Electrocatalytic reduction of nitric oxide and other substrates on hydrogel triblock copolymer Pluronic films containing hemoglobin or myoglobin based on protein direct electrochemistry

Protein-Pluronic film modified electrodes were constructed by casting the mixture of hemoglobin (Hb) or myoglobin (Mb) and triblock copolymer poly(ethylene oxide)₁₀₀-poly(propylene oxide)₆₅-poly(ethylene oxide)₁₀₀ (Pluronic) solutions onto the surface of pyrolytic graphite (PG) electrodes. A pair of well-defined and nearly reversible CV peaks at about −0.35 V versus SCE in pH 7.0 buffers was observed for protein-Pluronic films, characteristic of the protein heme Fe(III)/Fe(II) redox couples. The films were characterized by scanning electron microscopy (SEM), UV-vis and IR spectroscopy, as well as electrochemistry. The heme proteins were quite stable and retained their near-native structure in Pluronic films. Oxygen, hydrogen peroxide, nitrite and nitric oxide were electrochemically catalyzed by protein-Pluronic films with significant lowering of reduction overpotential. Both nitrite and hydrogen peroxide underwent dismutation in electrocatalysis. The reduction of NO₂⁻ and H₂O₂ on protein-Pluronic films was actually the catalytic reduction of their disproportionation products, NO and O₂, respectively. The biocatalytic mechanism of these substrates at protein-Pluronic film electrodes were discussed and speculated.     

Direct electrochemistry and electrocatalysis of heme-proteins immobilized in porous carbon nanofiber/room-temperature ionic liquid composite film

The combination of porous carbon nanofiber (PCNF) and room-temperature ionic liquid (RTIL) provided a suitable microenvironment for heme-proteins to transfer electron directly. Hemoglobin, myoglobin, and cytochrome c incorporated in PCNF/RTIL films exhibited a pair of well-defined, quasi-reversible cyclic voltammetric peaks at about −0.28 V vs. SCE in pH 7.0 buffers, respectively, characteristic of the protein heme Fe(III)/Fe(II) redox couples. The cyclic voltammetry and electrochemical impedance spectroscopy were used to characterize the modified electrode. The heme/PCNF/RTIL/CHIT films were also characterized by UV-vis spectroscopy, indicating that heme-proteins in the composite film could retain its native structure. Oxygen, hydrogen peroxide, and nitrite were catalytically reduced at the heme/PCNF/RTIL/CHIT film modified electrodes, showing the potential applicability of the films as the new type of biosensors or bioreactors based on direct electrochemistry of the redox proteins.     

Interaction between myoglobin and hyaluronic acid in layer-by-layer structures—An electrochemical study

The layer-by-layer (LBL) self-assembled film construction of the biocompatible polymer hyaluronic acid (HA) and single heme redox protein, myoglobin (Mb) is described. The films were built upon gold electrode substrates, both gold quartz crystal electrodes and bulk gold (Au(bulk)) electrodes, and formation of the LBL films was gravimetrically monitored by an electrochemical quartz crystal microbalance. The electrochemical properties of the hyaluronic acid/myoglobin films ({HA/Mb}_n) were investigated after each deposition step using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The CV response presented an oxidation peak at +0.3 V vs. SCE, not characteristic of the redox protein myoglobin, and, the peak current decreased slightly with each additional bilayer. CV at Au(bulk) electrodes in pH 5.0 acetate buffer solution, containing Mb, presented the same oxidation peak as observed at {HA/Mb}_n modified electrodes, confirming the presence of the same electroactive species. The Mb oxidation peak current depends linearly on scan rate, characteristic of adsorbed thin-layer electrochemical systems, attributed to free adsorbed heme. Impedance spectra, recorded after deposition of each bilayer, were in agreement with the cyclic voltammetry observations.     

X-ray voltabsorptometry on redox proteins

Biological X-ray absorption spectroscopy (BioXAS) is able to describe the metal environment in a metalloprotein and is sensitive to metal oxidation state. Coupling of BioXAS and electrochemistry permits the characterization of different oxidation states and avoids uncontrolled protein redox state changes due to X-ray beam irradiation. XAS spectroelectrochemistry requires electrochemical cells specifically designed to meet the requirements of both XAS measurements and electrochemical effectiveness in potential control. In this context, this paper describes a new cell tested with different types of working electrodes developed for BioXAS, in particular for in situ studies of redox proteins. The XAS electrochemical measurements of a relatively high-molecular-weight protein (Cu,Zn superoxide dismutase) for which it is difficult to observe direct electrochemistry have been achieved.New working electrodes, capable of fast and unmediated electron transfer, are described. The cell permits to isolate protein redox states and to measure X-ray absorption intensity during a potential scan (X-ray voltabsorptometry).     

Effects of microwave radiation on electrodeposition processes at tin-doped indium oxide (ITO) electrodes

In situ microwave activation is investigated for the electrodeposition of a metal (gold) and for a metal oxide (hydrous Ti(IV) oxide) onto tin-doped indium oxide (ITO) film electrodes. It is demonstrated that localized microwave heating of the ITO film can be exploited to affect electrodeposition processes.The electrochemically reversible and temperature sensitive one-electron redox system Fe(CN)₆^3−/4− was employed in aqueous solution in order to calibrate the average surface temperature at the ITO film electrode. In the presence of microwave radiation the average electrode surface temperature reached ca. 363 K whereas under the same conditions the bulk solution temperature reached ca. 313 K. Therefore localized heating of the ITO film appears to be important.The rate of electrodeposition of gold from an aqueous 1 mM tetrachloroaurate(III) solution in 0.1 M KCl (adjusted to pH 2) is enhanced by microwave activation. However, the morphology of deposits remains un-effected. Hydrous titanium (IV) oxide films were electrodeposited from an aqueous solution of 1 mM TiCl₃ in 0.1 M acetate buffer pH 4.7. Dense films with blocking character were obtained with conventional heating but a fibrous more open deposit forms in the presence of microwaves.     

Electrochemical study of chitosan films deposited from solution at reducing potentials

We report the electrochemical characterization of chitosan films deposited at gold electrodes from an acidic solution at reducing potentials. Cyclic voltammetry was used to characterize the deposition and electroactivity of chitosan coated gold electrodes. Chitosan films were found to deposit at gold electrodes at potentials more negative than −1.0 V versus Ag/AgCl, a potential associated with the onset of water reduction and increase in pH near the electrode. The chitosan films are electrochemically inactive; similar background charging currents are observed at bare gold and chitosan coated electrodes. The chitosan films are permeable to both cationic [Ru(NH₃)₆^3+/2+] and anionic [Fe(CN)₆^3−/4−] redox couples, but anionic complexes are retained in the chitosan film. Semiintegral analysis was used to examine adsorbed redox species at the chitosan coated electrode surface. Electrochemical parameters, including apparent diffusion coefficients for the redox probes at the electrodeposited chitosan modified electrodes are presented and are comparable to values reported for cast chitosan films.     

Electrochemical behaviour of metal hexacyanoferrate converted to metal hydroxide films immobilized on indium tin oxide electrodes—Catalytic ability towards alcohol oxidation in alkaline medium

In this work, we demonstrate a simple method to modify indium tin oxide (ITO) electrodes in order to perform electro-catalytic oxidation of alcohols in alkaline medium. Metal hexacyanoferrate (MHCF) films such as nickel hexacyanoferrate (NiHCF) and copper hexacyanoferrate (CuHCF) were successfully immobilized on ITO electrodes using an electrochemical method. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were employed to characterize the structural and morphological aspects of MHCF films. Cyclic voltammetry (CV) was used to study the redox properties and to determine the surface coverage of these films on ITO electrodes. Electrochemical potential cycling was carried out in alkaline medium in order to alter the chemical structure of these films and convert to their corresponding metal hydroxide films. SEM and XPS were performed to analyze the structure and morphology of metal hydroxide modified electrodes. Electro-catalytic oxidation ability of these films towards methanol and ethanol in alkaline medium was investigated using CV. From these studies we found that metal hydroxide modified electrodes show a better catalytic performance and good stability for methanol oxidation along with the alleviation of CO poisoning effect. We have obtained an anodic oxidation current density of ∼82 mA cm⁻² for methanol oxidation, which is at least 10 fold higher than that of any metal hydroxide modified electrodes reported till date. The onset potential for methanol oxidation is lowered by ∼200 mV compared to other chemically modified electrodes reported. A plausible mechanism was proposed for the alcohol oxidation based on the redox properties of these modified electrodes. The methodology adapted in this work does not contain costlier noble metals like platinum and ruthenium and is economically viable.     

Characterisation of hydrophobic carbon nanofiber-silica composite film electrodes for redox liquid immobilisation

Carbon (50-150 nm diameter) nanofibers were embedded into easy to prepare thin films of a hydrophobic sol-gel material and cast onto tin-doped indium oxide substrate electrodes. They promote electron transport and allow efficient electrochemical reactions at solid|liquid and at liquid|liquid interfaces. In order to prevent aggregation of carbon nanofibers silica nanoparticles of 7 nm diameter were added into the sol-gel mixture as a “surfactant” and homogeneous high surface area films were obtained. Scanning electron microscopy reveals the presence of carbon nanofibers at the electrode surface. The results of voltammetric experiments performed in redox probe—ferrocenedimethanol solution in aqueous electrolyte solution indicate that in the absence of organic phase, incomplete wetting within the hydrophobic film of carbon nanofibers can cause hemispherical diffusion regime typical for ultramicroelectrode like behaviour.The hydrophobic film electrode was modified with two types of redox liquids: pure tert-butylferrocene or dissolved in 2-nitrophenyloctylether as a water-insoluble solvent and immersed in aqueous electrolyte solution. With a nanomole deposit of pure redox liquid, stable voltammetric responses are obtained. The presence of carbon nanofibers embedded in the mesoporous matrix substantially increases the efficiency of the electrode process and stability under voltammetric conditions. Also well-defined response for diluted redox liquids is obtained. From measurements in a range of different aqueous electrolyte media a gradual transition from anion transfer dominated to cation transfer dominated processes is inferred depending on the hydrophilicity of the transferring anion or cation.     

Synthesis and characterization of silica MCM-48 as carrier of size-confined nanocrystalline metal oxides particles inside the pore system

The interpenetrating 3-dimensional channel system of silica MCM-48 has been selected for the deposition of Cu/Zn/O mixed metal oxide particles. With the wet impregnation technique aqueous solutions of metal acetates have been used to load the calcined form of the mesoporous silica. Successive impregnation yielded metal contents of ca. 9 wt.%. Calcination of the composite transformed the acetates to the metal oxides. X-ray powder diffraction and solid-state MAS NMR showed the uptake of the metal salt inside the pore system. N₂-adsorption, X-ray diffraction and TEM confirmed the mesoporous structure. XPS measurements and EXAFS analysis (Cu K- and Zn K-edges) confirmed the metal uptake. Whereas nano-disperse CuO particles have been obtained ZnO shows no regular structure and seems to have reacted with the silicate channel surface by coating the channel wall.     

Photo-induced Surface Property on Transparent Mesoporous Silica Thin Films Containing Single-site Photocatalyst

The transparent Ti-containing or Cr-containing mesoporous silica thin films can be prepared on quartz plate using a spin-coating sol–gel method. The spectroscopic characterization has revealed that these thin films contain isolated and tetrahedrally coordinated titanium oxide moieties and chromium oxide moieties in their frameworks, respectively. These mesoporous silica thin films containing metal oxide moieties have demonstrated a strong hydrophilic surface property even before UV-light irradiation and the appearance of the super-hydrophilic property after UV-light irradiation. Especially, the Cr-containing mesoporous silica (CMS) thin film showed the super-hydrophilic property even under visible light irradiation. In the presence of ethylene gas, the polyethylene was successfully formed on the surface of the CMS thin film under UV-light irradiation. After the formation of polyethylene the surface property of the CMS was converted into hydrophobic while keeping its transparency. The isolated and tetrahedrally coordinated titanium oxide moieties and chromium oxide moieties are responsible for these photo-induced surface reactions.     

Electrochemical preparation and surface properties of gold nanowire arrays formed by the template technique

A new procedure for preparing free-standing nanowire arrays is described. This is based on a template method which entails electrochemical metal deposition into nanometer-wide parallel pores of porous anodic oxide films on aluminum. By varying the preparation conditions and by applying electrochemical posttreatment methods, gold nanowire arrays are prepared, whose morphological and dimensional properties and oxygen content in the nanowire surface can be varied and investigated by SEM and XPS-characterization. The voltammetric behaviour of such nanowire arrays shows a significant enhancement of the ratio between the total electrolytically exposed surface area and the diffusionally accessible surface area in comparison with macroscopic flat gold electrodes.     

Metastability and electrocatalytic activity of ruthenium dioxide cathodes used in water electrolysis cells

Ruthenium dioxide-coated titanium electrodes when used as cathodes in water electrolysis cells are metastable systems which display high catalytic activity and unexpected stability. At the potentials involved the oxide films clearly should undergo reduction; the barrier to the latter process seems to arise due to hydroxylation of the outer layers of the thermally prepared oxide; the reduction of hydrous oxide films in general is inhibited by the intervention (as the primary reduction product) of a high energy state of the metal. The catalysis of the hydrogen gas evolution reaction at the oxide/solution interface is attributed to the involvement of a metastable cyclic redox mediator system involving multivalent Ru(OH)_x surface species.     

Preparation of ordered mesoporous nickel oxide film electrodes via lyotropic liquid crystal templated electrodeposition route

A novel electrochemical route to fabricate ordered mesoporous metal oxide film electrodes has been investigated with particular reference to nickel oxide. Ordered mesoporous nickel oxide films are successfully synthesized by templated electrodeposition of H_I-e nickel hydroxide and followed by heat-treatment in air at various temperatures. The films are characterized physically by thermogravimetry (TG), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The applicability of this film as inexpensive and high-performance supercapacitor electrode material is demonstrated by the electrochemical characterization using cyclic voltammetry (CV) and chronopotentiometry technique. The specific capacitance of the nickel oxide film depends on the annealing temperature, showing a maximum value of 590 F g⁻¹ when the as-deposited film is heat-treated at 250 °C for 1.5 h.     

Spectroelectrochemistry of two-layered composites of polyaniline and poly(o-aminophenol)

Electropolymerization of aniline on poly(o-aminophenol)(POAP)-coated gold and indium-doped tin oxide (ITO)-coated glass electrodes yields polyaniline(PANI)/POAP two-layer composite films, exhibiting reversible redox functions in aqueous acidic solution. The PANI deposition on the POAP-coated electrodes was monitored by cyclic voltammetry (CV) and in situ UV-vis spectroelectrochemistry. CV results show that PANI/POAP composite films exhibit better stability as compared to PANI films during potential cycling in aqueous acidic solutions. Characteristic UV-vis and Raman features of the composite films have been identified and their dependencies on the electrode potential are discussed. They were significantly different from the corresponding spectral characteristics of PANI and POAP films alone.     

Electrochemical oxidation of ammonia (NH4/NH3) on thermally and electrochemically prepared IrO2 electrodes

The electrochemical oxidation of ammonia (NH₄⁺/NH₃) in sodium perchlorate was investigated on IrO₂ electrodes prepared by two techniques: the thermal decomposition of H₂IrCl₆ precursor and the anodic oxidation of metallic iridium. The electrochemical behaviour of Ir(IV)/Ir(III) surface redox couple differs between the electrodes indicating that on the anodic iridium oxide film (AIROF) both, the surface and the interior of the electrode are electrochemically active whereas on the thermally decomposed iridium oxide films (TDIROF), mainly the electrode surface participates in the electrochemical processes.On both electrodes, ammonia is oxidized in the potential region of Ir(V)/Ir(IV) surface redox couple activity, thus, may involve Ir(V). During ammonia oxidation, TDIROF is deactivated, probably by adsorbed products of ammonia oxidation. To regenerate TDIROF, it is necessary to polarize the electrode in the hydrogen evolution region. On the contrary, AIROF seems not to be blocked during ammonia oxidation indicating its fast regeneration during the potential scan. The difference between both electrodes results from the difference in the activity of the iridium oxide surface redox couples.     

The bioelectrocatalytic properties of cytochrome C by direct electrochemistry on DNA film modified electrode

The direct electrochemistry of cytochrome C can be performed in weak acidic and basic aqueous solutions. Cytochrome C can be deposited as a stable and electrochemically active film on a deoxyribonucleic acid (DNA) modified glassy carbon electrode. These films can also be produced on gold, platinum, and transparent semiconducting tin oxide electrodes. Two-layer modified electrodes containing cytochrome C and a DNA film were prepared by the deposition of cytochrome C on a DNA film modified electrode. The cytochrome C/DNA film was electrocatalytically oxidation active for l-cysteine in a pH 8.3 tris(hydroxymethyl)aminomethane (TRIS)-buffered aqueous solution through both Fe^III and Fe^IV species. The electrocatalytic oxidation current developed from the anodic peak of the redox couple. The electrocatalytic oxidation properties of ascorbic acid, NH₂OH, N₂H₄, and SO₃²⁻ by a cytochrome C/DNA film were also determined, and shown to be electrocatalytically active. An electrochemical quartz crystal microbalance, cyclic voltammetry, and direct spectroelectrochemistry were used to study in situ DNA deposition on a gold disc electrode and cytochrome C deposition on DNA/Au and DNA/GC films. The direct electrochemistry of cytochrome C can also be performed, and it can be deposited as a stable and electrochemically active film on polyvinyl sulfonate, polystyrene sulfonate, TiO₂, and polyethylene glycol modified glassy carbon electrodes. The results show that cytochrome C interacts with, and deposits on, a DNA film modified electrode, and that the cytochrome C (Fe^III) oxidized form is more easily deposited on a DNA film than the cytochrome C (Fe^II) reduced form.     

Modified electrodes for probing the metal-reductase activity of metalloproteins: the reduction of iron(III) catalyzed by Desulfovibrio vulgaris Hildenborough cytochrome c3

Cytochromes c₃ are polyheme c-type cytochromes characterized by low redox potentials, that have been shown to develop metal-reductase activity. In this paper, different strategies are explored to immobilize one of them, Desulfovibrio vulgaris Hildenborough cytochrome c₃, a highly basic tetraheme cytochrome, including adsorption, covalent bonding, imprisonment in a layer-by-layer assembly, and entrapment within cast films or a dialysis membrane. The performance and efficiency of modified (carbon or gold) electrodes have been evaluated using electrochemical (cyclic and square-wave voltammetry, current-time curves) techniques in the presence of a soluble Fe(III) complex, ammonium Fe(III) citrate acting as the soluble substrate, and chosen as a model system. The advantages and drawbacks of each strategy are discussed with the view of further extension of environmental interest to more toxic metal contaminants.     

Electron-transfer processes of cytochrome C at interfaces. New insights by surface-enhanced resonance Raman spectroscopy

The heme protein cytochrome c acts as an electron carrier at the mitochondrial-membrane interface and thus exerts its function under the influence of strong electric fields. To assess possible consequences of electric fields on the redox processes of cytochrome c, the protein can be immobilized to self-assembled monolayers on electrodes and studied by surface-enhanced resonance Raman spectroscopy. Such model systems may mimic some essential features of biological interfaces including local electric field strengths. It is shown that physiologically relevant electric field strengths can effectively modulate the electron-transfer dynamics and induce conformational transitions.     

A membrane-bound cytochrome c3: a type II cytochrome c3 from Desulfovibrio vulgaris Hildenborough

A new tetraheme cytochrome c3 was isolated from the membranes of Desulfovibrio vulgaris Hildenborough (DvH). This cytochrome has a molecular mass of 13.4 kDa and a pI of 5.5 and contains four heme c groups with apparent reduction potentials of -170 mV, -235 mV, -260 mV and -325 mV at pH 7.6. The complete sequence of the new cytochrome, retrieved from the preliminary data of the DvH genome, shows that this cytochrome is homologous to the "acidic" cytochrome c3 from Desulfovibrio africanus (Da). A model for the structure of the DvH cytochrome was built based on the structure of the Da cytochrome. Both cytochromes share structural features that distinguish them from other cytochrome c3 proteins, such as a solvent-exposed heme 1 surrounded by an acidic surface area, and a heme 4 which lacks most of the surface lysine patch proposed to be the site of hydrogenase interaction in other cytochrome c3 proteins. Furthermore, in contrast to previously discovered cytochrome c3 proteins, the genes coding for these two cytochromes are adjacent to genes coding for two membrane-associated FeS proteins, which indicates that they may be part of membrane-bound oxidoreductase complexes. Altogether these observations suggest that the DvH and Da cytochromes are a new type of cytochrome c3 proteins (Type II: TpII-c3) with different redox partners and physiological function than the other cytochrome c3 proteins (Type I: TpI-c3). The DvH TpII-c3 is reduced at considerable rates by the two membrane-bound [NiFe] and [NiFeSe] hydrogenases, but catalytic amounts of TpI-c3 increase these rates two- and fourfold, respectively. With the periplasmic [Fe] hydrogenase TpII-c3 is reduced much slower than TpI-c3, and no catalytic effect of TpI-c3 is observed.     

Activity of anodic oxide films on metal and cermet anodes in cryolite-alumina melts

The activity of anodic oxide films on nickel, iron and copper metal anodes in cryolite-alumina melts was calculated using values of the reversible potential obtained from a polarization scan of the corresponding metal electrode. The best results were obtained with prepolarized electrodes. The anodic oxide layer formed on nickel becomes thick and dense and remains adherent during the period of prepolarization. Similar activity calculations were made for selected nickel-, iron- and copper-cermet compositions containing either a MnZn ferrite or a nickel ferrite ceramic phase. Large activities were observed for a NiO type corrosion product on both the nickel and nickelcermet electrodes. The results suggest that a dense surface layer containing a NiO phase is formed on nickel-cermet electrodes. This layer may help lower corrosion by minimizing electrolyte penetration of the anode surface.