Electrochemical oxidation of histidine at an anodic oxidized boron-doped diamond electrode in neutral solution

Electrochemical oxidation of histidine (His) at an anodic oxidized boron-doped diamond electrode (AOBDDE) was performed. A significant peak of His oxidation is observed at about +1.5 V vs. Ag/AgCl, however, the response current was inhibited due to strong His-oxidized product adsorption onto the electrode surface. The characteristics of the His-oxidized product adsorbed onto the electrode surface were investigated by studying the electrochemical behavior of the Fe(CN)₆⁴⁻ redox reaction using cycle voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Both CV and EIS results showed a decrease in the sum of transfer coefficients and an increase in the electron transfer resistance, which indicate that the adsorption film is a non-conductive film. The most possible active site locations for the AOBDDE for His oxidation are within these low-lying polycrystallite AOBDDE surface regions. The results from Raman and X-ray photoelectron spectroscopy offer strong evidence of the imidazole ring reaction from His. Experiments confirmed that the adsorbed film can be removed and the electrode surface reactivated using brief polarization at +2.5 V.     

The effects of nitrogenation on the electrochemical properties of nanocrystalline diamond films

The effect of the nitrogenation on the electrochemical properties of nanocrystalline diamond films produced by microwave plasma CVD in CH₄–Ar–H₂–N₂ gas mixtures was studied systematically, using cyclic voltammetry and electrochemical impedance spectroscopy measurements, for the first time. Differential capacitance, kinetic parameters of reactions in [Fe(CN)₆]^3-/4-redox system and potential window were found to be sensitive to the nitrogen concentration in the process gas. With its increase (from 0 to 25%), a transition of the NCD film behavior from “poor conductor” to metal-like character takes place. The heavily N-doped nanocrystalline diamond films have satisfactory electrochemical properties to be used as electrodes.     

Fabrication of boron doped diamond microband electrodes for electrochemical detection in a microfluidic channel

The manufacturing and electrochemical characterisation of an array of 20 boron doped nanocrystalline diamond (BNCD) microband electrodes for use in a poly(dimethylsiloxane) (PDMS) based microfluidic system are described. The electrodes were fabricated by plasma etching of a silicon oxide- and BNCD thin film coated silicon wafer and the resulting surface structured silicon wafer was subsequently bonded to the PDMS so that the BNCD microband electrodes were located within the PDMS microchannel. The electrochemical performance of the BNCD electrodes was studied and the electrodes were found to exhibit significantly better stability than previously employed gold microband arrays.     

Preparation and characterization of boron doped diamond electrodes on diamond anvil for in situ electrical measurements under high pressure

A layer of boron doped diamond (BDD) film was deposited selectively on a diamond anvil and employed as electrodes for measuring the electrical resistivity of matter under high pressure. Both heavily doped and lightly doped electrodes were characterized by Raman spectroscopy and scanning electron microscopy. Though the BDD film electrodes contain sp² carbon, it is still suitable for in situ high pressure electrical measurements. The dependability of diamond film electrodes was tested at high pressure up to 36 GPa, by measuring the electric resistance of C60 fullerene powder, and no damage of the electrodes was observed.     

AC impedance studies of anodically treated polycrystalline and homoepitaxial boron-doped diamond electrodes

The electrochemical properties of several types of diamond electrodes, including polycrystalline and homoepitaxial films, that underwent anodic treatment were examined with the electrochemical impedance spectroscopic (EIS) technique, as well as with capacitance-potential measurements. From an analysis of the impedance behavior, it was found that an additional capacitance element, which is apparent in the relatively high-frequency range (100-1000 Hz), was generated on the polycrystalline and (1 0 0) homoepitaxial diamond electrodes after anodic treatment. This capacitive element can be characterized as being non-Faradaic, because it has negligible dependence on the applied potential. Acceptor densities and depth profiles were calculated from the Mott-Schottky plots, and the acceptor densities in the near-surface region of the anodically treated surfaces were found to be extremely low. These results indicate that passive layers were generated on the diamond surfaces by the anodic treatment. The capacitance-potential behavior was also consistent with a model consisting of a semiconductor with a passive surface film. The passive film is proposed to arise as a result of the removal of hydrogen acting as an acceptor in the subsurface region, leaving hydrogen that is paired essentially quantitatively with the boron dopant, effectively neutralizing it.     

Electroanalytical application of modified diamond electrodes

Metal-modified diamond electrodes were fabricated by using ion implantation method for electroanalytical applications. Nickel and copper ion were implanted at a different film of boron-doped diamond (BDD) with a dose of 5×10¹⁴ cm⁻² for each type of ion. The electrochemical behavior has been studied for glucose oxidation in alkaline media by using cyclic voltammetry and flow injection analysis. Those electrodes exhibited high catalytic activity and excellent electrochemical stability with low background current even after strong ultrasonication in the cleaning process. The results indicate that metal-implanted method could be a promising method for controlling the electrochemical properties of diamond electrodes.     

Electrochemical studies of ganciclovir at boron-doped nanocrystalline diamond electrodes

The electrochemical oxidation of ganciclovir was investigated at boron-doped nanocrystalline diamond (BDND) electrodes by the use of cyclic voltammetry and differential pulse voltammetry. The optimization of the experimental variables including supporting electrolyte and pH value was studied, and the 0.04-M Britton-Robinson buffer solution (pH 2.5) was selected. The relationship of the oxidation peak potential to scan rate and pH value was also investigated, and 2 electron transfer and 2 proton participation for the oxidation process of ganciclovir at BDND electrode were obtained. Compared with boron-doped microcrystalline diamond and glassy carbon electrodes, the BDND electrode demonstrated the wider linear range of 0.5-350 μM, lower limit of detection of 0.2 μM, and higher reproducibility and stability for the determination of ganciclovir under the optimum conditions. For the analysis of ganciclovir in human serum at the BDND electrodes, precision and accuracy were checked by recovery experiments.     

Direct electrochemistry of blue copper proteins at boron-doped diamond electrodes

Boron-doped diamond (BDD) is a promising electrode material for use in the spectro-electrochemical study of redox proteins and, in this investigation, cyclic voltammetry was used to obtain quasi-reversible electrochemical responses from two blue copper proteins, parsley plastocyanin and azurin from Pseudomonas aeruginosa. No voltammetry was observed at the virgin electrodes, but signals were observed if the electrodes were anodised, or abraded with alumina, prior to use. Plastocyanin, which has a considerable overall negative charge and a surface acidic patch which is important in forming a productive electron transfer complex with its redox partners, gave a faradaic signal at pre-treated BDD only in the presence of neomycin, a positively charged polyamine. The voltammetry of azurin, which has a small overall charge and no surface acidic patch, was obtained identically in the presence and absence of neomycin. Investigations were also carried out into the voltammetry of two site-directed mutants of azurin, M64E azurin and M44K azurin, each of which introduce a charge into the protein's surface hydrophobic patch. The oxidizing and cleaning effects of the BDD electrode pre-treatments were studied electrochemically using two inorganic probe ions, Fe(CN)₆³⁻ and Ru(NH₃)₆³⁺, and by X-ray photoelectron spectroscopy (XPS). All of the electrochemical results are discussed in relation to the electrostatic and hydrophobic contributions to the protein/diamond electrochemical interaction.     

XPS investigations of thermally prepared RuO2 electrodes in reductive conditions

The surface composition of a thermally prepared RuO₂ electrode was studied by X-ray photoelectron spectroscopy (XPS) to investigate structural and surface composition changes occurring during the hydrogen evolution reaction. This was done using an electrochemical cell (EC) attached directly to the ultra high vacuum (UHV) chamber of the spectrometer, allowing the direct transfer of the electrode from the EC to the UHV chamber without exposure to ambient conditions. All the treatments have been performed in this cell, using 1N H₂SO₄. After a polarization of the electrode to −0.5 V versus standard calomel electrode (SCE), the XPS spectrum showed no shift in the binding energy of the Ru core level peaks, indicating that no reduction of Ru(IV) occurs. Further analysis of the O 1s core level spectrum also revealed that the adsorption of sulfate anions is maximum at −0.5 V versus SCE.     

EDTA modified glassy carbon electrode: Preparation and characterization

EDTA-phenoxyamide modified glassy carbon electrode (EDTA-GC) was prepared at a glassy carbon electrode by surface synthesis. In the first step, nitrophenyl was grafted to the glassy carbon (GC) surface via the electrochemical reduction of its tetraflouroborate diazonium salt. In the second step, nitrophenyl-modified electrode (NP-GC) was subjected to the cathodic potential scan to reduce the nitro to amine group. p-Aminophenyl modified glassy carbon electrode (AP-GC) was dipped into a EDTA solution containing 1-ethyl-3(3-(dimethlyamino)propyl)-carbodiimide (EDC) as an activating agent. Thus formed ((2-anilino-2-oxoethyl){2-[bis(carboxymethyl)amino]-ethyl}amino)acetic acid modified GC electrode was denoted as EDTA-GC and characterized by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), ellipsometry and X-ray photoelectron spectroscopy (XPS). Complexation of the EDTA-GC surface with Pb²⁺ ions was investigated if this electrode could be used as a metal sensor.     

Concatenation of electrochemical grafting with chemical or electrochemical modification for preparing electrodes with specific surface functionality

Surface modified electrodes are used in electro-analysis, electro-catalysis, sensors, biomedical applications, etc. and could also be used in batteries. The properties of modified electrodes are determined by the surface functionality. Therefore, the steps involved in the surface modification of the electrodes to obtain specific functionality are of prime importance. We illustrate here bridging of two routes of surface modifications namely electrochemical grafting, and chemical or electrochemical reduction. First, by electrochemical grafting an organic moiety is covalently immobilized on the surface. Then, either by chemical or by electrochemical route the terminal functional group of the grafted moiety is transformed. Using the former route we prepared lithium alkyl carbonate (–O(CH₂)₃OCO₂Li) modified carbon with potential applications in batteries, and employing the latter we prepared phenyl hydroxyl amine (–C₆H₄NHOH) modified carbon which may find application in biosensors. Benzyl alcohol (–C₆H₄CH₂OH) modified carbon was prepared by both chemical as well as electrochemical route. We report combinations of conjugating the two steps of surface modifications and show how the optimal route of terminal functional group modification depends on the chemical nature of the moiety attached to the surface in the electrochemical grafting step.     

Anodic stripping voltammetry of zinc at boron-doped diamond electrodes in ammonia buffer solution

This paper describes the deposition of zinc(II) with anodic stripping voltammetry on the boron-doped diamond electrode. We illustrate the dependency of several parameters on the magnitude of the oxidation peak and try to optimize the method. The supporting electrolyte was found to influence the oxidation peak magnitude. Compared with acetic acid, the most frequently used supporting electrolyte, ammonia buffer solution leads to a four times higher signal. We assume that the formation of zinc complexes, primarily tetraaminezinc(II), are responsible for the better response. Further factors studied and assessed include buffer pH, buffer concentration, deposition potential, deposition time and scan rate. With the improved conditions, a final detection limit of 5 ppb was accomplished.     

Electrochemical impedance spectroscopy of oxidized and hydrogen-terminated nitrogen-induced conductive ultrananocrystalline diamond

We have studied the electrochemical impedance spectroscopy of conductive ultrananocrystalline diamond (UNCD) modified by either oxidation or hydrogenation surface treatments. The impedance was measured in the frequency range from 0.1 Hz to 40 kHz at different DC voltages and the results fitted to an equivalent electrical circuit. Despite the complexity of the conductive UNCD surface, composed of sp³-bonded grains and grain boundaries with a high content of sp²-bonded carbon atoms, a Randles circuit with a constant phase element (CPE) for the capacitive element provided a reasonable model for both terminations. However, the parameters of the CPE were very different for each termination. Taking into account the results obtained, we propose that the interfacial impedance of oxidized UNCD is dominated by the oxidized sp²-bonded carbon atoms present at the grain boundaries, and the interfacial impedance of hydrogen-terminated UNCD is governed by both the grain boundaries and the grains.     

Characterization of boron-doped diamond electrodes by electrochemical impedance spectroscopy

Charge transfer on boron doped diamond (BDD) electrodes was studied by cyclic voltammetry and electrochemical impedance spectroscopy. The diamond films of 5 μm thickness and boron content between 200 ppm and 3000 ppm were prepared by the hot filament CVD technique on niobium substrate and mounted in a Teflon holder as rotating disk electrodes. The electrochemical measurements were carried out in aqueous electrolyte solutions of 0.5 M Na₂ SO ₄ + 5 mM K₃[Fe(CN)₆]/K₄[Fe(CN)₆]. Significant deviation in the redox behaviour of BDD and active Pt electrodes was indicated by a shift of the peak potentials in the cyclic voltammograms with increasing sweep rate and lower limiting diffusion current densities under rotating disk conditions. In the impedance spectra an additional capacitive element appeared at high frequencies. The potential and rotation dependence of the impedance spectra can be described quantitatively in terms of a model based on diffusion controlled charge transfer on partially blocked electrode surfaces. Direct evidence for the non-homogeneous current distribution on the diamond surface was obtained by SECM measurements.     

X-ray photoelectron spectroscopy study of sodium reactions in carbon cathode blocks of aluminium oxide reduction cells

Aluminium oxide reduction was performed in a laboratory electrolysis cell with different industrial carbon cathode blocks (semi-graphitic, graphitic, and graphitized blocks). During electrolysis, sodium species migrate from the bath into the carbon cathode. Consequences of this migration include expansion of the blocks—the so-called sodium swelling—that may lead to failure of the cell. Characterisation of the blocks by XPS indicated that in addition to ionic sodium species (e.g. NaF and NaHCO₃), two different types of metallic sodium were present in the cathodes. One type of metallic sodium is associated with a degradation of the graphitic structure, suggesting that this sodium is intercalated between the graphene layers, whereas the other type of metallic sodium was most probably present in micropores. Both types of metallic sodium were detected in semi-graphitic blocks while only the “micropore” sodium was found in graphitic and graphitized blocks. The metallic sodium was remarkably stable in the laboratory atmosphere, probably due to the fact that, after electrolysis, the entire porosity of the carbon cathode is filled with penetrated bath. This limits the access of oxygen and humidity to the metallic sodium.     

Residual stresses and crystalline quality of heavily boron-doped diamond films analysed by micro-Raman spectroscopy and X-ray diffraction

X-ray diffraction analysis and micro-Raman spectroscopy measurements have been used for stress studies on HFCVD diamond films with different levels of boron doping. The boron incorporation in the film varied in the range 10¹⁸-10²¹ boron/cm³. The grain size, obtained from SEM images, showed grains with 2-4-μm average size, which decreases when the doping level increases. The thickness of the films obtained by SEM cross-section view decreased from 8 to 5 μm as the doping level increased from 0 (undoped film) to 10²¹ boron/cm³. The total residual stress was determined by measuring, for each sample, the (331) diamond Bragg diffraction peak for Ψ-values ranging from −60° to +60°, and applying the sin² ψ method. For the micro-Raman spectroscopy the spectral analysis performed on each sample allowed the determination of the residual stress, from the diamond Raman peak shifts, and also the diamond purity, which decreases from 99 to 75% as the doping level increases. The type and magnitude of the residual stress obtained from X-ray and micro-Raman measurements agreed well only for undoped film, disagreeing when the doping level increased. We attributed this discrepancy to the domain size characteristic of each technique.     

Structural and electrochemical characterization of fullerene-based surfaces of C60 mono- or bis-adducts grafted onto self-assembled monolayers

Single layers of C₆₀ mono- and bis-adducts have been obtained by functionalizing an acid-terminated self-assembled monolayer (SAM) of thiols on gold. X-ray photoelectron spectroscopy demonstrated the grafting onto the SAM by covalent bonding, via the formation of an amide bond, while cyclic voltammetry and electrochemical impedance spectroscopy provided information on the redox properties of the C₆₀-films, as well as on structural characteristics.     

Plasma etching treatment for surface modification of boron-doped diamond electrodes

Boron-doped diamond (BDD) thin film surfaces were modified by brief plasma treatment using various source gases such as Cl₂, CF₄, Ar and CH₄, and the electrochemical properties of the surfaces were subsequently investigated. From X-ray photoelectron spectroscopy analysis, Cl and F atoms were detected on the BDD surfaces after 3 min of Cl₂ and CF₄ plasma treatments, respectively. From the results of cyclic voltammetry and electrochemical AC impedance measurements, the electron-transfer rate for Fe(CN)₆^3−/4− and Fe^2+/3+ at the BDD electrodes was found to decrease after Cl₂ and CF₄ plasma treatments. However, the electron-transfer rate for Ru(NH₃)₆^2+/3+ showed almost no change after these treatments. This may have been related to the specific interactions of surface halogen (C-Cl and C-F) moieties with the redox species because no electrical passivation was observed after the treatments. In addition, Raman spectroscopy showed that CH₄ plasma treatment of diamond surfaces formed an insulating diamond-like carbon thin layer on the surfaces. Thus, by an appropriate choice of plasma source, short-duration plasma treatments can be an effective way to functionalize diamond surfaces in various ways while maintaining a wide potential window and a low background current.     

Efficient electrochemical decomposition of perfluorocarboxylic acids by the use of a boron-doped diamond electrode

The electrochemical decomposition of environmentally persistent perfluorooctanoic acid (PFOA) was achieved by the use of a boron-doped diamond (BDD) electrode. The PFOA decomposition follows pseudo-first-order kinetics, with an observed rate constant (k₁) of 2.4 × 10^− 2 dm³ h^− 1. Under the present reaction conditions, k₁ increased with increasing current density and saturated at values over 0.60 mA cm^− 2. Therefore, the rate-limiting step for the electrochemical decomposition of PFOA was the direct electrochemical oxidation at lower current densities. In the proposed decomposition pathway, direct electrochemical oxidation cleaves the C-C bond between the C₇F₁₅ and COOH in PFOA and generates a C₇F₁₅ radical and CO₂. The C₇F₁₅ radical forms the thermally unstable alcohol C₇F₁₅OH, which undergoes F⁻ elimination to form C₆F₁₃COF. This acid fluoride undergoes hydrolysis to yield another F⁻ and the perfluorocarboxylic acid with one less CF₂ unit, C₆F₁₃COOH. By repeating these processes, finally, PFOA was able to be totally mineralized to CO₂ and F⁻. Moreover, whereas the BDD surface was easily fluorinated by the electrochemical reaction with the PFOA solution, medium pressure ultraviolet (MPUV) lamp irradiation in water was able to easily remove fluorine from the fluorinated BDD surface.