Direct Heterogeneous Electron Transfer Research Articles

Layer-by-Layer Assembly of Glucose Oxidase on Carbon Nanotube Modified Electrodes.

The use of enzymatically modified electrodes for the detection of glucose or other non-electrochemically active analytes is becoming increasingly common. Direct heterogeneous electron transfer to glucose oxidase has been shown to be kinetically difficult, which is why electron transfer mediators or indirect detection is usually used for monitoring glucose with electrochemical sensors. It has been found, however, that electrodes modified with single or multi-walled carbon nanotubes (CNTs) demonstrate fast heterogeneous electron transfer kinetics as compared to that found for traditional electrodes. Incorporating CNTs into the assembly of electrochemical glucose sensors, therefore, affords the possibility of facile electron transfer to glucose oxidase, and a more direct determination of glucose. This chapter describes the methods used to use CNTs in a layer-by-layer structure along with glucose oxidase to produce an enzymatically modified electrode with high turnover rates, increased stability and shelf-life.

Electron Transfer of Myoglobin Immobilized in Au Electrodes Modified with a RAFT PMMA-Block-PDMAEMA Polymer

Myoglobin was immobilized with poly(methyl methacrylate)-block-poly[(2-dimethylamino)ethyl methacrylate]PMMA-block-PDMAEMA polymer synthesized by reversible addition-fragmentation chain transfer technique (RAFT). Cyclic voltammograms gave direct and slow quasireversible heterogeneous electron transfer kinetics between Mb-PMMA-block-PDMAEMA modified electrode and the redox center of the protein. The values for electron rate constant (Ks) and transfer coefficient (α) were0.055±0.01·s−1and0.81±0.08, respectively. The reduction potential determined as a function of temperature (293–328 K) revealed a value of reaction center entropy ofΔS0of351.3±0.0002 J·mol−1·K−1and enthalpy change of-76.8±0.1 kJ·mol−1, suggesting solvent effects and charge ionization atmosphere involved in the reaction parallel to hydrophobic interactions with the copolymer. The immobilized protein also exhibits an electrocatalytical response to reduction of hydrogen peroxide, with an apparentKmof114.7±58.7 μM. The overall results substantiate the design and use of RAFT polymers towards the development of third-generation biosensors.

Catalytic Current Based on Direct Electron Transfer Reactions of Enzymes Immobilized onto Carbon Nanotubes

Multi-walled and single-walled carbon nanotubes were synthesized on platinum plate (MWCNTs/pt) and gold wire (SWCNTs/Au) electrodes, respectively, using a chemical vapor deposition (CVD) method. These carbon nanotube-modified electrodes were immersed into solutions of glucose oxidase (GOX) and D-fructose dehydrogenase (FDH) to immobilize these enzymes onto the electrode surfaces. After GOX was immobilized onto the MWCNT/Pt electrode, the well-defined catalytic oxidation current was increased from ca. -0.45 V (vs. Ag/AgCl/saturated KCl), which was close to the redox potential of flavin adenine dinucleotide as a prosthetic group of GOX under physiological pH values. Furthermore, catalytic oxidation currents of fructose based on direct heterogeneous electron transfer reactions between FDH and the SWCNT/Pt electrode were observed.

D-Fructose detection based on the direct heterogeneous electron transfer reaction of fructose dehydrogenase adsorbed onto multi-walled carbon nanotubes synthesized on platinum electrode

Multi-walled carbon nanotubes (MWCNTs) were synthesized on platinum plate electrodes by the chemical vapor deposition (CVD) method. From the results of X-ray photoelectron spectroscopy and voltammetric investigation, the iron nanoparticles used as a catalyst for the MWCNT synthesis were enclosed with MWCNTs. The MWCNTs synthesized on the Pt plate (MWCNTs/Pt) electrode were immediately immersed into solutions of d-fructose dehydrogenase (FDH) to immobilize the enzyme onto the MWCNTs/Pt electrode surfaces. After the FDH was immobilized onto the MWCNTs/Pt electrode, a well-defined catalytic oxidation current based on FDH was observed from ca. −0.15 V (versus Ag/AgCl/sat’d KCl), which was close to the redox potential of heme c as a prosthetic group of FDH. From an analysis of a plot of the catalytic current versus substrate, the calibration range for the fructose concentration was up to ca. 40 mmol dm −3, and the apparent Michaelis–Menten constant was evaluated to be 11 ± 1 mmol dm −3.

Bioelectrocatalytic current based on direct heterogeneous electron transfer reaction of glucose oxidase adsorbed onto multi-walled carbon nanotubes synthesized on platinum electrode surfaces

Multi-walled carbon nanotubes (MWCNTs) were synthesized on platinum plate electrodes by the chemical vapor deposition (CVD) method. The MWCNTs synthesized on the Pt plate (MWCNTs/Pt) electrode were immediately immersed into solutions of glucose oxidase (GOX) to immobilize these enzymes onto the MWCNTs/Pt electrode surfaces. After the GOX was immobilized onto the MWCNTs/Pt electrode, a well-defined catalytic oxidation current was increased from ca. −0.45 V (vs. Ag/AgCl/saturated KCl), which was close to the redox potential of flavin adenine dinucleotide (FAD) as a prosthetic group of GOX under physiological pH values.

Direct heterogeneous electron transfer reactions and molecular orientation of fructose dehydrogenase adsorbed onto pyrolytic graphite electrodes

Abstract Direct heterogeneous electron transfer reactions and the molecular orientation of d -fructose dehydrogenase (FDH, from Gluconobacter sp.) adsorbed onto electrodes were investigated. Catalytic oxidation currents based on the direct electron transfer reactions of FDH adsorbed onto basal-plane, highly oriented pyrolytic graphite (basal-plane HOPG), plastic formed carbon plate and tin-doped indium oxide electrodes were observed from a potential around −0.1 V (vs. Ag/AgCl/saturated KCl) in phosphate solution (pH 5.0) in the presence of fructose as a substrate for FDH. The catalytic current for the FDH adsorbed onto basal-plane HOPG electrodes indicated a pH dependence. The catalytic oxidation currents were only observed in acidic solutions (pH ⩽ 6), and were not observed in neutral and alkaline solutions. The reason why the catalytic current was not obtained in neutral and alkaline solutions was because the FDH complex decomposed in neutral and alkaline solutions based on the AFM measurements. The differential pulse voltammetric investigations for FDH adsorbed onto basal-plane HOPG electrode together with the AFM and electrophoresis results indicated that the flavin-containing subunit in FDH accepts electrons from d -fructose, and transfers these electrons to the heme c -containing subunit, and then the direct electron transfer reaction of FDH occurred at the heme c site.

Direct heterogeneous electron transfer reactions of fungal laccases at bare and thiol-modified gold electrodes

Mediatorless (direct) electron transfer between bare and thiol-modified gold electrodes and fungal laccases from different sources has been demonstrated. The electrochemical activity of the enzymes from basidiomycetes Trametes hirsuta, Trametes ochracea, and Cerrena maxima under aerobic and anaerobic conditions can clearly be observed using cyclic voltammetry and spectroelectrochemistry. Bioelectroreduction of oxygen by T. hirsuta laccase immobilized on amino-thiophenol-modified gold electrodes, starting at +625 mV vs. NHE, is demonstrated and differences in bioelectrocatalysis of the enzyme immobilized on bare and thiol-modified electrodes are shown. It was found that hydrogen peroxide was one of the products of oxygen electroreduction on gold electrodes modified with fungal laccases, whereas no significant peroxide formation was observed for T. hirsuta laccase immobilised on thiol-modified gold electrodes. Thus, a hypothesis about two different mechanisms of oxygen electroreduction by fungal laccases adsorbed on bare and thiol-modified electrodes is proposed.

Direct heterogeneous electron transfer reactions of bilirubin oxidase at a spectrographic graphite electrode

Mediatorless (direct) electron transfer between Myrothecium verrucaria bilirubin oxidase and spectroscopic graphite electrode has been demonstrated. The electrochemical activity of the enzyme under aerobic and anaerobic conditions is clearly shown using cyclic voltammetry. It is concluded that the T1 site of the enzyme is the first electron acceptor, both in solution (homogenous case) and when the bilirubin oxidase is adsorbed on the surface of the graphite electrode (heterogeneous case).

Direct heterogeneous electron transfer of theophylline oxidase

Direct electron transfer (DET) was shown between the heme containing enzyme theophylline oxidase (ThO) and the surface of both graphite and gold electrodes. As proof on graphite a steady state current for theophylline was recorded using the electrode modified with adsorbed ThO. The electrode showed a Michaelis–Menten-like response to theophylline with a detection limit of 0.2 mM and a Michaelis–Menten constant equal to 3.2 mM. These initial results open up a possibility for the development of reagentless third generation biosensor based on heterogeneous DET between ThO and an electrode. On gold DET between ThO and the surface of aldrithiol modified gold was studied with spectroelectrochemical measurements. DET was observed for soluble ThO as a change of its spectrum in a gold capillary responding to a change in the applied potential. It was shown that the redox conversion of the heme domain of the enzyme is directly (mediatorlessly) driven by the potential applied at the gold electrode. The measurements enabled an estimation of the formal potential ( E°′) of the redox process equal to −275±50 mV versus Ag|AgCl sat at pH 7.0. The experimentally determined number of the electrons involved in this heterogeneous electron transfer process was estimated to be equal to 0.53. The low precision in determination of the E°′ and the value of the number of electrons lower than one indicate that kinetic restrictions disturbed the evaluation of the true thermodynamic values from relatively fast spectroelectrochemical measurements.

Direct electron transfer of heme- and molybdopterin cofactor-containing chicken liver sulfite oxidase on alkanethiol-modified gold electrodes.

Direct heterogeneous electron transfer (ET) of sulfite oxidase (SOx), a heme- and molybdopterin cofactor-containing intermembrane enzyme, was studied on alkanethiol-modified Au electrodes both with SOx entrapped between the modified Au electrode and a permselective membrane and with SOx adsorbed at the electrode surface, in the absence of any membrane. SOx in direct electronic communication with the electrode surface gave a quasi-reversible electrochemical signal with a midpoint potential of--120 mV vs Ag/AgCl corresponding to the redox transformations of the heme domain of SOx and with a heterogeneous ET constant in the order of 15 s(-1). The efficiency of the bioelectrocatalytic 2e- oxidation of sulfite catalyzed by SOx in direct ET exchange with the electrode was shown to depend essentially on the nature of the alkanethiol layer. Adsorption and orientation of SOx on an 11-mercapto-1-undecanol (MuD-OH) self-assembled monolayer, i.e., terminally functionalized with OH groups, provided efficient catalytic oxidation of sulfite, contrary to nonfunctionalized alkanethiols, e.g., 1-decanethiol, or alkanethiol layers terminally functionalized with NH2 groups. Comparative studies with short-chain alkanethiols, e.g., cysteamine and 2-mercaptoethanol, revealed an evidently different mode of adsorption of SOx on these layers, onto which SOx was not catalytically active. Coadsorption of MuD-OH and 11-mercapto-1-undecanamine improved the surface properties of the SAM, resulting in a higher surface coverage with bioelectrocatalytically active SOx but not in an increased apparent catalytic rate constant, kcat, ranging in the order of 18-24 s(-1) at pH 7.4. The achieved efficiency of SOx bioelectrocatalysis in direct ET reaction between the modified electrode and the enzyme approached the rates characteristic for the catalysis mediated by cytochrome c, the natural redox partner of SOx, thus implying the retention of the biological function of SOx under the heterogeneous electrode reaction conditions. Results obtained enable the development of a third-generation biosensor for sulfite monitoring.

Direct electron transfer of adrenodoxin—a [2Fe–2S] protein—and its mutants at modified gold electrode

Adrenodoxin can adsorb on polyallylamine modified surfaces at low ionic strength, because of local acidic surface. After a gold electrode was modified with a multilayer of mercaptoundecanoic acid and polyallylamine the direct heterogeneous electron transfer between adrenodoxin and the electrode has been achieved. The apparent electron transfer rate constant k et o was estimated to be ∼0.14 s −1 and the midpoint redox potential to be −0.46 V±10 mV. Peak current is proportional to scan rate up to 150 mV s −1 indicating that wt-Adx was adsorbed on the electrode surface. Two mutants were investigated which show at the same modified electrode a redox potential shift of about 30 mV in the negative direction. An increase in the heterogeneous electron transfer rate and efficiency of cytochrome c reduction were observed with both mutants as compared to wild type adrenodoxin.

F-Centre luminescence from oxide-covered aluminium cathode induced by two-step reduction of peroxydisulfate anions

S. Kulmala, T. Ala-Kleme, A. Hakanen and K. Haapakka, J. Chem. Soc., Faraday Trans., 1997, 93, 165 DOI: 10.1039/A604025J

Direct Electrochemistry for the Imidazole Complex of Microperoxidase-11 in Dimethyl Sulfoxide Solution at Naked Electrode Substrates Including Glassy Carbon, Gold, and Platinum

The effect of the total water content on the persistence and rate of direct heterogeneous electron transfer between the imidazole complex of microperoxidase-11 (im-MP-11) and naked gold, platinum, and glassy carbon (GC) in dimethyl sulfoxide (DMSO) solutions containing 0.1 M tetra-n-butylammonium perchlorate was investigated using cyclic voltammetry. Electron transfer between im-MP-11 and Au, Pt, and GC has been found to be persistent for more than 1 h and at least quasi-reversible [k(s)' = (8.7 ± 0.1) × 10(-4) cm/s (Au), k(s)' = (7.2 ± 1.3) × 10(-4) cm/s (Pt), and k(s)' = (5.7 ± 1.0) × 10(-4) cm/s (GC)] in dimethyl sulfoxide containing an absolute water content between 0.1 and 1.8%. The heterogeneous electron transfer rate constant is independent of the total water content of the DMSO solution when between 0.1 and 1.8% water is present.

First direct interfacial electron transfer between a biomolecule and a solid electrode in non-aqueous media: Direct electrochemistry of microperoxidase-11 at glassy carbon in dimethyl sulfoxide solution

The direct heterogeneous electron transfer between the imidazole complex of microperoxidase-11 (im-MP-11) and naked glassy carbon electrodes in dimethyl sulfoxide (1–3.6% H 2O) containing 0.1 M tetra- n-butylammonium perchlorate has been accomplished using cyclic voltammetry. Homogeneous solutions of im-MP-11 in dimethyl sulfoxide were prepared by weak complexation with 18-crown-6. Heterogeneous electron transfer between im-MP-11 and glassy carbon has been shown to be persistent and at least quasi-reversible in dimethyl sulfoxide solution. The redox potential for im-MP-11 in dimethyl sulfoxide has been found to be −274 ± 5 mV vs. Ag/AgCl — significantly less negative than the potential (−389 mV vs. Ag/AgCl) for this same complex in aqueous 75 mM phosphate buffer, pH 7.0, containing 0.1 M NaClO 4. The calculated diffusion coefficient (1.8 × 10 −7 cm 2 s −1) and the rate of electron transfer in dimethyl sulfoxide (2.7 ± 0.3 × 10 −4 cm s −1) are approximately one order of magnitude lower than that for the same complex in 0.02 M aqueous phosphate buffer, pH 7.0.

Thermodynamic and kinetic studies of cytochrome c from different species

The direct heterogeneous electron transfer reactions of cytochrome c from different vertebrate species are compared in this report. A kinetic study revealed a biphasic temperature dependence at neutral pH for the formal heterogeneous electron transfer rate constants of these cytochromes. The properties of the electrode-solution interface are believed to cause this biphasic kinetic effect. This effect does not correlate with the conformational transitions evident in both formal potential and circular dichroism (CD) measurements as a function of temperature. The conformational transitions seen in the formal potential and CD measurements arise from the alkaline isomerization and heme crevice changes. These are subtle effects compared with the structural changes associated with unfolding of the protein that are evident in differential scanning calorimetric experiments. The conformational stabilities of cytochrome c between species relate to the differences in their amino acid sequences. The amino acid interactions which stabilize the structure of these different cytochrome c molecules have been evaluated and will be discussed.

Laser-induced electron ejection at metal surfaces Evidence for photoelectroreduction of cobalt(III) and chromium(III) ammines by direct heterogeneous electron transfer

Photocurrents for the reduction of selected Co(III) and Cr(II) ammine complexes at silver, gold, and mercury electrodes induced by laser irradiation at 514.5 and 647.1 nm have been measured as a function of electrode potential and double-layer structure, and compared with corresponding thermal electroreduction data in order to provide information diagnostic of the photoelectroreduction mechanism. Photocurrents were observed at potentials considerably (up to 2 V) positive of the threshold for conventional photoemission. Similar responses were obtained in the photo- and thermal currents for outer-sphere Co(NH 3) 3+ 6 reduction brought about by variations in the reactant double-layer concentration by means of specific ionic adsorption at silver and mercury electrodes. This suggests that the effective electron-tunneling distance is similarly small (⪅ 0.6–0.8 nm) for the photoassisted as well as unassisted electroreduction pathways. A relationship was seen between the electrode potentials at which photoassisted and unassisted reduction occurred for the various reactants, including those following inner- as well as outer-sphere thermal reduction mechanisms. The photoelectroreduction mechanism appears to involve direct transfer of photoexcited electrons at the metal surface to acceptor species located within the double-layer region. The photocurrent-potential behavior is consistent with the presence of only a small Gibbs energy barrier, due to nuclear reorganization, as a result of the high reaction exoergicity. The possible value of such photoassisted pathways for examining highly exoergic electrode reactions is pointed out, along with complications from their presence when utilizing laser spectroscopic methods for examining thermal electrochemical processes.

Direct electrochemical oxidation of Clostridium pasteurianum ferredoxin : Identification of facile electron-transfer processes relevant to cluster degradation

Rapid oxidation processes relevant to the degradation of [4Fe4S] clusters in Clostridium pasteurianum ferredoxin were studied via direct (unmediated) heterogeneous electron transfer at a pyrolytic graphite electrode. Differential-pulse voltammograms of native [4Fe4S] ferredoxin showed two well-defined oxidation peaks corresponding to apparent E-values of +793 and +1120 mV at 5°C. Direct involvement of the cluster was established through parallel experiments with the 2[4Fe4Se] derivative for which peak positions were shifted. Square-wave voltammetry showed that the product of the first electron transfer, which may correspond to the ‘super-oxidised’ [4Fe4S] 3+ oxidation level, undergoes rapid degradation ( t 1 2 < 1.6 ms at 5°C). The second oxidation process, as characterised by a significant (⪢100 mV) negative shift upon selenium substitution, very likely represents oxidation of S(Se) still associated with the protein and possibly contained within the remaining FES(Se) substructure.

Electrogenerated Chemiluminescence: XXXIII . The Production of Excited States by Direct Heterogeneous Electron Transfer from Semiconductor Electrodes

Electrogenerated chemiluminescence (ECL) arising from direct electron transfer from semiconductor electrodes to solution species was examined. The reduction of the oxidized forms of several luminescent species [, thianthrene, 9,10‐diphenylanthracene, and rubrene] on several n‐type semiconductor electrodes (, , , , ) was investigated. Only for reduction of rubrene radical cation on and was unequivocal evidence found for production of an excited state (triplet) by direct heterogeneous electron transfer at potentials where homogeneous redox processes producing excited states were not possible. Current interruption techniques were employed to study the emission decay and an unusual emission at open circuit following a cathodic potential step was found. A model for direct triplet generation based on reduction via surface states within the bandgap region and quenching of excited states by the electrode is proposed and constraints on the experimental observation of direct excited state formation are suggested.

Zur elektrochemischen lumineszenz an halbreitern

The electrochemical luminescence of aromatic compounds in aprotic solvents was investigated using the following semiconductor electrodes: n-GaP, n-ZnO and n-SnO 2. Electrochemical luminescence by direct heterogeneous electron transfer to the excited singlet or triplet state and subsequent luminescence could in no case be detected. The discussion of the mechanism of the redox processes involved is based on experimental observations and energetic correlations which were partly obtained by measuring the capacity of the semiconductor—solvent systems used. We presume that the directly excited states are deactivated by radiationless transitions due to a heavy atom effect. This conclusion was confirmed to the extent that the naphthalin luminescence was quenched by these semiconductors at 77 K.