET between MWCNTs and the T3 binuclear copper centers have been further investigated by studying the influence of two tyrosinase inhibitors: benzoic acid and cyanide. As previously reported for “blue” multicopper oxidases, such as bilirubin oxidase and laccase, tyrosinase can achieve oxygen reduction via DET between MWCNT and its T3 binuclear copper center, representing an alternative in the design of oxygen biocathodes for biofuel cells. [Copyright &y& Elsevier]