Electrochemical oxidation of underivatized-nucleic acids at highly boron-doped diamond electrodes

Boron-doped diamond (BDD) electrodes have been examined for the electrochemical oxidation of underivatized-nucleic acids in terms of single stranded and double stranded DNA. Cyclic voltammetry and square wave voltammetry have been used to study the oxidation reactions and to detect DNA without derivatization or hydrolysis steps. At the diamond electrode, at least two well-defined voltammetric peaks were observed for both single stranded and double stranded DNA. Diamond electrode is the first material to show a well-defined voltammetric peaks for adenine group oxidation directly in the helix structure of nucleic acid due to its wide potential window. For single stranded DNA, a third peak, related to the pyrimidine group oxidation was also observed. As-deposited diamond film with predominantly hydrogen-terminated surface exhibited superior performance over oxygen-terminated diamond in terms of sensitivity. However, by optimizing the ionic strength, sensitivity of O-terminated films could be improved. Linear calibration results have shown linearity of current with concentration in the range 0.1-8 microg mL(-1) for both guanine and adenine residues at as-deposited BDD. Detection limits (S/N = 3) of 3.7 and 10 ng mL(-1) for adenine and guanine residue in single stranded DNA, respectively, and 5.2 and 10 ng mL(-1) for adenine and guanine residue in double stranded DNA, respectively, were observed. This work shows the promising use of diamond as an electrochemical detector for direct detection of nucleic acids. The results also show the possibility of using the oxidation peak current of adenine group that is more sensitive for the direct detection of nucleicacids.     

The electrooxidation of organic acids at boron-doped diamond electrodes

The electrooxidation of citric acid, malic acid, alanine and cysteine at boron-doped diamond (BDD) electrodes and glassy carbon (GC) electrodes was investigated by use of cyclic voltammetry. Well-defined, irreversible peaks were obtained for the oxidation of citric acid and cysteine. Malic acid and alanine exhibit discernible responses. This preliminary study has shown that BDD has better sensitivity than GC for these compounds. Except for cysteine, none of the studied compounds exhibits a recognizable oxidation peak at GC electrodes at millimolar concentration levels.     

Electrochemical generation of ferrate in acidic media at boron-doped diamond electrodes

An extremely strong oxidant, ferrate (Fe(VI) or FeO4(2-), has been produced electrochemically in an acidic aqueous medium for the first time.     

Electrochemical characterization of boron-doped polycrystalline diamond thin-film electrodes

The electrochemical characterization of boron-doped polycrystalline diamond thin-film (BDF) electrodes was studied using the anodic scan after concentrating lead in 0.1 mol/L KCl – 41 mol/L Hg(NO₃)₂ and 0.1 mol/L KNO₃ – 0.01 mol/L HNO₃ – 41 mol/L Hg(NO₃)₂; accumulation voltage was –0.90 V. The results obtained were compared with those given by glassy carbon (GC) electrodes and proved that the BDF electrodes offered high sensitivity, good precision and extreme stability over a 2-month period. These electrodes provided good resolving power for the determination of lead and cadmium and gave satisfactory results in the analysis of a pure water sample.     

Electrochemical characterization of boron-doped polycrystalline diamond thin-film electrodes

The electrochemical characterization of boron-doped polycrystalline diamond thin-film (BDF) electrodes was studied using the anodic scan after concentrating lead in 0.1 mol/L KCl - 41 micromol/L Hg(NO(3))(2) and 0.1 mol/L KNO(3) - 0.01 mol/L HNO(3) - 41 micromol/L Hg(NO(3))(2); accumulation voltage was -0.90 V. The results obtained were compared with those given by glassy carbon (GC) electrodes and proved that the BDF electrodes offered high sensitivity, good precision and extreme stability over a 2-month period. These electrodes provided good resolving power for the determination of lead and cadmium and gave satisfactory results in the analysis of a pure water sample.     

Electrochemical detection of sugar-related compounds using boron-doped diamond electrodes

Electrochemical detection of sugar-related compounds was conducted using a boron-doped diamond (BDD) electrode as a detector for flow-injection analysis (FIA). Sugar-related compounds oxidize at high applied potentials, for which the BDD electrode is suitable for electrochemical measurements. Conditions for an FIA system with a BDD detector were optimized, and the following detection limits were achieved for sugar-related compounds: monosaccharides, 25-100 pmol; sugar alcohols, 10 pmol; and oligosaccharides, 10 pmol. The detection limit for monosaccharide D-glucose (Glu) was 105 pmol (S/N = 3). A linear range was acquired from the detection limit to 50 nmol, and the relative standard deviation was 0.65% (20 nmol, n = 6). A high-performance liquid chromatography (HPLC) column was added to the system between the sample injector and the detector and detection limits to the picomole level were achieved, which is the same for the HPLC system and the FIA system. The electrochemical oxidation reaction of Glu was examined using cyclic voltammetry with the BDD detector. The reaction proved to be irreversible, and proceeded according to the following two-step mechanism: (1) application of a high potential (2.00 V vs. Ag/AgCl) to the electrode causes water to electrolyze on the electrode surface with the simultaneous generation of a hydroxyl radical on the surface, and (2) the hydroxyl radical indirectly oxidizes Glu. Thus, Glu can be detected by an increase in the oxidation current caused by reactions with hydroxy radicals.     

Fast electrochemical triple-interface processes at boron-doped diamond electrodes

Two important mechanisms for electron transfer processes at boron-doped diamond electrodes involving the oxidation of tetramethylphenylenediamine (TMPD) dissolved in aqueous solution and the oxidation of tetrahexylphenylenediamine (THPD) deposited in the form of microdroplets and immersed into aqueous eletrolyte solution are reported. For TMPD, the first oxidation step in aqueous solution follows the equation: Remarkably slow heterogeneous kinetics at a H-plasma-treated boron-doped diamond electrode are observed, consistent with a process following a pathway more complex than outer-sphere electron transfer. At the same boron-doped diamond electrode surface a deposit of THPD undergoes facile oxidation following the equation: This oxidation and re-reduction of the deposited liquid material occurs at the triple interface organic droplet|diamond|aqueous electrolyte and is therefore an example of a facile high-current-density process at boron-doped diamond electrodes due to good electrical contact between the deposit and the diamond surface. Received: 3 February 2000 / Accepted: 18 February 2000     

Mediatorless catalytic oxygen reduction at boron-doped diamond electrodes

Most approaches to electron conduction from electrode to the enzyme requires the use of mediators – molecular relays which can take electrons from the electrode and deliver them to the redox sites of the enzyme. In the present paper, the biocatalytic reduction of oxygen to water in the presence of laccase is shown to proceed on the boron-doped diamond at highly positive potentials and without any additional mediator. The onset of catalytic reduction current appears at 0.805 V vs. NHE in solutions of pH 5.2. Laccase is either dissolved in the solution or trapped on the BDD electrode in a thin film of lipidic cubic phase. The remarkable stability of the modified electrode, avoiding the use of mediators and positive potential of the dioxygen reduction process make the BDD–laccase system especially interesting for applications in electrochemical sensing and microbiofuel cells.     

Direct cytochrome c electrochemistry at boron-doped diamond electrodes

Highly boron-doped diamond electrodes are characterized voltammetrically employing Ru(NH₃)₆^3+/2+, Fe(CN)₆^3−/4−, benzoquinone/hydroquinone, and cytochrome c redox systems. The diamond electrodes, which are polished to nanometer finish, are initially `activated' electrochemically and then pretreated by oxidation, reduction, or polishing. All electrodes give reversible cyclic voltammetric responses for the reduction of Ru(NH₃)₆³⁺ in aqueous solution.Redox systems other than Ru(NH₃)₆^3+/2+ show characteristic electrochemical behavior as a function of diamond surface pretreatment. In particular, the horse heart cytochrome c redox system is shown to give reversible voltammetric responses at Al₂O₃ polished boron-doped diamond electrodes. No voltammetric response for cytochrome c is detected at anodically pretreated diamond electrodes. The observations are attributed to preferential interaction of the polished diamond surface with the reactive region of the cytochrome c molecule and low interference due to a lack of protein electrode fouling.     

Direct oxidation of haemoglobin at bare boron-doped diamond electrodes

The communication reports the direct oxidation of human haemoglobin at a bare boron-doped diamond electrode under moderately alkaline conditions with detection limit of 0.4 microM.     

Microwave-enhanced electro-deposition and stripping of palladium at boron-doped diamond electrodes

In situ microwave activation has been applied to the electro-deposition and stripping of palladium metal (which is widely used as a catalyst) at cavitation resistant boron-doped diamond electrodes. Focused microwave radiation leading to heating, boiling, and cavitation is explored as an option to improve the speed and sensitivity of the analytical detection procedure. The deposition and anodic stripping of palladium by linear sweep voltammetry in 0.1 M KCl (pH 2) solution and at boron-doped diamond electrodes is shown to be strongly enhanced by microwave activation due to both (i) the increase in mass transport and (ii) the increase in the kinetic rate of deposition and stripping.The temperature at the electrode surface is calibrated with the reversible redox couple Fe(CN)₆⁴⁻/Fe(CN)₆³⁻ and found to be reach 380 K. In the presence of microwave radiation, the potential of onset of the deposition of palladium is strongly shifted positive from −0.4 to +0.1 V versus SCE. The optimum potential for deposition in the presence of microwaves is −0.4 V versus SCE and the anodic stripping peak current is shown to increase linearly with deposition time. Under these conditions, the stripping peak current varies linearly with the palladium concentration down to ca. 2 μM. At concentration lower than this a logarithmic variation of the stripping peak current with concentration is observed down to ca. 0.1 μM (for 5 min pre-concentration in presence of microwave radiation).     

Recent electroanalytical applications of boron-doped diamond electrodes

This review overviews recent reports on the electroanalytical applications of boron-doped diamond (BDD) electrodes. Because BDD electrodes have excellent features for electroanalysis, such as wide potential window, low background current, electrochemical stability, and fouling resistance, they can be useful for sensitive and stable detection of various substances, including drugs, bio-related substances, metal ions, and organic pollutants. Many articles have reported high-sensitivity detection of real samples, demonstrating that this electrode material is practically applicable. Surface modification of the BDD electrodes using metal nanoparticles, nanocarbons, and polymers can increase the sensitivity of the electrochemical detection. Furthermore, research on the electroanalytical device equipped with BDD electrodes will be expanded by combining peripheral technologies related to the device fabrication.     

Detection of aniline at boron-doped diamond electrodes with cathodic stripping voltammetry

Boron-doped diamond (BDD) electrodes were used to investigate the possibility of detecting aniline by linear-sweep cathodic stripping voltammetry. It was found that the dimeric species (p-aminodiphenylamine and benzidine) formed by anodic oxidation of aniline during the accumulation period are involved in electrochemically reversible redox processes and, in acidic media, the shape of the stripping voltammetric response is suitable for aniline detection in the micromolar concentration range. The low background current of conductive diamond is an advantage compared to other electrode materials and allows a detection limit of 1 μM. Weak adsorption properties and the extreme electrochemical stability are additional advantages of BDD and it was found that, even after long-time measurements, the electrode surface can regain its initial activity by an anodic polarization in the potential region of water decomposition.     

Electrochemical behavior of nitrogen gas species adsorbed onto boron-doped diamond (BDD) electrodes

The adsorption of nitrogen species, in neutral electrolyte solutions, onto boron-doped diamond (BDD) electrode surfaces from dissolved NO2, NO, and N2O gases was induced at 0 V/SCE. Modified BDD electrode surfaces showed a different electrochemical response toward the hydrogen evolution reaction than did a nonmodified electrode surface in electrolyte base solution. The formation of molecular hydrogen and nitrogen gaseous species was confirmed by the online differential electrochemical mass spectrometry (DEMS) technique. Among the three nitrogen oxides gases, NO2 substantially modifies the electrolyte via hydrolysis leading to the formation of NO3- and its adsorption on the BDD electrode surface. The BDD/(NO3-) interface was the only N2O and N2 species generating system.     

Interaction between co-deposited metals during stripping voltammetry at boron-doped diamond electrodes

Electrochemical processes, which underlie the use of conductive diamond electrodes for the simultaneous detection of two or more metal ions in solution by anodic stripping voltammetry (ASV), have been investigated. The model analyte system studied contains the two metal species, Ag+(aq) and Pb2+(aq), and the experimental techniques employed include cyclic and square wave voltammetries, along with X-ray photoelectron spectroscopy and secondary electron microscopy. Although the bulk metallic forms of Ag and Pb are immiscible, several interactions in the system between the two metal species present are observed, which significantly influence the electrodeposition and electrodissolution processes which underlie ASV. The subsequent nucleation and growth of a given metal on the electrode surface is enhanced by the presence of the second metal on the surface. The encapsulation of one metal by the other, within the metal particulates that form on the electrode surface, significantly reduces the stripping yield at the potentials characteristic of the individual metals. The stripping potentials are also influenced by bonding interactions between deposited Ag and Pb, which broaden the characteristic stripping peaks in cyclic voltammetry, as well as producing underpotential deposition and stripping. Given these interactions, the extent to which ASV at diamond electrodes can be used to determine the solution concentrations of Ag+(aq) and Pb2+(aq) is considered.     

Electrochemical evaluation and determination of antiretroviral drug fosamprenavir using boron-doped diamond and glassy carbon electrodes

Fosamprenavir is a pro-drug of the antiretroviral protease inhibitor amprenavir and is oxidizable at solid electrodes. The anodic oxidation behavior of fosamprenavir was investigated using cyclic and linear sweep voltammetry at boron-doped diamond and glassy carbon electrodes. In cyclic voltammetry, depending on pH values, fosamprenavir showed one sharp irreversible oxidation peak or wave depending on the working electrode. The mechanism of the oxidation process was discussed. The voltammetric study of some model compounds allowed elucidation of the possible oxidation mechanism of fosamprenavir. The aim of this study was to determine fosamprenavir levels in pharmaceutical formulations and biological samples by means of electrochemical methods. Using the sharp oxidation response, two voltammetric methods were described for the determination of fosamprenavir by differential pulse and square-wave voltammetry at the boron-doped diamond and glassy carbon electrodes. These two voltammetric techniques are 0.1 M H₂SO₄ and phosphate buffer at pH 2.0 which allow quantitation over a 4 × 10⁻⁶ to 8 × 10⁻⁵ M range using boron-doped diamond and a 1 × 10⁻⁵ to 1 × 10⁻⁴ M range using glassy carbon electrodes, respectively, in supporting electrolyte. All necessary validation parameters were investigated and calculated. These methods were successfully applied for the analysis of fosamprenavir pharmaceutical dosage forms, human serum and urine samples. The standard addition method was used in biological media using boron-doped diamond electrode. No electroactive interferences from the tablet excipients or endogenous substances from biological material were found. The results were statistically compared with those obtained through an established HPLC-UV technique; no significant differences were found between the voltammetric and HPLC methods.     

Indirect electrochemical oxidation of reverse osmosis membrane concentrates at boron-doped diamond electrodes

The treatment or disposal of concentrates generated from the filtrative treatment of water is rapidly becoming a factor of major environmental concern. This preliminary study discusses a novel approach in the abatement of reverse osmosis membrane retentate i.e. electrochemical oxidation. The recalcitrant organic constituents as well as the ammonia nitrogen in the retentate could be readily oxidised using boron-doped diamond electrodes. From the model fitted to these data, a constant removal rate and current efficiency was calculated. Analysis of the inorganic chlorinated species revealed that the oxidation mechanism was mainly due to the indirect oxidative action of electrogenerated hypochlorite.     

Electrochemical oxidation of phenol on boron-doped diamond electrode

The process of phenol oxidation on a boron-doped diamond electrode (BDD) is studied in acidic electrolytes under different conditions of generation of active oxygen forms (AOFs). The scheme of phenol oxidation known from the literature for other electrode materials is confirmed. Phenol is oxidized through a number of intermediates (benzoquinone, carboxylic acids) to carbon dioxide and water. Comparative analysis of phenol oxidation rate constants is performed as dependent on the electrolysis conditions: direct anodic oxidation, with oxygen bubbling, and addition of H₂O₂. A scheme is confirmed according to which active radicals (OH^·, HO₂^·, HO₂⁻) are formed on a BDD anode that can oxidize the substrate which leads to formation of organic radicals interacting with each other and forming condensation products. Processes with participation of free radicals (chain-radical mechanism) play an important role in electrochemical oxidation on BDD. Intermediates and polymeric substances (polyphenols, quinone structures, and resins) are formed. An excess of the oxidant (H₂O₂) promotes a more effective oxidation of organic radicals and accordingly inhibition of the condensation process.