Electrochemical Characterization of Epigallocatechin Gallate Using Square‐Wave Voltammetry

Electrochemical oxidation of (?)‐epigallocatechin gallate (EGCG), the main monomer flavanol found in green tea, has been investigated over a wide pH range at a glassy‐carbon electrode using square‐wave voltammetry (SWV). Square‐wave voltammograms of (?)‐epigallocatechin (EGC) and gallic acid have been studied as well. The I–E profile of EGCG, i.e. the oxidation potentials and the current responses of the first and the second peak, is pH dependent. The oxidation of EGCG is a quasireversible process over the studied pH range, which was also confirmed by the non‐linear relationship between the peak currents and squre root of frequency. The best SWV responses for EGCG were obtained at pH 2.0, frequency of 100 Hz, step of 2 mV and amplitude of 50 mV. Under these conditions, linear responses for EGCG were obtained for concentrations from 1×10^?7 M to 1×10^?6 M, and calculated LOD and LOQ for the first oxidation peak were 6.59×10^?8 M and 2.19×10^?7 M, respectively. The proposed electroanalytical procedure was applied for the determination of EGCG content in green tea. Developed SWV methodology represents a potential analytical tool in determination of catechins in tea samples.     

Trace Determination of Chromium by Square‐Wave Adsorptive Stripping Voltammetry on Bismuth Film Electrodes

This works reports the use of adsorptive stripping voltammetry (AdSV) for the trace determination of chromium on a rotating‐disk bismuth‐film electrode (BFE). During the reductive accumulation step, all the chromium species in the sample were reduced to Cr(III) which was complexed with cupferron and the complex was accumulated by adsorption on the surface of a preplated BFE. The stripping step was carried out by using a square‐wave (SW) potential‐time voltammetric signal. Electrochemical cleaning of the bismuth film was employed, enabling the same bismuth film to be used for a series of measurements in the presence of dissolved oxygen. The experimental variables as well as potential interferences were investigated and the figures of merit of the method were established. Using the selected conditions, the 3σ limit of detection for chromium was 100 ng L^?1 (for 120 s of preconcentration) and the relative standard deviation was 3.6% at the 2 μg L^?1 level (n=8). Finally, the method was applied to the determination of chromium in real samples with satisfactory results.     

Determination of Rutin in Pharmaceutical Compounds and Tea Using Cathodic Adsorptive Stripping Voltammetry

A new sensitive electrochemical procedure is presented for the determination of Rutin based on the accumulation of Rutin onto the surface of hanging mercury drop electrode for 80 s. Then the preconcentrated Rutin‐complex was analyzed by cathodic stripping square‐wave voltammetry. The effect of various parameters such as pH, concentration of copper, accumulation potential, accumulation time and scan rate on the sensitivity were studied. The optimum conditions for determination of Rutin include pH 6.0, 6.0 ng mL^?1 copper(II) concentration, accumulation potential of ?0.30 V and scan rate of 0.40 V s^?1. Under the optimum conditions and for an accumulation time of 80 s, the measured peak current at about ?0.03 V is proportional to the concentration of Rutin over the range of 2.0–85.0 nM. The practical limit of detection is 0.5 nM. The relative standard deviations for six replicate analyses of 10 and 50 nM Rutin are 1.8% and 1.7%, respectively. The method was applied to the determination of Rutin in synthetic, tea, and pharmaceutical samples with satisfactory results. The diffusion coefficient, complex formation constant and number of electron transfer of Rutin–Cu(II) complex was also estimated.     

Square‐Wave Voltammetry of Electroinactive Surfactants

Under the influence of previously published and some new theoretical results, potential‐ dependent adsorption and desorption of model electroinactive surfactants Triton X‐100 (T‐X‐100 or polyethylene glycol p‐(1,1,3,3‐tetramethylbutyl)‐phenyl ether) and sodium dodecyl sulfate (SDS) on the static mercury drop electrode (SMDE) were studied by square‐wave voltammetry (SWV). Although (according to the theory) the resulting current – potential curve should consist of two highly separated peaks, only desorption signal could be seen on each experimentally obtained voltammogram, most probably because of the limitations concerning the available potential range. Different properties of the recorded peak are in good agreement with the theory indicating that square‐wave voltammetry could be treated as a potential tool for tensammetric studies of electroinactive surface active substances.     

Electroanalytical Determination of Promethazine Hydrochloride in Pharmaceutical Formulations on Highly Boron‐Doped Diamond Electrodes Using Square‐Wave Adsorptive Voltammetry

The electrochemical oxidation of promethazine hydrochloride was made on highly boron‐doped diamond electrodes. Cyclic voltammetry experiments showed that the oxidation mechanisms involved the formation of an adsorbed product that is more readily oxidized, producing a new peak with lower potential values whose intensity can be increased by applying the accumulation potential for given times. The parameters were optimized and the highest current intensities were obtained by applying +0.78 V for 30 seconds. The square‐wave adsorptive voltammetry results obtained in BR buffer showed two well‐defined peaks, dependent on the pH and on the voltammetric parameters. The best responses were obtained at pH 4.0, frequency of 50 s^?1, step of 2 mV, and amplitude of 50 mV. Under these conditions, linear responses were obtained for concentrations from 5.96×10^?7 to 4.76×10^?6 mol L^?1, and calculated detection limits of 2.66×10^?8 mol L^?1 (8.51 μg L^?1) for peak 1 and of 4.61×10^?8 mol L^?1 (14.77 μg L^?1) for peak 2. The precision and accuracy were evaluated by repeatability and reproducibility experiments, which yielded values of less than 5.00% for both voltammetric peaks. The applicability of this procedure was tested on commercial formulations of promethazine hydrochloride by observing the stability, specificity, recovery and precision of the procedure in complex samples. All results obtained were compared to recommended procedure by British Pharmacopeia. The voltammetric results indicate that the proposed procedure is stable and sensitive, with good reproducibility even when the accumulation steps involve short times. It is therefore very suitable for the development of the electroanalytical procedure, providing adequate sensitivity and a reliable method.     

Determination of Trace Manganese by Square‐Wave Stripping Voltammetry

A novel method of determining trace manganese by square‐wave stripping voltammetry with simultaneous plating mercury electrode is described in this paper. Well‐defined stripping peaks of manganese are obtained in H₃BO₄‐NaOH buffer (pH 8.0) and peak currents are in good linear relationship with manganese concentrations in two ranges: from 0.36 nM to 36 nM and from 73 nM to 909 nM. The obtained detection limit is 0.18 nM and relative standard deviation is 2.3% (n=7) under a predeposition time of 90 s. This proposed method has been used for the determination of trace manganese in real plasma samples with satisfiable results and good matching with the results of graphite furnace atomic absorption spectrometry.     

Simultaneous Square‐Wave Voltammetric Determination of Paracetamol,Caffeine and Orphenadrine in Pharmaceutical Formulations Using a Cathodically Pretreated Boron‐Doped Diamond Electrode

An electroanalytical method for the simultaneous determination of paracetamol (PAR), caffeine (CAF), and orphenadrine (ORPH) using the square‐wave voltammetry (SWV) and a cathodically pretreated boron‐doped diamond electrode was developed. The method exhibits linear responses to PAR, CAF, and ORPH in the concentration ranges 5.4×10^?7–6.1×10^?5 M, 7.8×10^?7–3.5×10^?5 M, and 7.8×10^?7–3.5×10^?5 M, respectively, with detection limits of 2.3×10^?7 M, 9.6×10^?8 M, and 8.4×10^?8 M, respectively. The proposed method was successfully applied in the simultaneous determination of these analytes in pharmaceutical formulations.     

Square‐Wave Voltammetric Determination of Paraquat at Carbon Paste Electrode Modified with Hydroxyapatite

The voltammetric behavior of paraquat was investigated at hydroxyapatite‐modified carbon paste electrode HAP‐CPE in K₂SO₄. A method was developed for the detection of the trace of this herbicide, based on their redox reaction. The reduction peaks of paraquat were observed around ?0.70 V and ?1.00 V (vs. SCE) in square‐wave voltammetry. Experimental conditions were optimized by varying the accumulation time, apatite loading and measuring solution pH. Calibration plots were linear under the optimized parameters over the herbicide's concentration range 8–200×10^?7 mol L^?1, with a detection and quantification limits about 1.5×10^?8 mol L^?1 and 6.4 10^?8 mol L^?1, respectively.     

Cathodic Stripping Voltammetry of Uracil. Experimental and Theoretical Study Under Conditions of Square‐Wave Voltammetry

The electrode mechanism of uracil at a hanging mercury drop electrode (HMDE) is studied under cathodic stripping square‐wave voltammetric mode owing to the cathodic dissolution of a sparingly soluble compound formed between the electrode material and uracil. The experimental results can be partly explained in the light of the recent theory for cathodic stripping processes of insoluble salts under conditions of square‐wave voltammetry. It is established that the electrode reaction is complicated by attractive interactions between the deposited species of the insoluble compound. To elucidate the electrode mechanism completely a novel theoretical model is developed considering adsorption of the reacting analyte and lateral interactions between species of the insoluble compound. With the help of numerical simulations the effect of interactions is studied in detail, emphasizing the properties of the response that can be used as diagnostic criteria for recognition of the type of interaction forces. Theoretically predicted voltammetric properties agree well with the experimental results enabling clarification of the complex electrode mechanism of uracil at HMDE.     

Square‐Wave Voltammetry: A Review on the Recent Progress

A review on the recent progress of square‐wave voltammetry is presented, covering the period of the last five years. The review addresses the new theoretical development of the technique as well as its application for mechanistic purposes, electrode kinetic measurements, biochemical and analytical applications. Besides, a few novel methodological modifications are proposed that might expand the scope and application of the technique.     

Square‐Wave Voltammetry of Sodium Copper Chlorophyllin on Glassy‐Carbon and Paraffin‐Impregnated Graphite Electrode

Electrochemical oxidation of sodium copper chlorophyllin (CHL) has been investigated at a glassy‐carbon (GC) and paraffin‐impregnated graphite electrode (PIGE) using square‐wave voltammetry (SWV). Square‐wave voltammograms of other two chlorin‐type compounds, namely chlorin e₆ and chlorophyll a, have been studied as well. The measurements were performed in the pH range between 7 and 11. The square‐wave frequency was changed between 8 and 1000 Hz. The oxidation of studied chlorins is a complex, pH‐independent, reversible or quasireversible process, followed by the chemical transformation of the product. The product of the EC reaction of CHL is an electroactive π? π dimer, which strongly adsorbs on the electrode surface and undergoes further oxidation at more positive potential. The electrooxidation of the adsorbed dimer is a pH‐independent irreversible process with the formation of an electroinactive film. The voltammetric behaviour of chlorin e₆ on PIGE was qualitatively similar to that of CHL. The SW voltammograms of chlorin e₆ recorded on GCE and of chlorophyll a recorded on PIGE consisted of only one peak. The SW responses of studied compounds strongly depend on the stabilization of the reaction intermediate by adsorption to the electrode surface.     

An Experimental Investigation of Quasireversible Maximum of Azobenzene on Mercury Electrode by Fourier Transformed Square‐Wave Voltammetry

An experimental investigation of quasireversible maximum (QRM) of azobenzene on mercury electrode by two methods, i.e., traditional square‐wave voltammetry (SWV) and fast Fourier transformed square‐wave voltammetry (FT‐SWV), was presented, and the influence factors on QRM of FT‐SWV were discussed. The results show that the rate constants derived from FT‐SWV agree with that of derived from traditional SWV with acceptable differences, showing a sound verification that the rate constants derived from FT‐SWV were reliable. In addition, some theoretical predictions on FT‐SWV were experimentally confirmed through the characterization of strongly adsorbed azobenzene on mercury film electrode. As a result, FT‐SWV is further proved to be a powerful technique in kinetic studies of the surface adsorbed processes.     

Study of the Electrochemical Behavior and Sensitive Detection of Pesticides Using Microelectrodes Allied to Square‐Wave Voltammetry

This work describes the application of gold and carbon fiber microelectrodes allied to square‐wave voltammetry for the study of the electrochemical behavior of the organophosphorous insecticides (methyl parathion and dichlorvos) and bipyridilium herbicides (paraquat and diquat), and the development of the sensitive methodology for their analytical determinations in natural water samples. The microelectrodes were lab‐made constructed and their electrochemical behavior was characterized by measuring the electrochemical response with a solution of potassium ferricyanide. The experimental and voltammetric conditions to obtain the best analytical signal, in terms of intensities and profile of the peak voltammetric, for four pesticides were optimized and the results were used to evaluate the type of the electrochemical redox process and to appraise the number of electrons covered in each reduction process that occurred for pesticides and also, to propose a possible redox mechanism for a reduction process of pesticides at microelectrodes. Analytical curves were constructed and presented the linear relationships between the peak currents and the concentration of pesticides, for this, the detection limits for pure water (laboratory samples) for four pesticides were calculated and presented values under 15 μg L^?1, lower than maximum limit for drinking water (100 μg L^?1) permitted by Brazilian Council for groundwater, indicating that the methodology could be employed to analyze those pesticides in natural water samples.     

Differential Pulse Voltammetric Determination and Application of Square‐Wave Voltammetry of yRNA on a CPB‐Cellulose Modified Electrode

Electrochemical behavior of remarkably low levels of Ribonucleic acid yeast (yRNA) is studied through differential pulse voltammetry (DPV), and kinetic parameters of the electrochemical reaction have also been calculated through square‐wave voltammetry (SWV), based on immobilization of yRNA on the surface of a CPB‐cellulose modified electrode. YRNA/ CPB‐cellulose/ITO conductive glass electrode is demonstrated by Infrared reflect (IR) and electrochemical impedance spectroscopy (EIS). The oxidation peak potential of yRNA shifts negatively with increasing pH. The peak currents decrease gradually in successive scans and no corresponding reduction peaks occur, indicating that oxidation process of yRNA is completely irreversible. Variables influencing DPV response of yRNA, such as pH, pulse amplitude and electrolyte concentration, are explored and optimized. Peak currents are proportional to the concentration of yRNA in the range of 0.1 μg mL^?1–1.0 μg mL^?1, and the linear regression coefficient equals 0.9923. The detection limit for yRNA is 1.0×10^?10 g mL^?1. Interferences of L ‐cysteine, L ‐alanine, Hemoglobin, Uridine 5′‐monophosphate, Guanosine 5′‐monophosphate, Adenosine 5′‐triphosphate and some metal ions (Co³⁺, Cr³⁺, Ni²⁺, Hg²⁺, Zn²⁺, etc) are negligible. The methods adopted here are more sensitive and selective than currently applied techniques and overcome the drawback of absorption spectroscopy arising from a strong interference due to other UV‐absorbing substances.     

Square‐Wave Voltammetry of Quasi‐Reversible CE Reactions at Spherical Microelectrodes

A mathematical model for the CE mechanism in which the chemical together with the electrochemical reactions are quasi‐reversible at the surface of spherical macro and micro‐electrodes is presented for the case of square‐wave voltammetry. The analysis of voltammometric responses considers the influence of rate and equilibrium constants, together with the electrode radius, and their dependence on the square‐wave frequency (f). Both kinetics and the sphericity effect act synergistically on the electrochemical response. Also, the apparent electrode sphericity and the reversibility of the chemical as well as the electrochemical reactions are jointly affected by the variation of f. Disregarding the sphericity contribution in the calculation of kinetic parameters at a microelectrode may introduce errors even higher than one order of magnitude. The model allows the analysis of a more realistic and complex electrochemical system that requires not only the dependence of experimental responses on f, but also their fit with theoretical voltammograms, in order to provide some useful mechanistic information. Finally, concentration profiles are also studied to realize how the chemical contribution is buffering the absences of oxidized species at the electrode surface, and how those profiles are modified for the case of spherical macro and micro‐electrodes.     

Oxidation of Protopine at a Pyrolytic Graphite Electrode Using Cyclic and Square‐Wave Voltammetry

This paper describes oxidation of the isoquinoline alkaloid, protopine (PR) at a pyrolytic graphite electrode (PGE) using cyclic and square‐wave voltammetry. In the alkaline range (pH 7.5–10.5) of a Britton–Robinson (B–R) buffer, a PR oxidation can be observed as a well‐developed voltammetric peak around +0.9 V (vs. Ag|AgCl|3 M KCl). With increasing pH of the B–R buffer, the PR peak is shifted to less positive potentials. The acquired voltammetric data suggest that PR strongly adsorbs onto the surface of the pyrolytic graphite where it is subjected to irreversible electrochemical oxidation in its uncharged free (tricyclic) base form. The results are discussed in connection with the electrochemical oxidation of other isoquinoline alkaloids and the potential applications of these data.     

Determination of Melatonin Hormone in Bulk Form,Tablets and Human Serum by Square‐Wave Cathodic Adsorptive Stripping Voltammetry

Melatonin hormone plays an important role in many distinct physiological functions. A fully validated, sensitive and reproducible square‐wave cathodic adsorptive stripping voltammetric procedure was described for determination of melatonin in bulk form, tablets and human serum. The procedure was based on the reduction of the adsorptive hormone onto a hanging mercury drop electrode. Reduction behavior of melatonin was studied in both Britton‐Robinson (pH 2–11) and acetate (4.5–5.5) buffers. Acetate buffer of pH 5.0 was found reasonable as a supporting electrolyte for assay of the drug. The square‐wave cathodic adsorptive stripping voltammogram of melatonin showed a single well‐defined peak at ?1.45 V (vs. Ag/AgCl/KCl_s) using an accumulation potential of ?0.65 V. This peak may be attributed to the reduction of C?O double bond of the amide functional group of the reactant molecule. A mean recovery for 1×10^?8 M melatonin in bulk form followed 30 s accumulation of 98.87%±0.78 and a detection limit of 3.13×10^?10 M were achieved. The proposed procedure was successfully applied for the determination of the drug in tablets and human serum with mean recoveries of 97.68%±0.57 and 101.67%±0.85, respectively. A detection limit of 8.80×10^?10 M was achieved for the determination of the drug in human serum. Results of the proposed method were comparable and coincided with those obtained by reported method. Vitamin B₆ and common excipients, which are co‐formulated with melatonin, did not interfere. Also the effect of some interfering compounds such as serotonin, tryptophan and 5‐hydroxytryptophan on the determination of melatonin in human serum was studied, and all have no significant effect on the assay recovery.     

Square‐Wave Voltammetric Detection of Dopamine at a Copper‐(3‐Mercaptopropyl) Trimethoxy Silane Complex Modified Electrode

Square‐wave voltammetric detection of dopamine was studied at a copper (Cu)‐(3‐mercaptopropyl) trimethoxy silane (MPS)‐complex modified electrode (Cu‐MPS). The modification of the electrode was based on the attachment of MPS onto an electrochemically activated glassy carbon electrode (GCE) by the interaction between methoxy silane groups of MPS and surface hydroxyl groups and followed by the complexation of copper with the thiol groups of MPS. The surface of the modified electrode was further coated by a thin layer of Nafion film. The surface of the Nafion coated MPS‐Cu complex modified electrode (Nafion/Cu‐MPS) was characterized using cyclic voltammetry, electrochemical quartz crystal microbalance (EQCM), scanning electron microscope (SEM), X‐ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FT‐IR) spectrometry. The modified electrode exhibited an excellent electrocatalytic activity towards the oxidation of dopamine, which was oxidized at a reduced potential of +0.35 V (vs. Ag/AgCl) at a wider pH range. Various experimental parameters, such as the amount of copper, the pH, and the temperature were optimized. A linear calibration plot was obtained in the concentration range between 8.0×10^?8 M and 5.0×10^?6 M and the detection limit was determined to be 5.0×10^?8 M. The other common biological compounds including ascorbic acid did not interfere and the modified electrode showed an excellent specificity to the detection of dopamine. The Nafion/Cu‐MPS modified electrode can be used for about 2 months without any significant loss in sensitivity.     

Determination of Azidothymidine – an Antiproliferative and Virostatic Drug by Square‐Wave Voltammetry

The 3′‐azido‐3′‐deoxythymidine (AZT, Zidovudine) is an antiproliferative and virostatic drug widely used in human immunodeficiency virus type 1 (HIV‐1) infection treatment. With respect to side effects of high doses and a short half‐life of AZT, a fast and simple detection method for this agent could be helpful. The aim of our study was to determine AZT levels in natural samples (urine, serum, whole blood, and cell cultures, such as the HaCaT line of keratinocytes) without their mineralization and/or purification, by means of electrochemical methods using hanging mercury drop electrode (HMDE). On this electrode, AZT undergoes irreversible reduction at the peak potential near E_p?1.1 V (vs. Ag/AgCl/3 M KCl). Reduction AZT signals were measured by cyclic voltammetry (CV), differential pulse voltammetry (DPV), square‐wave voltammetry (SWV), and constant current chronopotentiometric stripping analysis (CPSA). In phosphate buffer (pH 8) the SWV yielded the best AZT signal with the detection limit of 1 nM. The determination of AZT concentration in biological materials is affected by electroactive components, such as proteins and DNA. For monitoring the influence of these compounds, AZT reduction was performed in the presence of 10 μg/mL calf thymus ssDNA and/or 100 μg/mL bovine serum albumin. In these cases, the detection limit increased to 0.25 μM. Also studied was the AZT concentration in keratinocyte cells (HaCaT line) during cell cultivation. It has been shown that the SWV may be considered as a useful tool for the determination of AZT concentration in cell cultures, and for monitoring AZT pharmacokinetics.     

Electroanalytical Determination of N‐Nitrosamines in Aqueous Solution Using a Boron‐Doped Diamond Electrode

The electrochemical methods cyclic and square‐wave voltammetry were applied to develop an electroanalytical procedure for the determination of N‐nitrosamines (N‐nitrosopyrrolidine, N‐nitrosopiperidine and N‐nitrosodiethylamine) in aqueous solutions. Cyclic voltammetry was used to evaluate the electrochemical behaviors of N‐nitrosamines on boron‐doped diamond electrodes. It was observed an irreversible electrooxidation peak located in approximately 1.8 V (vs. Ag/AgCl) for both N‐nitrosamines. The optimal electrochemical response was obtained using the following square‐wave voltammetry parameters: f=250 Hz, E_sw=50 mV and E_s=2 mV using a Britton–Robinson buffer solution as electrolyte (pH 2). The detection and quantification limits determined for total N‐nitrosamines were 6.0×10^?8 and 2.0×10^?7 mol L^?1, respectively.