Electroanalytical Methodology Research Articles

Voltammetric determination of ropinirole in the presence of levodopa at the surface of a carbon nanotubes based electrochemical sensor in pharmaceuticals and human serum

A nanostructured electrochemical sensor was developed to establish an innovative way to measure ropinirole and levodopa in real pharmaceutical and biological samples, which offers a high selectivity derived from the stable carbon nanotubes-Nafion polymer film. The surface morphology of the modified electrodes was characterized by scanning electron microscopy. The effect of surface modifications, supporting electrolyte, amount of carbon nanotubes suspension, accumulation time and potential were investigated. A sensitive electroanalytical methodology for the simultaneous determination of both drugs co-administrated in advanced Parkinson’s disease patients using adsorptive stripping square-wave voltammetry is presented. Under the optimized conditions, ropinirole and levodopa gave a linear response in the range 1×10−7–1×10−5M and 2.5×10−7–1×10−5M with detection limits 1.6×10−8M and 5.2×10−8M, respectively. The method was successfully utilised for their quantification in human serum samples and good recoveries were obtained without interference from endogenous dopamine, uric and ascorbic acid. In addition, the proposed sensor was successfully applied in the independent determination of ropinirole and levodopa content in pharmaceutical formulations, whose accuracy was attested by good agreement of the results with those, obtained using high performance liquid chromatography. The proposed sensor is characterized by high sensitivity and reproducibility, simple fabrication procedure, easy handling, resistance against surface fouling and low cost.

Forensic electrochemistry applied to the sensing of new psychoactive substances: electroanalytical sensing of synthetic cathinones and analytical validation in the quantification of seized street samples.

The electrochemical sensing of new psychoactive substance(s) (NPSs), synthetic cathinone derivatives also termed "legal highs", are explored with the use of metallic modified screen-printed electrochemical sensors (SPES). It is found that no significant electrochemical enhancement is evident with the use of either in situ bismuth or mercury film modified SPES compared to the bare underlying electrode substrate. In fact, the direct electrochemical reduction of the cathinone derivatives mephedrone (4-methylmethcathinone; 4-MMC) and 4'-methyl-N-ethylcathinone (4-methylethcathinone; 4-MEC) is found to be possible for the first time, without heavy metal catalysis, giving rise to useful voltammetric electroanalytical signatures in model aqueous buffer solutions. This novel electroanalytical methodology is validated toward the determination of cathinone derivatives (4-MMC and 4-MEC) in three seized street samples that are independently analyzed with high-performance liquid chromatography (HPLC) wherein excellent agreement between the two analytical protocols is found. Such an approach provides a validated laboratory tool for the quantification of synthetic cathinone derivatives and holds potential for the basis of a portable analytical sensor for the determination of synthetic cathinone derivatives in seized street samples.

Determination of Methylparaben by Square Wave Voltammetry at Boron-Doped Diamond Electrode

Parabens have been widely used as preservative in personal care products, foods and drugs. However, due to their endocrine disruption and carcinogenic possible action, the quantification of these pollutants in several matrices is necessary. In this way, the present work has as objective, the development of an electroanalytical methodology using square-wave technique. The target molecule is the methylparaben (MePa), which will be determined using the boron-doped diamond electrode (BDD) as the work electrode. Several operating conditions and voltammetric parameters were studied seeking greater efficiency of the technique. Under chosen conditions, the measurements were realized in an electrochemical cell containing Ag/AgCl as the reference system and a platinum foil as auxiliary electrode. Several electrolyte solutions were tested and the best results were obtained in 0.2 molL-1 phosphate buffer (pH 7.5). In such electrochemical cell, MePa showed a well-defined square wave voltammetric peak at a potential around 0.8 V, whose peak currents are linearly proportional to the MePa concentration in the electrolyte. Adequate linearity (0.9986), as shown in figure 1, in the range of 0.5 to 30 mgL-1, with limit of detection and quantification of 0.103 mgL-1 and 0.344 mgL-1, respectively, was found. The method was validated for the determination of MePa in a creek water sample, with low pollution content, obtained in Sao Carlos – SP county (S 21 59.670’ / W 047 54.183’). Spiked solutions of such water samples added to the buffer solution, showed recovery range between 68.40% a 106.15%, with a relative standard deviation (RSD) less than 6.8% (n=3). The method seemed to be sensitive to MePa and can be used for detection in others water samples. In future, we intended to use this method for detection of methylparaben in cosmetics creams.Figure 1. Method calibration curve for determination of MePa. Concentrations: 0.5, 0.75, 1.0, 5.0, 10.0, 15.0, 20.0, 25.0, 30.0 mgL-1.

Suitability of the voltammetry of immobilized microparticles to detect and discriminate lead compounds in microsamples of ancient black cosmetics

Lead compounds, in the form of galena, have been used as eye cosmetics since ancient times and they still appear in traditional products. The presence of some black pigments in several archaeological glass objects gave us the opportunity to study the nature of these products, in order to evaluate the results of an electrochemical method to characterise lead compounds when only heterogeneous microsamples are available and the material requires to be as less manipulated as possible. The present paper describes the data obtained with an electroanalytical methodology (Voltammetry of immobilised microparticles, VMP), and a complementary Scanning Electron Microscopy study, to detect these lead compounds. A simple and fast characterisation method is discussed in order to be able to monitor the presence of lead compounds without previous sample preparation, directly in the solid micro-sample, and using an affordable instrumentation. PbS could be identified and distinguished from other lead compounds in cosmetics by Differential Pulse VMP in 1M NaCl medium by a set of three peaks (−0.63V, −0.44V, +0.45V) in the anodic voltammogram and two peaks around −0.65V and +0.45V in the cathodic scan. The results showed the suitability of the electroanalytical method, offering valuable information about the chemical and mineral composition of the samples using a minimum amount of material, and illustrating the advantages in terms of time consuming, cost and accessibility of the laboratory facilities.

Enhanced simultaneous electroanalytical determination of two fluoroquinolones by using surfactant media and a peak deconvolution procedure

The use of anionic surfactants and the application of a deconvolution procedure were investigated as suitable methods to provide electrochemical peak separation in the simultaneous detection of the fluoroquinolones levofloxacin (LEVO) and norfloxacin (NOR). Concurrently, an electroanalytical methodology was developed based on the simultaneous determination of both fluoroquinolones using a mathematical treatment via baseline correction. We also developed a deconvolution procedure to separate overlapping peaks. These methods provided satisfactory peak separation and intensification of peak current, allowing efficient quantification of analytes in both tablets and biological samples with recovery values ranging from 88.1%–107.5%.

Enhancement of Charge Transport Rates By Incorporation of Platinum Nanoparticles to Iodine/Iodide Ionic Liquid Redox-Conducting Electrolytes

The iodine/iodide redox system has so far been the most commonly and most successfully used as a charge relay (mediator) in Dye Sensitized Solar Cells (DSSCs)[1]. The semi-liquid or semi-solid iodine/iodide-based systems have probably led the most promising results so far in the area of practical DSSCs. More recently, the iodide/iodine redox couple has been considered together with ionic liquids. The resulting redox-conducting electrolytes have several advantages: high conductivity, low vapor pressure, high iodide/iodine concentration and good electrochemical stability. Among disadvantages is their high viscosity that certainly contributes to the low diffusion coefficient of the oxidized form of the redox couple, iodine or triiodide, if the charge transport mechanism is truly physical. If charge transport involves electron hoping between I- and I, which seems to be operative as well, its effectiveness would require fast dissociation of iodine (I2), or triiodide (I3 -), molecule. Indeed, both interfacial and bulk (self-exchange) electron transfers involving the iodine/iodide redox system are somewhat complicated and appear slower than one would expect. There is a need to develop means of inducing the I-I bond breaking in the I3 - or I2 molecule. It has also been established that platinum (e.g. when deposited on the counter electrode of DSSC) catalyzes interfacial electron transfers in the iodine/iodide redox system. Surface chemistry data provide clear evidence that iodine (or iodides) chemisorb readily on platinum as monoatomic iodine. Strong interactions of Pt with iodine were reported and described; further, formation of the monolayer type coverages of strongly adsorbed monoatomic iodine together with weakly bound electroactive iodine/iodide was also postulated. In the present work, we explore the concept of three-dimensional distribution of nanostructured platinum within the electrolyte phase) to enhance dynamics of iodine/iodide electron transfers (electron self-exchange) as an attempt to develop a new generation of charge iodine-based relays for DSSC. The diagnostic experiments have been performed using the electroanalytical methodology developed for solid-state electrochemical measurements in the absence of external liquid supporting electrolyte. They have included measurements using the planar three-electrode cell utilizing an ultramicrodisk electrode and two-electrode type sandwich configuration [2,3]. In the present case of theiodine/iodide system dispersed together with Pt nanoparticles within ionic liquid phase, mutual compensations between physical and effective (electron self-exchange type) diffusion mechanisms are postulated. Regardless the mechanism, the charge propagation rates expressed in diffusional terms have been found on 10-6 cm2 s-1 level.[1] M. Graetzel, Acc. Chem. Res., 42 (2009) 1788[2] P.J. Kulesza, M.A. Malik in Wieckowski A (ed.), Interfacial Electrochemistry - Theory, Experiment and Applications, Solid-State Voltammetry 673, Marcel Dekker, New York (1999)[3] P.J. Kulesza, J.A. Cox, Electroanalysis 10 (1998) 73

Electroanalytical Methodology for the Direct Determination of 2,4-Dichlorophenoxyacetic Acid in Soil Samples Using a Graphite-Polyurethane Electrode

An electroanalytical methodology was developed for the direct determination of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) using a graphite-polyurethane composite electrode and square wave voltammetry (SWV). 2,4-D exhibited one reduction peak with characteristics of an irreversible process at −0.54 V (versus Ag/AgCl), which is controlled by the diffusion of the reagent on the electrode surface. After the experimental parameters optimization (pH 2.0,f=50 s−1,a=0.50 V, andΔEi=0.03 V), analytical curves were constructed in the range of 0.66 mg L−1to 2.62 mg L−1. Detection (LD) and quantification (LQ) limits were 17.6 μg L−1and 58.6 μg L−1, respectively. The methodology was successfully applied to measure the percolation of the herbicide 2,4-D in undisturbed soil columns of different granulometric compositions.

Development of an Electroanalytical Methodology for Determination of Pesticide Glyphosate in Environmental Samples

Development of an Electroanalytical Methodology for Determination of Pesticide Glyphosate in Environmental Samples

Eletrochemical reduction of patulin and 5-hydroxymethylfurfural in both neutral and acid non-aqueous media. Their electroanalytical determination in apple juices

The electro-reduction of patulin mycotoxin and 5-hydroxymethylfurfural at glassy carbon electrodes in acetonitrile +0.1molL−1 tetrabutylammonium perchlorate, in both the absence and the presence of different aliquots of trifluoroacetic acid is reported. 5-hydroxymethylfurfural is the most common interference in the determination of patulin in products derived from apples. The electrochemical techniques were cyclic and square wave voltammetries, and controlled potential bulk electrolysis. The number of electrons exchanged in the patulin electro-reduction of n=1 could be inferred from controlled potential bulk electrolysis measurements. Ultraviolet-visible and infrared spectroscopies were used to identify patulin electro-reduction product/s. A value of (2.1±0.1)×10−5cm2s−1 for the patulin diffusion coefficient was calculated from convoluted cyclic voltammograms. A method based on square wave voltammetry was developed for the quantitative determination of patulin in both fresh, and commercial apple juices in the presence of 5-hydroxymethylfurfural. Calibration curves obtained from solutions of the commercial reagent, and commercial apple juices were linear in the range from 3.0×10−7 to 2.2×10−5molL−1. The lowest concentration measured experimentally for a signal to noise ratio of 3:1 was 3×10−7molL−1 (45ppb) and a recovery percent of 84% was determined for commercial apple juices. This electroanalytical methodology appears as a good screening method for the determination of patulin in apple juices.

Virgin olive oil ortho-phenols—electroanalytical quantification

An electroanalytical methodology was developed for the determination of the total ortho-phenol content of virgin olive oil (VOO) with high sensitivity and reproducibility. The VOO ortho-phenol content depends on its freshness and is normally expressed as HT equivalent. Screen-printed electrodes were used with cyclic voltammetry to investigate the oxidation of catechol, phenol, hydroxytyrosol (HT), tyrosol, caffeic acid and ferulic acid. The oxidation of ortho-phenols and mono-phenols occurs following different mechanisms, and at different potentials. Using screen-printed electrodes and square wave voltammetry, an HT detection limit of 0.40μM, was obtained. The electroanalytical methodology developed was applied to the determination of ortho-phenol content in fresh and old VOO. The HT equivalent determined for a two-year-old VOO sample was 3mg/kg, for one-year-old samples was 6–7 mg/kg, and for a fresh VOO sample 30mg/kg, recoveries in the range of 78–93% of HT standard being obtained. The effect of VOO matrix components on the HT standard response was investigated.

Electroanalytical determination of carbendazim and fenamiphos in natural waters using a diamond electrode

In this study, a method for electroanalytical determination of carbendazim (CBZ) and fenamiphos (FNP) in natural and spiked water was developed using square wave voltammetry in Na2HPO4 0.1molL−1 as supporting electrolyte. The calibration curve for carbendazim detection presented good linearity in the concentration range of 0.50 to 15.0μM, with a sensitivity of 0.080A/molL−1 and a linearity of 0.998. The oxidation of fenamiphos on BDD electrode shows a dynamic range of concentration of 0.5 to 25.0μM and sensibility of 0.14A/molL−1. The recovery experiments showed values between 70 and 100% for spiked samples thus indicating the feasibility of the electroanalytical methodology to quantify CBZ and FNP in pure or natural waters.

Square wave voltammetric detection of electroactive products resulting from electrochemical nitrate reduction in alkaline media

The focus of this work is the simultaneous detection of the electroactive species resulting from electrochemical reduction of nitrate (ERN). A simple and fast electroanalytical methodology for the determination of electroactive products resulting from nitrate reduction at a copper electrode in an alkaline media is proposed on the basis of square wave voltammetry (SWV). The obtained results show that, using an adequate pH value, three electroactive species generated from ERN (NO2-, NH2–OH and NH4+) could be electrochemically detected at different applied potentials at a Pt-ring electrode. A comparison of the results obtained by SWV and our previously published cyclic hydrodynamic voltammetry (CHV) measurements was performed. In addition, using both techniques, the conditions for the voltammetric detection of electroactive species resulting from nitrate reduction were established, which will provide useful further information for the ERN process optimization.

Electrochemical Behavior of Chlorogenic Acid at a Boron‐Doped Diamond Electrode and Estimation of the Antioxidant Capacity in the Coffee Samples Based on Its Oxidation Peak

In this study, an electroanalytical methodology for the determination of chlorogenic acid (CGA) was achieved at a boron-doped diamond electrode under adsorptive transfer stripping voltammetric conditions. The values obtained for CGA were used to estimate the antioxidant properties of the coffee sample based on CGA oxidation. By using square-wave stripping mode, the compound yielded a well-defined voltammetric response at +0.49 V with respect to Ag/AgCl in Britton-Robinson buffer at pH 3.0 (after 120 s accumulations at a fixed potential of 0.40 V). At the optimum experimental conditions, linear calibration curve is obtained within the concentration range of 0.25 to 4.0 μg mL⁻¹ with the limit of detection 0.049 μg mL⁻¹ . The developed protocol was successfully applied for the analysis of antioxidant capacity in the coffee products such as Turkish coffee and instant coffee.

Use of Hg-Electroplated-Pt Ultramicroelectrode for Determining Elemental Sulphur in Naphtha Samples

This paper describes the applicability of a Hg-electroplated-Pt ultramicroelectrode in the quantification of elemental sulphur in naphtha samples by square-wave voltammetry. A reproducible deposition methodology was studied and is reported in this paper. This methodology is innovative and relies on the quality of the mercury stock solution to obtain reproducible surfaces required for the analytical methodology. All analyses were performed using a Hg-electroplated-Pt ultramicroelectrode (Hg-Pt UME) due to the low sensibility of such devices to ohmic drops in resistive solutions. The responses of the peak areas in voltammetric experiments were linear in all of the range studied. The method developed here is accurate and reproducible, with a detection limit of 0.010 mg L−1 and a good recovery range for both standard solutions of elemental sulphur (85 to 99%) and real naphtha sample (79%). These results attest to the potential for the application of this electroanalytical methodology in determining elemental sulphur in naphtha samples containing mercaptans and disulphides.

Direct electrochemical analysis of dexamethasone endocrine disruptor in raw natural waters

This paper describes an electroanalytical methodology using square-wave adsorptive voltammetry, which has been successfully applied for the direct determination of dexamethasone residues in raw natural waters used for the public supply of the Ceará State, Brazil. The obtained detection limits ranged from 7.47 × 10-9 to 1.80 × 10-8 mol L-1 for the three matrices of raw natural waters evaluated. High percentages of average recovery (98.86% ± 0.72), repeatability (0.32% ± 0.05) and reproducibility (0.91% ± 0.20) were obtained in these samples, reaffirming the sensitivity of the procedure. Energy of the LUMO orbitals and Mülliken’s atomic charges were calculated using the functional BLYP/DNP. The theoretical results allied to the diagnostic criteria of the square-wave voltammetry indicate that the dexamethasone redox mechanism is associated to the quasi-reversible and irreversible reduction process of the ketone groups located at C-20 and C-3, respectively.

Avaliação de metodologia eletroquímica no monitoramento da conversão de óleo de girassol em biodiesel

The development of analytical procedures to evaluate transesterification process is still a challenge in biodiesel production. Then, this paper shows an electroanalytical methodology to transesterification process assessment, proposing the application of nanostructured TiO2 electrodes. The results showed, for sunflower oil - methanol reaction catalyzed by KOH, a reduction peak in - 1050 mV and the gradual appearance of a second peak at - 1160 mV. This peak was observed as originated by the transesterification process and is probably related to intermediates. By measuring the intensity of this peak a kinetic profile was determined, showing that the conversion is almost finished in 2 h.

Square Wave Voltammetric detection of furan on platinum and platinum-based Screen Printed Electrodes

A rapid, precise and accurate electroanalytical methodology for the determination of furan, a possible human carcinogen, is proposed. The method is based on the use of Square Wave Voltammetry (SWV) on a platinum disk or on platinum based Screen Printed Electrodes, performed in acetonitrile with tetrabutylammonium perchlorate as supporting electrolyte. The oxidation peak height or area present good linear relationship with the furan concentration in the range 1.5 × 10 −5–1 × 10 −3 M, with all sorts of electrodes, ensuring a detection limit of 1–2 × 10 −5 M (1–2 ppm). The SWV parameters optimization and the use of Nafion membrane to protect SPE from fouling and reference electrode damage are also discussed. The application in a complex matrix is performed.

Analysis of Cardamonin by Square Wave Voltammetry

Several biochemical studies have already shown that cardamonin has health promoting properties, such is in agreement with typical characteristics of chalcones. Although being a very promising compound for the nutraceutical field there is a lack of studies concerning its electroanalytical properties. To develop an electroanalytical methodology for the quantification of cardamonin in cardamom. Cardamonin was analysed electrochemically by means of a hanging mercury drop electrode (HMDE) using square wave voltammetry (SWV). It was extracted from cardamom spice and quantified thereafter using the standard additions method to overcome matrix effects. A limit of detection (LOD) of 0.15 mg/L and good linearity (r² = 0.9998) were obtained. Decoction using ethanol as the extraction solvent appears to be the simplest extraction technique. Spectrophotometric analysis (maximum absorbance peak was found in ethanol at 344 nm with a value of molar extinction coefficient of (2.8 ± 0.1) × 10⁴ L mol⁻¹ cm⁻¹) and mass spectrometry analysis by electrospray in the positive ion mode were also performed. Cardamonin was detected voltammetrically. The LOD and limit of quantification (LOQ) of the proposed voltammetric methodology are adequate for trace analysis of this compound in several phytochemical matrices.

Sensitive Detection of Capsaicin by Adsorptive Stripping Voltammetry at a Boron‐Doped Diamond Electrode in the Presence of Sodium Dodecylsulfate

AbstractIn the present paper, a sensitive electroanalytical methodology for the determination of capsaicin using adsorptive stripping voltammetry (AdSV) at a boron‐doped diamond (BDD) electrode is presented. The voltammetric results indicate that in the presence of sodium dodecylsulfate (SDS) the BDD electrode remarkably enhances the oxidation of capsaicin which leads to an improvement of the peak current with a shift of the peak potential to less negative values. A linear working range of 0.05 to 6.0 µg mL−1 (0.16–20 µM) with a detection limit of 0.012 µg mL−1 (0.034 µM) has been obtained using BDD electrode by AdSV.

Modification of electrodes with catalytic, size-exclusion films

The application of electroanalytical methodology to the study of liquid-phase samples can be complicated by the adsorption of sample components on the electrode surface. Macromolecules are particularly problematic in this regard. An early means of addressing this problem was to use a membrane permeable to the analyte as a barrier between the sample phase and the electrochemical cell. Amperometric determination of oxygen in biological fluids is a historically important example. This approach was refined by modifying electrodes with semi-permeable, conducting films applied directly to the surface of the working electrode. Cellulose acetate is an example of a conductive material that blocked adsorption of compounds in biological samples but was permeable to analytes such as hydrogen peroxide. Modification of electrodes with ion-exchange films and, more recently, porous sol–gel films was an expansion of this methodology. A complicating factor was that oxidation or reduction of most analytes requires a catalyst. The development of films that are size-exclusion barriers to interferents and incorporate an electron-transfer catalyst is described.