Evidence of self-protonation on the electrodic reduction mechanism of an anti-Helicobacter pylori metronidazole isotere

The electrochemical reduction mechanism of 1-(2-ammoniumethyl)-2-methyl-5-nitroimidazole bromide (2) in DMSO + 0.1 mol l^?1 TBAP has been investigated by cyclic voltammetry and macroscale electrolysis, on a glassy carbon electrode, in comparison with metronidazole (1). The cyclic voltammogram of 2 is represented by three reduction waves, one of them at less negative potential, when compared to the first wave of metronidazole which indicates that it undergoes easier reduction. There is evidence for a self-protonation mechanism in the electroreduction of 2, represented by the absence of the first wave in the successive cyclic voltammogram, by the disappearance of the first reduction wave upon addition of base and increase of the same wave in the presence of exogenous proton donors. The stoichiometry of the reaction, at the first reduction wave, involves 0.8 mol electron mol^?1 and yields 0.2 mol of 4e^?/4H⁺ reduced derivative (probably an unstable hydroxylamine) and 0.8 mol of the amine derivative, the conjugated base of 2. The second and third waves are typical for nitroaromatic reduction and are related to the reduction of the nitro group in this aminoderivative.     

Reactions of the nitro radical anion of metronidazole in aqueous and mixed solvent: a cyclic voltammetric study

Cyclic voltammetry was used to investigate the electrochemical reduction of metronidazole (2-methyl-5-nitro-1H-imidazole-1-ethanol) at glassy carbon and gold electrodes at different pHs in aqueous solution as well as in mixed solvent viz., aqueous dimethyl formamide. The electrogenerated nitro radical anion undergoes a disproportionation reaction, the rate constant of which is dependent on the pH, solvent composition and electrode material. The interactions of the nitro radical anion with thymine and cytosine were also investigated using a cyclic voltammetric technique. Both the bases were found to react with metronidazole nitro radical anion. The rate constants for such reactions in aqueous solutions were 3.5 × 10³ and 3.0 × 10³ dm³ mol^?1 s^?1 for thymine and cytosine, respectively.     

Catechin antioxidant action at various pH studied by cyclic voltammetry and PM3 semi-empirical calculations

The mechanism of catechin electro-oxidation at various pH was studied using cyclic voltammetry (CV) on the glassy carbon (GC) electrode and PM3 semi-empirical calculations. The influence of activation of the surface of the GC electrode on CV results has been discussed. Mixed adsorption–diffusion control has been observed by applying mechanistic criteria of CV to the results obtained at the activated electrode. The calculated catechin diffusion coefficient D = 2.78 × 10^?6 cm² s^?1. A linear increase of the current peak has been observed with the increase of substrate concentration up to 40 μmol dm^?3 (surface coverage Γ ～ 10^?11 mol cm^?2). In the whole investigated pH range, the dE/dpH value is very close to the anticipated Nernstian dependence of ?59 mV/pH indicating that the slope is not affected by the different sequences of e^? and H⁺ transfer. Molecular modeling results show a decrease of ≈5 kcal mol^?1 in ΔHoF (between radical and parent molecule) and a decrease of ≈6 eV in IP (of the parent molecule) when the parent molecule is changed from neutral to monoanionic form of catechin showing that both processes – hydrogen and electron abstraction are facilitated by deprotonation. Electrochemical oxidation of catechin is known to proceed as a two step one-electron oxidation of the B-ring of o-phenolic groups. Upon an increase in the pH, the mechanistic pathway of catechin electro-oxidation in both oxidation steps changes from an eH to the He process. In the reaction with a free radical, this may induce the change from hydrogen to electron donation.     

Electroreduction of halogenated fumaric esters: a combined electrochemical and computational investigation

Diethyl fumarate and related halogenated derivatives have been studied by cyclic voltammetry and electrolysis, on mercury and glassy carbon electrodes, in DMF + 0.1 mol L^?1 TBAP and in acetonitrile + water (3:4) with 0.1 mol L^?1 NaCl or 0.1 mol L^?1 TEAP. For compounds with two reducible functionalities, the electron-deficient olefin and the C–X group, the grade of substitution on the halogenated carbon causes differences on the site involved in the first electron transfer thus determining the chemical reactivity. Enediester compounds with –CH₂– groups insulating the gem-trihalide groups (–CX₃, X=Br and Cl) suffer reduction, and the C–X cleavage is the reaction of choice. The reaction pathway also depends on the nature of the electrode and significant positive potentials shifts for compounds with a gem-tribromide group are evident in the cyclic voltammogram on a mercury electrode, in relation to results on a glassy carbon electrode. The formation of easily reduced organomercurials is the main reason for this intense positive shift in the first electron transfer. On a vitreous carbon electrode, factors directly related to the strength of the C–X bond play a determinant role. Concerning electrolysis, the gem-tribromo derivative furnishes H₂CCBr₂ and ethyl hydrogenfumarate, through hydrolysis produced by initial cleavage of the C–Br. For the other polyfunctional compounds (–CHX₂, –CH₂X), the olefin is reduced first to yield dimers and/or hydrogenated products depending upon the reaction conditions. Quantum chemical calculations performed on the anion radical of these compounds yield spin densities located on the –CBr₃/–CCl₃ group and on the enediester group for the –CCl₂/–CBr₂, –CH₂Br/–CH₂Cl derived compounds. Upon geometry optimisation, the C–Br bond in compounds containing –CBr₃ groups are longer, suggesting that this bond can easily be cleaved, yielding organomercurial compounds, in the case of the use of a mercury electrode or leading to bond cleavage, for inert electrodes. For the other substituents, the optimisation basically does not affect the geometry of the radical anion compared to the neutral substrate. These results have been confirmed by the calculated dissociation energy of the C–Br bond, which for the –CBr₃ group is approximately 100 kJ mol^?1 as against 240 kJ mol^?1 for the same bond in the –CH₂Br group. The coherence between the experimental and calculated data reinforces the usefulness of these computational tools in rationalising the reactivity of electrogenerated species as well as in predicting the electrochemical reaction outcome.     

Direct simultaneous determination of dihydroxybenzene isomers at C-nanotube-modified electrodes by derivative voltammetry

The voltammetric behavior of dihydroxybenzene isomers was studied with glassy carbon electrodes modified with multi-wall carbon nanotubes. In 0.1 mol L^?1 HAc + NaAc buffer solution (pH 5.5), the modified electrode showed a good electrocatalytic response towards dihydroxybenzenes. The peak currents increased significantly and their oxidation potentials shifted negatively. Through a derivative technique, the three oxidation peaks of dihydroxybenzene isomers can be separated, thus the method can be applied to direct simultaneous determination without previous separation. The linear calibration ranges were 2 × 10^?6–1 × 10^?4 mol L^?1 for hydroquinone and catechol, respectively, and 5 × 10^?6 to 8 × 10^?5 mol L^?1 for resorcinol, with detection limits of 6 × 10^?7, 6 × 10^?7 and 1 × 10^?6 mol L^?1, respectively. This method has been applied to the direct determination of dihydroxybenzene isomers in artificial wastewater, and the recovery was from 92% to 104%.     

Amperometric sensor based on ferrocene-doped silica nanoparticles as an electron transfer mediator for the determination of glucose in rat brain coupled to in vivo microdialysis

In this paper, microdialysis sampling was combined with an enzymatic assay of glucose level in rat brain. A novel amperometric glucose biosensor based on ferrocene-doped silica (FcDS) nanoparticles conjugated with a biopolymer chitosan (CHIT) membrane was developed. These uniform FcDS nanoparticles (about 15 ± 3 nm) were prepared by a water-in-oil (W/O) microemulsion method and were characterized by TEM and electrochemical technology. The nanosilica surface exhibited high biocompatability and the ferrocene doped inside maintained its high electron-transfer efficiency as a mediator. The glucose biosensor showed a detection limit of 2.0 × 10^?6 mol L^?1 with a linear range from 5.0 × 10^?6 to 1.2 × 10^?2 mol L^?1. Coupled to microdialysis, it was used to determine the glucose concentration in rat brain. The result was in satisfactory agreement with the literature.     

The electrochemistry and thin-layer luminescence spectroelectrochemistry of rhodamine 6G at a 4,4′-bipyridine-modified gold electrode

We report on the first direct electrochemistry and fluorescence spectroelectrochemistry of rhodamine 6G at a 4,4′-bipyridine-modified gold electrode. The value of n determined in spectropotentiostatic experiments at 1.87×10^?6 mol l^?1 of rhodamine 6G in 0.20 mol l^?1 KCl solution is 1.15, and the experimental value obtained for E⁰′ is ?0.787 V versus Ag ∣ AgCl ∣ KCl_sat, which agrees very well with the value (E⁰′=?0.791 V) obtained using cyclic voltammetry at a modified gold electrode. The values of the diffusion coefficients D_O and D_R for the oxidized and reduced forms of rhodamine 6G calculated from results of potential step and in situ fluorescence measurement experiments are 4.0×10^?6 cm² s^?1 and 4.2×10^?6 cm² s^?1, respectively. Cyclic voltammograms of rhodamine 6G show that the peak current I_p is proportional to the square root of the potential scan rate v^1/2, the ratio of the reduction to the oxidation peak height is about unity, and the separation of both reduction and reoxidation peak potentials ΔE_P is essentially constant at 135 mV at low scan rates. These results indicate that electrochemistry of rhodamine 6G at a 4,4′-bipyridine-modified gold electrode is a quasi-reversible one-electron electrode process.     

Carbon paste electrodes of the mixed oxide SiO2/Nb2O5 prepared by the sol–gel method: dissolved dioxygen sensor

A carbon paste of SiO₂/Nb₂O₅ material was used as the electrode in the development of a dissolved dioxygen sensor in 1.0 mol l^?1 KCl solution at pH 6.2. The material was prepared by the sol–gel method. In the investigation of its electrochemical properties, linear and cyclic voltammetric and chronoamperometric techniques were employed. Dioxygen reduction, which was diffusion controlled, occurred at ?280 mV vs. SCE by a two electron mechanism, producing peroxide. A linear response between the cathodic peak current intensity and the dissolved O₂ concentration was obtained for the region between 1.0 and 13.6 mg l^?1. The stability proved to be very good over successive voltammetric cycles. The electrode response time was about 5 s. The electron transfer reactions were explained as being to an n-type semiconductor of niobia dispersed in the silica surface.     

Novel studies on the electrochemical oxidation of 2-[4-(N,N-dimethylamino)phenyl]-6-methyl benzothiazole (DPMB) in acetonitrile at platinum electrodes

The electrochemical oxidation mechanism of 2-[4-(N,N-dimethylamino)phenyl]-6-methyl benzothiazole (DPMB) is studied in a 0.1 M N(C₄H₉)₄ClO₄ + acetonitrile (ACN) reaction medium by cyclic (CV) and square wave voltammetries (SWV) as well as by controlled potential bulk electrolysis at platinum electrodes. The primary radical cation formed by the one electron oxidation of DPMB undergoes a deprotonation process, which is the rate-determining step, followed by a radical–radical coupling. On the other hand, an initial quasi-reversible monoelectronic charge transfer mechanism is inferred from cyclic and square wave voltammograms recorded at scan rates and frequencies higher than 0.4 V s^?1 and 40 Hz, respectively. Diffusion coefficients of DPMB at different temperatures were calculated from the quasi-reversible convoluted cyclic voltammograms. DigiSim® and COOL software were used to fit the quasi-reversible cyclic and square wave voltammetric responses, respectively. Formal potentials, formal rate constants and positive transfer coefficients at different temperatures were evaluated from the fitting of cyclic voltammograms. The experimental activation parameters were also determined. The effects of the analytical concentration of the reagent and the temperature, as well as the addition of trifluoracetic acid and a strong base such as lutidine on the electrochemical responses are discussed. A general reaction mechanism as well as probable structures for dimeric products are proposed.Besides, the presence of an acid–base equilibrium in DPMB solutions is also studied by employing UV–Vis spectroscopic measurements at different trifluoracetic acid concentrations. An apparent value of (1.5 ± 0.2) × 10³ M^?1 was estimated for the DPMB basic constant at 20.0 °C     

The reduction of l-cystine hydrochloride at lead using static and rotating disc electrodes

The reduction of the disulphide, l-cystine hydrochloride to the l-cysteine hydrochloride thiol, in 0.1 mol dm^?3 HCl at 298 K, has been studied at pre-treated, circular, 0.50 cm² lead disc electrodes using steady state linear sweep voltammetry, non-steady state voltammetry and controlled potential coulometry. The diffusion coefficient for l-cystine hydrochloride was approximately 4.8 × 10^?10 m² s^?1 from the three techniques. Reduction of the disulphide was irreversible and hydrogen evolution occurred as a competitive reaction at approximately ?1.35 V vs. SCE. Analysis of the mixed control kinetics, using a Koutecky–Levich approach, allowed the relative roles of charge transfer and mass transport to be resolved. Anomalously high Tafel slopes, of typically ?183 mV, were observed due to disulphide adsorption. The charge transfer kinetics are consistent with the first electron gain being rate determining while reaction orders are +1 with respect to both the disulphide and proton concentrations. The mechanism of l-cystine hydrochloride reduction has been critically discussed.     

A piezoelectric gene-sensor using actinomycin D-functionalized nano-microspheres as amplifying probes

A gene-sensing system has been developed using actinomycin D-functionalized magnetic nano-microspheres, which can interact with double-stranded DNAs (dsDNAs) anchored on the gold film electrode of an electrochemical quartz crystal microbalance (EQCM). Actinomycin D acts as a guide that leads heavy microspheres onto the dsDNAs at the EQCM film. A magnetic separation shelf could separate unreacted microspheres conveniently. The modification and DNA hybridization at EQCM electrodes were examined by microgravimetric and electrochemical methods. In this way, an outstanding change in frequency decrease has been monitored owing to the mass increase on the EQCM electrodes. The limit for the determination of target DNA could be improved from 6.2×10^?8 to 2.0×10^?12 mol l^?1 by the amplifying technique.     

The electrochemical behavior of p-sulfonated calix[4]arene

The electrochemical behavior of p-sulfonated calix[4]arene was studied. In aqueous solution, p-sulfonated calix[4]arene can be oxidized when the potential is more than 0.7 V versus SCE. It is confirmed that the reaction is an irreversible two-electron transfer electrochemical reaction. The anodic peak potential, E_p, is affected by the acidity of the solution. E_p shifts in the negative direction when the pH increases. The electron transfer coefficient, α, is 0.65. At 25 °C, the diffusion coefficient of p-sulfonated calix[4]arene is 3.1 × 10^?5 cm² s^?1. The activation energy, E_a, for the electrochemical reaction is (18.8 ± 0.2) kJ mol^?1.     

Pseudopolarography of trace metals. Part II. The comparison of the reversible,quasireversible and irreversible electrode reactions

The theoretical and experimental pseudopolarographic curves of reversible, quasireversible and irreversible electrochemical reactions were compared and evaluated. The measurements were performed on a stationary mercury drop electrode (SMDE, PAR 303A), using differential pulse anodic stripping voltammetry (DPASV). A good agreement between the theoretical and the experimental shift of the half-wave potential with an increasing accumulation time was obtained for the reversible pseudopolarograms of 10^?7 mol dm^?3 Cd(II) (in 0.1 mol dm^?3 NaClO₄, pH ～2). As compared with the curve of the logarithmic analysis of the polarogram, the corresponding curve of the pseudopolarogram is steeper in the region of the half-wave potential. It has been shown that even though the pseudopolarograms are quasireversible or irreversible, there is a range at the foot of the curves with a reversible slope (usually below 10% of the total/limiting current). It has been verified that the range of this reversible slope can be extended by increasing the accumulation time, lowering the mercury drop size and diminishing the thickness of the diffusion layer. The estimated value for its approximative evaluation is about 1% of the total/limiting current. This is essential for the determination of the corresponding electrochemical parameters, such as: the formal potential (E°^′), transfer coefficient (α) and rate constant (k_s). From the experimentally obtained reversible slope of the (pseudo)polarographic curves of Zn(II) (in 1 mol dm^?3 NaClO₄, pH 4.7 ± 0.1), the parameters for the quasireversible electrochemical reactions were estimated as follows: E°^′=?0.964 ± 0.002 V, α=0.24 ± 0.02 and k_s～2–3×10^?3 cm s^?1. It is shown that an accurate transfer coefficient can be calculated from the curves of the logarithmic analysis of the quasireversible pseudopolarograms, which is not the case for the polarographic curves. The irreversible system, tested on the electrochemical reaction of the CdNTA complex (in 0.1 mol dm^?3 NaClO₄, pH 7.9 ± 0.1), shows relatively good agreement between the experimental and the theoretical dependences. The (pseudo)polarographic measurements enabled approximate estimation of the electrochemical parameters (E°^′=?0.835 ± 0.010 V, α=0.55 ± 0.02 and k_s=1.0 ± 0.4 × 10^?4 cm s^?1) which are in fairly good agreement with the literature data.     

Electrochemical investigation on the preparation of Lu–Co thin films in dimethylsulfoxide

Cyclic voltammetry and chronopotentiometric and chronoamperometric curves were used to study the electroreduction of Lu³⁺ in a LiCl + DMSO system. The electrode process of Lu³⁺ reduced on a Pt electrode occurs in one step: Lu³⁺ + 3e^? → Lu. The transfer coefficient and diffusion coefficient of Lu³⁺ in the 0.1 mol dm^?3 LuCl₃ + 0.1 mol dm^?3 LiCl + DMSO system were calculated as 0.141 and 2.86 × 10^?10 m² s^?1 at 305 K, respectively. The experimental results indicate that a Lu–Co thin film containing 5.65–56.30 wt% Lu was obtained by potentiostatic electrolysis in 0.1 mol dm^?3 LuCl₃ + 0.1 mol dm^?3 CoCl₃ + 0.1 mol dm^?3 LiCl + DMSO. The surface of the Lu–Co thin film observed by scanning electron microscopy (SEM) was uniform, adhesive and had a metallic luster. The Lu–Co thin films obtained were amorphous as proven by X-ray diffraction analysis (XRD).     

Electrochemical reduction of 2-fluorenecarboxaldehyde: A mechanism with a grandparent–grandchild reaction

The electrochemical reduction of 2-fluorenecarboxaldehyde, 1, has been investigated, principally in N,N-dimethylformamide. The initially formed anion radical undergoes an irreversible dimerization reaction with a rate constant of 2600 M^?1 s^?1. The very basic dimer dianion is protonated by the starting material. This “grandparent–grandchild” reaction well accounts for the amount of conjugate base of 1 that is formed. The effect of adding various OH, NH and CH acids was investigated. The enhancement of the dimerization rate observed with the OH and NH acids is interpreted in terms of the formation of hydrogen-bonded complexes with the anion radical which in turn undergo more rapid dimerization reactions than does the uncomplexed anion radical.     

The reduction of 2-bromomethyl-3-methyl- and 2,3-bis-bromomethyl-1,4-naphthoquinones,potential bioreductive alkylating agents. Electrochemical and computational studies

In the present work, 2,3-dimethyl-1,4-naphthoquinones, substituted at one or both side chains with bromine were prepared and submitted to electrochemical studies (cyclic voltammetry and electrolysis), in aprotic medium (DMF + 0.1 mol l^?1 TBAP), using different electrodes (Hg, GC and Au), to observe the role of bromide, as a good leaving group, in their electroreductions. The cyclic voltammograms are complex. Combined results from CV, chronoamperometry and analysis of the products of electrolysis, mainly dimers and the parent unsubstituted quinone, allowed the qualitative definition of the electrodic mechanism for the reduction of the brominated quinones. A reversible electronic transfer to the quinonoid group followed by the cleavage of C–Br, in an EC type mechanism, more specifically a reductive elimination, is suggested. The quinonoid radical is generated and suffers dimerization to electroactive dimers or a second electron uptake, furnishing the anion that can be protonated to yield 2,3-dimethyl-1,4-naphthoquinone, also electroactive. The additional waves are probably related to the reduction of quinomethide-derived products, upon comparison with a synthetic dimer. Computational studies corroborate the electrochemical observations. Despite the lack of unequivocal proof of quinonemethide generation, its intermediacy is highly probable and this has been proved to be essential for the biological activity of these compounds.     

Voltammetric determination of stability constants of iron(III)–glycine complexes in water solution

Determination of the stability constants of dissolved iron(III)–glycine system in water solution (I = 0.6 mol L^?1 in NaClO₄ at 25 ± 1 °C) using differential pulse cathodic voltammetry (DPCV) was performed on a static mercury drop electrode (SMDE). Iron(III) concentration of 2.5 × 10^?5 mol L^?1 and the pH range from 9.05 to 6.36 ensured the formation of enough concentration of iron(III)–glycine higher coordination complexes (1:2 and 1:3) to be measured by the applied method. The concentrations of total glycine varied from 0.1 to 0.5 mol L^?1. Cyclic voltammetry (CV) measurements were used to investigate reversibility of the iron(III)–glycine complexes which showed one-electron reversible character. The stability constants of iron(III) [Fe(Gly)₂]⁺ and Fe(Gly)₃ complexes, which had not been reported in the literature so far, were found to be log β₂ = 16.83 ± 0.47 and log β₃ = 18.64 ± 0.70, respectively. The model that best fitted the data gave two iron(II)–glycine stability constants for [FeGly]⁺ log K₁ = 3.69 ± 0.19 and for Fe(Gly)₂ log β₂ = 5.08 ± 0.60. According to the constants found, chemical distribution of iron(III) in glycine water solution, as a function of pH, was calculated and proposed.     

Cyclic voltammetric determination of free radical species from nitroimidazopyran: a new antituberculosis agent

PA-824 (2-nitro-6-(4-trifluoromethoxy-benzyloxy)-6,7-dihydro-5H-imidazo[2,1-b][1,3]oxazine) is being tested as antituberculosis drug. Little is known on the action mechanism of PA-824; however the reduction of the nitro group seems to be a key step in the metabolic activation, as is observed for the well-known bactericidal metronidazole. Consequently, this paper is focused on the cyclic voltammetric behavior of PA-824 with the aim of revealing the formation and stability of the corresponding nitro radical anion and its comparison with the metronidazole behavior.Both compounds PA-824 and metronidazole reveal, in aprotic medium (DMSO + 0.1 tetrabutylammonium hexafluorophosphate), a similar reduction pattern showing a well-resolved couple due to nitro reduction to form the corresponding nitro radical anion. The electrode reaction obeys an EC₂ mechanism with a dimerization reaction as the chemical step in aprotic medium. Using cyclic voltammetry theory for a dimerization reaction we have calculated the second-order decay constants, k_2,dim, and the half-life time, t_1/2, for the nitro radical anions formed from PA-824 and metronidazole. We have obtained k_2,dim values of 2.22 × 10² and 2.58 × 10⁴ M^?1s^?1 for metronidazole and PA-824, respectively. Our voltammetric results show that the PA-824 nitro radical anion requires more energy for formation (about 200 mV) and it is approximately 100 times less stable than the metronidazole radical anion.     

Interactions between DNA and a water-soluble C60 derivative studied by surface-based electrochemical methods

A novel electrochemical micromethod for the investigation of the interactions between DNA and non-electroactive species is described. The method was developed using the system of double-stranded DNA (dsDNA) modified gold electrodes (dsDNA/Au), a synthesized water-soluble C₆₀ derivative as a model, and [Co(phen)₃]^3+/2+ (phen=1,10-phenanthroline) as an electroactive indicator. Electrochemical studies with dsDNA-modified gold electrodes suggest that the C₆₀ derivative can interact strongly with dsDNA, with binding sites of the major groove of the double helix and phosphate backbone of dsDNA, a binding constant of (1.6 ± 0.2) × 10⁵ M^?1 obtained in 5 mM NaCl in Tris–HCl buffer, and a dissociation rate constant from the dsDNA/Au surface of (1.2 ± 0.1) × 10^?2 min^?1.     

Improvement of alternariol monomethyl ether detection at gold electrodes modified with a dodecanethiol self-assembled monolayer

The electro-oxidation of alternariol monomethyl ether (AME), one of the main metabolites of the Alternaria genus mycotoxins, is studied at 1-dodecanethiol (DDT)-modified gold electrodes, in acetonitrile (ACN) – aqueous phosphate buffer solutions of different pH values, by using cyclic (CV) and square-wave (SWV) voltammetries. The AME voltammetric response at the bare electrode suffers from two drawbacks: it appears at potentials close to the onset of gold oxide formation, and it is hampered by a fouling of the electrode surface due to the accumulation of oxidized products. These shortcomings are circumvented by the use of DDT-coated electrodes, since the intervening monolayer inhibits gold oxide formation and surface passivation by the electrochemical products, without affecting the oxidation kinetics of AME significantly. Diagnostic criteria based on the voltammetric peak parameters show that the electrochemical behavior of AME at the modified electrode is mainly controlled by reactant diffusion from solution, with a weak adsorption of both the mycotoxin and its oxidation products at monolayer defects. Calibration curves were constructed from the AME square-wave voltammetric response and a detection limit of 9.1 × 10^?8 mol dm^?3 was determined, which is about three times smaller than a previous estimate at platinum and glassy carbon electrodes, and about fifty times smaller than the limit derived from measurements carried out at a polyphenol oxidase-modified carbon paste electrode.