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.     

Study of the electrocatalytic reduction of nitrite with silicotungstic heteropolyanion

Three couples of reversible redox waves of the SiW₁₂O^4?₄₀ anion which are composed of two one-electron and one two-electron processes occur in the potential range +0.1 to ?0.7 V in aqueous solutions. The first (most positive) and third (most negative) redox waves exhibit good electrocatalytic activities for nitrite reduction in acid solutions with pH < 2 for the former and with $\text{pH}\text{? 4}$ for the latter. The behavior of the third catalytic wave, which is quite unusual, was studied in detail. The rate constant governing the reduction of NO^?₂ by the first wave of SiW₁₂O^4?₄₀ was measured by an ultramicrodisk electrode as 3.73 × 10³ M^?1 s^?1.     

Nitrite reduction in acidic solution at a GC/Eastman-AQ-Os(bpy)32+-PVP composite modified electrode

The negatively charged polymer polyester sulfonic acid (Eastman-AQ, abbreviated to AQ), positively charged polymer polyvinyl pyridine (PVP) and mediator Os(bpy)₃²⁺ were used to construct composite modified glassy carbon (GC) electrodes (abbreviated to GC/AQ-Os(bpy)₃²⁺-PVP). The reduction of NO₂^? in acidic solution was taken as a model reaction to explore the properties of the modified electrode. On the steady state polarization curve of NO₂^? reduction there were two well-developed waves with much enhanced plateau current densities and positively shifted half-wave potentials compared with bare GC electrodes. In 0.05 mol/l H₂SO₄ + 5 mmol/l NO₂^? the modified electrode exhibited plateau current densities of 723 and 1153 μA cm^?2 and half-wave potential shifts of 0.29 and 0.56 V for the first and second current wave, respectively, showing promising potential for nitrite detection. The catalytic activity for NO₂^? reduction did not decrease appreciably over 4 months. A number of relevant kinetic and thermodynamic parameters are estimated experimentally. NO₂^? reduction at the composite modified electrodes is suggested to follow the SR mechanism (pure kinetic conditions involving mutual compensation between a catalytic reaction and substrate diffusion in the film in addition to diffusion in the solution phase) according to the Savéant–Andrieux theory.     

Electrochemical studies of isolapachol with emphasis on oxygen interaction with its radical anions

Cyclovoltammetric studies, in aprotic medium (DMF+0.1 mol l^?1 TBAP or DMSO+0.1 mol l^?1 TEAP), on glassy carbon and/or platinum electrodes were performed with isolapachol [2-hydroxy-3-(3-methyl-1-butenyl)-1,4-naphthoquinone], in the absence and presence of oxygen, which were aimed at investigating its electrochemical reduction mechanism and possible oxygen interaction with its radical anion. The electrochemical behaviour is complex and similar to that observed for lapachol, and the first peak is related to the semiquinone formation, although complicated by the occurrence of self-protonation mechanisms and hydrogen-bonded intermediates formation. The observed positive shift in the potential of the first wave of isolapachol, in comparison to lapachol, is related to the higher acidity of the enolic group. The cyclovoltammetric results obtained in the presence of O₂ clearly indicate the consumption of the semiquinone anion-radicals of isolapachol by oxygen, in an EC type reaction, generating the deprotonated form of isolapachol and HOO^?. The observed generation of the superoxide radical, after electron transfer can be related to the mechanism of biological action and toxicity of isolapachol.     

Oxygen reduction mechanism on corroded zinc

The mechanism of oxygen reduction on the as-polished and corroded zinc specimens has been studied using a rotating ring disc electrode (RRDE) system. On the as-polished surface, oxygen was reduced into two distinct steps. In the first step, about 44% of O₂ was reduced to H₂O₂ in a 2-electron reaction with the rest being reduced to OH^? in a 4-electron reaction. On the other hand, in the second step, with the increase of overpotential O₂ was almost exclusively reduced to OH^? in a 4-electron reaction. The first step reduction occurred on an air-formed oxide-covered surface at more positive potential than ?1.2 V vs. Ag/AgCl and the second step reduction (E < ?1.2 V) took place on a semi-uniformly active surface. On the corroded surface, the second step was not distinctly observed on the polarization curve, because reduction of the zinc corrosion products simultaneously took place around ?1.2 V. The O₂ reduction in the first step was inhibited by deposition of the corrosion products, though the ratio of amount of O₂ reduced to OH^? in a 4-electron reaction was larger than that on the as-polished surface. The mechanism of oxygen reduction is discussed on the basis of results obtained from the RRDE experiment.     

Studies of the effect of anthranilic and thiosalicyclic acid on the two-step electroreduction of Zn(II) ions

The reduction of Zn(II) ions at the dropping mercury electrode in 1 mol dm^?3 NaClO₄ + 0.001 mol dm^?3 HClO₄ with the addition of anthranilic or thiosalicyclic acid is examined using an impedance method in wide potential and frequency ranges. It is concluded that the studied acids affect mainly the rate constant of the first electron trasnfer, catalyzing the process. On the other hand, the transfer of the second electron is inhibited. Catalytic activity of thiosalicyclic acid is higher than that of anthranilic acid, resulting mainly from double layer effects.     

Electrochemical evaluation of the reaction rate between methyl viologen mediator and diaphorase enzyme for the electrocatalytic reduction of NAD+ and digital simulation for its voltammetric responses

The electrocatalytic reduction of NAD⁺ using diaphorase enzyme was studied. Methyl viologen was used as an electron transfer mediator between an electrode and the enzyme. A catalytic wave for the reduction of NAD⁺ when all the species were in the solution was measured with cyclic voltammetry at a gold-amalgam electrode which showed low background currents at negative potentials. Steady-state currents could be obtained under the conditions of slow scan rate, low methyl viologen concentration, and high NAD⁺ concentration as the electrode reaction was converted to an electrochemical-catalytic (EC′) reaction. The bimolecular rate constant for the reaction of the reduced methyl viologen with the oxidized diaphorase was estimated as 7.5×10³ M^?1 s^?1 from the slope of the current versus [MV²⁺] plot. Another slope of the current against the square root of the enzyme concentration also gave a close value of 6.7×10³ M^?1 s^?1. In the calculation of the rate constant, the stoichiometric factor when it is not one-to-one was considered. With the evaluated rate constant, digital simulation using the suggested reaction mechanism was compared with the experimentally obtained voltammograms. Satisfactory agreement indicates that the evaluation methods of the rate constant and the suggested mechanism are appropriate for the mediated enzyme-catalyzed electrochemical reactions.     

Electrochemical behaviour of 2-(4′-hydroxybenzeneazo)benzoic-acid at pyrolytic graphite electrodes

The electrochemical reduction of 2-(4′-hydroxybenzeneazo)benzoic acid (I) has been studied at pyrolytic graphite electrodes in the pH range 2.0–10.4. The cyclic voltammetric behaviour clearly indicated an ECE mechanism in acidic medium in which the two-electron two-proton reduction of I gives the hydrazo derivative. The acid catalysed disproportionation of the hydrazo intermediate was also studied in the pH range 2.0–6.0 and the value of k′/[H⁺] was found to be 1.4 × 10^?2 1 mol^?1 s^?1 The products of the reduction have been isolated and characterized using IR, melting point, mass and related techniques.     

A monomer–dimer equilibrium in self-assembled monolayers of N-ethyl-N′-octadecylviologen on the electrode surface. The influence of water content and hexafluorophosphate ion

This paper describes the dimerization of self-assembled monolayers (SAMs) of N-ethyl-N′-octadecylviologen (1) on GC and Au electrode surfaces in the presence of 0.1 M NH₄PF₆ aqueous solutions. The ‘wet’ and ‘dry’ SAMs of 1 showed multiple redox peaks for the first reduction of 1 in the presence of NH₄PF₆, in contrast to the case of other supporting electrolytes (typically KCl, NaNO₃, Na₂SO₄ and NaClO₄) where both wet and dry SAMs of 1 exhibited a single redox wave for the first reduction. The dry SAM showed a well defined reduction peak at ?0.57 V along with a shoulder reduction peak at ?0.50 V and two oxidation peaks at ?0.50 and ?0.42 V. On the contrary, the wet SAM gave a very sharp reduction peak at ?0.50 V and a small shoulder peak at ?0.57 V in addition to two oxidation peaks like those observed for the dry SAM. The reduction peak of ?0.50 V was ascribed to the reduction of strongly hydrated dications of 1, while the reduction peak at more negative potential (?0.57 V) was attributed to the reduction of the dehydrated dications of 1. The two oxidation peaks at ?0.50 and ?0.42 V were ascribable to the oxidation of the usual radical cation monomer and the radical cation dimer, respectively. In the case of the wet SAM, upon continuous potential cycling, the sharp reduction peak of ?0.50 V clearly decreased, whereas the more negative reduction peak of ?0.57 V was highly stable. In this case, in the oxidation process, the monomer peak of ?0.50 V increased, while the dimer peak of ?0.42 V decreased. Thus it is reasonably assumed that in the wet SAM, initially the radical cations of 1 feel an aqueous environment in the monolayer where the dimerization is highly favored and at subsequent potential cycles, due to the entry of hydrophobic anions of PF₆^? into the monolayer, the pre-existent water molecules are expelled from the monolayer and under this circumstance the radical cations of 1 may feel the environment very similar to non-aqueous media where the dimerization is totally suppressed. The adsorption tendency of 1 on the electrode surface was also studied using the SAMs prepared by dissolving 1 in water+ethanol mixtures of different ratios. The appearance of multiple peaks was found to depend significantly on the alkyl chain length of asymmetric viologen. The inclusion/expulsion of solvents and anions into/from the SAM during the redox reaction were studied by the electrochemical quartz crystal microbalance (EQCM). It was found that in the presence of SO₄^2? ions ca. 17 water molecules per one SO₄^2? ion were transported to the SAM of 1 during the oxidation, whereas ca. five water molecules were transported in the presence of PF₆^? ions.     

Adsorption of atomic hydrogen on a polycrystalline Pt electrode surface studied by FT-IRAS: the influence of adsorbed carbon monoxide on the spectral feature

Adsorption of atomic hydrogen on a polycrystalline Pt electrode surface was studied by in situ infrared reflection absorption spectroscopy (IRAS). When the electrode potential was adjusted in a potential range where the underpotential-deposited (upd) hydrogen was formed, an absorption band assignable to the vibration of on-top CO (which would be formed by the reduction of a trace of CO₂) appeared at ca. 2010 cm^?1 even for highly purified 0.1 M (M=mol dm^?3) H₂SO₄ solution. An absorption band due to the on-top H was observed at ca. 2070 cm^?1 for a conventional acidic solution in a potential range as narrow as ca. 0.1 V just before the hydrogen evolution reaction (her) ascribable to the reduction of hydronium ions began. On the other hand, the on-top H band was observed unequivocally for a solution containing 1 mM H₂SO₄ and 99 mM Na₂SO₄ over a wide potential range where molecular hydrogen was formed by the reduction of hydronium ions. Even for a neutral solution such as 0.1 M KCl, the weak band ascribable to the on-top H was detected. The dependence of the spectral feature on the concentration of hydronium ions and the applied electrode potential strongly suggested that the on-top H is the intermediate in the electrochemical reduction of hydronium ions. We demonstrated that the adsorbed CO is readily formed by the reduction of CO₂ in the 0.1 M H₂SO₄ solution.     

Electrochemical reduction of 1,4-benzoquinone. Interaction with alkylated thymine and adenine nucleobases

The electrochemical reduction of 1,4-benzoquinone (Q) in the presence of 1-octylthymine and 9-octyladenine has been performed in dimethylsulfoxide on glassy carbon electrodes. The electrochemical behaviour shows that the semiquinone (Q^?) and the quinone dianion (Q^2?) interact with 1-octylthymine (TH) following a mechanism which involves association and protonation reactions. It is demonstrated by cyclic voltammetry and NMR experiments, that the protonated dianion (QH^?) is stabilised by an excess of TH, alternatively it disproportionates into an association complex of dianion-hydroquinone [Q,QH₂]^2?. The 1-octylthymine anion (T^?) produced in the proton transfer reactions activates a homogeneous chain mechanism allowing the consumption of benzoquinone. For the experiments carried out in the presence of 9-octyladenine (AH), it was observed that only a strong hydrogen bonding association takes place between the nucleobase and Q^2?.     

Oxygen reduction on copper in neutral NaCl solution

The reduction of oxygen on copper in neutral unbuffered 1 mol dm^?3 NaCl has been studied using rotating ring-disc electrodes at six oxygen concentrations equivalent to atmospheres of 2% O₂ + N₂ to 100% O₂. Steady-state potentiostatic measurements show that the reaction is first order with respect to [O₂] and that, following adsorption of O₂, the first electron transfer is rate determining. In 50% O₂ + N₂ and 100% O₂, a cathodic oxygen reduction peak is observed in both potentiodynamic and potentiostatic experiments at a disc potential of ?0.3 to ?0.4 V/SCE. The reaction is dominated by the overall four-electron reduction to OH^?, with only small amounts of peroxide detected by the ring electrode at disc potentials corresponding to the formation of the cathodic oxygen reduction peak. Tafel slopes increase with [O₂] and vary from ?0.13₅ V in 2% O₂ + N₂ to a limiting value of ?0.16 V to ?0.18 V in air, 50% O₂ + N₂ and 100% O₂.The results are explained by a mechanism involving oxygen reduction on two types of surface site with different reactivities. The most catalytic surface is believed to comprise Cu(0) and Cu(I) sites, where the Cu(I) species is stabilized as Cu(OH)_ads and/or submonolayer Cu₂O. The less catalytic site consists of Cu(0) only. Oxygen reduction is believed to proceed by a series pathway involving an adsorbed peroxide intermediate on both sites. Peroxide is reduced to OH^? prior to desorption at Cu(0) sites, but some is released before being reduced at Cu(0)/Cu(I) sites. Surface coverage by catalytic Cu(0)/Cu(I) species is favoured by a higher interfacial pH and more positive disc potentials.     

Complex electrochemistry of flavodoxin at carbon-based electrodes: results from a combination of direct electron transfer,flavin-mediated electron transfer and comproportionation

Staircase cyclic voltammetry (SCV) and differential pulse voltammetry on fully oxidized flavodoxin from Desulfovibrio vulgaris Hildenborough at the bare glassy carbon electrode give one redox couple at a potential of ?218 mV (standard hydrogen electrode (SHE)) at pH = 7.0 with an SCV peak current proportional to the scan rate. This response is caused by flavin mononucleotide (FMN), dissociated from the protein and adsorbed onto the electrode. The midpoint potential and the pK of 6.5 are equal to the values measured with free FMN in solution. When the cationic promoter neomycin is added, one additional and diffusion controlled response is observed. The midpoint potential is ?413 mV (SHE) at pH 7.0 with a redox-linked pK of 4.8 for the reduced form. The temperature dependence is ?1.86 mV K^?1, yielding ΔS° = ?179 J mol^?1 K^?1 and ΔH° = ?12.4 kJ mol^?1. Although the starting material was 100% quinone, no response was observed around the midpoint potential of the quinone to semiquinone reduction of ?113 mV (SHE) at pH 7.0, determined in an EPR-monitored titration with dithionite. Digital simulation shows that the peak currents of the second reduction couple approach a maximum value after a few cycles if comproportionation of fully reduced and fully oxidized flavodoxin occurs in solution and a small amount of semiquinone is either present initially or is generated by mediation of electrode-bound FMN. In the latter case the heterogeneous electron transfer rate between adsorbed FMN and flavodoxin is 6.3 × 10^?6 m s^?1. The implications of this anomalous behaviour for electrochemistry on flavin enzymes like glucose oxidase are discussed.     

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     

Comparative studies on the electrogenerated chemiluminescent and amperometric behavior of the Ru(bpy)32+ system on a paraffin-impregnated graphite electrode and a glassy carbon electrode

The electrogenerated chemiluminescent (ECL) and amperometric behavior of Ru(bpy)₃²⁺ system on a paraffin-impregnated graphite electrode (PIGE) and a glassy carbon electrode (GCE) was studied by different electrochemical techniques. Under conventional cyclic voltammetric (CV) conditions, two anodic ECL peaks (EP1 and EP2) (vs. SCE) were observed at 1.18 and 1.37 V for a PIGE, and at 1.20 and 1.44 V for a GCE. The EP1 normally occurred, but not the EP2. A complicated mechanism was involved in the formulation of EP2. A detection limit as low as 1×10^?9 mol l^?1 of C₂O₄^2? was obtained on the PIGE by the CV method at 100 mV s^?1. Strong ECL signals were found on both electrodes in either an oxalate-containing aqueous solution or an organic solution when a cyclic square wave (CSW), between two suitable potentials was used. However, the GCE showed higher reproducibility than the PIGE for continuous CSW (n=10) measurements. Chronoamperometry was also applied by using a potential step from 0.15 to 1.85 V. It took 389 ms on a PIGE and 837 ms on a GCE to reach each maximal ECL intensity.     

Investigation of potential inversion in the reduction of 9,10-dinitroanthracene and 3,6-dinitrodurene

In the case of two-electron reduction of a compound, potential inversion refers to the situation where introduction of the second electron occurs with greater ease than the first. That is E⁰₁?E⁰₂<0 where E⁰₁ and E⁰₂ are the standard potentials for the two steps of reduction. The extent of potential inversion in the reduction of 9,10-dinitroanthracene, 1, and 3,6-dinitrodurene, 2, has been assessed by steady-state microelectrode voltammetry, cyclic voltammetry under conditions of near-reversible behavior, cyclic voltammetry under conditions of quasireversible behavior and electrochemical impedance spectroscopy (EIS). The studies were conducted in acetonitrile at 298 K. Cyclic voltammetry under quasireversible conditions and EIS were most sensitive to small changes in E⁰₁?E⁰₂. The value of E⁰₁?E⁰₂ for 1 was found to be ?107 mV and that for 2 was ?280 mV.     

Vibrational study of adsorbed phosphate ions on rhodium single crystal electrodes

The adsorption of H₂PO₄^? ions was studied on low Miller index rhodium single crystal electrodes by in situ FTIR spectroscopy. It is found that for Rh(1 0 0) and Rh(1 1 0), H₂PO₄^? ions are the major species at low potentials, but at higher potentials, some of the H₂PO₄^? ions undergo a potential induced deprotonation and probably there is a mixture of H₂PO₄^? and HPO₄^2? ions. On Rh(1 1 1) the deprotonation starts at very low potentials and at higher potentials the H₂PO₄^? is fully converted to HPO₄^2?. The behavior of the band center and of the band intensity with the applied potential was also analyzed. It is found that the adsorption increases from 0.08 V vs. a Pd–H₂ electrode up to 0.5 V and then it decreases when the OH starts to be adsorbed.     

Kinetic study of oxygen reduction on nanoparticles of ruthenium synthesized by pyrolysis of Ru3(CO)12

Electrocatalytic activity for molecular oxygen reduction on nanoparticles of ruthenium mixed with Nafion^® in 0.5 M H₂SO₄ was studied. The electrocatalytic material was synthesized by pyrolysis of Ru₃(CO)₁₂ in sealed ampoules at 190 °C for 3 h. The product of the pyrolysis was characterized by DSC, FTIR, XRD, SEM (EDX), TEM and also electrochemically. X-ray diffraction and TEM studies indicated that the synthesized powder presents a nano-crystalline structure. The electrochemical results obtained by rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) techniques show that the oxygen reduction reaction take place by a multi-electronic charge transfer process (n=4e^?) with the formation of ca. 4.5% of hydrogen peroxide. A Tafel slope of ca. ?100 mV dec^?1 and an exchange current density of 7.9 ×  10^?7 mA cm^?2 were deduced. The kinetics of the reduction reaction were evaluated in the potential range of 0.7–0.0 V (SHE) through a simple Damjanovic model. Further interpretation of the analysis of the kinetic results in the light of Anastasijevic's procedure is also presented.     

Anion recognition and electrochemical characteristics of the self-assembled monolayer of nickel(II) azamacrocyclic complex

Anionic recognition of the self-assembled monolayer of dinickel(II) (2,2-bis(1,3,5,8,12-pentaazacyclotetradec-3-yl)-diethyl disulfide) perchlorate (1) was studied electrochemically. The dinickel(II) complex 1 adsorbs on gold electrodes from methanol solutions and yields stable, self-assembled electroactive monolayers (SEMs); the SEM of 1 shows a reversible redox wave at 0.82 V in aqueous 0.1 M NaNO₃ corresponding to the Ni^3+/2+ redox reaction. The surface coverage, Γ, of the self-assembly of 1 determined by cyclic voltammetry is constant (Γ=(1.4±0.08)×10^?10 mol cm^?2) with change in the deposition time (2–36 h) and the concentration of 1 in methanol solution (0.2–5 mM) and is equivalent to a monolayer coverage of the nickel macrocyclic complex. The capacitance of the monolayer of 1 was determined from the double-layer capacitance measurements by chronoamperometry; the monolayer of 1 is assigned to be well-solvated by observing that the dielectric constant of the self-assembly domain (?_film=74) is nearly equal to that of water (?_water=78). Electrochemical investigations reveal that the monolayer of 1 can sense electrochemically various non-electroactive anions, NO₃^?, CF₃COO^?, SO₄^2?, H₂PO₄^?, HPO₄^2?, ClO₄^?, PF₆^? and SCN^?, from the variation of the formal potential, E°′, in aqueous solutions of different anions. The E°′ of the monolayer of 1 is 0.82 V in aqueous 0.1 M NaNO₃ and shifts to a less positive potential, 0.55 V, in aqueous 0.1 M Na₂SO₄; the shift in the E°′ was reversible on exchanging the monolayer of 1 between 0.1 M NaNO₃ and 0.1 M Na₂SO₄. The shift in the E°′ of the monolayer has been explained by an axial coordination of electrolyte anions with the trivalent nickel ion. The redox reaction of the SEM of 1 is not observed in aqueous solutions of 0.1 M NaClO₄ and 0.1 M NaSCN; but the redox activity was retained on changing the monolayer electrode to an aqueous solution of 0.1 M Na₂SO₄ or 0.1 M NaNO₃. The monolayer of 1 could detect electrochemically the biologically important phosphate anion, adenosine triphosphate (ATP), at submillimolar concentrations; on addition of 1 mM ATP, the formal potential of the monolayer shifts towards the less positive potential region by about 250 mV. The CVs of the SEM of 1 were recorded in aqueous solutions containing different concentrations of NaH₂PO₄ or Na₂SO₄, keeping the ionic strength of the electrolyte solution constant with added NaNO₃. The E°′ of the monolayer shifts to the less positive potential region with an increase in the concentration of H₂PO₄^? or SO₄^2? anion in solution phase, and the analysis of cyclic voltammetric results reveals that the nickel(III) complex forms a 1:1 complex with SO₄^2? anion but a 1:2 complex with H₂PO₄^? anion.