Detection of trace levels of Pb in tap water at boron-doped diamond electrodes with anodic stripping voltammetry

Boron-doped diamond (BDD) electrodes were used to investigate the possibility of detecting trace levels of lead by linear-sweep anodic stripping voltammetry. The low limit of detection (2 nM) is an advantage compared to other electrode materials, and it was found that at low pH values, copper concentrations that are usually present in drinking water do not affect to a large extent the detection of lead. These findings recommend anodic stripping voltammetry at the BDD electrodes as a suitable mercury-free method for the determination of trace levels of lead in drinking water. The results obtained for the lead detection in tap water real samples are in excellent agreement with those found by inductively coupled plasma-mass spectrometry (ICP-MS), demonstrating the practical analytical utility of the method.     

Interaction of Pb and Cd during anodic stripping voltammetric analysis at boron-doped diamond electrodes

Highly boron-doped diamond (BDD) films were utilized for simultaneous electrochemical measurement of micromolar-level concentrations of Pb and Cd, and for the examination of their interactions. Differential pulse anodic stripping voltammetry (DPASV) was used for this detection. This approach can help to understand the possible detection of trace metals at BDD electrodes without the aid of mercury. These metals were found to strip at their characteristic potentials, in solutions containing Cd or Pb alone, and in those containing these metals together. The mixed solutions (concentration range: 1-5 μM) yielded well-separated stripping peaks for Pb and Cd and the differential stripping peak currents for the respective metals increased linearly with increasing metal concentration. There were mutual interferences due to Pb-Cd interactions, but these can be taken into account with the aid of three-dimensional calibration plots. A model has been developed to help explain the Pb-Cd interactions.     

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.     

Electrochemical oxidation of phenol at boron-doped diamond electrode in pulse current mode

In this study, a pulse current supply was initially used in a BDD anode system (pulse-BDD anode system) for electrochemical oxidation of phenol. The influences of operative parameters (current density, retention time, pulse duty cycle, power frequency) on the system performances were exmamined by response surface methodology (RSM). As for COD degradation efficiency (D_COD) and specific energy consumption (E_s), the influence of retention time was more important than current density and pulse duty cycle, while power frequency hardly presented significant influence. By the comparison with constant current mode, an obvious specific energy consumption reduction was achieved in the pulse-BDD anode system, though the D_COD was slightly lower. The significant E_s decrease might be attributed to the reduction of side reactions and concentration polarization in pulse current mode. The pulse-BDD anode system demonstrated an efficient technology to simultaneously obtain high pollutant degradation efficiency and low energy consumption.     

Highly sensitive and selective electrochemical determination of dopamine and ascorbic acid at Ag/Ag2S modified electrode

A biosensor electrode possessing highly sensitive and selective determination of dopamine (DA) is fabricated. This electrode, a silver (Ag) thin film on indium-tin-oxide glass, is treated with a silver sulfide (Ag₂S) film using electrochemical deposition. Active Ag ion is easier to form on Ag₂S than on pristine Ag, which prefers to attract ascorbic acid (AA). The Ag₂S layer reduces the oxidation potential of AA due to the electrostatic interaction, which results in well-separation of mixed oxidation responses to both of DA and AA. Besides, the Ag₂S-modified electrode exhibits dramatic electrocatalytic effect on the oxidation of DA in the presence of AA. In 0.1 M phosphate buffer solution at pH ∼ 7.0, the differential pulse voltammetric peak intensity linearly correlates with DA concentration in two regions, viz. 1.0–10, and 10–100 μM, with correlation coefficient of 0.998 and 0.995, respectively. The lowest concentration limit of 1.0 μM DA can be detected. The interference of AA effectively diminishes in the mixed solution. These features make the Ag₂S significant for selective and sensitive measurement of DA in the presence of excess AA.     

Simultaneous determination of acetaminophen and dopamine using SWCNT modified carbon–ceramic electrode by differential pulse voltammetry

A highly sensitive method was investigated for the simultaneous determination of acetaminophen (AC) and dopamine (DA) using single wall carbon nanotubes modified carbon–ceramic electrode (SWCNT/CCE). The SWCNT/CCE displayed excellent electrochemical catalytic activities towards the oxidation of AC and DA. Under optimized experimental conditions in differential pulse voltammetry technique, AC and DA gave linear response over the ranges 0.2–100.0 μM (R² = 0.996) and 0.4–150.0 μM (R² = 0.999), respectively. The detection limits (S/N = 3) were found to be 0.12 μM for AC and 0.22 μM for DA. The present method was applied to the determination of AC and DA in some commercial pharmaceutical samples.     

Characterization of slow charge transfer processes in differential pulse voltammetry at spherical electrodes and microelectrodes

The use of differential pulse voltammetry at spherical electrodes for the study of the kinetic of charge transfer processes is analyzed. An analytical solution is presented, valid for any value of the electrode radius, the heterogeneous rate constant and the transfer coefficient.Several reversibility criteria are established based on the variation of DPV peak with the duration of the potential pulses and the electrode radius. Moreover, general working curves for extraction of kinetic parameters from DPV experiments are given.The anomalous shape of DPV curves for quasireversible processes with small values of the transfer coefficient is reported. The effect of the presence of both electroactive species on DPV curves for quasireversible and irreversible systems is also studied.     

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 incineration of chloromethylphenoxy herbicides in acid medium by anodic oxidation with boron-doped diamond electrode

The electrochemical degradation of saturated solutions of herbicides 4-chloro-2-methylphenoxyacetic acid, 2-(4-chlorophenoxy)-2-methylpropionic acid and 2-(4-chloro-2-methylphenoxy)propionic acid in 1 M HClO₄ on a boron-doped diamond (BDD) thin film anode has been studied by chronoamperometry, cyclic voltammetry and bulk electrolysis. At low anodic potentials polymeric products are formed causing the fouling and deactivation of BDD. This is reactivated at high potentials when water decomposes producing hydroxyl radical as strong oxidant of organics. Electrolyses in a batch recirculation system at constant current density ≥8 mA cm⁻² yielded overall decontamination of all saturated solution. The effect of current density and herbicide concentration on the degradation rate of each compound, the specific charge required for its total mineralization and instantaneous current efficiency have been investigated. Experimental results have been compared with those predicted by a theoretical model based on a fast anodic oxidation of initial herbicides, showing that at 30 mA cm⁻² their degradation processes are completely controlled by mass transfer. Kinetic analysis of the change of herbicide concentration with time during electrolysis, determined by high-performance liquid chromatography, revealed that all compounds follow a pseudo first-order reaction. Aromatic intermediates and generated carboxylic acids have been identified using this technique and a general pathway for the electrochemical incineration of all herbicides on BDD is proposed.     

Capability of a carbon-polyvinylchloride composite electrode for the detection of dopamine, ascorbic acid and uric acid

In this work, the composite carbon-polyvinylchloride (C-PVC) was used as an electrode for the detection of dopamine, ascorbic acid, uric acid and their mixtures by differential pulse voltammetry (DPV). The results showed that the untreated C-PVC electrode was selective and stable for the oxidation of dopamine in a mixture containing uric acid and an excess of ascorbic acid in acidic medium. The pre-treated C-PVC electrode in a neutral medium exhibited good resolution of the mixture components in the micro molar concentration range of DA. The ageing of the C-PVC electrode during longer time periods did not affect the peak potential and the detection of dopamine, uric acid and ascorbic acid in 0.1 M H₂SO₄. The practical analytical utility of the C-PVC electrode was demonstrated by the measurement of uric acid in human urine and serum samples without any preliminary pre-treatment.     

Synthesis and characterization of layered Nb2C MXene/ZnS nanocomposites for highly selective electrochemical sensing of dopamine

MXenes, due to their exceptional properties, are tagged as materials of future in the field of two dimensional (2D) materials. Niobium carbide (Nb₂C) is an important member of MXene family having vast application in the field of lithium ion batteries and supercapacitors. However, its applications in the field of sensing have not been explored yet. This research work reports the synthesis and application of Nb₂C/ZnS nanocomposite for the sensing of dopamine (DA) for the first time. The etching of Nb₂C from parent MAX phase (Nb₂AlC) was performed at 55 °C. The application of Nb₂C electrode for the electrochemical sensing of DA was employed through differential pulse voltammetry (DPV). Zinc sulphide (ZnS) nanoparticles were synthesized hydrothermally to enhance the electrochemical properties of Nb₂C. The characterization of these prepared samples was done with the help XRD, SEM, EDS, and of FTIR spectroscopy. The MXene-ZnS nanocomposite modified glassy carbon electrode (GCE) proved to be a very effective electrode material to detect dopamine electrochemically with a wide linear detection range of 0.09–0.82 mM, a very low detection limit of 1.39 μM and excellent sensitivity of 12.1 μAμM^-1. The modified glassy carbon electrode also proved to be exceptionally selective towards dopamine in the presence of interfering agents like ascorbic acid, citric acid and glucose.     

On the performances of lead dioxide and boron-doped diamond electrodes in the anodic oxidation of simulated wastewater containing the Reactive Orange 16 dye

The performances of the Ti-Pt/β-PbO₂ and boron-doped diamond (BDD) electrodes in the electrooxidation of simulated wastewaters containing 85 mg L⁻¹ of the Reactive Orange 16 dye were investigated using a filter-press reactor. The electrolyses were carried out at the flow rate of 7 L min⁻¹, at different current densities (10-70 mA cm⁻²), and in the absence or presence of chloride ions (10-70 mM NaCl). In the absence of NaCl, total decolourisation of the simulated dye wastewater was attained independently of the electrode used. However, the performance of the BDD electrode was better than that of the Ti-Pt/β-PbO₂ electrode; the total decolourisations were achieved by applying only 1.0 A h L⁻¹ and 2.0 A h L⁻¹, respectively. In the presence of NaCl, with the electrogeneration of active chlorine, the times needed for total colour removal were markedly decreased; the addition of 50 mM Cl⁻ or 35 mM Cl⁻ (for Ti-Pt/β-PbO₂ or BDD, respectively) to the supporting electrolyte led to a 90% decrease of these times (at 50 mA cm⁻²). On the other hand, total mineralization of the dye in the presence of NaCl was attained only when using the BDD electrode (for 1.0 A h L⁻¹); for the Ti-Pt/β-PbO₂ electrode, a maximum mineralization of 85% was attained (for 2.0 A h L⁻¹). For total decolourisation of the simulated dye wastewater, the energy consumption per unit mass of dye oxidized was only 4.4 kWh kg⁻¹ or 1.9 kWh kg⁻¹ using the Ti-Pt/β-PbO₂ or BDD electrode, respectively. Clearly the BDD electrode proved to be the best anode for the electrooxidative degradation of the dye, either in the presence or absence of chloride ions.     

The improvement of boron-doped diamond anode system in electrochemical degradation of p-nitrophenol by zero-valent iron

Boron-doped diamond (BDD) electrodes are promising anode materials in electrochemical treatment of wastewaters containing bio-refractory organic compounds due to their strong oxidation capability and remarkable corrosion stability. In order to further improve the performance of BDD anode system, electrochemical degradation of p-nitrophenol were initially investigated at the BDD anode in the presence of zero-valent iron (ZVI). The results showed that under acidic condition, the performance of BDD anode system containing zero-valent iron (BDD-ZVI system) could be improved with the joint actions of electrochemical oxidation at the BDD anode (39.1%), Fenton's reaction (28.5%), oxidation–reduction at zero-valent iron (17.8%) and coagulation of iron hydroxides (14.6%). Moreover, it was found that under alkaline condition the performance of BDD-ZVI system was significantly enhanced, mainly due to the accelerated release of Fe(II) ions from ZVI and the enhanced oxidation of Fe(II) ions. The dissolved oxygen concentration was significantly reduced by reduction at the cathode, and consequently zero-valent iron corroded to Fe(II) ions in anaerobic highly alkaline environments. Furthermore, the oxidation of released Fe(II) ions to Fe(III) ions and high-valent iron species (e.g., FeO²⁺, FeO₄²⁻) was enhanced by direct electrochemical oxidation at BDD anode.     

Mixed first and zero order kinetics in the electrooxidation of sulfamethoxazole at a boron-doped diamond (BDD) anode

Pharmaceutical residues in the aquatic environment represent an emerging environmental problem, because many pharmaceuticals are refractory towards conventional waste water treatment. This study focussed on the oxidation of the sulfonamide antibiotic sulfamethoxazole (SMX) at a boron-doped diamond anode, at which reactive hydroxyl radicals are formed. Electrochemical oxidation led to mineralization with high current efficiency, but without the formation of known toxic products of partial oxidation. A “mixed” kinetic order with respect to substrate concentration was observed; the kinetics could be shifted in the direction of either diffusion control (first order in SMX) or current control (zero order in SMX) by adjusting the substrate concentration and current density. Alternatively, the electrooxidation could be described by a model, applicable to a wide range of reaction conditions, in which the kinetic orders with respect to current and initial substrate concentration were approximately 0.4 and 0.5, respectively.     

Sensitive determination of Cd and Pb by differential pulse stripping voltammetry with in situ bismuth-modified zeolite doped carbon paste electrodes

Here we investigated the analytical performances of the bismuth-modified zeolite doped carbon paste electrode (BiF-ZDCPE) for trace Cd and Pb analysis. The characteristics of bismuth-modified electrodes were improved greatly via addition of synthetic zeolite into carbon paste. To obtain high reproducibility and sensitivity, optimum experimental conditions for bismuth deposition were studied. Voltammetric responses of the BiF-ZDCPEs prepared with different ratios of zeolite, carbon powder, and silicone, were examined under same conditions. The in situ plated (zeolite/graphite powder/silicone, 10/190/80 w/w) BiF-ZDCPEs exhibited the most sensitive response to Cd and Pb in 0.10 M acetate buffer (pH 4.5). The detection limits of the modified electrode were 0.08 μg L⁻¹ for Cd(II) and 0.10 μg L⁻¹ for Pb(II) based on three times the standard deviation of the baseline with a preconcentration time of 120 s under optimal conditions, respectively. The modified electrode showed well linear response to both Cd(II) and Pb(II) over the concentration range from 1.0 to 20.0 μg L⁻¹. The BiF-ZDCPEs were successfully applied to the determination of Cd(II) and Pb(II) in real samples, and the results were in agreement with those of atomic absorption spectroscopy (AAS).     

Simultaneous determination of ascorbic acid, dopamine and uric acid by use of a MWCNT modified carbon-ceramic electrode and differential pulse voltammetry

Multi walled carbon nanotube modified carbon-ceramic electrode (MWCNT/CCE) was employed for the simultaneous determination of ascorbic acid (AA), dopamine (DA) and uric acid (UA). The MWCNT/CCE displayed very good electrochemical catalytic activities with respect to CCE. The oxidation over-potentials of AA, DA and UA decreased dramatically, and their oxidation peak currents increased significantly at MWCNT/CCE compared to those obtained at the bare CCE. Differential pulse voltammetry was used for the simultaneous determination of AA, DA and UA in their ternary mixture. The peak separation between AA and DA, and DA and UA was large up to 205 and 160 mV, respectively. The calibration curves for AA, DA and UA were obtained in the range of 15.00-800.00, 0.50-100.00, and 0.55-90.00 μM, respectively. The detection limits (S/N = 3) were 7.71, 0.31, and 0.42 μM for AA, DA and UA, respectively.The present method was applied to the determination of AA, DA and UA in human serum and some commercial pharmaceutical samples, using standard adding method and the results were quite promising.     

Determination of picomolar silver concentrations by differential pulse anodic stripping voltammetry at a carbon paste electrode modified with phenylthiourea-functionalized high ordered nanoporous silica gel

This study introduces the design of an anodic stripping voltammetric (ASV) method for the silver ion determination at a carbon paste electrode (CPE), chemically modified with phenylthiourea-nanoporous silica gel (Tu-SBA-15-CPE). The electroanalytical pro includes two steps: preconcentration of metal ions at an electrode surface, followed by quantification of the accumulated species by differential pulse anodic stripping voltammetric methods. Factors affecting the performance of the anodic stripping were investigated, including the modifier quantity in the paste, the electrolyte concentrations, the solution pH and the accumulation potential or time. The most sensitive and reliable electrode contained 10% Tu-SBA-15 and 90% carbon paste. The accumulation potential and time were set at, −200 mV and 300 s, respectively, and the scan rate at 50 mV s⁻¹ in the scan range of −200 to 700 mV. The resulting electrode demonstrated a linear response over range of silver ion concentration of 8.0-80 pmol/L with detection limit (S/N = 3) of 5 pmol/L. The prepared electrodes were used for the silver determination in sea and tap water samples and very good recovery results were obtained. The accuracy was assessed through recovery experiments and independent analysis by graphite furnace atomic absorption spectrometry.     

Electrochemical oxidation of acid black 210 dye on the boron-doped diamond electrode in the presence of phosphate ions: Effect of current density, pH, and chloride ions

The electrochemical oxidation of acid black 210 dye (AB-210) on the boron-doped diamond (BDD) was investigated under different pH conditions. The best performance for the AB-210 oxidation occurred in alkaline phosphate solution. This is probably due to oxidizing agents such as phosphate radicals and peroxodiphosphate ions, which can be electrochemically produced with good yields on the BDD anode, mainly in alkaline solution. Under this condition, the COD (chemical oxygen demand) removal was higher than that obtained from the model proposed by Comninellis. Electrolyses performed in phosphate buffer and in the presence of chloride ions resulted in faster COD and color removals in acid and neutral solutions, but in alkaline phosphate solution, a better performance in terms of TOC removal was obtained in the absence of chloride. Moreover, organochloride compounds were detected in all electrolyses performed in the presence of chloride. The AB-210 electrooxidation on BDD using phosphate as supporting electrolyte proved to be interesting since oxidizing species generated from phosphate ions were able to completely degrade the dye without producing organochloride compounds.     

Selective and sensitive detection of dopamine in the presence of ascorbic acid by molecular sieve/ionic liquids composite electrode

A simple, sensitive, and reliable method based on a molecular sieve/ionic liquids composite electrode has been successfully developed for selective determination of dopamine (DA). The electrochemical behavior of dopamine (DA) at the modified electrode was investigated by cyclic voltammetry (CV) and square wave voltammetry (SWV). The influence of experimental parameters including pH of solution, amount of modifier, accumulation potential and time on the response of DA was investigated. At the optimum conditions, the peak current of DA was linear with the concentration of DA in the wide range of 5.0 × 10⁻⁸ mol L⁻¹ to 8 × 10⁻⁴ mol L⁻¹, with the correlation coefficient of 0.9982. The detection limit was 1.0 × 10⁻⁸ mol L⁻¹ (S/N = 3) in the presence of 0.2 mM ascorbic acid (AA). The interference studies showed that the modified electrode had excellent selectivity. What's more, the modified electrode also exhibited good reproducibility and stability for determination of DA, and could be applied to determine human serum samples.     

Identification of aromatic systems by differential pulse voltammetry in the study of the hydropyrolysis of pyrene and coal asphaltenes

Differential pulse voltammetry at the hanging mercury drop electrode has proved effective in determining the identity of the aromatic structures present in the complex mixtures resulting from the hydropyrolysis of pyrene and coal-derived asphaltenes. Voltammetric results are consistent with g.c.-m.s. analyses for low molecular mass fractions, and the method is also applicable to asphaltenes.