Gold sputtered electrode surfaces enhance direct electron transfer reactions of human cytochrome P450s

We immobilized human cytochrome P450 (CYP), a membrane-bound enzyme, onto both smooth and nanostructured surfaces of gold electrodes via a naphthalene thiolate monolayer film. Rapid electron transfer of CYP with an electrode as a redox partner took place when the enzyme was immobilized onto an electrode surface with nanostructures. This structure was easily prepared by conventional sputtering techniques. A well-defined pair of peaks was observed at ? 0.175 V (vs. SHE) with the largest heterogeneous electron transfer rate constant of 340 s^? 1 for human CYP. The positive redox potential shift of 45 mV upon drug (testosterone) binding was clearly detected, which corresponded to a change in the spin states of heme iron in CYP. The present study showed that gold sputtered surfaces are very useful for direct electron transfer reactions of human CYP isoforms.     

The qualitative electrochemical determination of clarithromycin and spectroscopic detection of its structural changes at gold electrode

The aim of the present study was the qualitative determination of the pure clarithromycin using a gold electrode in neutral electrolyte by cyclic linear sweep voltammetry. It was shown that in the range of −1.2 V to 1.0 V vs. SCE in 0.05 M NaHCO₃, a gold electrode is successfully employed for the qualitative determination of clarithromycin by detection of the reproductive four anodic and one cathodic peaks. After the potentiostatic measurements at the potential values corresponded to current peaks, the bulk electrolyte was analyzed by FTIR spectroscopy to show the changes in molecular structure of clarithromycin. FTIR analysis of the bulk electrolyte after 4 h of holding the potential at −0.61 V vs. SCE (cathodic peak) showed the apparent changes in clarithromycin molecule structure: in the ester bond of the lactone and in ethers and acetal bonds.     

Mediated electron transfer with P450cin

P450cin stereoselectively hydroxylated its natural substrate 1,8-cineole to 2β-hydroxy-1,8-cineole in an electrochemical cell which allowed for substitution of the natural cofactor NADPH by artificial redox mediators. Cobalt sepulchrate, phenosafranine, safranine T, FAD and FMN enabled artificial electron transfer from the platinum electrode to P450cin via the redox partner protein cindoxin. The highest product formation, 6.50 ± 0.60 nmol (product) nmol (P450)^?1 min^?1 cm^?2, was achieved using cobalt sepulchrate. Surprisingly, phenosafranine and safranine T enabled electron transfer even in the absence of NADPH, cindoxin, and cindoxin reductase, thereby illustrating that none of the natural redox partners is needed for product formation.     

Gelatin-functionalized carbon nanotubes for the bioelectrochemistry of hemoglobin

In this paper, we compared the use of gelatin-functionalized carbon nanotubes (CNTs) as substrates for Hemoglobin (Hb) immobilization and as electrodes for electrochemical reduction of the absorbed Hb. The non-covalently gelatin-functionalized CNTs possessed an improved solubility in aqueous solution and may have an enhanced biocompatibility with Hb. The characteristics of Hb/gelatin-CNTs composite films were studied by using UV–vis spectroscopy, FTIR spectroscopy and electrochemical methods. The immobilized Hb showed a couple of quasi-reversible redox peaks with a formal potential of −0.35 V (vs. SCE) in 0.10 M pH 7.0 phosphate buffer solution (PBS). The surface concentration of electroactive Hb immobilized on gelatin-CNT/GC electrode was about 4.34 × 10⁻¹⁰ mol cm⁻².     

Anodic electron shuttle mechanism based on 1-hydroxy-4-aminoanthraquinone in microbial fuel cells

In microbial fuel cells (MFCs), the electron transfer from microorganisms to the cell anode is a decisive factor on the power output. Though quinone derivatives can function as electron shuttles, the electron shuttle pathways have so far not been demonstrated. In this paper, the mechanism of electron shuttle via an exogenous mediator was studied in MFCs using Geobacter metallireducens (G. metallireducens). 1-hydroxy-4-aminoanthraquinone was labeled by fluorescamine and the product (HAQ-F) showed strong and stable fluorescence. The addition of HAQ-F into MFCs increased cell voltage from 170 mV to 290 mV, suggesting that the redox mediator could facilitate electron transfer from bacteria to anode. Further, confocal laser scanning microscopy imaging indicated that HAQ-F was present in microbial cells, demonstrating that the redox mediator shuttled across the membranes to get reduced within cells.     

Immobilization and electrochemical behavior of gold nanoparticles modified leukemia K562 cells and application in drug sensitivity test

A new strategy for immobilization of tumor cells on electrode surface and accelerating electron transfer between electrode and the immobilized cells was proposed to study the electrochemical behavior of cells and the effect of antitumor drug on cell viability. The leukemia K562 cells immobilized in a microporous cellulose membrane were firstly modified with colloidal gold nanoparticles to retain efficiently the activity of immobilized living tumor cells and promote electron transfer between electroactive centers of the cells and the electrode, exhibiting a well-defined anodic peak of guanine at +0.830 V at 50 mV s⁻¹. The electrochemical response could be used to describe cell growth and evaluate the effectiveness of antitumor drug methotrexate on tumor cells. The proposed method offered potential advantages for drug sensitivity test with little usage of cells. It could be developed as a convenient means for the study of the tumor cells growth and the cytotoxicity of antitumor drugs.     

Bioelectrocatalytic oxidation of glucose by hexose oxidase directly wired to graphite

Glucose-oxidizing enzymes are widely used in electrochemical biosensors and biofuel cells; in most applications glucose oxidase, an enzyme with non-covalently bound FAD and low capability of direct electronic communications with electrodes, is used. Here, we show that another glucose-oxidizing enzyme with a covalently bound FAD center, hexose oxidase (HOX), adsorbed on graphite, exhibits a pronounced non-catalytic voltammetric response from its FAD, at − 307 mV vs. Ag/AgCl, pH 7, characterized by the heterogeneous electron transfer (ET) rate constant of 29.2 ± 4.5 s^− 1. Direct bioelectrocatalytic oxidation of glucose by HOX proceeded, although, with a 350 mV overpotential relative to FAD signals, which may be connected with a limiting step in biocatalysis under conditions of the replacement of the natural redox partner, O₂, by the electrode; mediated bioelectrocatalysis was consistent with the potentials of a soluble redox mediator used. The results allow development of HOX-based electrochemical biosensors for sugar monitoring and biofuel cells exploiting direct ET of HOX, and, not the least, fundamental studies of ET non-complicated by the loss of FAD from the protein matrix.     

Electropolymerized biotinylated poly (pyrrole–viologen) film as platform for the development of reagentless impedimetric immunosensors

We describe herein the synthesis and electrochemical polymerization of a viologen monomer functionalized by a biotin group and the use of its redox properties for the impedimetric sensing of protein binding. The electrochemical oxidation of the pyrrole group at 0.85 V in CH₃CN led to a biotinylated polypyrrole film allowing the successive anchoring of an avidin layer and a biotinylated cholera toxin layer. EIS performed at ?0.45 V/SCE without redox probe in solution showed the specific immobilization of the anti-cholera toxin antibody.     

Role of direct microbial electron transfer in corrosion of steels

It has recently been discovered that many microbial species have the capacity to connect their metabolism to solid electrodes, directly exchanging electrons with them through membrane-bound redox compounds, nevertheless such a direct electron transfer pathway has been evoked rarely in the domain of microbial corrosion. Here was evidenced for the first time that the bacterium Geobacter sulfurreducens is able to increase the free potential of 304 L stainless steel up to 443 mV in only a few hours, which represents a drastic increase in the corrosion risk. In contrast, when the bacterial cells form a locally well-established biofilm, pitting potentials were delayed towards positive values. The microscopy pictures confirmed an intimate correlation between the zones where pitting occurred and the local settlement of cells. Geobacter species must now be considered as key players in the mechanisms of corrosion.     

Direct electrochemical response of Myoglobin using a room temperature ionic liquid, 1-(2-hydroxyethyl)-3-methyl imidazolium tetrafluoroborate,as supporting electrolyte

Direct electrochemical response of Myoglobin (Myb) at the basal plane graphite (BPG) electrode was observed when a room temperature ionic liquid (RTIL), 1-(2-hydroxyethyl)-3-methyl imidazolium tetrafluoroborate ([HEMIm][BF₄]), was used as the supporting electrolyte. In a 0.17 M [HEMIm]BF₄ aqueous solution, a couple of well-defined redox peaks of Myb could be obtained, whose anodic and cathodic peak potentials were at −0.158 and −0.224 V (vs. Ag/AgCl), respectively. Both anodic and cathodic peak currents increased linearly with the potential scan rate. Compared with the supporting electrolyte of phosphate buffer, [HEMIm][BF₄] played an obvious promotion for the direct electron transfer between Myb and the BPG electrode. Further investigation suggested that Myb was adsorbed tightly on the surface of the BPG electrode in the presence of [HEMIm][BF₄] to form a stable, approximate monolayer Myb film. Myb adsorbed on the BPG electrode surface showed a remarkable electrocatalytic activity for the reduction of oxygen in a [HEMIm][BF₄] aqueous solution. Based on these, a third-generation biosensor could be constructed to directly detect the concentration of oxygen in aqueous solution with a limit of detection of 2.3 × 10⁻⁸ M.     

Direct electrochemistry and electrocatalysis of hemoglobin in sodium alginate film on a BMIMPF6 modified carbon paste electrode

A room temperature ionic liquid (RTIL) modified carbon paste electrode was constructed based on the substitute of paraffin with 1-butyl-3-methyl-imidazolium hexafluorophosphate (BMIMPF₆) as binder for carbon paste. Direct electrochemistry and electrocatalytic behaviors of hemoglobin (Hb) entrapped in the sodium alginate (SA) hydrogel film on the surface of this carbon ionic liquid electrode (CILE) were investigated. The presence of IL in the CILE increased the electron transfer rate and provided a biocompatible interface. Hb remained its bioactivity on the surface of CILE and the SA/Hb modified electrode showed a pair of well-defined, quasi-reversible cyclic voltammetric peaks with the apparent standard potential (E^0′) at about −0.344 V (vs. SCE) in pH 7.0 Britton–Robinson (B–R) buffer solution, which was attributed to the Hb Fe(III)/Fe(II) redox couple. UV–Vis absorption spectra indicated that heme microenvironment of Hb in SA film was similar to its native status. Hb showed a thin-layer electrochemical behavior in the SA film with the direct electron transfer achieved on CILE without the help of electron mediator. Electrochemical investigation indicated that Hb took place one proton with one electron electrode process and the average surface coverage of Hb in the SA film was 3.2 × 10⁻¹⁰ mol/cm². The immobilized Hb showed excellent electrocatalytic responses to the reduction of H₂O₂ and nitrite.     

Single microparticle applied in magnetic-switchable electrochemistry

In this work we demonstrated the micromanipulation of a single magnetic microparticle (Fe₃O₄) modified with Prussian blue (PB) for use in magnetic-switchable electrochemistry. A single Fe₃O₄-PB microparticle with 120 μm was isolated in an electrochemical microcell (20 μL), in which a fine control of PB electrochemical process on carbon electrode (Ø = 4.0 mm) was obtained. For cyclic voltammetry, redox processes attributed to PB/PW (Prussian blue/Prussian white) one electron redox couple were observed, however the capacitive currents were very high. On the other hand, by using differential pulse voltammetry, a maximum faradaic current for anodic peak of 200 nA cm^− 2 at 0.06 V was observed. Several and high stable chronoamperograms were obtained by “switch on” and “switch off” magnetic commutative states for a single microparticle, showing that the system developed here can be very promising for application in electrochemistry.     

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.     

Activity of Pt anode catalyst modified by underpotential deposited Pb in a direct formic acid fuel cell

We characterized the electrocatalytic activity of platinum electrode modified by underpotential deposited lead (PtPb_upd) for a formic acid (HCOOH) oxidation and investigated the influence on the power performance of direct formic acid fuel cells (DFAFC). Based on the electrochemical analysis using cyclic voltammetry and chronoamperometry, PtPb_upd electrode modified by underpotential deposition (UPD) exhibited significantly enhanced catalytic activity for HCOOH oxidation below anodic overpotential of 0.4 V (vs. SCE). Multi-layered PtPb_upd electrode structure of Pt/Pb_upd/Pt resulted in more stable and enhanced performance using 50% reduced loading of anode catalyst. The performance of multi-layered PtPb_upd anode is about 120 mW/cm² at 0.4 V and it also showed a sustainable cell activity of 0.52 V at an application of constant current loading of 110 mA/cm².     

Permanent electrochemical promotion of C3H8 oxidation over thin sputtered Pt films

The effect of “permanent electrochemical promotion of catalysis” (P-EPOC) was studied for the first time in the catalytic deep oxidation of C₃H₈ over a thin (～ 150 nm) sputtered Pt film on YSZ, under excess of oxygen at 350 °C. Short positive potential application (+ 1 V) resulted in a 5.6-fold increase of the catalytic rate, where C₃H₈ conversion reached 33%, while the apparent Faradaic efficiency was ～ 330. After positive current interruption the catalytic rate remained in a highly active steady-state, determined by the total charge of the anodic polarization step. Restoration of the catalytic activity to the initial value occurred only by a similar negative potential imposition. This new stable steady-state after current interruption can be interpreted by storage of a non-reactive oxygen species upon anodic polarization at the proximity of the Pt/YSZ interface and subsequent enhanced migration of spillover O^δ? species from the electrolyte support to the Pt/gas interface under open-circuit conditions.     

Direct electron transfer-type four-way bioelectrocatalysis of CO2/formate and NAD+/NADH redox couples by tungsten-containing formate dehydrogenase adsorbed on gold nanoparticle-embedded mesoporous carbon electrodes modified with 4-mercaptopyridine

Tungsten-containing formate dehydrogenase from Methylobacterium extorquens AM1 (FoDH1) catalyzes formate oxidation with NAD⁺. FoDH1 shows little direct communication with carbon electrodes, including mesoporous Ketjen Black-modified glassy carbon electrode (KB/GCE); however, it shows well-defined direct electron transfer (DET)-type bioelectrocatalysis of carbon dioxide reduction, formate oxidation, NAD⁺ reduction, and NADH oxidation on gold nanoparticle (AuNP)-embedded KB/GCE treated with 4-mercaptopyridine. Microscopic measurements reveal that the AuNPs (d = 5 nm) embedded on the KB surface are uniformly dispersed. Electrochemical data indicate that the pyridine moiety on the AuNPs plays important roles in facilitating the interfacial electron transfer kinetics and increasing the probability of productive orientation of FoDH1. The formal potential of the electrochemical communication site, which is most probably an ion‑sulfur cluster, is evaluated as − 0.591 ± 0.005 V vs. Ag | AgCl | sat. KCl from Nernst analysis of the steady-state catalytic waves.     

Accelerated anaerobic corrosion of electroactive sulfate-reducing bacteria by electrochemical impedance spectroscopy and chronoamperometry

Many microbial species can use cathodes as electron donors for metabolism. This direct electron transfer (DET) pathway has rarely been proposed in biocorrosion processes. DET from Q235 carbon steel to the sulfate-reducing bacteria (SRB) Desulfovibrio caledoniensis and its effect on the corrosion behavior of carbon steel were investigated in the present study. Electroactive SRB biofilm was found to play a key role in the ennoblement of the corrosion potential (E_corr) and in the acceleration of the corrosion rate, indicating that SRB mainly affected the cathodic reaction of low carbon steel corrosion. In addition, SRB biofilm obtained electrons from carbon steel electrode polarized at − 0.74 V. These findings present new evidence for DET between SRB biofilm and carbon steels.     

Gold doped by palladium: Building of Au–Pd electrodes showing exceptional capability for achieving electrocatalytic reductions

Smooth gold is easily palladized by means of very thin galvanostatic deposits of Pd⁰. Such chemically modified materials are mainly tested in acetonitrile in the presence of tetraalkylammonium salts (TBABF₄). All exhibit large usable electrochemical windows: between ? 1.8 V and + 1.5 V vs. saturated calomel electrode (SCE). When employed as cathodic material, they show exceptional catalytic capabilities especially for achieving the one-electron cleavage of organic halides (RX). Thus, these modified surfaces, found to be chemically and electrochemically stable, lead to amazing potential shifts compared to those obtained with glassy carbon, smooth platinum, or gold cathodes under the same experimental conditions. Specifically, these palladium-doped gold interfaces were preliminarily used for the reduction of alkyl iodides RIs. The presence of gold is expected to boost up the catalytic effect of palladium already reported with such compounds.     

pH effects on the rate of heterogeneous electron transfer across a fluorine doped tin oxide/monolayer interface

Spontaneously adsorbed monolayers of [Ru(bpy)₂PIC](PF₆)₂ have been formed on fluorine doped tin oxide macro- and microelectrodes, bpy is 2,2′-bipyridyl and PIC is 2-(4-carboxyphenyl)imidazo[4,5-f][1,10]phenanthroline. These monolayers exhibit well-defined, almost ideal electrochemical responses over a wide range of voltammetric scan rates. The formal potential of the Ru^2+/3+ process shifts by less than 30 mV upon immobilization suggesting that the monolayers are well solvated. Significantly, chronoamperometry, conducted on a microsecond timescale, reveals that protonation of the PIC bridging ligand modulates the rate of interfacial electron transfer. The heterogeneous electron transfer rate constant, measured at an overpotential of +50 mV, decreases from 7.0 ± 1.1 × 10⁵ to 0.7 ± 0.1 × 10⁵ s⁻¹ as the pH of the supporting electrolyte is increased from 1.7 to 9.3. These observations are consistent with the redox mechanism occurring via a heterogeneous electron transfer process, the rate of PIC which depends on the energy difference between the metal dπ-orbitals and the lowest unoccupied molecular orbital (LUMO) of the bridge. Protonation of the bridging ligand decreases this energy gap, resulting in an overall increase in the rate of the redox reaction. Significantly, despite the close proximity of the luminophores to a conducting surface, the monolayers remain luminescent suggesting that the electronically excited state is only weakly coupled to the electrode surface. This is consistent with bipyridyl as the site of the excited state in the metal complex.     

Electrochemical synthesis of a ferrocenecarboxylic acid–C60 composite and its electrocatalysis to hydrogen peroxide

A new ferrocenecarboxylic acid–C₆₀ composite (Fc–C₆₀) has been synthesized by controlled potential electrolysis. A composite modified glassy carbon electrode has been prepared based on its good electrochemical activity. The modified electrode in 0.1 M NaClO₄ solution shows a reversible oxidation wave at E_1/2 = 0.32 V (vs. SCE) attributed to the oxidation of the ferrocene entity and a quasi-reversible reduction wave of C₆₀ entity at E_1/2 = ?0.54 V (vs. SCE). Electrocatalytic studies show that Fc–C₆₀ at the modified electrode can mediate the reduction of hydrogen peroxide (H₂O₂), and a broad linear range from 1.2 μM to 21.9 mM for H₂O₂ were obtained with a determination limit of 2.5 × 10^?7 M by amperometry.