Adsorptive Cathodic Stripping Research Articles

Identification of homogeneous [Co4(H2O)4(HPMIDA)2(PMIDA)2]6− as an effective molecular-light-driven water oxidation catalyst

Transition-metal complexes, especially cobalt complexes, are used as precatalysts in homogeneous water oxidation catalysis. However, a practical method of identifying the effective active species in cobalt-complex homogeneous catalysts under highly oxidizing conditions have not yet been established. In this work, (H3O)6·[Co4(H2O)4(HPMIDA)2(PMIDA)2]·2H2O (1) was introduced, and its photocatalytic performance was evaluated. The compound exhibited effective water oxidation catalysis, with a turnover number of 661.5 at pH 9.0 using [Ru(bpy)3](ClO4)2 as photosensitizers and sodium persulfate as sacrificial electron acceptor. Accordingly, 1 was used as target catalysts, and the active species was thoroughly investigated under catalytic conditions. The stability of 1 was further tested and confirmed by a series of experiments (cyclic voltammetry (CV), ultraviolet–visible (UV–vis) spectrometry, dynamic light-scattering (DLS), aging experiments, and extraction techniques). Results showed that 1 maintained its structural integrity under the given photocatalytic conditions. Furthermore, cathodic adsorptive stripping voltammetry and inductively coupled plasma-mass spectrometry were used to quantify the amount of Co2+ ions released from 1 into borate buffer after oxidation. The pH dependence of the oxygen-evolution performance of 1 and other cobalt species were then compared. Data collectively revealed that 1 was stable during water oxidation; it did not release free Co2+ ions and was not hydrolyzed to cobalt oxide or hydroxide, thereby confirming that 1 was the effective active species in water-oxidation catalysis.

Determination of the Side-Reaction Coefficient of Desferrioxamine B in Trace-Metal-Free Seawater

Electrochemical techniques like adsorptive cathodic stripping voltammetry with competitive ligand equilibration (ACSV-CLE) can determine total concentrations of marine organic ligands and their conditional binding constants for specific metals, but cannot identify them. Individual organic ligands, isolated from microbial cultures or biosynthesized through genomics, can be structurally characterized via NMR and tandem MS analysis, but this is tedious and time-consuming. A complementary approach is to compare known properties of natural ligands, particularly their conditional binding constants, with those of model organic ligands, measured under suitable conditions. Such comparisons cannot be meaningfully interpreted unless the side-reaction coefficient (SRC) of the model ligand in seawater is thoroughly evaluated. We conducted series of potentiometric titrations, in non-coordinating medium at seawater ionic strength (0.7 M NaClO4) over a range of metal:ligand molar ratios, to study complexation of the siderophore desferrioxamine B (DFOB) with Mg and Ca, for which it has the highest affinity among the major seasalt cations. From similar titrations of acetohydroxamic acid in the absence and presence of methanesulfonate (mesylate), it was determined that Mg and Ca binding to this common DFOB counter-ion is not strong enough to interfere with the DFOB titrations. Stability constants were measured for all DFOB complexes with Mg and Ca including, for the first time, the bidentate complexes. No evidence was found for Mg and Ca coordination with the DFOB terminal amine. From the improved DFOB speciation, we calculated five SRCs for each of the five (de)protonated forms of DFOB in trace-metal-free seawater, yet we also present a more convenient definition of a single SRC that allows adjustment of all DFOB stability constants to seawater conditions, no matter which of these forms is selected as the 'component' (reference species). An example of Cd speciation in seawater containing DFOB illustrates the non-trivial use of different SRCs for polyprotic, polydentate organic ligands.

Fe-Binding Dissolved Organic Ligands in the Oxic and Suboxic Waters of the Black Sea

In the oxygen-rich layer of the Black Sea, above the permanent halocline, the Fe and nitrate concentrations are low where fluorescence is relatively high , indicating uptake by phytoplankton. In this study we used ligand exchange adsorptive cathodic stripping voltammetry (CLE-aCSV), using 2-(2-Thiazolylazo)-p-cresol (TAC) as measuring ligand, to investigate the role of Fe-binding dissolved organic ligands in keeping Fe in the dissolved phase and potentially biologically available. The conditional stability constant, logK´, was between 21 and 22 in most samples, which is on average lower than in ocean water. The Fe-binding dissolved organic ligand concentrations varied between 0.35 and 4.81 nEq of M Fe, which was higher than the dissolved concentration of Fe (DFe) as found in most samples. At two stations ligands were saturated in the surface. At one station ligands were saturated near the oxycline, where ligand concentrations seemed to increase, indicating that they play a role in keeping Fe in the dissolved phase across the redox gradient. At the fluorescence maximum (between 40 and 50 m), the dissolved organic ligand binding capacity (alphaFeL=K´*[L´]) of Fe was at its highest while the concentration DFe was at its lowest. Here, we find a statistically significant, positive relationship between fluorescence and the logarithm of alphaFeL, along with fluorescence and the ratio of the total ligand concentration over DFe. These relationships are best explained by phytoplankton utilizing Fe from Fe-binding organic ligands, resulting in an increase in free Fe-binding ligands.

Voltammetric Investigation of Hydrothermal Iron Speciation

Hydrothermal vent fluids are highly enriched in iron (Fe) compared to ambient seawater, and organic ligands may play a role in facilitating the transport of some hydrothermal Fe into the open ocean. This is important since Fe is a limiting micronutrient for primary production in large parts of the world`s surface ocean. We have investigated the concentration and speciation of Fe in several vent fluid and plume samples from the Nifonea vent field, Coriolis Troughs, New Hebrides Island Arc, South Pacific Ocean using competitive ligand exchange - adsorptive cathodic stripping voltammetry (CLE - AdCSV) with salicylaldoxime (SA) as the artificial ligand. Our results for total dissolved Fe (dFe) in the buoyant hydrothermal plume samples showed concentrations up to 3.86 µM dFe with only a small fraction between 1.1% and 11.8% being chemically labile. Iron binding ligand concentrations ([L]) were found in µM level with strong conditional stability constants up to log K[L],Fe3+ of 22.9. Within the non-buoyant hydrothermal plume above the Nifonea vent field, up to 84.7% of the available Fe is chemically labile and [L] concentrations up to 97 nM were measured. [L] was consistently in excess of Felab, indicating that all available Fe is being complexed, which in combination with high Felab values in the non-buoyant plume, signifies that a high fraction of hydrothermal dFe is potentially being transported away from the plume into the surrounding waters, contributing to the global oceanic Fe budget.

Influence of Ocean Acidification on the Organic Complexation of Iron and Copper in Northwest European Shelf Seas; a Combined Observational and Model Study

The pH of aqueous solutions is known to impact the chemical speciation of trace metals. In this study we conducted titrations of coastal seawaters with iron and copper at pH 7.91, 7.37 and 6.99 (expressed on the total pH scale). Changes in the concentration of iron and copper that complexed with the added ligands 1-nitroso-2-napthol and salicylaldoxime respectively were determined by adsorptive cathodic stripping voltammetry - competitive ligand equilibrium (AdCSV-CLE). Interpretation of the results, assuming complexation by a low concentration of discrete ligands, showed that conditional stability constants for iron complexes increased relative to inorganic iron complexation as pH decreased by approximately 1 log unit per pH unit, whilst those for copper did not change. No trend was observed for concentrations of iron and copper complexing ligands over the pH range examined. We also interpreted our titration data by describing chemical binding and polyelectrolytic effects using non-ideal competitive adsorption in Donnan-like gels (NICA-Donnan model) in a proof of concept study. The NICA-Donnan approach allows for the development of a set of model parameters that are independent of ionic strength and pH, and thus calculation of metal speciation can be undertaken at ambient sample pH or the pH of a future, more acidic ocean. There is currently a lack of basic NICA-Donnan parameters applicable to marine dissolved organic matter (DOM) so we assumed that the measured marine dissolved organic carbon could be characterized as terrestrial fulvic acids. Generic NICA-Donnan parameters were applied within the framework of the software program visual MINTEQ and the metal –added ligand concentrations [MeAL] calculated for the AdCSV-CLE conditions. For copper, calculated [MeAL] using the NICA-Donnan model for DOM were consistent with measured [MeAL], but for iron an inert fraction with kinetically inhibited dissolution was required in addition to the NICA-Donnan model in order to approximate the trends observed in measured [MeAL]. We calculated iron and copper speciation in Northwest European shelf water samples at ambient alkalinity and projected increased pCO2 concentrations as a demonstration of the potential of the approach.

Occurrence of copper-complexing ligands in the coastal sediments of eastern Red Sea

The speciation of copper in seawater is greatly influenced by the presence of organic ligands that find their way from various sources. The occurrence of copper-complexing ligands in the coastal sediment of eastern Red Sea and their potential resuspension under the impact of physical parameters such as wind and currents are investigated. The competitive ligand exchange and detection of copper by adsorptive cathodic stripping voltammetry (ACSV) were used to determine the dissolved concentration of copper-complexing ligands and their conditional stability constants after suspension of marine sediments in UV-irradiated seawater. The laboratory experiments of suspended marine sediments in UV-irradiated seawater followed by the measurements of copper in the filtrate have indicated the presence of two ligands with concentrations in the range of 3.53–25.58 nM g−1 for L1 and 8.33–28.35 nM g−1 for L2 whereas the log conditional stability constants ranged between 12.59 and 13.87 for log K 1 and 11.79 and 12.96 for log K 2. Comparison of log K 2 with values for log K of copper complexes with thiols from previous studies suggests that thiols consist of the major part of copper-complexing ligands in the coastal marine sediments of eastern Red Sea. Relatively, positive and good correlations are found for copper-complexing ligands with total copper and organic content in the sediments. Calculation of the flux of copper-complexing ligands and their contribution to the total budget in the coastal water south of Jeddah indicate that the impact of sediments as potential source is less than 13 %. It is therefore that other processes such as in situ production and input from sewage effluents have to be considered to account for the majority of dissolved copper-complexing ligands in the area.

Microfabricated chip integrating a bismuth microelectrode array for the determination of trace cobalt(II) by adsorptive cathodic stripping voltammetry

This work presents the determination of trace Co(II) by adsorptive cathodic stripping voltammetric (AdCSV) using a 3-electrode microdevice featuring an integrated bismuth microelectrodes array (Bi-MEA). The microdevice was fabricated through sputtering and photolithography and consisted of thin Ag and Pt films (serving as reference and counter electrode, respectively), while the working transducer was a microelectrode array composed of 625 bismuth microdisks (10μm in radius and interelectrode distance of 200μm). Exploiting the superior performance of Bi-MEA in quiescent solutions at low supporting electrolyte concentration, the AdCSV determination of Co(II) after its complexion with dimethylglyoxime (DMG) was conducted in the presence of only 1.0×10−3molL−1 ammonia buffer and 30s preconcentration without stirring. The microfabricated Bi-MEAs were characterized by cyclic voltammetry and scanning electron microscopy, while their improved analytical characteristics (low ohmic drop and high mass transport) were revealed after a thorough comparison with bismuth macro-electrodes. Besides, other experimental variables related to Co(II) determination (the DMG concentration, the preconcentration potential, the preconcentration time) were studied in detail. The limit of detection for Co(II) was 0.18μgL−1. These microdevices were successfully applied to the analysis of Co(II) in a spiked mineral water sample and a certified lake water sample. This ready-to-use microsensor overcomes the drawbacks of existing electroplated electrodes, enabling AdCSV determination of trace Co(II) for on-site monitoring.

Electrocatalytic Quantification of Pantaprazole

A polyaniline-zinc oxide-modified glassy carbon electrode (PANI-ZnO/GCE) has been developed for the determination of pantaprazole. The morphology of fabricated PANI-ZnO/GCE was characterized by scanning electron microscopy. The electrochemical behavior of pantaprazole on PANI-ZnO/GCE was investigated using square-wave cathodic adsorptive stripping voltammetry (SWCAdSV). Results showed that PANI-ZnO/GCE modified sensor exhibited excellent electrocatalytic activity to pantaprazole with a detection limit of 45.59 ng/mL, a reproducibility of 2.07% relative standard deviation, and a recovery from 99.76% to 99.81%. The ease of fabrication, high electroactive surface area and excellent electrochemical performance are the promising feature of the sensor.

Ultratrace Determination of Indium in Natural Water by Adsorptive Stripping Voltammetry in the Presence of Cupferron as a Complexing Agent

A novel and highly sensitive adsorptive voltammetric procedure for direct determination of In(III) in natural water samples was introduced. The method is based on cathodic adsorptive stripping voltammetry of the In(III)-cupferron complex at a renewable mercury film silver based electrode (Hg(Ag)FE). Experimental conditions were established and included: 0.1 mol L−1 acetate buffer (pH = 5.5), 5 × 10−5 mol L−1 cupferron, accumulation potential 0 V. The calibration graph was linear from 5 × 10−10 mol L−1 to 2 × 10−8 mol L−1 with the correlation coefficient of 0.9993. Limit of detection (LOD) and limit of quantification (LOQ) for preconcentration time of 30 s were 1.5 × 10−10 mol L−1 and 5 × 10−10 mol L−1, respectively. The interference of surface active substances and humic substances on the indium voltammetric signal was precisely examined, and effectively minimalized. The application of the procedure to the analysis of natural water samples was validated by the determination of indium in certificate reference materials SPS-WW1 Wastewater and SPS-SW1 Surface Water, river water, rain water and tap water.

Rapid Screening Method for Detecting Ethinyl Estradiol in Natural Water Employing Voltammetry

17α-Ethinyl estradiol (EE2), which is used worldwide in the treatment of some cancers and as a contraceptive, is often found in aquatic systems and is considered a pharmaceutically active compound (PhACs) in the environment. Current methods for the determination of this compound, such as chromatography, are expensive and lengthy and require large amounts of toxic organic solvents. In this work, a voltammetric procedure is developed and validated as a screening tool for detecting EE2 in water samples without prior extraction, clean-up, or derivatization steps. Application of the method we elaborate here to EE2 analysis is unprecedented. EE2 detection was carried out using differential pulse adsorptive cathodic stripping voltammetry (DP AdCSV) with a hanging mercury drop electrode (HMDE) in pH 7.0 Britton-Robinson buffer. The electrochemical process of EE2 reduction was investigated by cyclic voltammetry at different scan rates. Electroreduction of the hormone on a mercury electrode exhibited a peak at −1.16 ± 0.02 V versus Ag/AgCl. The experimental parameters were as follows: −0.7 V accumulation potential, 150 s accumulation time, and 60 mV s−1 scan rate. The limit of detection was 0.49 μg L−1 for a preconcentration time of 150 s. Relative standard deviations were less than 13%. The method was applied to the detection of EE2 in water samples with recoveries ranging from 93.7 to 102.5%.

Towards a zero-blank, preconcentration-free voltammetric method for iron analysis at picomolar concentrations in unbuffered seawater

A method with negligible blank values for the determination of total iron at the ultratrace level in seawater has been optimized and validated exploring for the first time the performance and limitations of Adsorptive Cathodic Stripping Voltammetry (AdCSV) in non-buffered solutions. The method is based on the CSV determination of the Fe-dihydroxynaphthalene (DHN) complex using atmospheric oxygen to catalytically enhance the signal via hydrogen peroxide formation at the electrode/solution interface. The accumulation of hydroxyl ions, the by-product of the hydrogen peroxide formation, increased the pH in the diffusion layer in the absence of buffer bringing it to 9, the optimum for the analytical performance of the method. Voltammograms in UV digested seawater showed no stability or reproducibility drawbacks. The negligible, lower than 5pM, blank level, is due to the simplicity of the procedure requiring no sample manipulation and a maximum of three reagents only, necessarily the ligand DHN and a base only for those samples previously acidified to raise the pH to circumneutral values (here HCl and NH3 according to common trace metals protocols). These reagents do not require cleaning before use, further simplifying the overall procedure. Analysis of seawater previously acidified at pH ~1.5 with HCl and neutralized with ammonia showed interferences due to the buffering properties of the NH3/NH4Cl couple and the transient formation of a volatile electroactive interference that can be easily removed by simply allowing a set time before analysis. In general, the proposed method features several advantages, including high sample throughput, an excellent limit of detection at 12pM, minimum sample handling (no preconcentration or change of matrix is required), cost effectiveness and mainly a negligible blank. The method was successfully validated using open ocean consensus samples (SAFe D2 and S).

Electrochemistry of the COMT Enzyme Inhibitor Entacapone and Its Determination in Pharmaceutical Formulation and Human Blood-Pharmacokinetic Study

Entacapone (ENT) is a potent inhibitor of catechol-O-methyltransferase enzyme. ENT is reduced via two irreversible cathodic waves over the pH range 2.0–11.5. It has strong interfacial adsorptive behavior onto Hg electrode surface. Langmuir isotherm indicated monolayer adsorption and the equilibrium parameter RL value (0.40 – 0.93) indicated that sorption process was favorable. A monolayer surface coverage of 6.06 × 10−10 mol cm−2 was estimated and each adsorbed ENT molecule occupied an area of 0.27 nm2. Taking advantage of the adsorption features of ENT, a sensitive and selective square-wave adsorptive cathodic stripping voltammetric (SW-AdCSV) method was developed for trace determination of ENT in pure form, commercial formulations (in Comtan tablets; as a single active drug and in Stalevo 200 tablets; as tertiary mixture of ENT, levodopa and carbidopa) and in human serum. Limits of quantification (LOQ) of 5.0 × 10−10 and 1.3 × 10−9 mol L−1 ENT were achieved in pure form and in spiked human serum, respectively. This analysis was carried out without addition of any surfactant, extraction of the drug and interference from some common excipients, co-formulated and co-administrated drugs. The proposed method has also been successfully applied for assay of ENT in real plasma samples and seems to be suitable for therapeutic drug monitoring purposes.

Adsorptive stripping voltammetric determination of anticancer drug lomustine in biological fluids using in situ mercury film coated graphite pencil electrode

Abstract In situ mercury film coated graphite pencil electrode (MF/GPE) was prepared and applied for selective and sensitive electrochemical determination of anticancer drug lomustine (LMT). Parameter optimization for the preparation of MF/GPE was examined. The bare and modified GPEs were characterized by scanning electron microscopy (SEM), cyclic voltammetry and square wave voltammetry. With pH 5.0, Britton–Robinson (BR) buffer and in the presence of 0.5 M sulfate ions as indifferent supporting electrolyte, LMT yields a well defined and sensitive reduction peak at the MF/GPE. The electrochemical parameters such as the charge value (Q), surface concentration (Γ), electron transfer coefficient (α) and the standard rate constant (ks) for the reduction of LMT at the MF/GPE were calculated. The achieved limits of detection and quantification were 8.13 × 10− 8 M and 2.71 × 10− 7 M by square wave cathodic adsorptive stripping voltammetry (SWCASV), respectively. The modified MF/GPE was used as a sensor for the detection of LMT in human blood and urine samples with good accuracy and precision.

Rapid and sensitive determination of cobalt by adsorptive cathodic stripping voltammetry using tin–bismuth alloy electrode

Tin-bismuth alloy electrode (SnBiE) was used for trace cobalt determination for the first time. Compared to the bismuth bulk electrode, the SnBiE possesses the advantages of higher hydrogen overpotential, easier manufacture, and lower cost. In addition, there is no need for a pretreatment (in terms of modification) of the electrode before measurements. The analysis of Co(II) was made by the adsorptive accumulation of the cobalt-nioxime complex from solution containing 60 mu M nioxime and 0.1 M ammonia at pH 9.4 and followed by the reduction of the accumulated complex. The electroanalysis results show that the optimal sensitivity can be obtained by using nitrite as the auxiliary reagent. The calibration plot for Co(II) quantification was linear from 0.2 to 20 nM with a correlation coefficient of 0.998. Meanwhile, a detection limit of 44 pM was obtained in connection with an accumulation time of 60 s, which is more sensitive than that of the mercury, bismuth, lead, and lead-copper electrodes. The practical applications of SnBiE have been performed for the determination of Co(II) in real water samples, and the results are consistent with those results by use of inductively coupled plasma-mass spectrometry (ICP-MS).

A new strategy for 2,4,6-Trinitrotoluene adsorption and electrochemical reduction on poly(melamine)/graphene oxide modified electrode

A poly(melamine)/graphene oxide (PM/GO) layer modified on a glassy carbon electrode (GCE) was used for the adsorption and electrochemical detection of 2,4,6 trinitrotoluene (TNT). The surface morphology and electrochemical behaviour of the PM/GO/GCE were characterized by scanning electron microscopy (SEM), fourier transform infrared spectroscopy (FT-IR), cyclic voltammetry (CV) and adsorptive cathodic stripping voltammetry (AdCSV). The PM/GO/GCE exhibited excellent adsorption and electrochemical reduction of TNT via the AdCSV technique with two linear ranges, 1–90μgL−1 and 100–1000μgL−1, a detection limit of 0.34μgL−1 and a quantitation limit of 1.14μgL−1. The PM/GO/GCE provided for a high sensitivity, good repeatability and selectivity. This modified electrode was successfully applied to detect TNT in soil samples with good recoveries that ranged from 93 to 99%.

Electrochemical behavior and assay of anti-Parkinson drug selegiline using cathodic adsorptive stripping square wave voltammetry in bulk form

Electrochemical behavior and assay of anti-Parkinson drug selegiline using cathodic adsorptive stripping square wave voltammetry in bulk form Priyanka Sinha, Anjuman Shekhawat, Digish K Sharma Electrochemical Research Laboratory, Department of Chemistry, University of Rajasthan, Jaipur, India Abstract: The electrochemical reduction behavior of selegiline was investigated by cyclic voltammetry using glassy carbon electrode and validated by square wave cathodic adsorptive stripping voltammetry. Selegiline gave one well-defined reduction peak in the potential range −0.4 to −0.5 V versus Ag/AgCl electrode. The reduction process is irreversible and partial diffusion controlled. Various chemical and instrumental parameters affecting the electroanalytical response for the determination of selegiline were investigated and optimized. Under optimized conditions, the adsorptive stripping peak current is found to be linear over the concentration range of 3.0×10–7–2.5×10–5 M with a detection limit of 3.04×10–8 M and a lower limit of quantification of 1.01×10–7 M. Keywords: selegiline, anti-Parkinson drug, glassy carbon electrode, square wave cathodic adsorptive stripping voltammetry, diffusion controlled

English

English

Square Wave Cathodic Adsorptive Stripping Voltammetric Determination of the Anticancer Drugs Flutamide and Irinotecan in Biological Fluids Using Renewable Pencil Graphite Electrodes

AbstractA sensitive electrochemical method based on square wave cathodic adsorptive stripping voltammetry (SWCASV) using pencil graphite electrodes (PGE) was developed for the individual and simultaneous determination of the anticancer drugs flutamide (Flu) and irinotecan (Irino) in biological fluids. Calibration curves showed an excellent linear response with limits of detection of 1.68×10−9 and 1.55×10−8 M Irino and Flu, respectively. The statistical evaluation of within‐day repeatability (n=5) and day to day precision (n=5) showed satisfactory accuracy and precision. SWCASV using a PGE for individual and simultaneous determination of both drugs in bulk form, human urine and serum samples was demonstrated.

Adsorptive Cathodic Stripping Voltammetric Method for Determination of Gallium Using an In Situ Plated Lead Film Electrode

AbstractThe in situ plated lead film electrode was proposed for the first time for adsorptive stripping voltammetric determination of gallium in water samples. The method was based on simultaneous lead film formation and Ga(III)‐cupferron complex preconcentration at −0.7 V and its cathodic stripping during the potential scan. The composition of the supporting electrolyte, cupferron concentration, conditions of lead film formation, potential and time of accumulation were studied in detail. Under optimum conditions the limit of detection was 3.8×10−9 mol L−1. The proposed procedure was validated in the course of Ga(III) determination in waste water certified reference materials.

Selective and Sensitive Determination of Uranyl Ions in Complex Matrices by Ion Imprinted Polymers‐Based Electrochemical Sensor

AbstractWe report on the design of a UO22+‐selective electrode based on the use of UO22+ imprinted polymer nanoparticles (IP‐NPs), and its application for the differential pulse adsorptive cathodic stripping voltammetry determination of uranyl ions. A carbon paste electrode was modified with the IP‐NPs, and differential pulse adsorptive cathodic stripping voltammetry was applied as the detection technique after open‐circuit sorption of UO22+ ions. The modified electrode responses to UO22+ was linear in the 0.1 µg L−1 to 10 µg L−1 and in the 0.01 mg L−1 to 10 mg L−1. The method detection limit of the sensor was 0.03 µg L−1.