Transient electrochemistry: beyond simply temporal resolution.

Some physicochemical intrigues for which transient electrochemistry was necessary to solve the problem are summarized in this feature article. First, we highlight the main constraints to be aware of to access to low time scales, and particularly focus on the effects of stray capacitances. Then, the electron transfer rate constant measured for redox molecules in a self-assembled monolayer configuration is compared to the conductance measured through the same systems, but at the single molecule level. This evidences strong conformational changes when molecules are trapped in the nanogap created between both electrodes. We also report about dendrimers, for which a short electrochemical perturbation induces creation of a diffusion layer within the molecule, allowing the electron hopping rate to be measured and analyzed in terms of molecular motions of the redox centers. Finally, we show that transient electrochemistry provides also useful information when coupled to other methodologies. For example, when an ultrasonic field drives very fast movements of a bubble situated above the electrode surface, the motion can be detected indirectly through a modification of the diffusion flux. Another field concerns pulse radiolysis, and we describe how the reactivity (at the electrode or within the solution) of radicals created by a radiolytic pulse can be quantified, widening the possibilities of electrochemistry to operate in biological media.

Numerical simulation of diffusion processes at recessed disk microelectrode arrays using the quasi-conformal mapping approach.

In this work, we present a theoretical analysis of diffusion processes at arrays of recessed microelectrodes and evaluate the dependence of these processes on the main geometrical parameters (distance between electrodes in the array and slope of side walls of conical recesses) of this complex system. To allow for faster computation time and excellent accuracy, numerical simulations were performed upon transforming the real space allowed for diffusion using a quasi-conformal mapping introduced for this array geometry in our previous work (Amatore, C., Oleinick, A. I. and Svir, I. J. Electroanal. Chem. 2006, 597, 77-85). The applied quasi-conformal mapping is perfectly suited to the considered microelectrode array geometry and ensures that the abrupt change of boundary conditions reflecting the contorted geometries of the considered microelectrode array are treated efficiently and precisely in the simulations.

Pd-catalyzed homocoupling reaction of arylboronic acid: insights from density functional theory.

The key step in the mechanism of the Palladium-catalyzed homocoupling of arylboronic acids ArB(OH)(2)(Ar = 4-Z-C(6)H(4) with Z = MeO, H, CN) in the presence of dioxygen, leading to symmetrical biaryls, has been elucidated by using density functional theory. In particular, by starting from the peroxo complex O(2)PdL(2)(L = PPh(3)), generated in the reaction of dioxygen with the Pd(0) catalyst, the fundamental role played by an intermediate formed by coordination of one oxygen atom of the peroxo complex to the oxophilic boron atom of the arylboronic acid has been pointed out. This adduct reacts with a second molecule of arylboronic acid to generate a cis-Ar-Pd(OOB(OH)(2))L(2) complex that can form the stable intermediate trans-Ar-Pd(OH)L(2) (experimentally characterized) through a sequence of hydrolysis and isomerization reactions. All theoretical insights are in agreement and do substantiate the experimentally postulated mechanism. Furthermore, direct comparison of experimental and computed spectroscopic parameters (here, (31)P chemical shifts) allows us to confirm the formation of the intermediate.

Relaxation of the electrical double layer after an electron transfer approached by Brownian dynamics simulation.

In this paper, the dynamical properties of the electrochemical double layer following an electron transfer are investigated by using Brownian dynamics simulations. This work is motivated by recent developments in ultrafast cyclic voltammetry which allow nanosecond time scales to be reached. A simple model of an electrochemical cell is developed by considering a 1:1 supporting electrolyte between two parallel walls carrying opposite surface charges, representing the electrodes; the solution also contains two neutral solutes representing the electroactive species. Equilibrium Brownian dynamics simulations of this system are performed. To mimic electron transfer processes at the electrode, the charge of the electroactive species are suddenly changed, and the subsequent relaxation of the surrounding ionic atmosphere are followed, using nonequilibrium Brownian dynamics. The electrostatic potential created in the center of the electroactive species by other ions is found to have an exponential decay which allows the evaluation of a characteristic relaxation time. The influence of the surface charge and of the electrolyte concentration on this time is discussed, for several conditions that mirror the ones of classical electrochemical experiments. The computed relaxation time of the double layer in aqueous solutions is found in the range 0.1 to 0.4 ns for electrolyte concentrations between 0.1 and 1 mol L(-1) and surface charges between 0.032 and 0.128 C m(-2).

The effects of vesicular volume on secretion through the fusion pore in exocytotic release from PC12 cells.

Many spikes in amperometric records of exocytosis events initially exhibit a prespike feature, or foot, which represents a steady-state flux of neurotransmitter through a stable fusion pore spanning both the vesicle and plasma membranes and connecting the vesicle lumen to the extracellular fluid. Here, we present the first evidence indicating that vesicular volume before secretion is strongly correlated with the characteristics of amperometric foot events. L-3,4-dihydroxyphenylalanine and reserpine have been used to increase and decrease, respectively, the volume of single pheochromocytoma cell vesicles. Amperometry and transmission electron microscopy have been used to determine that as vesicle size is decreased the frequency with which foot events are observed increases, the amount and duration of neurotransmitter released in the foot portion of the event decreases, and vesicles release a greater percentage of their total contents in the foot portion of the event. This previously unidentified correlation provides new insight into how vesicle volume can modulate the activity of the exocytotic fusion pore.

Spatially resolved electrochemiluminescence on an array of electrode tips.

An array of electrode tips with 6-microm center-to-center spacing, fabricated through chemical etching of an optical fiber bundle, and coated with gold, was used for initiating electrochemiluminescence (ECL) in an aqueous solution of Ru(bpy)3(2+) and tri-n-propylamine (TPrA). ECL generated at the tips of the electrodes in the array was detected with a CCD camera and exhibited both high sensitivity and high resolution. In the case in which the ECL signal could not be distinguished from the background, ECL signals could be obtained by pulsing the array and summing multiple CCD images. The behavior of this array was compared to a second array that consisted of individual electrodes insulated with an electrophoretic paint.

Characterization of the electrochemical oxidation of peroxynitrite: relevance to oxidative stress bursts measured at the single cell level.

The electrochemical signature of peroxynitrite oxidation is reported for the first time, and its mechanism discussed in the light of data obtained by steady-state and transient voltammetry at microelectrodes. Peroxynitrite is an important biological species generated by aerobic cells presumably via the near diffusion-limited coupling of nitric oxide and superoxide ion. Its production by living cells has been previously suspected during cellular oxidative bursts as well as in several human pathologies (arthritis, inflammation, apoptosis, ageing, carcinogenesis, Alzheimer disease, AIDS, etc.). However, this could only be inferred on the basis of characteristic patient metabolites or through indirect detection, or by observation of follow-up species resulting supposedly from its chemical reactions in vivo. In this work, thanks to the independent knowledge of the electrochemical characteristics of ONO2- oxidation, the kinetics and intensity of this species released by single human fibroblasts could be established directly and quantitatively based on the application of the artificial synapse method. It was then observed and established that fibroblasts submitted to mechanical stresses produce oxidative bursts, which involve the release within less than a tenth of a second of a complex cocktail composed of several femtomoles of peroxynitrite, hydrogen peroxide, nitric oxide, and nitrite ions.

Mapping concentration profiles within the diffusion layer of an electrode: application to redox catalysis.

Catalytic reductions of some aromatic halides were performed at a millimetric electrode with several redox mediators. The resulting concentration profiles were monitored amperometrically by placing an ultramicroelectrode inside the diffusion layer produced at the former electrode. The features of redox catalysis and the subsequent structuring of the diffusion layer were investigated experimentally under steady-state conditions imposed by the spontaneous convection of the solution. The concentration profiles established from the probe measurements were in agreement with our theoretical predictions, based on fast kinetics of redox catalysis. Under these conditions, very similar to preparative electrosynthesis, the diffusion layer separates into two domains where pure diffusion takes place and the concentration profiles therein are mainly linear. We demonstrate that the limit between these two zones does not depend on kinetics, but is rather fixed by the product of the ratio of the bulk concentrations of each species and the ratio of their diffusion coefficients.

Rate and mechanism of the oxidative addition of phenyl iodide to Pd0 ligated by triphenylarsine: evidence for the formation of a T-shaped complex.

The oxidative addition of phenyl iodide to the palladium(o) generated from [Pd0(dba)2] and n equivalents of AsPh3 (the most efficient catalytic precursor in Stille reactions) proceeds from [(solv)Pd0(AsPh3)2] (solv= solvent). However, the latter is present only in trace concentrations because it is involved in an equilibrium with the major, but nonreactive, complex [Pd0(dba)(AsPh3)2]. As regards the phosphine ligands, dba has a decelerating effect on the rate of the oxidative addition by decreasing the concentration of the reactive species. Relative to PPh3, the effect of AsPh3 is to increase the rate of the oxidative addition of PhI by a factor ten in DMF and seven in THF, independent of the value of n, provided that n > or = 2. In contrast to PPh3, the addition of more than two equivalents of AsPh3 to [Pd0(dba)2] (dba= trans,trans-dibenzylideneacetone) does not affect the kinetics of the oxidative addition because of the very endergonic displacement of dba from [Pd0(dba)(AsPh3)2] to form [Pd0(AsPh3)3]. The complex trans-[PhPdI(AsPh3)2], formed in the oxidative addition, is involved in a slow equilibrium with the T-shaped complex [PhPdI(AsPh3)] after appreciable decomplexation of one AsPh3. Under catalytic conditions, that is, in the presence of a nucleophile, such as CH2=CH-SnBu3 which is able to coordinate to [Pd0(AsPh3)2], a new Pd0 complex is formed: [Pd0(eta2-CH2=CHSnBu3)(AsPh3)2]; however, this complex does not react with PhI. Consequently, CH2=CH-SnBu3 slows down the oxidative addition by decreasing the concentration of the reactive species [(solv)Pd0(AsPh3)2]. This demonstrates that a nucleophile may be not only involved in the transmetallation step, but may also interfere in the kinetics of the oxidative addition step by decreasing the concentration of reactive Pd0.

Precise adjustment of nanometric-scale diffusion layers within a redox dendrimer molecule by ultrafast cyclic voltammetry: an electrochemical nanometric microtome.

Performing cyclic voltammetry at scan rates into the megavolt per second range allows the exploration of the nanosecond time scale as well as the creation of nanometric diffusion layers adjacent to the electrode surface. This latter property is used here to adjust precisely the diffusion layer width within the outer shell of a fourth-generation dendrimer molecule decorated by 64 [Ru(II)(tpy)2] redox centers (tpy = terpyridine). Thus the shape of the dendrimer molecule adsorbed onto the ultramicroelectrode surface can be explored voltammetrically in a way reminiscent of an analysis with a nanometric microtome. The quantitative analysis developed here applied to the experimental voltammograms demonstrates that in agreement with previous scanning tunneling microscopy (STM) studies the adsorbed dendrimer molecules are no more spherical as they are in solution but resemble more closely hemispheres resting onto the electrode surface on their diametrical planes. The same quantitative analysis gives access to the apparent diffusion coefficient featuring electron hopping between the [Ru(II)/ Ru(III)(tpy)2] redox centers distributed on the dendrimer surface. Based on the electron hopping rate constant thus measured and on a Smoluchowski-type model developed here to take into account viscosity effects during the displacement of the [Ru(II)/Ru(III)(tpy)2] redox centers around their equilibrium positions, it is shown that the [Ru(II)/Ru(III)(tpy)2] redox centers are extremely labile in their potential wells so that they may cross-talk considerably more easily than they would do in solution at an equivalent concentration.

Rate and mechanism of the reversible formation of cationic (eta3-allyl)-palladium complexes in the oxidative addition of allylic acetate to palladium(0) complexes ligated by diphosphanes.

The oxidative addition of the allylic acetate, CH2=CH-CH2-OAc, to the palladium(o) complex [Pd0(P,P)], generated from the reaction of [Pd(dba)2, with one equivalent of P,P (P,P = dppb = 1,4-bis(diphenylphosphanyl)butane, and P,P = dppf = 1,1'-bis(diphenylphosphanyl)ferrocene), gives a cationic (eta3-allyl)palladium(II) complex, [(eta3-C3H5)Pd(P,P)+]. with AcO as the counter anion. This reaction is reversible and proceeds through two successive equilibria. The overall equilibrium constants have been determined in DMF. Compared with PPh3, the overall equilibrium lies more in favor of the cationic (eta3-allyl)palladium(II) complex when bidentate P,P ligands are considered in the order: dppb > dppf > PPh3. The reaction proceeds via a neutral intermediate complex [(eta2-CH=CH-CHCH2-OAc)Pd0(P,P)], which has been kinetically detected. The rate constants of the successive steps have been determined in DMF by UV spectroscopy and conductivity measurements. The overall complexation step of the Pd0 by the allylic acetate C=C bond is faster than the oxidative addition/ionization step which gives the cationic (eta3-allyl)palladium(II) complex.

Photochemical generation of cyclopentadienyliron dicarbonyl anion by a nicotinamide adenine dinucleotide dimer analogue.

Irradiation of the absorption band of an NAD (nicotinamide adenine dinucleotide) dimer analogue, 1-benzyl-1,4-dihydronicotinamide dimer, (BNA)(2), in acetonitrile containing a cyclopentadienyliron dicarbonyl dimer, [CpFe(CO)(2)](2), results in generation of 2 equiv of the cyclopentadienyliron dicarbonyl anion, [CpFe(CO)(2)](-), accompanied by the oxidation of (BNA)(2) to yield 2 equiv of BNA(+). The studies on the quantum yields, the electrochemistry, and the transient absorption spectra have revealed that the photochemical generation of [CpFe(CO)(2)](-) by (BNA)(2) proceeds via photoinduced electron transfer from the triplet excited state of (BNA)(2) to [CpFe(CO)(2)](2).

Ultrafast voltammetry of adsorbed redox active dendrimers with nanometric resolution: an electrochemical microtome.

Megavolt-per-second cyclic voltammetry is used to control the expansion with time of a diffusion layer created within the redox outer shell of a fourth generation dendrimer molecule adsorbed onto an electrode; one quarter of the dendrimer is shown in the picture. This allows the measurement of the degree of communication between the ruthenium(II/III) bis(terpyridyl) ("LRuL") redox centers borne at the extremity of the dendrimer tethers, as well as the characterization of the shape of the adsorbed dendrimer molecule.

Oxidative addition of allylic carbonates to palladium(0) complexes: reversibility and isomerization

The oxidative addition of a cyclic allylic carbonate to the palladium(0) complex generated from a [Pd(dba)2]+2 PPh3 mixture affords a cationic pi-allylpalladium(II) complex with the alkyl carbonate as the counter-anion. This reaction is reversible and proceeds with isomerization of the allylic carbonate at the allylic position. The equilibrium constant has been determined in DMF. The influence of the precursor of the palladium(0) is discussed.

Oxidative addition of palladium(0) complexes generated from

The major complex formed in solution from [[Pd0(dba)2]+1P-N] mixtures is [Pd0(dba)(P-N)] (dba=trans,trans-dibenzylideneacetone; P-N=PhPN, 1-dimethylamino-2-diphenylphosphinobenzene; FcPN, N,N-dimethyl-1-[2-(diphenylphosphino)ferrocenyl]methylamine; OxaPN, 4,4'-dimethyl-2-(2-diphenylphosphinophenyl)-1,3-oxazoline). Each complex consists of a mixture of isomers involved in equilibria: two 16-electron rotamer complexes [Pd0(eta2-dba)(eta2-P-N)] and one 14-electron complex [Pd0(eta2-dba)(eta1-P-N)] observed for FcPN and OxaPN. [Pd0(dba)(PhPN)] and [SPd0(PhPN)] (S solvent) react with PhI in an oxidative addition: [SPd0(PhPN)] is intrinsically more reactive than [Pd0(dba)(PhPN)]. This behavior is similar to that of the bidentate bis-phosphane ligands. When the PhPN ligand is present in excess, it behaves as a monodentate phosphane ligand, since [Pd0(eta2-dba)(eta1-PhPN)2] is formed first by preferential cleavage of the Pd-N bond instead of the Pd olefin bond. [Pd0(eta1-PhPN)3] is also eventually formed. [Pd0(dba)(FcPN)] and [Pd0(dba)(OxaPN)] are formed whatever the excess of ligand used. [SPd0(FcPN)] and [SPd0)(OxaPN)] are not involved in the oxidative addition. The 16-electron complexes [Pd0(eta2-dba)(eta2-FcPN)] and [Pd0(eta2-dba)(eta2-OxaPN)] are found to react with PhI via a 14-electron complex as has been established for [Pd0(eta2-dba)(eta1-OxaPN)]. Once again, the cleavage of the Pd-N bond is favored over that of Pd-olefin bond. This work demonstrates the higher affinity for [Pd0(P-N)] of dba compared with the P-N ligand, and emphasizes once more the important role of dba, which either controls the concentration of the most reactive complex, [SPd0(PhPN)], or is present in the reactive complexes, [Pd0(dba)(FcPN)] or [Pd0(dba)(OxaPN)], and thus contributes to their intrinsic reactivity.

Interplay between membrane dynamics, diffusion and swelling pressure governs individual vesicular exocytotic events during release of adrenaline by chromaffin cells.

Release of adrenaline by chromaffin cells occurs through a process involving docking and then fusion of a secretory vesicle to the cytoplasmic membrane of the cell. Fusion proceeds in two main stages. The first one leads to the creation of a stable fusion pore passing through the two membranes and which gives a constant release flux of neurotransmitter (pore-release stage). After a few milliseconds, this initial stage which is not investigated here proceeds through a sudden enlargement of the initial pore (full-fusion stage) up to the complete incorporation of the vesicle membrane into that of the cell and total exposure of the initial matrix vesicle core to the extracellular fluid. The precise time-resolved dynamics of the release and of the vesicle membrane during the full-fusion phase can be extracted with a precision never achieved so far by de-convolution of experimental chronoamperometric currents monitored during individual exocytotic secretion events. The peculiar dynamics of the vesicle membrane proves that exocytotic events are powered by the swelling of the matrix polyelectrolyte core of the vesicle, although they are kinetically regulated by diffusion in the matrix and by the dynamics of the vesicle and cell membranes. Two simple theoretical models based on the dynamics of pores are developed to account for these dynamics and are shown to predict behaviors which are essentially identical to the experimental ones. This offers a new view of the kinetic grounds which control the full-fusion stage, and therefore provides a new interpretation of the sudden transition between the pore-release and the full-fusion stages. This transition occurs when the increasing membrane surface tension energy due to the refrained internal swelling pressure overcomes the edge energy of the pore, so that the initial fusion pore becomes unstable and is disrupted. This new view predicts that secretory vesicles which contain matrixes energetically similar to those of the adrenal cells investigated here can be separated into two classes according to their radius and catecholamine content. Small vesicles (less than ca. 25 nm radius, and containing less than ca. 20000 molecules) should always release through pores. Larger vesicles should always end into fusing except if another mechanism closes the pore before ca. 10000 molecules of catecholamines have been released.

Micrometrically controlled surface modification of Teflon by redox catalysis: electrochemical coupling between Teflon and a gold band ultramicroelectrode

Carbon-fluorine bonds of Teflon (polytetrafluoroethylene, PTFE) can be reduced electrochemically with the purpose of modifying its adhesive and wetting surface properties by micrometrically controlled surface carbonization of the material. This can be performed adequately by redox catalysis provided that the redox mediator couple has a sufficiently negative reduction potential. The process is investigated kinetically with benzonitrile as the mediator and a gold-band ultramicroelectrode mounted adjacent to a PTFE block, though separated from it by an insulating micrometric mylar gap. For moderate fluxes of reduced mediator, the whole device behaves as a generator-collector double-band assembly with a constant current amplification factor. This is maintained over long periods of time, during which the carbonized PTFE zones extends over distances that are much wider than the slowly expanding cylindrical diffusion layer generated at the gold-microband electrode. This establishes that the overall redox catalysis proceeds through electronic conduction in the n-doped carbonized material. Thus, carbonization progresses at the external edge of the freshly carbonized surface in a diffusion-like fashion (dependence on the square root of time), while the redox-mediator oxidized form is regenerated at the carbonized PTFE edge facing to the gold ultramicroelectrode, so that the overall rate of carbonization is controlled by solution diffusion only. For larger fluxes of mediator, the heterogeneous rate of reduction and doping of PTFE becomes limiting, and the situation is more complex. A conceptually simple model is developed which predicts and explains all the main dynamic features of the system under these circumstances and allows the determination of the heterogeneous rate constant of carbon-fluorine bonds at the interface between the carbonized zone and the fresh PTFE. This model can be further refined to account for the effect of ohmic drop inside the carbonized zone on the heterogeneous reduction rate constants and henceforth gives an extremely satisfactory quantitative agreement with the experimental data.

Anionic Pd(0) and Pd(II) intermediates in palladium-catalyzed Heck and cross-coupling reactions.

The anions of PdCl(2)L(2) and Pd(OAc)(2), precursors of palladium(0) used in cross-coupling and Heck reactions, play a crucial role in these reactions. Tricoordinated anionic complexes Pd(0)L(2)Cl(-) and Pd(0)L(2)(OAc)(-) are the effective catalysts instead of the usually postulated Pd(0)L(2) complex. The anion ligated to the palladium(0) affects the kinetics of the oxidative addition to ArI as well as the structure and reactivity of the arylpalladium(II) complexes produced in this reaction. Thus, pentacoordinated anionic complexes are formed, ArPdI(Cl)L(2)(-) or ArPdI(OAc)L(2)(-), the precursor of neutral trans-ArPd(OAc)L(2), instead of the usually postulated trans-ArPdIL(2) complex (L = PPh(3)).