Square-wave Voltammetric Measurements Research Articles

Catalytic voltammetric determination of Mo(VI) ultratraces at the tellurium film electrode using the Mo(VI)-mandelic acid-chlorate system

An in-situ plated tellurium film electrode at the glassy carbon substrate (TeF-GCE) was applied for a sensitive catalytic voltammetric determination of molybdenum in the presence of 0.08 mM of mandelic acid (HA), 0.1 M of KClO3, 0.01 M of HCl and 150 μg L−1 of Te(IV). The performed cyclic and square-wave voltammetric measurements indicate that the investigated catalytic system at the TeF-GCE involves electrocatalysis of the second kind, in which a composite complex between the catalyst Mo(V)-A and ClO3− ions forms, and the subsequent irreversible electrochemical reaction yields the catalyst and electroinactive ClO2− ions. The applied analytical procedure was based on 120 s of deposition of Te film at −0.6 V, followed by differential pulse polarization of the electrode from 0 V to −0.6 V. The sensitivity of the method was 4.02 μA/(μg L−1) and the catalytic voltammetric response was proportional to the concentration of molybdenum within the range from 0.02 to 0.14 μg L−1. The limit of detection was found to be 0.004 μg L−1 of Mo(VI), which makes the catalytic voltammetric method suitable for the quantification of Mo ultratraces in surface waters.

Improved electroanalytical characteristics for the determination of pesticide metobromuron in the presence of nanomaterials

In the present work, bare ultra trace graphite electrode (UTGE), UTGE modified with multi–walled carbon nanotubes (UTGE–MWCNTs), and UTGE modified with graphene nanoplatelets (UTGE–GNPs) were considered as working electrodes. For the first time, the UTGEs were modified with MWCNTs and GNPs by simple and fast drop-casting approach (the whole procedures take no more times than ca. 30 min). The comprehensive microscopic and electrochemical characterization of the unmodified and the modified UTGEs was conducted by means of atomic force microscopy (AFM), electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV) techniques. The prepared electrodes were further applied for the analytical purposes, and the procedures for the square–wave voltammetric (SWV) determination of pesticide metobromuron (Mbn) using the bare UTGE, the UTGE–MWCNTs, and the UTGE–GNPs were developed. For the first time, this compound was electrochemically investigated. The SWV measurements were performed in Britton–Robinson buffer (B–R) solution at pH 2.0 as a supporting electrolyte. SWV parameters, i.e. amplitude, frequency, and step potential, were optimized. The linear relationships between peak current vs. increasing concentrations of Mbn were defined using the bare UTGE, the UTGE–MWCNTs, and the UTGE–GNPs, and the limits of detection were calculated (0.13, 0.11, 0.048 μmol L−1, respectively). The analytical parameters determined from calibration curves indicate similar sensitivity on all tested electrodes, however, the widest linearity range as well as the lowest LOD and LOQ values were achieved on the UTGE modified with GNPs. The utility of the proposed method with the UTGE–GNPs was verified by the quantitative analysis of Mbn in soil samples with satisfactory results (recovery of 99.1%). Furthermore, the impact of possible interferences was tested and evaluated, and obtained results proved good selectivity of the proposed method.

Synthesis and electrochemical characterization of nanostructured magnetic molecularly imprinted polymers for 17-β-Estradiol determination

The authors report the synthesis, characterization and electrochemical study of a nanostructured magnetic molecularly imprinted polymer (Fe3O4-MIP) for the determination of 17-β-Estradiol (17-β-E2). The synthesized magnetic nanoparticles (Fe3O4-NPs) and Fe3O4-MIP were investigated using High Resolution Transmission Electron Microscopy (HRTEM), High Resolution Scanning Electron Microscopy (HRSEM), X-Ray Diffraction (XRD) and Fourrier Transform Infrared spectroscopy (FT-IR) as characterization techniques. The electrochemical characterization was studied by cyclic voltammetry and electrochemical impedance spectroscopy Several important parameters controlling the performance of the Fe3O4-MIP based sensor such as the choice of electrochemical analysis technique, the frequency, the effect of pH and the incubation time were investigated. Under the optimal conditions, the oxidation peak current was linearly related to 17-β-E2 concentration in the range of 0.05–10μM. The Fe3O4-MIP based sensor amplifies the oxidation current in square-wave voltammetric measurements which allows reaching 20nM as detection limit. Furthermore, the Fe3O4-MIP sensing system exhibits a high selectivity towards 17-β-E2 and was applied successfully for its determination in river water samples.

Versatile Immunosensor Using a Quantum Dot Coated Silica Nanosphere as a Label for Signal Amplification

A versatile immunosensor using a CdTe quantum dots (QDs) coated silica nanosphere (Si/QD) as a label was proposed for ultrasensitive detection of a biomarker. In this approach, silica nanospheres with good monodispersity and uniform structure were employed as the carrier for immobilization of QDs and antibodies. Rabit IgG served as a model protein to demonstrate the performance of the immunosensor. Goat antirabbit IgG antibody was covalently bound to CdTe QDs on the surface of silica nanospheres. CdTe QDs coated with a silica nanosphere label (Si/QD/Ab2) were attached onto the gold electrode surface through a subsequent "sandwich" immunoreaction. This reaction was confirmed by scanning electron microscopic (SEM) and fluorescence microscopic images. Due to signal amplification from the high loading of CdTe QDs, 6.6- and 5.9-fold enhancements in electrochemiluminescent (ECL) and square-wave voltammetric (SWV) signals for IgG detection were achieved compared to the unamplified method. The detection limits for IgG were 1.3 and 0.6 pg mL(-1) for ECL and SWV measurements, respectively. The resulting versatile immunosensor possesses high sensitivity, satisfactory reproducibility and regeneration, and good precision. This simple and specific strategy has vast potential to be used in other biological assays.

Real-time electrochemical monitoring of drug release from therapeutic nanoparticles

An electrochemical protocol for real-time monitoring of drug release kinetics from therapeutic nanoparticles (NPs) is described. The method is illustrated for repetitive square-wave voltammetric measurements of the reduction of doxorubicin released from liposomes at a glassy-carbon electrode. Such operation couples high sensitivity down to 20 nM doxorubicin with high speed and stability. It can thus monitor in real time the drug release from NP carriers, including continuous measurements in diluted serum. Such direct and continuous monitoring of the drug release kinetics from therapeutic NPs holds great promise for designing new drug delivery NPs with optimal drug release properties. These NPs can potentially be used to deliver many novel compounds such as marine-life derived drugs and hydrophobic drugs with limited water solubility that are usually difficult to be characterized by traditional analytical tools.

Electrodeposition of Uranium and Transuranics Metals (Pu) on Solid Cathode

The results from a study of U and Pu metal electrodeposition from molten eutectic LiCl-KCl on a solid inert cathode are presented. This study has been conducted using ~50 g of U-Pu together with rare earths (mostly Nd) and 1.5 kg of salt. The introduction of a three-electrode probe with an Ag/AgCl reference electrode has allowed voltammetric measurement during electrolysis and control of the cathode potential versus the reference. Cyclic and square-wave voltammetric measurements proved to be very useful tools for monitoring the electrolysis as well as selecting the cathode versus reference potential to maximize the separation between actinides and rare earths. The voltammetric data also highlighted the occurrence of back reactions between the cathode deposit and oxidizing equivalents formed at the anode that remained in the molten salt electrolyte. Any further electrolysis test needs to be conducted continuously and followed by immediate removal of the cathode to minimize those back reactions.

Resistance to Surfactant and Protein Fouling Effects at Conducting Diamond Electrodes

AbstractBoron‐doped diamond thin‐film electrodes display negligible fouling effects in the presence of high levels of surface‐active materials, including proteins. Dramatic improvements in the stability of the analyte response (compared to common glassy carbon and carbon paste electrodes) are illustrated using bovine serum albumin (BSA), gelatin, and Triton X‐100 in connection with repetitive square‐wave voltammetric (SWV) measurements. The voltammetric response of ascorbic acid at the diamond electrode exhibits negligible shifts in peak potentials and minimal depressions of current signals over a wide range of surfactant concentrations (0–750 ppm). For example, the diamond electrode exhibited 70, 50 and 60 mV potential shifts for 10 repetitive voltammetric scans in the presence of 100 ppm BSA, gelatin and Triton X‐100, respectively, compared to 120, 190, and 280 mV shifts observed at the glassy carbon electrode. Furthermore, only 4.3 and 6.2% of the initial current decays were observed in the presence of 100 ppm Triton X‐100 and gelatin, respectively (compared to 45.2 and 34.4% diminutions at the glassy carbon electrode). Such improved performance was also confirmed from the SWV measurements of uric acid, dihydroxyphenylacetic acid, and catechol. The greatly improved resistance to surfactant interference reflects the fact that the as‐grown diamond thin film, composed of oxide‐free and hydrogen‐terminated surface, has a relatively lower surface energy and minimal electrostatic attributes, either specific or general, so that little adsorption of surface‐active agents occurs. The topographic AFM images of the diamond electrode surface confirm a negligible BSA fouling effect after repetitive SWV measurements. Such enhanced antifouling features make diamond electrodes very attractive for numerous real‐life electroanalytical applications.

Metal- and ligand-directed one-pot syntheses, crystal structures, and properties of novel oxo-centered tetra- and hexametallic clusters.

Starting from closely related metal-ligand combinations, completely different oligomeric metal clusters are synthesized. Whereas, picoline-tetrazolylamide HL(1) (1) and zinc or nickel acetate afforded [2x2] grids [M(4)(L(1))(8)] (2), slightly different N-(2-methylthiazole-5-yl)-thiazole-2-carboxamide HL(2) (5 a) and nickel acetate yielded the monometallic complex [Ni(L(2))(2)(OH(2))(2)] (6). In contrast, reaction of 5 a with zinc acetate produced the tetrametallic zinc cluster [Zn(4)O(L(2))(4)(OAc)(2)] (7). Even more surprising, when 3-methyl-substituted HL(3) (5 b) instead of 2-methyl-substituted HL(2) (5 a) was allowed to react under identical conditions with zinc acetate, the cluster [Zn(4)O(L(3))(4)Cl(2)] (8) crystallized from dichloromethane. Clusters 7 and 8 are isostructural. As for 7, in 8 two of the edges of the tetrahedron of zinc ions are doubly bridged, two are singly bridged, and the other two are nonbridged. On the other hand, when iron(II) acetate under aerobic conditions was allowed to react with 5 a, the unprecedented complex [[Fe(3)O(L(2))(2)(OAc)(4)](2)O] (9) was isolated. Cluster 9 is composed of two trimetallic, triangular mu(3)-O(2-)-centered [Fe(3)O(L(2))(2)(OAc)(4)](+) modules, linked by an almost linear mu(2)-O(2-) bridge. The Mössbauer spectrum together with cyclic voltammetric and square-wave voltammetric measurements of 9 are reported, and 6-9 were characterized unequivocally by single-crystal X-ray structure analyses.

Redox characteristics of manganese and cobalt complexes obtained from pyridine N-oxide

Manganese and cobalt complexes, using pyridine N-oxide as ligand, have been synthesized, and their cyclic and square-wave voltammetric measurements have been carried out. The results reveal that the complexes exhibit different voltammetric pattern, which suggests that the redox processes are most probably metal-centered. In both complexes, extra redox activity is observed once the potential exceeds certain value of the voltage. The observation of an oxidation wave in manganese complex at +0.75 V vs. Ag/AgCl or +0.95 V vs. NHE strongly suggests that this complex can bring about oxidation of water and can, thus, serve as a synthetic analogue of water oxidizing complex (WOC) of PS II.

Ultratrace Measurements of Nucleic Acids by Baseline-Corrected Adsorptive Stripping Square-Wave Voltammetry

The direct and reliable electrochemical detection of DNA is of paramount importance to the development of modern DNA hybridization chips, for the detection of nucleic acids following their electrophoretic separations, or for the sensing of DNA damage and interactions. Such solid electrode voltammetric measurements of nucleic acids have been traditionally hampered by the large solvent decomposition background current that obscures the oxidation signals of the purine nucleobases. This paper reports on the use of adsorptive stripping square-wave voltammetry, in connection with the "moving average baseline correction" approach, for monitoring ultratrace levels of DNA and RNA. Compared to other baseline-fitted or background-subtraction protocols, the moving average baseline scheme is particularly effective in isolating the small purine nucleobase peaks, which appear as small shoulders on the steep background discharge contribution. The remarkably flat baseline thus obtained (up to extreme potentials) leads to a dramatic lowering of the detection limits to the femtomole level and to a performance that compares favorably with that of computerized chronopotentiometric measurements of nucleic acids. Combined with the speed of square-wave voltammetric measurements, such developments should expand the role of voltammetry in DNA diagnostics and nucleic acid research.

Influence of the HEPES Buffer on Electrochemical Reaction of the Copper(II)-Salicylaldoxime Complex

The effect of the 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) on the redox process of copper(II)-salicylaldoxime complex [Cu(II)-SA] was studied. The measurements were performed on a hanging mercury drop electrode (HMDE) using various electrochemical techniques. The presence of the HEPES buffer (pH 8.0) doubled the reduction current of the Cu(II)-SA complex, whereas the reduction potential shifted to more positive values (about 50 mV). The results obtained indicate that the presence of the HEPES buffer facilitates the electron transfer between the mercury electrode and the adsorbed Cu(II)-SA complex during the reduction process. The experiments carried out using chronocoulometry with a double step potential revealed that the surface coverage of the Cu(II)-SA complex remained unchanged (in the range between 9 and 10.5 〈 10 π11 mol cm π2 ), regardless of the presence of the HEPES buffer. Square-wave voltammetric measurements also indicate that the redox process of the Cu(II)-SA complex is electrochemically reversible with the reactant adsorption, in spite of the presence of the HEPES buffer .

Study of uranyl(VI) ion reduction at various ionic strengths of sodium perchlorate

To determine the number of water molecules involved in the electrode reaction of the uranyl(VI)-(V) redox pair, square-wave voltammetric measurements of uranyl(VI) ion reduction in non-complexing NaClO4 media at pH 2 and different ionic strengths between 0.1 and 8.5 mol 1−1 were performed. The available literature water activity data were compared and taken into account. The loss of two water molecules was established.

Square-wave voltammetry of uranyl-humate complex

Square-wave voltammetric measurements were applied to the identification and determination of the uranyl-humate complex in 0.55 mol 1 −1 NaCl solution and in artificial and natural sea water. The sensitivity of the square-wave voltammetric response can be enhanced by the accumulation of the complex because of its strong adsorption at the electrode surface. The detection limit for the uranyl-humate complex after an accumulation period of 10 min at a potential 0.0 V vs. a sodium chloride-saturated Ag/AgCl electrode was found to be (5.0 ± 0.2) X 10 −9 mol 1 −1 and the maximum surface concentration of the adsorbed reactant was Γ s=(7.1±0.1)X10 −11 mol cm −2. The mechanism of the electrode reaction was established. The interferences of various cations and surface-active substances were studied.

Square-wave voltammetry of europium(III)-salicylate complex

Square-wave voltammetric measurements of the Eu(III)-salicylate (Eu-Sal) complex in 0.55 mol 1 −1 NaCl at pH 5.3 were made. At various europium concentration levels (from 10 −8 to 10 −6 mol 1 −1) the redox process of the Eu-Sal complex is reversible, with adsorption of the reactant of the redox reaction. The detection limit with an accumulation of 5 min at 0 V vs. Ag/AgCl was 10 −8 mol 1 −1 Eu(III). A maximum surface concentration Γ max=(2.31±0.46)×10 −11 mol cm −2 was found.