Electrochemical Redox Behavior of Omeprazole Using a Glassy Carbon Electrode

The electrochemical redox behavior of omeprazole (OMZ), a gastric acid pump inhibitor, was investigated at a glassy carbon electrode using cyclic, differential pulse and square‐wave voltammetry over a wide pH range. The pH‐dependent oxidation occurs in two irreversible consecutive charge transfer reactions. Adsorption of the nonelectroactive product was also observed. The first oxidation involves removal of one electron, followed by deprotonation and leads to the formation of a hydroxylated species. The second oxidation process is related to the hydroxyl and amino groups in the benzimidazole moiety. The reduction is irreversible, also pH‐dependent, and occurs in a single step at the sulfoxide group in a diffusion‐controlled mechanism. The diffusion coefficient of omeprazole was calculated to be D_OMZ=2.31×10^?6 cm² s^?1.     

Electrooxidation of Nitrite Mediated by Cu‐x‐Tetraaminophenylporphyrin (x=2, 3, and 4) Glassy Carbon‐Modified Electrodes: Effect of Substituent Position

A study of glassy carbon electrodes modified with poly‐Cu‐x‐tetraaminophenylporphyrin (TAPP) (x=2, 3, and 4) toward the electrooxidation of nitrite is reported. The position of the amino group influences the degree of electropolymerization and the response toward the electrooxidation of nitrite. The influence of pH on the electrocatalytic oxidation of nitrite was also studied, finding that the activity is strongly pH‐dependent. For each system (polymeric porphyrins, poly‐Cu‐2‐TAPP, poly‐Cu‐3‐TAPP, and poly‐Cu‐4‐TAPP), the behavior of the modified electrode as amperometric nitrite sensor was studied at the best pH response, finding a linear correlation over a wide range of concentrations for poly‐Cu‐3‐TAPP. With poly‐Cu‐2‐TAPP, the range is smaller, and there is no linear relationship for poly‐Cu‐4‐TAPP. The plot of I_p vs. the square root of the scan rate shows a linear relationship for all the polymer systems, indicating that the electrooxidation of nitrite in the modified electrodes is a process controlled by diffusion. The Tafel slope was calculated, indicating that the determining step in the electrooxidation of nitrite varies depending on the position of the substituent, showing a dramatic change in the nature of the active sites.     

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.     

Nanomolar Determination of Methyldopa in the Presence of Large Amounts of Hydrochlorothiazide Using a Carbon Paste Electrode Modified with Graphene Oxide Nanosheets and 3‐(4′‐Amino‐3′‐hydroxy‐biphenyl‐4‐yl)‐acrylic Acid

A novel electrochemical sensor for sensitive detection of methyldopa at physiological pH was developed by the bulk modification of carbon paste electrode (CPE) with graphene oxide nanosheets and 3‐(4′‐amino‐3′‐hydroxy‐biphenyl‐4‐yl)‐acrylic acid (3,′AA). Applying square wave voltammetry (SWV), in phosphate buffer solution (PBS) of pH 7.0, the oxidation current increased linearly with two concentration intervals of methyldopa, one is 1.0×10^?8–1.0×10^?6 M and the other is 1.0×10^?6–4.5×10^?5 M. The detection limit (3σ) obtained by SWV was 9.0 nM. The modified electrode was successfully applied for simultaneous determination of methyldopa and hydrochlorothiazide. Finally, the proposed method was applied to the determination of methyldopa and hydrochlorothiazide in some real samples.     

Effect of 3‐O‐Galloyl Substitution on the Electrochemical Oxidation of Quercetin and Silybin Galloyl Esters at Glassy Carbon Electrode

The galloyl substitution effect on the antioxidant potential of quercetin‐3‐O‐gallate (QG) and silybin‐3‐O‐gallate (SBG), and the oxidation of QG and SBG were studied by cyclic, differential and square‐wave voltammetry using a glassy carbon electrode, and compared with their structural components, quercetin (Q), silybin (SB), gallic acid and gallic acid methyl ester. Their multi‐step pH‐dependent anodic behaviour, first oxidation followed by oxidation of the hydroxyl groups at ring A, is similar to Q and SB. The galloyl substitution significantly improved the antioxidant potential of SB compared to Q, and brought useful knowledge about the antioxidant activity of Q and SB monogalloyl esters.     

Oxidation Mechanism of Fluorescein at Glassy Carbon Electrode

This study investigates redox properties of fluorescein (FLSC), a fluorescent tracer with many applications in several areas, markedly in biochemical research and health care diagnosis, on glassy carbon electrode (GCE) at a wide interval of pH by using voltammetric techniques. Three peaks were observed at different potentials. The investigation revealed that FLSC is irreversibly electroxidized under a diffusion‐controled and pH–dependent process. The oxidation process in acid and physiological media occurs in two consecutive steps with formation of a main electroactive oxidation product in acid medium. Both oxidation steps involve the transfer of one electron and one proton, corresponding to the oxidation of phenolic groups with formation of ortho‐quinone derivatives, which are reversibly reduced to form catechol derivatives, and/or polymeric products. One electron and one proton are removed from the phenolic group at the position C6’ at the first step and at position C3’ at the second step. The diffusion coefficient of FLSC was assessed in pH=7.0 phosphate buffer (9.77×10⁻⁵ cm² s⁻¹). A differential pulse voltammetric method for determination of FLSC in physiological medium was also proposed.     

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.     

Adsorptive Stripping Voltammetric Determination of Trace Level Ricin in Castor Seeds Using a Boron‐doped Diamond Electrode

Ricin, (Ricinus communis agglutinin, RCA) is one of the most poisonous of naturally occurring substances and has great potential for bioterrorism because no antidote exists. Fast detection at low concentrations is a challenge, and vital to the development of proper countermeasures. In this study, a square wave adsorptive stripping voltammetric (SWAdSV) method for determining RCA using a cathodically polarized boron‐doped diamond (BDD) electrode is presented. An irreversible electrochemical RCA oxidation peak was identified on the BDD electrode by different voltammetric techniques using both direct and adsorptive stripping modes. An adsorption‐controlled (slope log Ip vs log v of 0.80) pH‐dependent process was observed. For values of 1.0≤pH≤9.0, the numbers of protons and electrons associated with the oxidation reaction were estimated (ca. 1.0) by differential pulse voltammetry. The RCA oxidation step may correspond to the oxidation of tryptophan amino acid residues, and occurs in a complex mechanism. The excellent analytical performance of the cathodically polarized BDD electrode in combination with the stripping mode ramp was verified with RCA by using a short deposition time in an open circuit potential (120 s). Under optimized analysis conditions, a linear response in the range of (3.3–94.0)×10⁻⁹ mol L⁻¹ (r²=0.9944) and a limit of detection of 6.2×10⁻¹⁰ mol L⁻¹ were estimated. This LOD is lower than several methods found in the literature. For example, it is 168 times lower than that obtained by using square wave voltammetric with a glassy carbon electrode. Moreover, an even lower LOD might be achieved by using the SWAdSV method with a higher pre‐concentration time. In addition, trace levels of RCA were successfully determined in different castor seed cultivars with an overall average recovery from 99.2±1.6 % for the three different RCA‐A concentration levels. The high accuracy of the analytical data highlights the use of the proposed method for determining RCA in other samples.     

Voltammetry of Benzo[a]pyrene in Aqueous and Nonaqueous Media: Adsorptive Stripping Voltammetric Determination at Pencil Graphite Electrode

A detailed study of the electrochemical oxidation of Benzo[a]pyrene (BaP) at the glassy carbon and pencil graphite electrodes was carried out in aqueous and nonaqueous media. Using square‐wave stripping mode, the compound yielded a well‐defined voltammetric response at pencil graphite electrode in acetate buffer, pH 4.8 at +1.13 V (vs. Ag/AgCl) (a preconcentration step being carried out at a fixed potential of +0.70 V for 180 s). The process could be used to determine BaP concentrations in the range 0.25–1.25 μM, with a detection limit of 0.027 μM (6.82 μg L^?1). The applicability to assay of spiked human urine samples was also illustrated.     

Electroanalytical Oxidation of p‐Coumaric Acid

Abstract

The mechanism of the electrochemical oxidation of p‐coumaric acid on a glassy carbon electrode was investigated using cyclic, differential pulse, and square wave voltammetry at different pHs. The oxidation of p‐coumaric acid is irreversible over the whole pH range. After successive scans, the p‐coumaric acid oxidation product deposits on the electrode surface, forming a polymeric film that undergoes reversible oxidation at a lower potential than p‐coumaric acid. This polymeric film increases in thickness with the number of scans, covering the electrode surface, and impeding the diffusion of the p‐coumaric acid and its oxidation on the electrode. The oxidation of p‐coumaric acid is pH dependent up until values close to the pK_a. For pHs higher than pK_a, the p‐coumaric acid oxidation process is pH independent. An electroanalytical determination procedure of p‐coumaric in pH 8.7 0.2 M ammonium buffer was developed, and a detection limit, LOD=83 nM, and the limit of quantification, LOQ=250 nM, were obtained.     

Electrochemical Determination of Vitamin D3 in Pharmaceutical Products by Using Boron Doped Diamond Electrode

A sensitive square‐wave voltammetry method was developed to determine cholecalciferol (vitamin D₃) in pharmaceutical products at boron‐doped diamond electrode as a working electrode. Vitamin D₃ provided a well‐defined voltammetric peak at around +1.00 V (vs. Ag/AgCl, 3.5 mol dm^?3) in 0.02 mol dm^?3 Britton‐Robinson buffer pH 5.0 prepared in 50 % ethanol. The influence of various factors such as type and pH of the supporting electrolyte, scan rate and square‐wave parameters were studied and optimized. Under optimum conditions, the oxidation peak current increased linearly with the concentration of vitamin D₃ over the range of 2 to 200 μmol dm^?3. The calculated limit of detection and limit of quantitation were 0.17 μmol dm^?3 and 0.51 μmol dm^?3, respectively. The boron‐doped diamond electrode exhibited specific recognition capability for cholecalciferol amongst possible interferences, and the determination of vitamin D₃ was possible in samples such as commercial pharmaceutical products without complicated sample pretreatments.     

Electroanalytical Determination of Carbaryl in Natural Waters on Boron Doped Diamond Electrode

The anodic voltammetric behavior of carbaryl on a boron‐doped diamond electrode in aqueous solution is reported. The results, obtained by square‐wave voltammetry at 0.1 mol L^?1 Na₂SO₄ and pH 6.0, allow the development of a method to determine carbaryl, without any previous step of extraction, clean‐up, preconcentration or derivatization, in the range 2.5–30.0×10^?6 mol L^?1, with a detection limit of 8.2±0.2 μg L^?1 in pure water. The analytical sensitivity of this electrochemical method diminished slightly, from 3.07 mA mmol^?1 L to 2.90 mA mmol^?1L, when the electrolyte was prepared with water samples collected from two polluted points in an urban creek. In these conditions, the recovery efficiencies obtained were around 104%. The effect of other pesticides (fenthion and 4‐nitrophenol) was evaluated and found to exert a negligible influence on carbaryl determination. The square‐wave voltammetric data obtained for carbaryl were typical of an irreversible electrode process with mass transport control. The combination of square‐wave voltammetry and diamond electrodes is an interesting and desirable alternative for analytical determinations.     

Renewable Silver‐Amalgam Film Electrode for Rapid Square‐Wave Voltammetric Determination of Thiamethoxam Insecticide in Selected Samples

The paper presents the use of a renewable silver‐amalgam film electrode (Hg(Ag)FE) for the determination of the insecticide thiamethoxam (TMO) in Britton‐Robinson buffer pH 7.0 (LOD=0.25 µg mL^?1, LOQ=0.70 µg mL^?1) by direct cathodic square‐wave voltammetry (SWV). The voltammetric response for TMO obtained at this electrode was the same as that obtained with a hanging mercury drop electrode, represented by two distinct reduction peaks. Since the electron transfer processes are coupled with chemical reactions involving protons, the SWV signals strongly depend on the pH of the supporting electrolyte. The developed Hg(Ag)FE‐SWV method was tested for the determination of TMO in spiked honey and river water samples, as well as for the determination of its content in the commercial formulation Actara 25 WG.     

On the Electrochemical Oxidation of Resveratrol

Resveratrol (3,5,4′‐trihydroxystilbene) is an organic metabolite produced by plants in response to fungal infection. It is found in various plant fruits and is abundant in the skins of unripe grapes and related products. This photosensitive molecule exists in two isomeric forms, trans and cis‐resveratrol. The antioxidant activity of resveratrol, both trans and cis forms, was evaluated by means of cyclic, differential pulse and square‐wave voltammetry over a wide pH range, using a glassy carbon electrode. Voltammograms of resveratrol presented two oxidation peaks; the first oxidation peak corresponds to the oxidation of the phenol group and was shown to undergo an irreversible oxidation reaction. The second oxidation peak corresponds to the oxidation of the resorcinol moiety and is also irreversible. The influence of pH on the electrochemical oxidation process of resveratrol was investigated.     

Electrochemical Oxidation of Sulfasalazine at a Glassy Carbon Electrode

Sulfasalazine (SSZ) is a pharmaceutical compound used for the treatment of rheumatoid arthritis. The electrochemical oxidation of SSZ at a glassy carbon electrode was studied by cyclic, differential pulse and square wave voltammetry in a wide pH range. For electrolytes with pH<11.0, the oxidation is an irreversible, diffusion‐control, pH‐dependent process that involves the transfer of one electron and one proton from the hydroxyl group of the salicylic moiety. For pH>11.0 the oxidation is pH‐independent, and a pK_a≈11 was determined. The formation of a quinone‐like oxidation product that undergoes two electrons and two protons reversible redox reaction was observed. Also, UV‐vis spectra of SSZ were recorded as a function of supporting electrolytes pH. An electrochemical oxidation mechanism was proposed.     

Effect of Basic Amino Acids on Nickel Ion Reduction at a Mercury Electrode

In a 0.02 M borax solution (pH 8.5), basic amino acids (arginine, lysine, and ornithine) react with Ni²⁺ to form a mono‐ligand complex that is reduced at a mercury electrode at about ?0.85 V vs. Ag|AgCl|KCl (3 M). At a long time scale (staircase voltammetry; scan rate<50 mV s^?1), the complex reduction is a catalytic (EC′) process, the rate‐determining step being the regeneration of the reducible species by the reaction of the amino acid with free Ni²⁺. At a short time scale (differential pulse voltammetry or higher scan rate staircase voltammetry), the reaction rate is controlled by the diffusion of the complex. Although the same kind of complexation occurs with either basic amino acids or glycine, the last one does not induce a similar process. The peculiar effect of basic amino acids is due to the side chain that causes the ligand molecule to adopt a favorable orientation at the electrode surface. The differential pulse voltammetry peak current is proportional to the total amino acid concentration over the concentration range from 2 to 100 μM. Hence a voltammetric method for arginine determination in nutritional supplements was developed and validated using HPLC as reference method.     

A Comparison of Glucose‐ and Glucosamine‐Related Inhibitors: Probing the Interaction of the 2‐Hydroxy Group with Retaining β‐Glucosidases

The inhibition of the β‐glucosidases from sweet almonds and Caldocellum saccharolyticum at varying pH values by the glucosamine‐related inhibitors 1 – 7 has been compared to the inhibition by the known glucose analogues 8 – 14 . The amino derivatives 3 , 4 , 6 , and 7 were prepared in one step from the known 15 – 18 (Scheme 1), and the amino‐1,2,3‐triazole 5 by a variant of the synthesis leading to the glucose analogue 12 (Scheme 2). The key step for the preparation of the aminoimidazole 1 and of the amino‐1,2,4‐triazole 2 is the regioselective cleavage of the benzyloxy group at C(2) of the gluconolactam 35 and the mannonolactam 57 , respectively, by BCl₃ and Bu₄NBr (Schemes 3 and 4, resp.). The pH optimum for the inhibition by the amines is lower than their pK_HA values, evidencing that they are bound as ammonium salts and that H‐bonding between C(2)−NH and the cat. base B⁻ contributes more strongly to binding than any possible H‐bond to the NH₂−C(2) group. The influence of the ammonium group on the inhibitory strength correlates with the basicity of the `glycosidic heteroatom'. The strongest increase of the inhibitory strength is observed for the amines lacking a `glycosidic heteroatom' (ΔΔG(OH→NH)=−1.5 to −2.9 kcal/mol). The increase is less pronounced for the amino derivatives 3 – 4 , which possess a weakly basic `glycosidic heteroatom' (ΔΔG(OH→NH)=−0.6 to −1.1 kcal/mol); the amino compounds 1 and 2 , which possess a strongly basic `glycosidic heteroatom', are weaker inhibitors than the corresponding hydroxy compounds, as expressed by ΔΔG(OH→NH) between +4.3 and +4.7 kcal/mol for the amino‐imidazole 1 , and between +2.3 and 2.8 kcal/mol for the amino‐1,2,4‐triazole 2 , denoting the dominant detrimental influence of a C(2)−NH group on the H‐bond acceptor properties of a sufficiently basic `glycosidic heteroatom'.     

Electrochemical Oxidation of Quercetin

The mechanism of electrochemical oxidation of quercetin on a glassy carbon electrode has been studied using cyclic, differential pulse and square‐wave voltammetry at different pH. It proceeds in a cascade mechanism, related with the two catechol hydroxyl groups and the other three hydroxyl groups which all present electroactivity, and the oxidation is pH dependent. Quercetin also adsorbs strongly on the electrode surface; and the final oxidation product is not electroactive and blocks the electrode surface. The oxidation of the catechol 3′,4′‐dihydroxyl electron‐donating groups, occurs first, at very low positive potentials, and is a two electron two proton reversible reaction. The hydroxyl group oxidized next was shown to undergo an irreversible oxidation reaction, and this hydroxyl group can form a intermolecular hydrogen bond with the neighboring oxygen. The other two hydroxyl groups also have an electron donating effect and their oxidation is reversible.     

Electrocatalytic Determination of 6‐Tioguanine at a p‐Aminophenol Modified Carbon Paste Electrode

A p‐aminophenol modified carbon paste electrode (p‐APMCPE) was constructed for determination of an anticancer drug 6‐thioguanine (6‐TG). The cyclic voltammogram showed that the electrocatalytic oxidation of 6‐TG at the surface of p‐APMCPE occurs at a potential about 840 mV less positive than at an unmodified electrode. Square‐wave voltammetry results presented that the electrocatalytic oxidation peak currents of 6‐TG in pH 9.0 had two linear dynamic ranges in the range of 0.2 to 8.0 and 8.0 to 350.0 μM 6‐TG with a detection limit of 0.08 μM. The kinetic parameters such as electron transfer coefficient (α) and rate constant were determined for the chemical reaction between 6‐TG and p‐aminophenol. Finally, this method was evaluated for the determination of 6‐TG in 6‐thioguanine tablets and urine samples.