A laccase-wiring redox hydrogel for efficient catalysis of O2 electroreduction

Laccase was earlier wired to yield an O2 electroreduction catalyst greatly outperforming platinum and its alloys. Here we describe the design, synthesis optimization of the composition, and characterization of the +0.55 V (AgAgCl) laccase-wiring redox hydrogel, with an apparent electron diffusion coefficient (D(app)) of 7.6 x 10(-7) cm2 s(-1). The high D(app) results in the tethering of redox centers to the polymer backbone through eight-atom-long spacer arms, which facilitate collisional electron transfer between proximal redox centers. The O2 flux-limited, true-area-based current density was increased from the earlier reported 560 to 860 microA cm(-2). When the O2 diffusion to the 7-microm-diameter carbon fiber cathode was cylindrical, half of the O2 flux-limited current was reached already at 0.62 V and 90% at 0.56 V vs Ag/AgCl, merely -0.08 and -0.14 V versus the 0.7 V (Ag/AgCl) reversible O2/H2O half-cell potential at pH 5.     

Characteristics of a miniature compartment-less glucose-O2 biofuel cell and its operation in a living plant

We report the temperature, pH, glucose concentration, NaCl concentration, and operating atmosphere dependence of the power output of a compartment-less miniature glucose-O(2) biofuel cell, comprised only of two bioelectrocatalyst-coated carbon fibers, each of 7 micro m diameter and 2 cm length (Mano, N.; Mao, F.; Heller, A. J. Am. Chem. Soc. 2002, 124, 12962). The bioelectrocatalyst of the anode consists of glucose oxidase from Aspergillus niger electrically "wired" by polymer I, having a redox potential of -0.19 V vs Ag/AgCl. That of the cathode consists of bilirubin oxidase from Trachyderma tsunodae "wired" by polymer II having a redox potential of +0.36 V vs Ag/AgCl (Mano, N.; Kim, H.-H.; Zhang, Y.; Heller, A. J. Am. Chem. Soc. 2002, 124, 6480. Mano, N.; Kim, H.-H.; Heller, A. J. Phys. Chem. B 2002, 106, 8842). Implantation of the fibers in the grape leads to an operating biofuel cell producing 2.4 micro W at 0.52 V.     

An assessment of the relative contributions of redox and steric issues to laccase specificity towards putative substrates

Laccases catalyze the one-electron oxidation of a broad range of substrates coupled to the 4 electron reduction of O2 to H2O. Phenols are typical substrates, because their redox potentials (ranging from 0.5 to 1.0 V vs. NHE) are low enough to allow electron abstraction by the T1 Cu(II) that, although a relatively modest oxidant (in the 0.4-0.8 V range), is the electron-acceptor in laccases. The present study comparatively investigated the oxidation performances of Trametes villosa and Myceliophthora thermophila laccases, two enzymes markedly differing in redox potential (0.79 and 0.46 V). The oxidation efficiency and kinetic constants of laccase-catalyzed conversion of putative substrates were determined. Hammett plots related to the oxidation of substituted phenols by the two laccases, in combination with the kinetic isotope effect determination, confirmed a rate-determining electron transfer from the substrate to the enzyme. The efficiency of oxidation was found to increase with the decrease in redox potential of the substrates, and the Marcus reorganisation energy for electron transfer to the T1 copper site was determined. Steric hindrance to substrate docking was inferred because some of the phenols and anilines investigated, despite possessing a redox potential compatible with one-electron abstraction, were scarcely oxidised. A threshold value of steric hindrance of the substrate, allowed for fitting into the active site of T. villosa laccase, was extrapolated from structural information provided by X-ray analysis of T. versicolor lac3B, sharing an identity of 99% at the protein level, thus enabling us to assess the relative contribution of steric and redox properties of a substrate in determining its susceptibility to laccase oxidation. The inferred structural threshold is compatible with the distance between two phenylalanine residues that mark the entrance to the active site. Interaction of the substrate with other residues of the active site is commented on.     

Oxygen is electroreduced to water on a "wired" enzyme electrode at a lesser overpotential than on platinum

The first enzyme-based catalyst that is superior to platinum in the four-electron electroreduction of oxygen to water is reported. The smooth Pt cathode reached half and 90% of the mass transport-limited current density at respective overpotentials of -0.4 and -0.58 V in 0.5 M sulfuric acid, and only at even higher overpotentials in pH 7.2 phosphate buffer. In contrast, the smooth "wired" bilirubin oxidase cathode reached half and 90% of the mass transport-limited current density at respective overpotentials as low as -0.2 and -0.25 V. The mass transport-limited current density for the smooth "wired" enzyme cathode in PBS was twice that with smooth Pt in 0.5 M sulfuric acid. Under 1 atm O2 pressure, O2 was electroreduced to water on a polished carbon cathode, coated with the "wired" BOD film, in pH 7.2 saline buffer (PBS) at an overpotential of -0.31 V at a current density of 9.5 mA cm-2. At the same overpotential, the current density of the polished platinum cathode in 0.5 M H2SO4 was 16-fold lower, only 0.6 mA cm-2.     

聚芳酰胺-多壁碳纳米管混合物固定漆酶电极的电化学行为

以聚芳酰胺-多壁碳纳米管混合物为载体,利用漆酶表面氨基与聚芳酰胺主链端羧基的共价偶联以及碳纳米管与漆酶间的疏水作用,构筑了具有较高稳定性和电催化活性的漆酶修饰电极.并对该固酶修饰电极的固酶量、酶活力、电化学行为及其电催化氧还原的性能进行了表征.对漆酶分子具有亲和力的聚芳酰胺芳环结构及聚芳酰胺端羧基与漆酶表面氨基的共价偶联避免了漆酶的脱落和变性.而碳纳米管与聚芳酰胺的混合使得该三维修饰电极具有良好的电子导电性,并成功地实现了漆酶的氧化还原活性位与电极之间的直接电荷转移,这一点可由在0.73和0.38V附近观察到漆酶的T1和T2(漆酶的T1,T2铜活性位的形式电位分别为0.78和0.39V(vsNHE))铜活性位的两对氧化还原峰确认.漆酶的担载量为56.0mg·g-1,具有电化学活性的漆酶占总担载漆酶量的68%.在pH=4.4磷酸盐缓冲溶液中,该修饰电极上氧气还原的起始电位为0.55V,其对氧气的米氏常数KM为55.8μmo·lL-1,对氧气的检测限为0.57μmo·lL-1.在4℃下保存两个月后能实现直接电荷转移的漆酶量仅下降了14%左右而氧还原超电势提高了约50mV.结果表明该修饰电极有望用作酶基生物燃料电池的阴极和电流型氧气传感器.     

Electrochemical impedance spectroscopy of mixed conductors under a chemical potential gradient: a case study of Pt|SDC|BSCF

The AC impedance response of mixed ionic and electronic conductors (MIECs) exposed to a chemical potential gradient is derived from first principles. In such a system, the chemical potential gradient induces a gradient in the carrier concentration. For the particular system considered, 15% samarium doped ceria (SDC15) with Ba(0.5)Sr(0.5)Co(0.8)Fe(0.2)O(3-delta) (BSCF) and Pt electrodes, the oxygen vacancy concentration is a constant under the experimental conditions and it is the electron concentration that varies. The resulting equations are mapped to an equivalent circuit that bears some resemblance to recently discussed equivalent circuit models for MIECs under uniform chemical potential conditions, but differs in that active elements, specifically, voltage-controlled current sources, occur. It is shown that from a combination of open circuit voltage measurements and AC impedance spectroscopy, it is possible to use this model to determine the oxygen partial pressure drop that occurs between the gas phase in the electrode chambers and the electrode|electrolyte interface, as well as the interfacial polarization resistance. As discussed in detail, this resistance corresponds to the slope of the interfacial polarization curve. Measurements were carried out at temperatures between 550 and 650 degrees C and oxygen partial pressure at the Pt anode ranging from 10(-29) to 10(-24) atm (attained using H(2)/H(2)O/Ar mixtures), while the cathode was exposed to either synthetic air or neat oxygen. The oxygen partial pressure drop at the anode was typically about five orders of magnitude, whereas that at the cathode was about 0.1 atm for measurements using air. Accordingly, the poor activity of the anode is responsible for a loss in open circuit voltage of about 0.22 V, whereas the cathode is responsible for only about 0.01 V, reflecting the high activity of BSCF for oxygen electro-reduction. The interfacial polarization resistance at the anode displayed dependences on oxygen partial pressure and on temperature that mimic those of the electronic resistivity of SDC15. This behavior is consistent with hydrogen electro-oxidation occurring directly on the ceria surface and electron migration being the rate-limiting step. However, the equivalent resistance implied by the oxygen partial pressure drop across the anode displayed slightly different behavior, possibly indicative of a more complex reaction pathway.     

血红蛋白在壳聚糖修饰碳纳米管上的电化学特性及对过氧化氢的电催化分析

用壳聚糖对多壁碳纳米管进行修饰,构建了一种用于固定血红蛋白的新型复合材料,并研究了血红蛋白在该碳纳米管上的电化学性质及其对过氧化氢的电催化活性.扫描电镜结果表明,壳聚糖修饰的多壁碳纳米管呈单一的纳米管状,并能均匀分散在玻碳电极表面.紫外光谱分析表明血红蛋白在该复合膜内能很好地保持其原有的二级结构.将该材料固定在玻碳电极上后,血红蛋白能成功地实现其直接电化学.根据峰电位差随着扫描的变化,计算得到血红蛋白在壳聚糖修饰的碳纳米管膜上的电荷转移系数为0.57,表观电子转移速率常数为7.02 s-1.同时,该电极对过氧化氢显示出良好的催化性能,电流响应信号与H2O2浓度在1.0×10-6 ～1.5×10-3 mol/L间呈线性关系,检出限为5.0×10-7 mol/L.修饰电极显示了良好的稳定性.     

Bioelectrocatalytic Oxygen Reduction by Laccase Immobilized on Various Carbon Carriers

Laccase is an enzyme that is used for fabricating cathodes of biofuel cells. Many studies have been aimed at searching the ways for enhancing specific electrochemical characteristics of cathode with the laccase- based catalyst. The electroreduction of oxygen on the electrode with immobilized laccase proceeds under the conditions of direct electron transfer between the electrode and active enzyme center. In this work, the effect of oxygen partial pressure on the electrocatalytic activity of laccase is studied. It is shown that, at the concentrations of oxygen dissolved in the electrolyte higher than 0.28 mM, the process is controlled by the kinetics of the formation of laccase–oxygen complex, whereas at lower concentrations and a polarization higher than 0.3 V, the process is limited by the oxygen diffusion. A wide range of carbon materials are studied as the carriers for laccase immobilization: carbon black and nanotubes with various BET specific surface areas. The conditions, which provide the highest surface coverage of carbon material with enzyme in the course of spontaneous adsorptive immobilization and the highest specific characteristics when using a “floating” electrode simulating a gas-diffusion electrode, are determined: 0.2 M phosphate-acetate buffer solution; oxygen atmosphere; the carrier material (nanotubes with a BET surface area of 210 m²/g and a mesopore volume of 3.8 cm³/g); and the composition of active mass on the electrode (50 wt % of carbon material + 50 wt % of hydrophobized carbon black).     

Effects of electron kinetic energy and ion-electron inelastic collisions in electron capture dissociation measured using ion nanocalorimetry

Ion nanocalorimetry is used to measure the effects of electron kinetic energy in electron capture dissociation (ECD). With ion nanocalorimetry, the internal energy deposited into a hydrated cluster upon activation can be determined from the number of water molecules that evaporate. Varying the heated cathode potential from -1.3 to -2.0 V during ECD has no effect on the average number of water molecules lost from the reduced clusters of either [Ca(H2O)15]2+ or [Ca(H2O)32]2+, even when these data are extrapolated to a cathode potential of zero volts. These results indicate that the initial electron kinetic energy does not go into internal energy in these ions upon ECD. No effects of ion heating from inelastic ion-electron collisions are observed for electron irradiation times up to 200 ms, although some heating occurs for [Ca(H2O)17]2+ at longer irradiation times. In contrast, this effect is negligible for [Ca(H2O)32]2+, a cluster size typically used in nanocalorimetry experiments, indicating that energy transfer from inelastic ion-electron collisions is negligible compared with effects of radiative absorption and emission for these larger clusters. These results have significance toward establishing the accuracy with which electrochemical redox potentials, measured on an absolute basis in the gas phase using ion nanocalorimetry, can be related to relative potentials measured in solution.     

以泡沫镍载NiCo2O4纳米线阵列为阴极催化剂的Al-H2O2半燃料电池

研究了以泡沫镍载NiCo2O4纳米线阵列为阴极催化剂的Al-H2O2半燃料电池的性能. 以无模板生长法制备了泡沫镍载NiCo2O4纳米线阵列阴极材料, SEM测定结果表明, NiCo2O4纳米线几乎垂直于泡沫镍载体表面生长. 以电压和功率密度-电流密度曲线研究了H2O2浓度、电解液流速和温度对电池性能的影响, 结果显示, 以铝片为阳极, 0.6 mol/L H2O2为氧化剂的电池的开路电压约为1.40 V; 在室温和57 ℃下, 电流密度为98和172 mA/cm2时, 最大功率密度分别达到79和120 mW/cm2.  在5000 s的测试时间内, 0.70 V的恒电流密度和75 mA/cm2 的恒电压值几乎为一常数, 这表明以泡沫镍载NiCo2O4纳米线阵列为催化剂电还原H2O2具有很好的活性、稳定性和传质性能.     

Design of a bioelectrocatalytic electrode interface for oxygen reduction in biofuel cells based on a specifically adapted Os-complex containing redox polymer with entrapped Trametes hirsuta laccase

The design of the coordination shell of an Os-complex and its integration within an electrodeposition polymer enables fast electron transfer between an electrode and a polymer entrapped high-potential laccase from the basidiomycete Trametes hirsuta. The redox potential of the Os^3+/2+-centre tethered to the polymer backbone (+ 720 mV vs. NHE) is perfectly matching the potential of the enzyme (+ 780 mV vs. NHE at pH 6.5). The laccase and the Os-complex modified anodic electrodeposition polymer were simultaneously precipitated on the surface of a glassy carbon electrode by means of a pH-shift to 2.5. The modified electrode was investigated with respect to biocatalytic O₂ reduction to H₂O. The proposed modified electrode has potential applications as biofuel cell cathode.     

On the stability of the "wired" bilirubin oxidase oxygen cathode in serum

Oxygen is electroreduced to water on the "wired" bilirubin oxidase (w-BOD) catalyst at a considerably lesser potential than on pure platinum. The w-BOD catalyst could be of value in an implantable glucose-O2 biofuel cell, operating living tissue, if it were stable in serum. We found, however, that w-BOD loses its activity in a few hours in the combined presence of the urate and O2, both of which are normal serum constituents (Bioelectrochemistry, 2004, 65, 83-88). Here we report a second major instability: When the disconnected w-BOD cathode is allowed, in the absence of urate, to poise itself at the potential of the O2/H2O half cell at pH 7.2, it loses its activity rapidly. Unlike the urate/O2 caused loss, this loss can be avoided either by applying a potential that is reducing relative to the O2/H2O half-cell potential, or by excluding O2 and adding a mildly reducing reagent, such as urate. The w-BOD cathode can be stored, therefore, in deoxygenated serum, which contains urate.     

Direct electrochemistry and electrocatalysis of hemoglobin entrapped in semi-interpenetrating polymer network hydrogel based on polyacrylamide and chitosan

Semi-interpenetrating polymer network (semi-IPN) hydrogel based on polyacrylamide (PAM) and chitosan was prepared to immobilize redox protein hemoglobin (Hb). The Hb-PAM-chitosan hydrogel film obtained has been investigated by scanning electron microscopy (SEM) and UV-VIS spectroscopy. UV-VIS spectroscopy showed that Hb kept its secondary structure similar to its native state in the Hb-PAM-chitosan hydrogel film. Cyclic voltammogram of Hb-PAM-chitosan film-modified glass carbon (GC) electrode showed a pair of well-defined and quasi-reversible redox peaks for Hb Fe(III)/Fe(II), indicating that direct electron transfer between Hb and GC electrode occurred. The electron-transfer rate constant was about 5.51 s(-1) in pH 7.0 buffers, and the formal potential (E degrees ') was -0.324 V (vs. SCE). The dependence of E degrees ' on solution pH indicated that one-proton transfer was coupled to each electron transfer in the direct electron-transfer reaction. Additionally, Hb in the semi-IPN hydrogel film retained its bioactivity and showed excellent electrocatalytic activity toward H(2)O(2). The electrocatalytic current values were linear with increasing concentration of H(2)O(2) in a wide range of 5-420 microM. The unique semi-IPN hydrogel would have wide potential applications in direct electrochemistry, biosensors and biocatalysis.     

A membrane-less enzymatic fuel cell with layer-by-layer assembly of redox polymer and enzyme over graphite electrodes

Layer-by-layer (LBL) assembly of alternate osmium redox polymers and glucose oxidase, at anode, and laccase, at cathode, using graphite electrodes form a membrane-less glucose/O(2) enzymatic fuel cell providing a power density of 103 μW cm(-2) at pH 5.5.     

Electricity from low-level H2 in still air--an ultimate test for an oxygen tolerant hydrogenase

We demonstrate an extreme test of O(2) tolerance for a biological hydrogen-cycling catalyst: the generation of electricity from just 3% H(2) released into still, ambient air using an open fuel cell comprising an anode modified with the unusual hydrogenase from Ralstonia metallidurans CH34, that oxidizes trace H(2) in atmospheric O(2), connected via a film of electrolyte to a cathode modified with the fungal O(2) reductase, laccase.     

A Four‐Electron Sulfur Electrode Hosting a Cu2+/Cu+ Redox Charge Carrier

The elemental sulfur electrode with Cu²⁺ as the charge carrier gives a four‐electron sulfur electrode reaction through the sequential conversion of S?CuS?Cu₂S. The Cu‐S redox‐ion electrode delivers a high specific capacity of 3044 mAh g^?1 based on the sulfur mass or 609 mAh g^?1 based on the mass of Cu₂S, the completely discharged product, and displays an unprecedently high potential of sulfur/metal sulfide reduction at 0.5 V vs. SHE. The Cu‐S electrode also exhibits an extremely low extent of polarization of 0.05 V and an outstanding cycle number of 1200 cycles retaining 72 % of the initial capacity at 12.5 A g^?1. The remarkable utility of this Cu‐S cathode is further demonstrated in a hybrid cell that employs an Zn metal anode and an anion‐exchange membrane as the separator, which yields an average cell discharge voltage of 1.15 V, the half‐cell specific energy of 547 Wh kg^?1 based on the mass of the Cu₂S/carbon composite cathode, and stable cycling over 110 cycles.     

Direct Electrochemistry of Hemoglobin Immobilized in Sodium Alginate Film on the Ionic Liquid [BMIM]PF6 Modified Carbon Paste Electrode

In recent years the direct electron transfer of redox protein on electrode surface has attracted great attentions1. Different kind of modified electrode and various supporting films for immobilization of proteins had been proposed. But most of them are ba…     

Kinetics of redox polymer-mediated enzyme electrodes

Oxygen-reducing enzyme electrodes are prepared from laccase of Trametes versicolor and a series of osmium-based redox polymer mediators covering a range of redox potentials from 0.11 to 0.85 V. Experimentally obtained current density generated by the film electrodes is analyzed using a one-dimensional numerical model to obtain kinetic parameters. The bimolecular rate constant for mediation is found to vary with mediator redox potential from 250 s(-1) M(-1) when mediator and enzyme are close in redox potential to 9.4 x 10(4) s(-1) M(-1) when the redox potential difference is large. The value of the bimolecular rate constant for the simultaneously occurring laccase-oxygen reaction is found to be 2.4 x 10(5) s(-1) M(-1). The relationship between mediator-enzyme overpotential and bimolecular rate constant is used to determine the optimum mediator redox potential for maximum power output of a hypothetical biofuel cell with a planar cathode and a reversible hydrogen anode. For laccase of T. versicolor (E(e)(0) = 0.82), the optimum mediator potential is 0.66 V (SHE), and a molecular structure is presented to achieve this result.     

Physical Characterization and Reactivity of the Uranyl Peroxide [UO(2)(η(2)-O(2))(H(2)O)(2)]·2H(2)O: Implications for Storage of Spent Nuclear Fuels

The unusual uranyl peroxide studtite, [UO(2)(η(2)-O(2))(H(2)O)(2)]·2H(2)O, is a phase alteration product of spent nuclear fuel and has been characterized by solid-state cyclic voltammetry. The voltammogram exhibits two reduction waves that have been assigned to the U(VI/V) redox couple at -0.74 V and to the U(V/IV) redox couple at -1.10 V. This potential shows some dependence upon the identity of the cation of the supporting electrolyte, where cations with larger ionic radii exhibit more cathodic reduction potentials. Raman spectroelectrochemistry indicated that exhaustive reduction at either potential result in a product that does not contain peroxide linkers and is likely to be UO(2). On the basis of the reduction potentials, the unusual behavior of neptunium in the presence of studtite can be rationalized. Furthermore, the oxidation of other species relevant to the long-term storage of nuclear fuel, namely, iodine and iodide, has been explored. The phase altered product should therefore be considered as electrochemically noninnocent. Radiotracer studies with (241)Am show that it does not interact with studtite so mobility will not be retarded in repositories. Finally, a large difference in band gap energies between studtite and its dehydrated congener metastudtite has been determined from the electronic absorption spectra.     

Glucose and choline on-line biosensors based on electropolymerized Meldola's blue

Electropolymerized film of Meldola's blue (MB) was prepared and demonstrated as electron shuttle between the immobilized horse peroxidase (HRP) and glassy carbon electrode (GCE) for sensing hydrogen peroxide (H(2)O(2)) produced by enzyme catalytical reactions. Electrochemical polymerization of Meldola's blue was carried out by cyclic voltammetry (CV) in a phosphate buffer solution (pH 7.00) in a potential window from -0.60 to +1.30 V. The pH of the electropolymerization solution was found to be closely related to the resulted polymeric MB and the best polymeric film was obtained in a pH 7.00 phosphate buffer. The polymeric MB was demonstrated to shuttle the electron transfer between the immobilized HRP and GCE and utilized as a mediator for HRP immobilized biosensor for biocatalytical reduction of H(2)O(2) at a potential of -0.30 V (versus AgCl/Ag). The H(2)O(2) sensing system was applied to construct glucose and choline on-line sensors by wiring H(2)O(2) produced by enzyme oxidase catalytical reaction. The possibility of these sensors as on-line detectors for on-line and continuous measurement was explored off-line. The operating potential, interference, and lifetime of these sensors were also examined.