Direct electron transfer between the heme of cellobiose dehydrogenase and thiol modified gold electrodes

Cellobiose dehydrogenase (CDH) is an extracellular fungal enzyme with two domains, one containing flavin adenine dinucleotide (FAD) and one containing heme. The electrochemistry of CDH, as well as its cleaved FAD- and heme-subunits, was studied using a membrane electrode, i.e. the enzyme was trapped under a permselective membrane on a cystamine or 3-mercaptopropionic acid modified gold electrode. Direct un-mediated electron transfer (ET) between the heme of CDH and thiol modified gold electrodes was demonstrated using cyclic voltammetry. At low sweep rate (10 mV s^?1) and low pH (pH 4.3) up-hill ET from heme to FAD in CDH was observed. The formal potential of the heme in CDH and in the cleaved heme-subunit was found to be the same and equal to ?41 mV versus Ag ∣ AgCl at pH 5.1. The dependence of the formal potential on the pH (in the pH range 3.6–6.0) indicates the presence of one redox-linked ionisable functional group. Entropy and enthalpy changes were determined in variable temperature experiments as follows, ΔS°′=?194±14 J mol^?1 K^?1 and ΔH°′=?74±6 kJ mol^?1. The electrocatalytic behaviour of the CDH electrodes was demonstrated by addition of the enzyme substrate, cellobiose. The catalytic current was shown to decrease upon increased pH, in accordance with previous kinetic data in solution. The model of electron transport from the substrate (cellobiose) to FAD, and then through the heme domain to the electrode was confirmed in the experiments.     

Enhanced electron transfer for hemoglobin in poly (ester sulfonic acid) films on pyrolytic graphite electrodes

Stable films made from the ionomer poly (ester sulfonic acid) Eastman AQ55 on pyrolytic graphite (PG) electrodes gave reversible voltammetry for the incorporated protein hemoglobin (Hb). Cyclic voltammetry of Hb-AQ films showed a pair of well-defined, reversible peaks at about ?0.04 V versus SHE at pH 5.5. Compared to solutions, electron transfer between Hb and PG electrodes was greatly facilitated in the AQ films. Soret absorption band positions suggest that Hb retains a near native conformation in AQ films in the medium pH range. The formal potential of the Hb heme Fe(III)/Fe(II) couple in AQ films shifted linearly between pH 4 and 11 with a slope of ?52 mV pH^?1, suggesting that one proton transfer is coupled to each electron transfer in the electrochemical reaction. Square wave voltammograms of Hb-AQ films were fit by nonlinear regression analysis using a model featuring dispersion of formal potentials. Hb can act as an enzyme-like catalyst in these films as demonstrated by catalytic reduction of trichloroacetic acid with significant decreases in the electrode potential required.     

Mechanistic study of the autoxidation of reduced flavin and quinone compounds

The mechanism of the autoxidation of reduced flavin and quinone compounds was investigated through semiquantitative analysis of voltammetric waves of the reduction of dioxygen (O₂) catalyzed by flavin and quinone adsorbed on electrode surfaces, where the redox equilibrium among the oxidized, (flavo)semiquinone and fully reduced states is easily controlled by the electrochemical method. The analysis provided evidence that the semiquinone radical plays a predominant role in the autoxidation. The generation of a superoxide anion radical as a product of the one-electron transfer from the semiquinone to O₂ was confirmed by the reduction of ferricytochrome c. The fact that the catalytic reduction wave of O₂ increases with pH was ascribed to the increase in the semiquinone formation constant. The mechanism of the reoxidation of reduced flavoprotein glucose oxidase with O₂ was also examined. The result supports the semiquinone-dependent one-electron transfer as the mechanism.     

Electrochemical behavior of quercetin: Experimental and theoretical studies

Electrochemical oxidation of quercetin, as important biological molecule, has been studied in 0.1 M phosphate buffer solution, using cyclic voltammetry, chronoamperometry, rotating disk electrode voltammetry as well as quantum mechanical calculations. The heterogeneous charge transfer rate constant, k′, transfer coefficient, α, and exchange current density, j₀, for oxidation of quercetin at the glassy carbon electrode are determined as 4.84 × 10^?2 cm s^?1, 0.65 ± 0.01 and (1.17 ± 0.39) × 10^?7 A cm^?2, respectively. The formal potential, E^0′, of quercetin is pH dependent with a slope of ?60.1 mV per unit of pH which is close to the anticipated Nernstian value of ?59 mV for a two electrons and two protons process. The standard formal potential, E⁰, of quercetin was found to be equal with 558 mV versus saturated calomel electrode (SCE). The mechanism of oxidation was deduced from voltammetric data in various pHs and also in different concentrations of quercetin. The diffusion coefficient of quercetin was calculated as 3.18 × 10^?6 cm² s^?1 for the experimental condition, using chronoamperometric results. The results of density functional theory (DFT) calculations for the oxidation of quercetin in aqueous solution, are also presented. The theoretical standard electrode potential of quercetin is obtained to be 568 mV versus SCE, which is in good agreement with the experimental value. The discrepancy between theoretical and experimental values is only 10 mV. The agreement verifies the accuracy of experimental method and the validity of mathematical model.     

Electrocatalytic oxidation of reduced nicotinamide adenine dinucleotide (NADH) at a chlorogenic acid modified glassy carbon electrode

The redox response of chlorogenic acid solution at an inactivated glassy carbon electrode was investigated and an ECE mechanism was proposed for the electrode process. It has been shown that the oxidation of chlorogenic acid at an activated glassy carbon electrode leads to the formation of a deposited layer of about 4.5×10^?10 mol cm^?2 at the surface of the electrode. Cyclic voltammetry was used for the deposition process and the resulting modified electrode retains the activity of the quinone/hydroquinone group anticipated for a surface-immobilized redox couple. The properties of the electrodeposited films, during preparation under different conditions, and the stability of the deposited film were also examined. The pH dependence of the redox activity of these films was found to be 57 mV per pH unit, which is very close to the anticipated Nernstian dependence of 59 mV per pH unit. The modified electrode exhibits potent and persistent electrocatalysis for NADH oxidation in phosphate buffer solution (pH 7.0) with a diminution of the overpotential of about 430 mV and an increase in peak current. The electrocatalytic current increases linearly with NADH concentration from 0.1 to 1.0 mM. The apparent electron transfer rate constant, k_s, and the heterogeneous rate constant for electrooxidation of NADH, k_h, were also determined using cyclic voltammetry and rotating disk electrode voltammetry, respectively.     

EQCM study of electropolymerization and redox cycling of 3,4-polyethylenedioxythiophene

Polyethylenedioxythiophene films were electropolymerized potentiostatically (E=1.1–1.3 V/SHE) at a gold electrode covering an EQCM from solutions containing monomer, lithium perchlorate and non-ionic surfactant (polyoxyethylene-10-laurylether). The ion exchange was studied by EQCM during redox cycling at a scan rate of 20–50 mV s^?1 in 0.5 M LiClO₄+0.5 M LiCl+0.5 M Na toluenesulfonate solutions in the potential range ?0.5<E<0.8 V (SHE). Mass versus charge curves display a hysteresis. During the reduction scan the ion content of the film is in equilibrium and is defined by the potential, whereas during oxidation the mass lags behind the charge due to slow water exchange. The reduction scan can be described by equilibrium theory supposing that about 40% of counter-ions remain bound in the film. During polymerization at E≥1.2 V, the mass gain calculated from the frequency change slows down more rapidly than is estimated from the current. This may be attributed to the viscosity of the growing film with acoustic decay length of 2.45 μm.     

Effect of pH on direct electron transfer between graphite and horseradish peroxidase

The effect of pH on the kinetics of the electroreduction of H₂O₂ catalysed by horseradish peroxidase (HRP) has been studied with LSV in the potential range from 700 to ?50 mV versus SCE (under steady-state conditions and with an RDE system) and at ?50 mV versus Ag/AgCl on HRP-modified graphite electrodes placed in a wall-jet flow-through electrochemical cell. Increasing [H₃O⁺] was shown to enhance significantly the current of the bioelectroreduction of H₂O₂ due to direct electron transfer (ET) between graphite and the enzyme over the potential range involved. It is demonstrated that at high overvoltages (E<0.2 V) H₃O⁺ does not affect the rate of the enzymatic reduction of H₂O₂, but it increases the rate of direct ET between graphite and HRP. The values of the apparent rate constant of heterogeneous ET between HRP and graphite, k_s, changed from a value of 0.54±0.05 s^?1 in phosphate buffer solution (PBS) at pH 7.9, to a value of 11.0±1.7 s^?1 in PBS at pH 6.0. Analysing the pH rate profile and the variation of the k_s with increasing [H₃O⁺] made it possible to consider the reaction mechanism as implying the participation of a proton in the limiting step of charge transfer.     

Direct electron transfer between Alcaligenes eutrophus Z-1 hydrogenase and glassy carbon electrodes

In this work we report on the ability of the Alcaligenes eutrophus Z-1 hydrogenase immobilized on glassy carbon electrodes to support the electro-enzymatic reduction of NAD⁺ at ?700 mV vs. standard calomel electrode without external promoters or electron mediators.The electron-transfer process seems to be coupled to a relaxation of the quaternary structure of the enzyme molecules in contact with the electrode surface.     

Analysis of the temperature dependence of the α and β coefficients of the electrode reactions in the Cr(III)/Cr(II) and Cr(II)/Cr(Hg) systems

The temperature dependences of the four transfer coefficients for cathodic and anodic electrode reactions of two different types of systems, namely Cr(III)/Cr(II) and Cr(II)/Cr(Hg), were determined in 5.0 M NaClO₄ solution. The results were obtained for a wide range of temperatures from ?20 to 90°C. The α parameter determined from the log k_fh on E dependence corresponding to the Cr³⁺+e^?→Cr²⁺ electrode process shows the highest dependence on temperature (?α/?T=1.1±0.3×10^?3 K^?1), whereas the β parameter corresponding to the Cr²⁺→Cr³⁺+e^? electrode process is virtually independent of temperature (?β/?T=0). The same results were obtained when the transfer coefficients were determined from dependence of the enthalpy and entropy of activation on overpotential. The observed temperature dependence of α is attributed to the changes of the ?₂ potential. The transfer coefficients for the Cr^2+/0 electrode reaction also show some dependence on temperature (?α/?T=8.7±2.0×10^?4 K^?1 and ?β/?T=3.2±1.4×10^?4 K^?1), which probably results from the influence of the ?₂ potential.     

Photocorrosion of polycrystalline CdSe in aqueous KOH: an in situ study by photocalorimetry

Photocalorimetry in conjunction with thermodynamical analysis has been employed to characterize the photocorrosion reaction of a semiconductor electrode. The potential dependence of the heat-monitoring signal and of the photocurrent has been measured at a CdSe electrode (polycrystalline film on Ti) in 0.1 mol l^?1 aqueous KOH under 600 nm illumination. From these measurements a characteristic potential (E*=?0.21±0.02 V versus SHE) is obtained which connects the reversible potential and the Peltier coefficient of the corrosion reaction. These quantities are specific for a given reaction. Values of E* have been calculated for thermodynamically possible corrosion reactions of CdSe from the respective standard potential, standard entropy change and heat of transport. Good agreement between the measured and calculated values of E* and the photocalorimetric detection of a light-absorbing corrosion film indicate that CdSe+2OH^?→Cd(OH)₂+Se+2e^? is the predominating corrosion reaction under the present conditions.     

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.     

Diffusionless electron transfer of microperoxidase-11 on gold electrodes

Microperoxidase-11, MP-11, is made by proteolytic digestion of cytochrome c, cyt. c. It consists of a polypeptide of 11 amino residues attached covalently to the heme. Given that MP-11 has a more exposed heme than the complete protein, it would seem that electron transfer, ET, between immobilized MP-11 and electrodes would be at least as fast as for intact cyt. c. However, while the maximal heterogeneous ET rate for immobilized cyt. c is around 1000 s^?1, that reported previously for immobilized MP-11 does not exceed 20 s^?1. This work attempts to understand this difference in measured ET rates. The MP-11 was immobilized on gold electrodes using several protocols: (electrode A) the immobilization was done following a previously published carbodiimide based recipe yielding ET rates of the order of 20 s^?1; (B) MP-11 was bound to gold electrodes by Lomant’s reagent and gave an ET rate close to 4000 s^?1; (C) physisorbed MP-11 on gold electrodes with a self assembled monolayer, SAM, of alkane thiols gave an ET rate approaching 2000 s^?1 for the shortest length alkane thiol. Inspection of the immobilization chemistries suggests that the procedure employed in producing electrodes B and C are likely to lead to a monolayer or less of immobilized MP-11 while the procedure employed for electrode A may lead to a film comprised of a multilayer of MP-11. The presence of such a film on electrode A complicates the ET process since the MP-11 in the layer adjacent to the electrode could have fast ET rates while the MP-11 in the outer layers may have significantly slower ET rates. The net result would be an apparent ET rate constant which is much smaller than the value for the first layer. The measurements and calculations are presented in support of such an interpretation.     

Electrochemical recognition properties of 13- and 16-membered azo- and azoxycrowns in solution

The voltammetric reduction of 13- and 16-membered azo- and azoxycrowns is investigated in the presence of alkali metal cations (Na⁺ or K⁺ for 13- or 16-membered azocrowns, respectively). Two types of recognition behaviour have been observed. The first type consists in a positive shift of the redox potential (100–150 mV) upon addition of metal cation (single peak behaviour). The second type consists in the appearance of two separated redox peaks (150–320 mV more positive, two-wave situation). Better electrochemical recognition results were obtained using SW voltammetry with the peak potential for the complex up to 0.32 V (for the azoxycrown derivative) more positive of the original azocrown compound. For azo- and azoxycrowns exhibiting ‘two-wave’ behaviour, the binding enhancement (ratio of binding constants for reduced and neutral species) was 10²–10⁵ M^?1. For other azocrown compounds with single peak recognition behaviour the binding constants for reduced species were estimated to be 10⁴–10⁵ M^?1. In general, K⁺ binds to 16-membered neutral azocrowns more strongly than Na⁺ to 13-membered ones, whilst binding to the anion-radical has the opposite trend.     

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.     

The kinetics of neutral methyl viologen in acidic H2O+DMF mixed solutions studied by cyclic voltammetry

The chemistry of the two-electron reduction product of viologen (1,1′-dialkyl-4,4′-bipyridinium, V²⁺) neutral species, is important in understanding the electrochemical behavior of viologens and their utilization. The kinetics for the reactions of neutral methyl viologen (V⁰) in the presence of H⁺ (from HCl), CH₃COOH (pK_a=4.75), ClCH₂CH₂COOH (pK_a=4.00), HCOOH (pK_a=3.75) in aqueous media was examined by cyclic voltammetry according to the EEC_i mechanism. To avoid the electrodeposition of V⁰, we used a 9:1 (v/v%) H₂O+DMF mixture as the solvent medium. To evaluate the rate constants for the chemical reaction followed by the second electron transfer step of V²⁺, the ratio of the anodic and cathodic peak current (I_pa2/I_pc2) corresponding to V⁰–e^??V⁺ was plotted against log τ, where τ is the time between E_1/2 and the switching potential, at various scan rates of 0.02–3.5 V s^?1. The chemical reaction was found to be a parallel reaction consisting of H⁺-catalyzed and general-acid (HA) catalyzed reactions. The second-order rate constants are determined as k_H⁺=3.5×10³ M^?1 s^?1, k_CH3COOH=5.7 M^?1 s^?1, k_HCOOH=4.6×10¹ M^?1 s^?1, and k_ClCH2CH2COOH=3.2×10¹ M^?1 s^?1 using the Nicholson–Shain method and k_H2O was estimated as <3×10^?6 M^?1 s^?1. The CVs were digitally simulated under the assumption of a two-step reaction of V⁰ following the two-step electrode reactions of V²⁺ to V⁰. The simulated CVs show good agreement with those obtained experimentally, when the first-step reaction of V⁰ is a relatively fast reversible reaction and the second-step reaction is a slow irreversible one. Based on these results, we propose that V⁰ is in pseudo-equilibrium with H⁺ or HA to produce VH⁺ which undergoes a reaction with H₂O.     

The ion selectivity of monensin incorporated phospholipid/alkanethiol bilayers

Monensin was incorporated into phospholipid/alkanethiol bilayers on the gold electrode surface by a new, paint-freeze method to deposit a lipid monolayer on the self-assembled monolayers (SAMs) of alkanethiol. The advantages of this assembly system with a suitable function for investigating the ion selective transfer across the mimetic biomembrane are based on the characteristics of SAMs of alkanethiols and monensin. On the one hand, the SAMs of alkanethiols bring out their efficiency of packing and coverage of the metal substrate and relatively long-term stability; on the other hand, monensin improves the ion selectivity noticeably. The selectivity coefficients K_Na+,K+, K_Na+,Rd+, and K_Na+,Ag+ are 6×10^?2, 7.2 × 10^?3 and 30 respectively. However, the selectivity coefficient K_Na+,Li+ could not be obtained by a potentiometric method due to the specific interaction between Li⁺ and phospholipid and the lower degree of complexion between Li⁺ and monensin. The potential response of this bilayer system to monovalent ions is fairly good. For example, the slope of the response to Na⁺ is close to 60 mV per decade and its linearity range is from 10^?1 to 10^?5m with a detection limit of 2 × 10^?6m. The bilayer is stable for at least two months without changing its properties. This monensin incorporated lipid/alkanethiol bilayer is a good mimetic biomembrane system, which provides great promise for investigating the ion transfer mechanism across the biomembrane and developing a practical biosensor.     

Synthetic semiconductor diamond electrodes: kinetics of some redox reactions

Linear-sweep voltammograms were recorded on boron-doped diamond thin-film electrodes in K₃Fe(CN)₆, K₄Fe(CN)₆, quinone, and hydroquinone solutions; transfer coefficients and rate constants for anodic and cathodic reactions were determined. The reaction rates were compared to the diamond samples' electrical characteristics measured by the impedance spectroscopy techniques. The polarization (faradaic) resistance of a redox reaction at its equilibrium potential was shown to be proportional to the specific resistance of diamond samples. Reaction rate constants for the quinone/hydroquinone system appeared to be significantly less than those for the Fe(CN)₆^3?/4? system.     

The role of the potential distribution at the electrode interface in determining the electron transfer rate constant

Simultaneous electrochemical impedance (EI) and electroreflectance (ER) measurements were performed on ‘bare’ gold, N-acetyl-l-cysteine(NAC) modified gold, and cytochrome c (cyt c) adsorbed on a NAC modified gold electrode. The electrical impedance of the electrode interface was modeled by a series connection of a constant phase element (CPE), a capacitor, and a resistor. The analysis of the combined EI and ER data yielded the heterogeneous electron transfer (ET) rate constant at each frequency of the modulating potential. This is in contrast to previous techniques which used the frequency dependence of the EI or ER response to obtain a value of the rate constant. Assuming that the Faradaic potential was equal to the potential at the nodes of the three element impedance model yielded frequency dependent rate constants. A small modification of the three element impedance model was needed to obtain a frequency independent rate constant of 850±80 s^?1. This suggests that the distribution of the potential at the electrode interface (reflected in the choice of the electrical impedance model) is an important factor in the determination of the ET rate constant.     

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.