Electrochemical Characterization and Quantification of the Strong Antioxidant and Antitumor Agent Pomiferin

The electrochemical behavior of pomiferin, a natural isoflavone with significant antioxidant, antidiabetic and antitumor properties, is reported here for the first time at a glassy carbon electrode (GCE). In order to understand the redox processes of this compound, its response was compared with the nonantioxidant isoflavone osajin. Based on cyclic and square‐wave voltammetric methods it was observed that pomiferin presents a quasireversible anodic peak, which was attributed to the oxidation of the catechol group, and that is strongly influenced by pH. This anodic process yields a well‐defined DPV response, which can be used for the analytical determination of this potential pharmotherapeutic isoflavone.     

Simultaneous Voltammetry Determination of Dihydroxybenzene Isomers by Nanogold Modified Electrode

A convenient electrochemical deposition method to prepare nanogold/glassy carbon modified electrode (nano‐Au/GCE) is adopted. In 0.1 mol/L HAc‐NaAc buffer solution (pH 4.61), the nano‐Au/GCE shows an excellent electrocatalytical behavior for the redox of dihydroxybenzene. A simple, rapid and highly selective voltammetry for simultaneous determination of dihydroxybenzene isomers, hydroquinone, catechol, and resorcinol, is developed using the nano‐Au/GCE. This method has been applied to the direct determination of the three dihydroxybenzene isomers in artificial wastewater.     

Anodic Behaviour of Flavonoids Orientin,Eriodictyol and Robinin at a Glassy Carbon Electrode

Orientin, eriodictyol and robinin are polyphenolic compounds, and their oxidation mechanism is pH‐dependent, in two steps, involving a different number of electrons and protons. Orientin and eriodictyol first oxidation occurs at a lower potential, corresponding to the reversible oxidation of the catechol group, and is followed by an irreversible oxidation on the ring‐A at more positive potential. Robenin oxidation is irreversible, with the formation of electroactive products, and occurs at ring‐A and ring‐B. The electrochemical characterization of their redox behaviour brought useful data about their chemical stability, antioxidant and pro‐oxidant activity, enabling a comprehensive understanding of their redox mechanism.     

Simultaneous Determination of Hydroquinone and Catechol at an Activated Glassy Carbon Electrode

A selective and very simple electrochemical method, based on anodization of a glassy carbon electrode (GCE), was developed for the simultaneous detection of hydroquinone (HQ) and catechol (CT). It was found that the activated GCE showed an excellent catalytic behavior and enhanced reversibility towards the oxidation of both HQ and CT. The redox responses from the mixture of HQ and CT were easily resolved at an activated GCE. The detection limits for HQ and CT were calculated to be 0.16 and 0.11 μM, respectively. The activated GCE was successfully examined for real sample analysis with tap water and it showed a stable and reliable recovery data.     

Electrochemical behaviour of rosmanol 9-ethyl ether,a diterpene lactone antioxidant isolated from sage

The electrochemical behaviour and mechanism of the redox process of the natural antioxidant rosmanol 9-ethyl ether, isolated fromSaliva officinalis L., were studied. The cyclic voltammograms of rosmanol 9-ethyl ether (R9EE), at characteristic pH values, and the electrochemical parameters for all investigated pH values were measured. Three characteristic pH regions, each with different behaviour of R9EE, were identified. In regions of pH < 4 and pH > 5 only one anodic peak appeared, whereas in the solutions of pH 4–5 two anodic peaks could be noted. The overall oxidation mechanism at pH < 4 is an e.H.e.H. oxidation mechanism, which as a final product gives a quinonic molecule. The influence of pH on the second oxidation peak potential tends towards zero in accordance with the preceding dissociation of the one of phenolic groups, thus suggesting an e.e.H. mechanism at pH > 5. This means that at the pH values expected in plant cells, R9EE has an unexpected structure, making this substance a potent antioxidant. Electrochemical and spectrophotometric measurements enabled us to establish an extremely low pK _a value (4.35) for R9EE.     

Nucleophilic Addition of Thiaproline to Electrochemically Derived o‐Quinone,Application to the Sensitive Voltammetric Detection of Thiaproline

The mechanism of electrochemical behavior of catechol in the presence of thiaproline is investigated by cyclic voltammetry, controlled‐potential coulometry and spectrophotometric tracing of the reaction coordinate. The results indicate that thiaproline participate in with an ECEC mechanism in a nucleophilic (Michael) addition to o‐quinone. Effect of pH of buffer solution on reaction pathway is studied and showed that addition of thiaproline to the o‐quinone is performed only in solutions with pHs higher than 5. These results indicate that the addition of thiaproline is occurred first from amine functional group. In the second step, the addition of carboxylate group of thiaproline to C‐5 of catechol results the final product with a lactone ring in its structure. Observation of two isosbestic point in absorption spectrum during the progress of the electrolysis together with the FT‐IR results for final product can be presented as evidence for two step addition of thiaproline to catechol. Final product, due to the electron donor property of thiaproline, more easily oxidized respect to the former catechol and as a result, a new redox couple is obtained for this compound in lower potentials. The easier anodic oxidation of addition product (relative to catechol) caused to an increase in anodic current for catechol, which is proportional to the thiaproline concentration. The differential pulse voltammetry (DPV) is applied as a sensitive voltammetric method for the detection of thiaproline. A linear range from 5×10^?8 to 5×10^?6 M with a detection limit of 1×10^?8 M is resulted for thiaproline. With respect to the reversibility of the electrochemical reactions in the mechanism, and also more facile oxidation of the addition products, the square‐wave voltammetry is presented as a method with considerably more sensitivity for the detection of sub‐micromolar amounts of thiaproline. The advantageous properties of the voltammetric method for thiaproline detection lie in its excellent catalytic activity, sensitivity and simplicity.     

Electrochemical Study of Catechol in the Presence of Dibuthylamine and Diethylamine in Aqueous Media: Part 1. Electrochemical Investigation

Electrochemical oxidation of catechol has been studied in the presence of secondary amines as nucleophiles in aqueous solution with various pH values using cyclic voltammetry and differential pulse voltammetry. Cyclic voltammetry of catechol in pure buffered solution (2.00 pH<9.00) shows one anodic and corresponding cathodic peak which relates to the transformation of catechol to corresponding o‐benzoquinone and vice versa within a quasi‐reversible two electron transfer process. Also, a little amount of o‐benzoquinone undergoes polymerization reaction. Cyclic voltammogram of catechol in the presence of nucleophilic amines, show one anodic peak in the first scan of potential but on the reverse scan the corresponding cathodic peak disappear and new peak is observed at less positive potential. In the second scan of potential also a new anodic peak is observed. On the other hand at high concentration of amines the redox peak attributable to formed polymer disappear showing that in this condition the polymerization reaction occurs at non‐measurable extent. On the basis of these observations we propose an ECE mechanism for the electrochemical oxidation of catechol in the presence of secondary amines.     

Electrochemical Reaction Mechanism of Phenacetin at a Carboxylated Multiwall Carbon Nanotube Modified Electrode and Its Analytical Applications

A novel type of carboxylated multiwalled carbon nanotube modified electrode（c-MWCNTs/GCE） was constructed and the electrochemical properties of phenacetin（PHE） at it were studied. In a buffer solution of 0.1 mol/L HAc-NaAc（pH=5.3）, PHE exhibited a couple of quasi-reversible redox peaks and an anodic peak in the poten- tial range of 0.2--1.2 V at c-MWCNTs/GCE. The peak current was proportional to the concentration of PHE in the range of 4.0× 10^-6_ 1.0 × 10^-4 mol/L with a detection limit of 1.0× 10^-6 mol/L（S/N=3）. The c-MWCNTs/GCE showed excellent repeatability and stability and the electrochemical reaction mechanism of PHE was proposed. This method was used to determine the content of PHE in medical tablets and the recovery was determined to be 96.5%--104.2% by means of a standard addition method.     

The Chemistry behind Catechol‐Based Adhesion

The adhesion of some marine organisms to almost any kind of surface in wet conditions has aroused increasing interest in recent decades. Numerous fundamental studies have been performed to understand the scientific basis of this behaviour, with catechols having been found to play a key role. Several novel bio‐inspired adhesives and coatings with value‐added performances have been developed by taking advantage of the knowledge gained from these studies. To date there has been no detailed overview focusing exclusively on the complex mode of action of these materials. The aim of this Review is to present recent investigations that elucidate the origin of the strong and versatile adsorption capacities of the catechol moiety and the effects of extrinsic factors that play important roles in the overall adhesion process, such as pH value, solvent, and the presence of metal ions. The aim is to detail the chemistry behind the astonishing properties of natural and synthetic catechol‐based adhesive materials.     

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.     

A sensor based on the carbon nanotubes-ionic liquid composite for simultaneous determination of hydroquinone and catechol

MWNTs-IL-Gel/GCE, a glassy carbon electrode modified with multiwalled carbon nanotubes (MWNTs) and ionic liquids (IL), was developed to serve as a sensor for simultaneous determination of Hydroquinone (HQ) and catechol (CC) in this paper. The modified GCE showed two well-defined redox waves for HQ and CC in both CV and DPV with a peak potential separation of ca. 0.1 V, which was large enough for simultaneous detection. The results revealed that the oxidation of HQ and CC with the enhancement of the redox peak current and the decrease of the peak-to-peak separation exhibit excellent electrocatalytic behaviors. A high sensitivity of 1.8×10(-7)M with detection limits of 6.7×10(-8)M and 6.0×10(-8)M (S/N=3) for HQ and CC were obtained. Moreover, the constants of apparent electron transfer rate of HQ and CC at MWNTs-IL-Gel/GCE were calculated as 7.402 s(-1) and 8.179 s(-1), respectively, and the adsorption quantity of HQ and CC was 1.408×10(-6) mol cm(-2) with chronocoulometry. The developed sensor can be applied to determinate directly of HQ and CC in aqueous solution.     

Voltammetric monitoring of laccase-catalysed mediated reactions

Six different compounds capable of mediating laccase-catalysed reactions have been tested by cyclic voltammetry. They exhibited quasi-reversible electrodic behaviour with formal redox potentials ranging from 150 to 800 mV (E(0)' vs. SCE). The immersion of a laccase-coated glassy carbon electrode (GCE) in mediator solutions generated large cathodic catalytic currents easily recorded by cyclic voltammetry at low-potential scan rates. This current showed two well-defined pH profiles, which correlated with the variation of the mediator redox potentials at the pH range tested. The relevant effect of temperature on the activity of laccase has been assessed here. Likewise, it was shown that the current record varied with the substrate concentration. This trend fitted Michaelis-Menten kinetics, which allowed us to give an estimation of the affinity of the fungal laccase for the different mediators.     

Electrochemical Determination and in silico Studies of Fludarabine on NH2 Functionalized Multiwalled Carbon Nanotube Modified Glassy Carbon Electrode

A sensitive voltammetric technique has been developed for the determination of Fludarabine using amine‐functionalized multi walled carbon nanotubes modified glassy carbon electrode (NH₂‐MWCNTs/GCE). Molecular dynamics simulations, an in silico technique, were employed to examine the properties including chemical differences of Fludarabine‐ functionalized MWCNT complexes. The redox behavior of Fludarabine was examined by cyclic, differential pulse and square wave voltammetry in a wide pH range. Cyclic voltammetric investigations emphasized that Fludarabine is irreversibly oxidized at the NH₂‐MWCNTs/GCE. The electrochemical behavior of Fludarabine was also studied by cyclic voltammetry to evaluate both the kinetic (k_s and E_a) and thermodynamic (ΔH, ΔG and ΔS) parameters on NH₂‐MWCNTs/GCE at several temperatures. The mixed diffusion‐adsorption controlled electrochemical oxidation of Fludarabine revealed by studies at different scan rates. The experimental parameters, such as pulse amplitude, frequency, deposition potential optimized for square‐wave voltammetry. Under optimum conditions in phosphate buffer (pH 2.0), a linear calibration curve was obtained in the range of 2×10^?7 M–4×10^?6 M solution using adsorptive stripping square wave voltammetry. The limit of detection and limit of quantification were calculated 2.9×10^?8 M and 9.68×10^?8 M, respectively. The developed method was applied to the simple and rapid determination of Fludarabine from pharmaceutical formulations.     

强碱溶液中阳极极化的玻碳电极特性

The surface properties of GCE treated by anodic polarization in strong alkaline solutions are makedly different from that of polished GCE. XPS results show thta more oxygenic groups as carbonyl and acid anhydride are formed on the electrode surface, the ratio of O/C increases, C1s and O1s peaks shift to higher binding energies. Moreover, the adsorptivity of the treated GCE decreases and shows no obvious adsorption for Fe2+ in H2SO4 solution. On the other hand, behaviour of the electrode reaction is also prominently improved. The background current is less, the electron transfer rate is greatly speeded up and the reverse of the electrode reaction for Fe3+/2+, Eu3+/2+ is effectively enhanced for treated GCE.     

Nanocellulose/Carbon Nanoparticles Nanocomposite Film Modified Electrode for Durable and Sensitive Electrochemical Determination of Metoclopramide

A novel electrochemical sensor based on nanocellulose‐carbon nanoparticles (NC‐CNPs) nanocomposite film modified glassy carbon electrode (GCE) is developed for the analysis of metoclopramide (MCP). Atomic force microscopy, scanning electron microscopy and electrochemical impedance spectroscopy were used to characterize the roughness, surface morphology and performance of the deposited modifier film on GCE. SEM image demonstrated that modifier nanoparticles are uniformly deposited on GCE, with an average size of less than 50 nm. The electrochemical behavior of MCP and its oxidation product is studied using linear sweep and cyclic voltammetry over a wide pH range on NC‐CNPs modified glassy carbon electrode. The results revealed that the oxidation of MCP is an irreversible and pH‐dependent process that proceeds in an adsorption‐controlled mechanism and results in the formation of a main oxidation product, which adsorbs on the surface of NC‐CNPs/ GCE. The modified electrode showed a distinctive anodic response towards MCP with a considerable enhancement (49 fold) compared to the bare GCE. Under the optimized conditions, the modified electrode exhibited a wide linear dynamic range of 0.06–2.00 µM with a detection limit of 6 nM for the voltammetric determination of MCP. The prepared modified electrode showed several advantages such as simple preparation method, high stability, reproducibility, and repetitive usability. The modified electrode is successfully applied for the accurate determination of trace amounts of MCP in pharmaceutical and clinical preparations.     

Electrochemical Detection of Alloxan on Reduced Graphene Oxide Modified Glassy Carbon Electrode

Alloxan is a toxic reagent that strongly induces the diabetes by destroying insulin‐producing β‐cells in the pancreas of living organisms. The reduction product of alloxan is dialuric acid, which is responsible for the intracellular generation of ROS to enhance the stress in living cells to cause kidney disease or diabetic nephropathy. Herein, we studied for the first time the electrochemical properties of alloxan on reduced graphene oxide modified glassy carbon electrode (rGO/GCE) in 0.1 M phosphate buffer solution (PBS) at pH 7. The obtained results were compared with graphene oxide modified GCE (GO/GCE) and bare GCE surfaces. The modified rGO/GCE showed well defined redox couple with 10 fold increase in both reduction as well as oxidation peak current for alloxan than that of GO/GCE and bare GCE. Differential pulse voltammetry (DPV) technique shows the linear increase in both oxidation and reduction peak current of alloxan in the range of 30 μM to 3 mM with LOD of 1.2 μM. An amperometric signal of alloxan is also increases with respect to each addition of 50 μM of alloxan on rGO/GCE at constant potential of ?0.05 V. The linear range of alloxan is observed between 50 μM to 750 μM (S/N=3). This kind of rGO/GCE surface is more suitable platform or sensor matrix for estimating unknown concentration of alloxan molecule in the real biological systems.     

Underpotential Deposition Preparation of Pt‐loading AuNPs/Reduced Graphene Oxide and Its Catalytic Detection of Catechol

Ultralow Pt‐loading Au nanoparticles have been fabricated on the surface of reduced graphene oxide (RGO) by using underpotential deposition (UPD) monolayer redox replacement process. The Pt/Au/RGO modified electrode exhibits an excellent electrocatalytic activity toward catechol and hydroquinone. Under the optimized condition, the separation of peak‐to‐peak between hydroquinone and catechol is 197 mV, which is wide enough to distinguish the isomers of benzenediol. Catechol is detected by the Pt/Au/RGO/GCE with a low detection limit in the presence of hydroquinone.     

Adsorption phenomena in the NAD+/NADH system at glassy carbon electrodes

A variety of electrochemical approaches has been used to investigate the adsorption of NAD⁺, NADH and the NAD-NAD dimer from aqueous solution at glassy carbon electrodes (GCE) with supplementary studies of adsorption at pyrolytic graphite and platinum electrodes from aqueous media and at GCE from DMSO solution. The following hypotheses are advanced concerning the adsorption orientation: at carbon electrodes, on which NADH is not adsorbed, NAD⁺ produced by anodic oxidation of the NADH is first rapidly adsorbed in a planar configuration relative to the electrode surface, which is probably bound to the surface through the adenine moiety; there is then a relatively slow reorientation of the adsorbed NADH molecules to a perpendicular orientation relative to the electrode surface, which adsorbate is more tightly bound to the surface than the planar oriented adsorbate and which likely involves interaction between parallel adenine and pyridinium rings. Reduction (one-electron process) of NAD⁺ at the GCE produces the NAD-NAD dimer, which, at a clean electrode surface, involves a diffusion-controlled process and an adsorption-controlled process; the latter is due to formation of adsorbed dimer, which is more strongly adsorbed than NAD⁺. The dimer is oxidized at the GCE only if it is adsorbed. The factors controlling and involved in the adsorption processes have been examined with particular reference to the use of anodic voltammetry for the analytical determination of NADH.     

A catechol biosensor based on a gold nanoparticles encapsulated-dendrimer

Tyrosinase has been immobilized on a Au nanoparticles encapsulated-dendrimer bonded conducting polymer on a glassy carbon electrode for the estimation of catechol. The modified electrode was characterized by cyclic voltammetry and AFM techniques. The principle of catechol estimation was based on the reduction of biocatalytically liberated quinone species at +0.2 V versus Ag/AgCl (3 M KCl), with good stability, sensitivity, and featuring a low detection limit (about 0.002 μM) and wide linear range (0.005 μM-120 μM). The electrochemical redox peak of catechol on the GCE/PolyPATT/Den(AuNPs)/tyrosinase was also investigated. A response time of 7 s, reusability up to 5 cycles and a shelf life of more than 2 months under refrigerated conditions were reported. Various parameters influencing biosensor performance have been optimized including pH, temperature, and applied potential. The utility and application of this nanobiosensor was tested in a real water samples.     

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.