Redox energetics and kinetics of uranyl coordination complexes in aqueous solution

Detailed voltammetric results for five uranyl coordination complexes are presented and analyzed using digital simulations of the voltammetric data to extract thermodynamic (E(1/2)) and heterogeneous electron-transfer kinetic (k(0) and alpha) parameters for the one-electron reduction of UO(2)(2+) to UO(2)(+). The complexes and their corresponding electrochemical parameters are the following: [UO(2)(OH(2))(5)](2+) (E(1/2) = -0.169 V vs Ag/AgCl, k(0) = 9.0 x 10(-3) cm/s, and alpha = 0.50); [UO(2)(OH)(5)](3-) (-0.927 V, 2.8 x 10(-3) cm/s, 0.46); [UO(2)(C(2)H(3)O(2))(3)](-) (-0.396 V, approximately 0.1 cm/s, approximately 0.5); [UO(2)(CO(3))(3)](4-) (-0.820 V, 2.6 x 10(-5) cm/s, 0.41); [UO(2)Cl(4)](2-) (-0.065 V, 9.2 x 10(-3) cm/s, 0.30). Differences in the E(1/2) values are attributable principally to differences in the basicity of the equatorial ligands. Differences in rate constants are considered within the context of Marcus theory of electron transfer, but no specific structural change(s) in the complexes between the two oxidation states can be uniquely identified with the underlying variability in the heterogeneous rate constants and electron-transfer coefficients.     

Influence of electronic and structural effects on the oxidative behavior of nickel porphyrins

With the aim of better understanding the electronic and structural factors which govern electron-transfer processes in porphyrins, the electrochemistry of 29 nickel(II) porphyrins has been examined in dichloromethane containing either 0.1 M tetra-n-butylammonium perchlorate (TBAP) or tetra-n-butylammonium hexafluorophosphate (TBAPF(6)) as supporting electrolyte. Half-wave potentials for the first oxidation and first reduction are only weakly dependent on the supporting electrolyte, but E(1/2) for the second oxidation varies considerably with the type of supporting electrolyte. E(1/2) values for the first reduction to give a porphyrin pi-anion radical are effected in large part by the electronic properties of the porphyrin macrocycle substituents, while half-wave potentials for the first oxidation to give a pi-cation radical are affected by the substituents as well as by nonplanar deformations of the porphyrin macrocycle. The potential difference between the first and second oxidations (Delta/Ox(2) - Ox(1)/) is highly variable among the 29 investigated compounds and ranges from 0 mV (two overlapped oxidations) to 460 mV depending on the macrocycle substituents and the anion of the supporting electrolyte. The magnitude of Delta/Ox(2) - Ox(1)/ is generally smaller for compounds with very electron-withdrawing substituents and when TBAP is used as the supporting electrolyte. This behavior is best explained in terms of differences in the binding strengths of anions from the supporting electrolyte (ClO(4)(-) or PF(6)(-)) to the doubly oxidized species. A closer analysis suggests two factors which are important in modulating Delta/Ox(2) - Ox(1)/ and thus the binding affinity of the anion to the porphyrin dication. One is the type of pi-cation radical (a proxy for the charge distribution in the dication), and the other is the conformation of the porphyrin macrocycle (either planar or nonplanar). These findings imply that the redox behavior of porphyrins can be selectively tuned to display separate or overlapped oxidation processes.     

Overoxidation of phenol by hexachloroiridate(IV)

It has been previously established that the aqueous oxidation of phenol by a deficiency of [IrCl(6)](2-) proceeds through the production of [IrCl(6)](3-) and phenoxyl radicals. Coupling of the phenoxyl radicals leads primarily to 4,4'-biphenol, 2,2'-biphenol, 2,4'-biphenol, and 4-phenoxyphenol. Overoxidation occurs through the further oxidation of these coupling products, leading to a rather complex mixture of final products. The rate laws for oxidation of the four coupling products by [IrCl(6)](2-) have the same form as those for the oxidation of phenol itself: -d[Ir(IV)]/dt = {(k(ArOH) + k(ArO(-))K(a)/[H(+)])/(1 + K(a)/[H(+)])}[ArOH](tot)[Ir(IV)]. Values for k(ArOH) and k(ArO(-)) have been determined for the four substrates at 25 °C and are assigned to H(2)O-PCET and electron-transfer mechanisms, respectively. Kinetic simulations of a combined mechanism that includes the rate of oxidation of phenol as well as the rates of these overoxidation steps show that the degree of overoxidation is rather limited at high pH but quite extensive at low pH. This pH-dependent overoxidation leads to a pH-dependent stoichiometric factor in the rate law for oxidation of phenol and causes some minor deviations in the rate law for oxidation of phenol. Empirically, these minor deviations can be accommodated by the introduction of a third term in the rate law that includes a "pH-dependent rate constant", but this approach masks the mechanistic origins of the effect.     

Electrochemistry of nanopore electrodes in low ionic strength solutions

The steady-state voltammetric behavior of truncated conical nanopore electrodes (20-200 nm orifice radii) has been investigated in low ionic strength solutions. Voltammetric currents at the nanopore electrode reflect both diffusive and migrational fluxes of the redox molecule and, thus, are strongly dependent on the charge of the redox molecule and the relative concentrations of the supporting electrolyte and redox molecule. In acetonitrile solutions, the limiting current for the oxidation of the positively charged ferrocenylmethyltrimethylammonium ion is suppressed at low supporting electrolyte concentrations, while the limiting current for the oxidation of the neutral species ferrocene is unaffected by the ionic strength. The dependence of the limiting current on the relative concentrations of the supporting electrolyte and redox molecule is accurately predicted by theory previously developed for microdisk electrodes. Anomalous values of the voltammetric half-wave potential observed at very small nanopore electrodes (<50 nm radius orifice radii) are ascribed to a boundary potential between the pore interior and bulk solution (i.e., a Donnan-type potential).     

Electrochemically triggered Michael addition on the self-assembly of 4-thiouracil: generation of surface-confined redox mediator and electrocatalysis

Generation of a surface-confined redox mediator (RM) by an electrochemically triggered Michael addition reaction and the electrocatalytic properties of the mediator are described. Electrogenerated o-quinone undergoes Michael addition reaction with the self-assembled monolayer (SAM) of 4-thiouracil (4-TU) on a gold (Au) electrode and yields a surface-confined RM, 1-(3,4-dihydroxyphenyl)-4-mercapto-1H-pyrimidin-2-one (DPTU). The Michael addition reaction depends on the electrolysis potential and time, solution pH, and concentration of catechol (CA) used in the reaction. The redox mediator, DPTU, exhibits reversible redox response, characterstic of a surface-confined species at approximately 0.22 V in neutral pH. The anodic peak potential of DPTU shifts by 58+/-2 mV while changing the solution pH by one unit, suggesting that protons and electrons taking part in the redox reaction are in the ratio of 1:1. The apparent rate constant (ksapp) for the heterogeneous electron-transfer reaction of the RM was determined to be 114+/-5 s-1. The surface coverage (Gamma) of DPTU on the electrode surface was 8.2+/-0.1x10(-12) mol/cm2. DPTU shows excellent electrocatalytic activity toward oxidation of reduced nicotinamide adenine dinucleotide (NADH) with activation overpotential, which is approximately 600 mV lower than that observed at the unmodified Au electrode. The dipositive cations in the supporting electrolyte solution amplify the electrocatalytic activity of DPTU. A 2.5-fold enhancement in the catalytic current was observed in the presence of Ca2+ or Ba2+ ions. The sensitivity of the electrode toward NADH in the presence and absence of Ca2+ ions was 0.094+/-0.011 and 0.04+/-0.0071 nA cm-2 nM-1, respectively. A linear increase in the catalytic current was obtained up to the concentration of 0.8 mM, and the electrode can detect amperometrically as low as 25 nM of NADH in neutral pH.     

Investigations on the kinetics of electron transfer reactions in magnetic fields

There is a controversial debate if a magnetic field can influence the rate of electron transfer (ET) reactions. In this paper, we report kinetic measurements of the ET rate constants for the redox couples [IrCl6]2-/[IrCl6]3-, [Fe(CN)6]3-/[Fe(CN)6]4-, and [Fe(H2O)6]3+/[Fe(H2O)6]2+ in magnetic fields up to 1 T. To reduce effects arising from magnetically induced mass transport (magnetohydrodynamic effect), disk microelectrodes with a diameter of 50 microm were used in potentiodynamic (cyclic and linear sweep voltammetry) and in electrochemical impedance spectroscopy experiments. None of the investigated redox couples showed a magnetic field effect on the ET rate constant.     

Application of voltammetric methods for the analysis of gold and other transition metals in quartzite rock

Simple, rapid and accurate voltammetric methods viz. DCP, DPP and DPASV have been presented for the trace determination of gold and other transition metals in quartzite rock sample. The analysis has been performed using 0.1 M(NH4)2 tartrate, 0.1 M NaClO4 and 1 M NaOH as supporting electrolyte with 0.001% gelatin as maximum suppressor. The results show the presence of CuII(10.70), CoII(4.72), FeIII(66.96), AuIII(0.066), ZnII(1.68) and CdII(0.62) mg x g(-1) metal ions from the sample. Gold produced a well defined wave/peak with E1,2/Ep = -0.61 V/-0.64 V vs SCE, in 1 M NaOH supporting electrolyte. The quantitative analysis of metal inos was carried out by the method of standard addition. Statistical treatment of the observed voltammetric data reveals high accuracy and good precision of determination. The observed voltammetric results are comparable with those obtained using AAS method.     

Cyclic control of the surface properties of a monolayer-functionalized electrode by the electrochemical generation of Hg nanoclusters

Hg(2+) ions are bound to a 1,4-benzenedimethanethiol (BDMT) monolayer assembled on a Au electrode. Electrochemical reduction of the Hg(2+)-BDMT monolayer to Hg(+)-BDMT (at E degrees =0.48 V) and subsequently to Hg(0)-BDMT (at E degrees =0.2 V) proceeds with electron-transfer rate constants of 8 and 11 s(-1), respectively. The Hg(0) atoms cluster into aggregates that exhibit dimensions of 30 nm to 2 microm, within a time interval of minutes. Electrochemical oxidation of the nanoclusters to Hg(+) and further oxidation to Hg(2+) ions proceeds with electron-transfer rate constants corresponding to 9 and 43 s(-1), respectively, and the redistribution of Hg(2+) on the thiolated monolayer occurs within approximately 15 s. The reduction of the Hg(2+) ions to the Hg(0) nanoclusters and their reverse electrochemical oxidation proceed without the dissolution of mercury species to the electrolyte, implying high affinities of Hg(2+), Hg(+), and Hg(0) to the thiolated monolayer. The electrochemical transformation of the Hg(2+)-thiolated monolayer to the Hg(0)-nanocluster-functionalized monolayer is characterized by electrochemical means, surface plasmon resonance (SPR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), atomic force microscopy (AFM), and contact-angle measurements. The Hg(0)-nanocluster-modified surface reveals enhanced hydrophobicity (contact angle 76 degrees ) as compared to the Hg(2+)-thiolated monolayer (contact angle 57 degrees ). The hydrophobic properties of the Hg(0)-nanocluster-modified electrode are further supported by force measurements employing a hydrophobically modified AFM tip.     

Ion-exchange voltammetry of dopamine at Nafion-coated glassy carbon electrodes: quantitative features of ion-exchange partition and reassessment on the oxidation mechanism of dopamine in the presence of excess ascorbic acid

The incorporation/exclusion features of dopamine (DA), ascorbic acid (AA) and uric acid (UA) are evaluated for Nafion (NA)-coated glassy carbon electrodes (GCE) of different thicknesses. The ion-exchange partition of DA(+) between the NA film and the sodium phosphate electrolyte is evaluated by determining the partition coefficient (k(D)) and the apparent diffusion coefficient (D(app)) in thick NA films which were 401 and 1.5 x 10(-9) cm(2) s(-1), respectively. The solution diffusion coefficient was found to be 6.0 x 10(-6) cm(2) s(-1). Also, the effect of NA loading and of the voltammetric timescale on DA voltammetry in the presence of excess AA is assessed, at physiologic like conditions. It is demonstrated that, although AA is excluded at the NA coating, a catalytic regeneration of DA, induced by AA, occurs at the interface NA film/electrolyte resulting from the diffusion of the o-quinone product of DA oxidation from the electrode surface to that interface. The interference of AA in the voltammetric signal of DA is eliminated using 18 microg mm(-2) NA films and v> or =0.5 V s(-1). Therefore, fast, selective and sensitive voltammetric analysis of DA at concentrations<100 microM in the presence of excess AA, e.g., 1 mM is achieved.     

Influence of redox molecules on the electronic conductance of single-walled carbon nanotube field-effect transistors: application to chemical and biological sensing

In an effort to develop sensitive nanoscale devices for chemical and biological sensing, we have examined, using liquid gating, the conductance of semiconducting single-walled carbon nanotube-based field-effect transistors (SWCNT-FETs) in the presence of redox mediators. As examples, redox couples K3Fe(CN)6/K4Fe(CN)6 and K2IrCl6/K3IrCl6 are shown to modulate the SWCNT-FET conductance in part through their influence via the electrolyte gate on the electrostatic potential of the solution, as described by Larrimore et al. (Nano Lett. 2006, 6, 3129-1333) and in part through electron transfer between the redox mediators and the nanotubes. In the latter case, the rate of electron transfer is determined by the difference in chemical potential between the redox mediator and the SWCNTs and by the concentrations of the oxidized and reduced forms of the redox couple. Furthermore, these devices can detect the activity of redox enzymes through their sensitivity to the change in oxidation state of the enzyme substrate. An example is given for the blue copper oxidase, Trametes versicolor laccase, in which the rate of change of the SWCNT device conductance is linearly proportional to the rate of oxidation of the substrate 10-(2-hydroxyethyl)phenoxazine, varied over 2 orders of magnitude by the laccase concentration in the picomolar range. The behavior described in this work provides a highly sensitive means with which to do chemical and biological sensing using SWCNTs that is different from the amperometric, capacitive, and field-effect type sensing methods previously described in the literature for this material.     

Fabrication of carbon microelectrodes with an effective radius of 1 nm

A method for producing insulated nanometer-sized carbon electrodes is presented. These electrodes are produced using electrochemical etching of carbon fibers followed by deposition of electrophoretic paint. A new deposition approach for insulating the tips, the so-called “inverted deposition” technique, is introduced. This technique allows complete insulation of the whole body of the etched carbon fiber except for the very tip, leaving an electrochemical active area with effective diameters as small as a few nanometers. The process overcomes pinhole formation that can be a problem with the normal electrophoretic paint deposition process. The fabricated electrodes show ideal steady-state voltammetric behavior. The voltammetric response corresponding to the reduction of hexacyanoferrate(III) and hexaammineruthenium(III) is investigated on these small electrodes in the absence and presence of supporting electrolyte. For these two multiple-charged ions the steady-state voltammetric behavior in the absence of supporting electrolyte is found to deviate from expected behavior, especially at very small electrodes.     

Studies of the inner reorganization energies of the cation radicals of 1,4-bis(dimethylamino)benzene, 9,10-bis(dimethylamino)anthracene, and 3,6-bis(dimethylamino)durene by photoelectron spectroscopy and reinterpretation of the mechanism of the electrochemical oxidation of the parent diamines

The inner reorganization energy of the cation radical of 1,4-bis(dimethylamino)benzene, 1, has been determined to be 0.72 +/- 0.02 eV by means of gas-phase photoelectron spectroscopy (PES). PES studies of 9,10-bis(dimethylamino)anthracene, 2, and 3,6-bis(dimethylamino)durene, 3, demonstrate that their reorganization energies are smaller than that of 1. The effect of lowering the inner reorganization energy on the rate constant for an electrochemical electron-transfer reaction is to increase the electron-transfer rate constant, k(s). However, voltammetric studies of the two-electron oxidation of 2 and 3 indicate that the values of k(s) for each step are smaller than those for 1, in contradistinction to the measured differences in reorganization energies. The voltammetric studies of 2 and 3 were reinterpreted according to a mechanism in which each step of oxidation was written as a two-step process, electron transfer with a small inner reorganization energy plus a chemical step of structural change. The agreement of simulations according to this mechanism with the experimental data was excellent. The new reaction scheme eliminated some suspicious features previously obtained with an analysis where electron transfer and structural change were considered to be concerted. In particular, all electron-transfer coefficients (alpha) were close to one-half, whereas the earlier treatment produced values of alpha much larger or smaller than one-half.     

Modulating the redox properties of an osmium-containing metallopolymer through the supporting electrolyte and cross-linking

Thin films of the perchlorate salt of an [Os(N,N'-alkylated-2,2'-biimidazole3)2+/3+-containing polymer have been formed on planar platinum microelectrodes. The electrochemical response associated with the Os2+/3+ couple occurs at -0.19 V. In aqueous perchlorate media at near-neutral pH the voltammetric response is close to that expected for an electrochemically reversible reaction involving a surface-confined reactant. Chronoamperometry conducted on a microsecond time scale indicates that the film and solution resistances are comparable for low concentrations of supporting electrolyte. However, for LiClO4 concentrations greater than 0.4 M, RFilm contributes less than 25% of the overall cell resistance. These results suggest that when the film is dehydrated and the density of redox centers is increased, electron or hole hopping dominates the rate of homogeneous charge transport through the film. The rate of homogeneous charge transport, characterized by D(CT)1/2Ceff, where DCT is the homogeneous charge transport diffusion coefficient and Ceff is the effective concentration of osmium centers within the film, depends weakly on the concentration of LiClO4 as supporting electrolyte decreasing from (8.1 +/- 0.16) x 10(-9) to (4.7 +/- 0.4) x 10(-9) mol cm(-2) s(-1/2) as the perchlorate concentration increases from 0.1 to 1.0 M. These values are about 2 orders of magnitude lower than those of the chemically cross-linked chloride salt of the polymer. The rate of heterogeneous electron transfer is unusually rapid in this system and increases from (5.2 +/- 0.4) x 10(-3) to (7.8 +/- 0.4) x 10(-3) cm s(-1) on going from 0.1 to 0.4 M LiClO4 before becoming independent of the supporting electrolyte concentration at (9.2 +/- 0.6) x 10(-3) cm s(-1) for [LiClO4] > or = 0.6 M.     

Voltammetric behavior of vitamin B(2) on the gold electrode modified with a self-assembled monolayer of l-cysteine and its application for the determination of vitamin B(2) using linear sweep stripping voltammetry

The voltammetric behavior of Vitamin B(2) (VB(2)) has been studied at the gold electrode modified with a self-assembled monolayer of l-cysteine. The voltammetric responses are evaluated with respect experimental conditions, such as composition and pH of the supporting electrolyte, concentration of VB(2), accumulation potential and accumulation time. On basis of the voltammetric behavior a highly sensitive method is present for the determination of VB(2) by using linear sweep stripping volammetry. The method is suitable for the determination of VB(2) concentrations between 5.0x10(-11) and 5.0x10(-6) mol l(-1). And the detection limit can be reached to 2.5x10(-11) mol l(-1). The method is applied to determine the concentration of VB(2) in the tablets with satisfactory results.     

Electron self-exchange in the solid-state: cocrystals of hydroquinone and bipyridyl triazole.

Solid-state voltammetry, spectroscopy, and microscopy studies have been used to probe the proton and electron conductivity within a self-assembled cocrystal, HQBpt. This crystallographically defined material contains 3,5-bis(pyridin-2-yl)-1,2,4-triazole, HBpt, dimers that are pi-stacked and hydrogen bonded to 1,4-hydroquinone, H(2)Q, in a herringbone arrangement. When deposited onto platinum microelectrodes, the cocrystal exhibits a well-defined voltammetric response corresponding to oxidation of H(2)Q to the quinone, Q, across a wide range of voltammetric time scales, electrolyte compositions, and pH values. Scanning electron microscopy reveals that redox cycling in aqueous perchlorate solutions in which the pH is systematically varied from 1 to 7 triggers electrocrystallization and the extensive formation of rodlike crystals. Fast scan rate voltammetry reveals that the homogeneous charge transport diffusion coefficient, D(app), is independent of the perchlorate concentration for 0.1 < [ClO(4)(-)] < 1.0 M (pH 6.6) at 3.14 +/- 0.11 x 10(-)(9) cm(2) s(-)(1). Moreover, D(app) is independent of the perchloric acid concentration for concentrations greater than approximately 2.0 M, maintaining a value of 4.81 +/- 0.07 x 10(-)(8) cm(2) s(-)(1). The observation that D(app) is independent of the supporting electrolyte suggests that the rate-determining step for homogeneous charge transport is not the availability of charge-compensating counterions or protons, but the dynamics of electron self-exchange between H(2)Q and Q. We have used the Dahms-Ruff formalism to determine electron self-exchange rate constants which are 2.84 +/- 0.22 x 10(9) and 9.69 +/- 0.73 x 10(10) M(-)(1) s(-)(1) for pH values greater than approximately 2.0 and less than -0.3, respectively. Significantly, these values are more than 2 orders of magnitude larger that those found for benzoquinone self-exchange reactions in aqueous solution. These results indicate that hydrogen bonds play an important role in supporting rapid electron transfer. The increase in D(app) between pH 1.0 and -0.3 is associated with protonation of the HBpt moieties, which triggers a reversible change in the material's structure.     

Quasi-octahedral complexes of pentamethylcyclopentadienyliridium(III) bearing bis(diphenylphosphinomethyl)phenylphosphine (dpmp)

Reaction of [Cp*IrCl2]2 (1) with dpmp in the presence of KPF6 afforded a binuclear complex [Cp*IrCl(dpmp-P1,P2;P3)IrCl2Cp*](PF6) (2) (dpmp =(Ph2PCH2)2PPh). The mononuclear complex [Cp*IrCl(dpmp-P1,P2)](PF6) (4) was generated by the reaction of [Cp*IrCl2(BDMPP)](BDMPP =PPh[2,6-(MeO)2C6H3]2) with dpmp in the presence of KPF6. These mono- and binuclear complexes have four-membered ring structures with a terminal and a central P atom of the dpmp ligand coordinated to an iridium atom as a bidentate ligand. Since there are two chiral centers at the Ir atom and a central P2 atom, there are two diastereomers that were characterized by spectrometry. Complexes anti-4 and syn-4 reacted with [Cp*RhCl2]2 or [(C6Me6)RuCl2]2, giving the corresponding mixed-metal complexes, anti- and syn- [Cp*IrCl(dppm-P1,P2;P3)MCl2L](PF6) (6: M = Rh, L = Cp*; 7: M = Ru, L = C6Me6). Treatment with AuCl(SC4H8) gave tetranuclear complexes, anti- and syn-8 [[Cp*IrCl(dppm-P1,P2;P3)AuCl]2](PF6)2 bearing an Au-Au bond. Reaction of anti- with PtCl2(cod) generated the trinuclear complex anti-9, anti-[[Cp*IrCl(dppm-P1,P2;P3)]2PtCl2](PF6)2. These reactions proceeded stereospecifically. The P,O-chelated complex syn-[Cp*IrCl(BDMPP-P,O)] (syn-10)(BDMPP-P,O = PPh[2,6-(MeO)2C6H3][2-O-6-(MeO)C6H3]2) reacted with dpmp in the presence of KPF6, generating the corresponding anti-complex as a main product as well as a small amount of syn-complex, [Cp*Ir(BDMPP-P,O)(dppm-P1)](PF6) (11). The reaction proceeded preferentially with inversion. The reaction processes were investigated by PM3 calculation. anti- was treated with MCl2(cod), giving anti-[Cp*Ir(BDMPP-P,O)(dppm-P1;P2,P3)MCl2](PF6)(14: M = Pt; 15: M = Pd), in which the MCl2 moiety coordinated to the two free P atoms of anti-11. The X-ray analyses of syn-2, anti-2, anti-4, anti-8 and anti-11 were performed.     

Simple electrochemical method for deposition and voltammetric inspection of silver particles at the liquid-liquid interface of a thin-film electrode

A novel experimental methodology for depositing and voltammetric study of Ag nanoparticles at the water-nitrobenzene (W-NB) interface is proposed by means of thin-film electrodes. The electrode assembly consists of a graphite electrode modified with a thin NB film containing decamethylferrocene (DMFC) as a redox probe. In contact with an aqueous electrolyte containing Ag(+) ions, a heterogeneous electron-transfer reaction between DMFC((NB)) and Ag(+)((W)) takes place to form DMFC(+)((NB)) and Ag deposit at the W-NB interface. Based on this interfacial reaction, two different deposition strategies have been applied. In the uncontrolled potential deposition protocol, the electrode is immersed into an AgNO(3) aqueous solution for a certain period under open circuit conditions. Following the deposition step, the Ag-modified thin-film electrode is transferred into an aqueous electrolyte free of Ag(+) ions and voltammetrically inspected. In the second protocol the deposition was carried out under controlled potential conditions, i.e., in an aqueous electrolyte solution containing Ag(+) ions by permanent cycling of the electrode potential. In this procedure, DMFC((NB)) is electrochemically regenerated at the electrode surface, hence enabling continuation and voltammetric control of the Ag deposition. Hence, the overall electrochemical process can be regarded as an electrochemical reduction of Ag(+)((W)) at the W-NB interface, where the redox couple DMFC(+)/DMFC acts as a mediator for shuttling electrons from the electrode to the W-NB interface. Ag-particles deposited at the W-NB interface affect the ion transfer across the interface, which provides the basis for voltammetric inspection of the metal deposit at the liquid-liquid interface with thin-film electrodes. Voltammetric properties of thin-film electrodes are particularly sensitive to the deposition procedure, reflecting differences in the properties of the Ag deposit. Moreover, this methodology is particularly suited to inspect catalytic activities of metal particles deposited at the liquid-liquid interface toward heterogeneous electron-transfer reactions occurring at the at the liquid-liquid interface.     

Integration of polyaniline/poly(acrylic acid) films and redox enzymes on electrode supports: an in situ electrochemical/surface plasmon resonance study of the bioelectrocatalyzed oxidation of glucose or lactate in the integrated bioelectrocatalytic systems

Electropolymerization of aniline in the presence of poly(acrylic acid) on Au electrodes yields a polyaniline/poly(acrylic acid) composite film, exhibiting reversible redox functions in aqueous solutions at pH = 7.0. In situ electrochemical-SPR measurements are used to identify the dynamics of swelling and shrinking of the polymer film upon the oxidation of the polyaniline (PAn) to its oxidized state (PAn(2+)) and the reduction of the oxidized polymer (PAn(2+)) back to its reduced state (PAn), respectively. Covalent attachment of N(6)-(2-aminoethyl)-flavin adenin dinucleotide (amino-FAD, 1) to the carboxylic groups of the composite polyaniline/poly(acrylic acid) film followed by the reconstitution of apoglucose oxidase on the functional polymer yields an electrically contacted glucose oxidase of unprecedented electrical communication efficiency with the electrode: electron-transfer turnover rate approximately 1000 s(-1) at 30 degrees C. In situ electrochemical-SPR analyses are used to characterize the bioelectrocatalytic functions of the biomaterial-polymer interface. The current responses of the bioelectrocatalytic system increase as the glucose concentrations are elevated. Similarly, the SPR spectra of the system are controlled by the concentration of glucose. The glucose concentration controls the steady-state concentration ratio of PAn/PAn(2+) in the film composition. Therefore, the SPR spectrum of the film measured upon its electrochemical oxidation is shifted from the spectrum typical for the oxidized PAn(2+) at low glucose concentration to the spectrum characteristic of the reduced PAn at high glucose concentration. Similarly, the polyaniline/poly(acrylic acid) film acts as an electrocatalyst for the oxidation of NADH. Accordingly, an integrated bioelectrocatalytic assembly was constructed on the electrode by the covalent attachment of N(6)-(2-aminoethyl)-beta-nicotinamide adenine dinucleotide (amino-NAD(+), 2) to the polymer film, and the two-dimensional cross-linking of an affinity complex formed between lactate dehydrogenase and the NAD(+)-cofactor units associated with the polymer using glutaric dialdehyde as a cross-linker. In situ electrochemical-SPR measurements are used to characterize the bioelectrocatalytic functions of the system. The amperometric responses of the system increase as the concentrations of lactate are elevated, and an electron-transfer turnover rate of 350 s(-1) between the biocatalyst and the electrode is estimated. As the PAn(2+) oxidizes the NADH units generated by the biocatalyzed oxidation of lactate, the PAn/PAn(2+) steady-state ratio in the film is controlled by the concentration of lactate. Accordingly, the SPR spectrum measured upon electrochemical oxidation of the film is similar to the spectrum of PAn(2+) at low lactate concentration, whereas the SPR spectrum resembles that of PAn at high concentrations of lactate.     

Enantioselective iridium-catalyzed carbonyl allylation from the alcohol or aldehyde oxidation level using allyl acetate as an allyl metal surrogate

Protocols for highly enantioselective carbonyl allylation from the alcohol or aldehyde oxidation level are described based upon transfer hydrogenative C-C coupling. Exposure of allyl acetate to benzylic alcohols 1a-i in the presence of an iridium catalyst derived from [IrCl(cod)]2 and (R)-BINAP delivers products of C-allylation 2a-i. Employing isopropanol as terminal reductant, exposure of allyl acetate to aryl aldehydes 3a-i in the presence of an iridium catalyst derived from [IrCl(cod)]2 and (-)-TMBTP delivers identical products of C-allylation 2a-i. In all cases examined, exception levels of enantioselectivity are observed. Thus, enantioselective carbonyl allylation is achieved from the alcohol or aldehyde oxidation level in the absence of any preformed allylmetal reagents. These studies define a departure from preformed organometallic reagents in carbonyl additions that transcend the boundaries of oxidation level.     

Solvent effects on charge transport through solid deposits of [Os(4,4'-diphenyl-2,2'-dipyridyl)2Cl2

Mechanically attached, solid-state films of [Os(4,4'-diphenyl-2,2'-dipyridyl)2Cl2] have been formed on gold macro- and microelectrodes and their voltammetric properties investigated. The voltammetric response of these films associated with the Os(2+/3+) redox reaction is reminiscent of that observed for an ideal reversible, solution phase redox couple only when the contacting electrolyte contains of the order of 40% v/v of acetonitrile (ACN). The origin of this effect appears to involve preferential solvation of the redox centres by acetonitrile which facilitates the incorporation of charge compensating counterions. Scanning electron microscopy reveals that voltammetric cycling in 40:60 ACN-H2O containing 1.0 M LiClO4 as the electrolyte induces the formation of microcrystals. Voltammetry conducted under semi-infinite linear diffusion conditions has been used to determine the apparent diffusion coefficient, Dapp, for homogeneous charge transport through the deposit. The dynamics of charge transport decrease with increasing film thickness but appear to increase with increasing electrolyte concentration. These observations suggest that ion transport rather than the rate of electron self-exchange limit the overall rate of charge transport through these solids. When in contact with 40:60 ACN-H2O containing 1.0 M LiClO4 as electrolyte, Dapp values for oxidation and reduction are identical at 1.7 +/- 0.4 x 10(-12) cm2 s(-1). In the same electrolyte, the standard heterogeneous electron transfer rate constant, k(o), determined by fitting the full voltammogram using the Butler-Volmer formalism, is 8.3 +/- 0.5 x 10(-7) cm s(-1). The importance of these results for the rational design of solid state redox active materials for battery, display and sensor applications is considered.