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If hydrogen can be stored and carried safely at a high density, hydrogen-fuel cells offer effective solutions for vehicles. The stable chemisorption of atomic hydrogen on single layer graphene (SLG) seems a perfect solution in this regard, with a theoretical maximum storage capacity of 7.7 wt %. However, generating hydrogenated graphene from H2 requires extreme temperatures and pressures. Alternatively, hydrogen adatoms can easily be produced under mild conditions by the electroreduction of protons in solid/liquid systems. Graphene is electrochemically inert for this reaction, but H-chemisorption on SLG can be carried out under mild conditions via a novel Pt-electrocatalyzed "spillover-surface diffusion-chemisorption" mechanism, as we demonstrate using dynamic electrochemistry and isotopic Raman spectroscopy. The apparent surface diffusion coefficient (∼10-5 cm2 s-1), capacity (∼6.6 wt %, ∼85.7% surface coverage), and stability of hydrogen adatoms on SLG at room temperature and atmospheric pressure are significant, and they are perfectly suited for applications involving stored hydrogen atoms on graphene.

International audience; A simple and readily applicable voltammetric approach is described to characterize and measure thesite-hopping surface diffusion of underpotential-deposited (UPD) metal adatoms at nanoelectrodes. UPD refers tothe deposition of atoms on foreign metal supports at potentials lower than those predicted by Nernst law for a bulkdeposition. Despite its importance in several fields of catalysis, advanced nanofabrication or even atomicnanoengineering by atomic layer epitaxy, diffusion of UPD adatoms is difficult to observe at micro- andmacroelectrodes. In fact, at electrodes of usual dimensions, UPD adatoms surface diffusion is masked by otherelectrochemical phenomena of greater relative amplitudes. Conversely, at nanowires electrodes sealed in glass,only an extremely small fraction of the surface of the wires is exposed to the electrolyte solution and is rapidlyloaded. This allows the spillage of UPD adatoms onto the much larger area of the nanowire rod which is immuneto Faradaic reactions due to its isolation from the electrochemical solution by the glass casing. Therefore,surprisingly for a UPD process, voltammetric peaks currents and integral desorption charges primarily reflect thesediffusional surface processes so that the integral charges vary linearly with the inverse of the square root of thescan rate. This is easily observable and measurable with usual bench-level electrochemical instrumentation. In thiswork, by using nanoelectrodes with diameters between 90 and 260 nm, we were able to establish the majorinvolvement of this site-hopping surface diffusion of UPD Pb adatoms on polycrystalline gold (Au) and characterizeit quantitatively. The equivalent surface diffusion coefficient of UPD Pb adatoms was determined to be ~4.4 × 10-11 cm2 s-1 at room temperature, corresponding to a Gibbs free energy activation barrier of ca. 18.72 kJ mol-1 (i.e.,0.19 eV per Pb adatom) for the reaction of inter-sites exchange of Pb adatoms on a polycrystalline Au surface.

International audience; This work validates a posteriori the use of the theory developed ca. 40 years ago by Amatore and Savéant for regular arrays of electroactive band or disk electrodes. This theory has been profitably used and widely applied in more than 900 research papers for the treatment of the voltammetric responses of arrays of nanoelectroactive objects that were necessarily random by construction. In this work we show that competitive diffusion interaction between the active sites enforce redistribution of individual diffusion layer which homogenize dimensions of the solution volumes 'feeding' each active site. For arrays of nanodisk electrodes, it also validates the corresponding approximation assuming that its voltammetric behavior is to that afforded by identical independent cylindrical unit cells performing in parallel.

International audience; Since the early work of Bernard Katz, the process of cellular chemical communication via exocytosis, quantal release, has been considered to be all or none. Recent evidence has shown exocytosis to be partial or 'subquantal' at single-cell model systems, but there is a need to understand this at communicating nerve cells. Partial release allows nerve cells to control the signal at the site of release during individual events, where the smaller the fraction released, the greater the range of regulation. Here we show that the fraction of the vesicular octopamine content released from a living Drosophila larval neuromuscular neuron is very small. The percentage of released molecules was found to be only 4.5% for simple events and 10.7% for complex (i.e., oscillating or flickering) events. This large content, combined with partial release controlled by fluctuations of the fusion pore, offers presynaptic plasticity that can be widely regulated. Two works published in 2010 suggested that the Katz principle, [1] was incorrect for all-or-none release and that only part of the chemical load of vesicles was released during exocytosis, at least as measured as a full spike during amperometry. [2] The combination of electrochemical methods to measure both release and vesicle content in 2015 added a wealth of information to support the concept of partial release in exocytosis. [3] Additionally, this has recently been supported by work with TIRF microscopy showing 'subquantal' release from vesicles in adrenal chromaffin cells and using super-resolution STED microscopy. [4] It appears that the full event generally involves release of only part of the load of chemical messenger in single-cell model systems like adrenal chromaffin and PC12 cells. Is this also true at living neurons in a nervous system and to what extent? To answer this critical question, we quantified the number of octopamine molecules in the neuromuscular neurons of Drosophila larvae by adapting an amperometric technique developed in our

International audience; Zinc, a suspected potentiator of learning and memory, is shown to affect exocytotic release and storage in neurotransmitter-containing vesicles. Structural and size analysis of the vesicular dense core and halo using transmission electron microscopy was combined with single-cell amperometry to study the vesicle size changes induced after zinc treatment and to compare these changes to theoretical predictions based on the concept of partial release as opposed to full quantal release. This powerful combined analytical approach establishes the existence of an unsuspected strong link between vesicle structure and exocytotic dynamics which can be used to explain the mechanism of regulation of synaptic plasticity by Zn 2+ through modulation of neurotransmitter release.

International audience; Reactive Oxygen/Nitrogen Species (ROS/RNS) produced by macrophages inside their phagolysosomes are closely related to immunity and inflammation by being involved in the removal of pathogens, altered cells, etc. The existence of a homeostatic mechanism regulating the ROS/RNS amounts inside phagolysosomes has been invoked to account for the efficiency of this crucial process but this could never be unambiguously documented. In this work, intracellular electrochemical analysis with platinized nanowires electrodes (Pt-NWEs) allowed monitoring ROS/RNS effluxes with sub-millisecond resolution from individual phagolysosomes randomly impacting onto the electrode inserted inside a living macrophage. This evidenced for the first time that the consumption of ROS/RNS by their oxidation at the nanoelectrode surface stimulates the production of significant ROS/RNS amounts inside phagolysosomes. These results established the existence of the long-time postulated ROS/RNS homeostasis and allowed quantifying its kinetics and efficiency. ROS/RNS concentrations may then be maintained at sufficiently high levels for sustaining proper pathogen digestion rates without endangering the macrophage internal structures.

International audience; This opinion paper details three typical cases in which new theoretical concepts need to be implemented in molecular electrochemistry in order to rationalize experimental results obtained in nanoscale cells or performed in new electrolytic media such as RTILs.

International audience; Reactive oxygen and nitrogen species (ROS and RNS) play important roles in various physiological processes (e.g., phagocytosis) and pathological conditions (e.g., cancer). The primary ROS/RNS, viz., hydrogen peroxide, peroxynitrite ion, nitric oxide, and nitrite ion, can be oxidized at different electrode potentials and therefore detected and quantified by electroanalytical techniques. Nanometer-sized electrochemical probes are especially suitable for measuring ROS/RNS in single cells and cellular organelles. In this article, we survey recent advances in localized measurements of ROS/RNS inside single cells and discuss several methodological issues, including optimization of nanoelectrode geometry, precise positioning of an electrochemical probe inside a cell, and interpretation of electroanalytical data.

International audience; The electron transfer/ion transfer mode of scanning electrochemical microscopy (SECM), in which a nanopipette containing a solvent immiscible with the outer solution is used as a tip to approach a microelectrode substrate, is investigated by simulations in order to analyze the effect of the electroactive species being simultaneously present inside nanopipette and in the external liquid phase. The simulations consider conventional transport modes of the species inside the nanopipette as well as in the bulk solution coupled with their transfer across the liquid-liquid nanointerface supported at the nanopipette tip. The shapes of the simulated approach curves (tip current vs. tip-substrate distance) are highly sensitive to the ratio of initial concentrations in the nanopipette and in the outer solution for a given partition coefficient value providing a direct method to determine its value. The effect of the 1 st and 2 nd order homogeneous reaction consuming the product electrogenerated at the microelectrode surface onto its collection by the nanopipette is also presented.

In order to increase signal-to-noise (S/N) performances, the current trend in electro(bio)analytical chemistry consists in developing arrays whose electroactive components are considerably reduced in size and already approach the very nanoscale. A comparable situation involving nanoscale electroactive or electrocatalytic nanoparticles randomly dispersed on a flat non-electroactive surface is already extremely common. Similarly, insulating self-assembled monolayers (SAMs) are often modified by dispersed ‘molecular nanoelectrodes’ consisting of nanopatches of insulating tethers bearing redox-head groups exposed to the analyzed solution with the purpose of mediating/catalyzing electron transfer kinetics between a substrate and the electrode. Finally, most SAMs present randomly distributed nano-sized pinholes through which direct electron transfer from the underlying electrode and a dissolved substrate may occur. It is therefore clear that these continuous developments as well as the increasingly facile and low-cost access to nanofabrication techniques will soon let (bio)electroanalytical chemists to resort more and more often to arrays of functionalized nanoelectrodes or nanoparticles. However, the theoretical analyses and stochastic simulations reported in this work demonstrate that reaching the nanoscale implies a complete change of theoretical electrochemical paradigms. This is of extreme importance as soon as one wishes to rationalize quantitatively measurements involving nano-scaled electroactive components. Indeed, based on Brownian simulations, we established that beyond a dimension of a few tens of nanometers, stochastic effects strongly alter the meaning of the kinetic and thermodynamic measurements vs. those based on classical electrochemical models.

International audience; Cyclic voltammetry has proven its versatile popularity compared with most other electroanalytical methods for investigating electrochemical mechanisms and kinetics. Many textbooks provide clues for understanding and quantify the main features displayed in a voltammogram, but this is unfortunately not often useful when confronted to intricate mechanisms as those presently investigated. This brief opinion article discloses a few theoretical challenges that need to be solved to investigate issues associated with modern applications of cyclic voltammetry.

International audience; Text This review presents the main principles underlying the theoretical description of the behavior of regular and random arrays of nanometric active sites. It is further shown how they can be applied for establishing a useful semi-analytical approximation of the arrays responses under diffusion limited conditions when they involve the common situation of active sites with identical sizes. This approximation is general and, as exemplified for different type of arrays, can be employed for describing the behavior of any array involving arbitrary distributions of their active sites onto the substrate surface. Furthermore, this efficient approach allows statistical characterization of active sites distributions of any array based on chronoamperometric data. Introduction Text Nanostructured electrochemical interfaces (NECI) are frequently encountered in practice. For instance, partially blocked electrodes [1], defects and pinholes in self-assembled monolayers [2], pores in synthetic or natural membranes [3, 4], catalytic nanoparticles (NP) or any type of catalytic site distributed at an inert or less reactive substrate [5] etc. are physical representation of NECI. In these systems, the location of the active sites cannot be controlled (excluding some specific situations) thus leading to disordered surface structures referred as random arrays of active sites. In order to model properly the response of such systems one has to deal simultaneously with multiple scales: electron transfer events at the nano-or microsized active site surface, varying distances between the disordered active sites and interaction of the diffusion layers originated at each sites, global diffusion layer propagation towards the bulk solution, eventual homogeneous kinetics etc. In addition, theoretical description of random arrays is complicated by the fact that taking into account exact positions of the active sites with respect to each other is either difficult or impossible, however, the electrochemical response of the system can be modeled employing statistical information about active sites dispersal. One of the earliest attempts to model disordered systems in the context of partially blocked surfaces was made by one of us [1]. In this work, the behaviors of random arrays with disk-or band-like electrochemically active surface structures were investigated by relying on regular arrays with uniform size of active sites. Such treatment simplified greatly description of the system, but at the same time, it has a good predictive power as confirmed by notable citation of this early work. Nevertheless, due to computer limitations at the time it was difficult to do

Many practical systems at micro- and nanoscale can be represented as arrays of active sites distributed randomly [1]. As shown previously these systems can be efficiently addressed theoretically by using Voronoi diagrams [2, 3] which allows facile tessellation of the system into the unit cells around each active sites. The overall current flowing in the system can then be evaluated by modelling diffusion-reaction processes inside every unit cell and summing the contributions from individual active sites. Although this approach is tempting by its simplicity and efficiency [3] one should bear in mind that Voronoi diagram representing the unit cells by polygonal prisms remains approximation and as each approximation remains valid only under certain conditions. In this work [4] we show that even for the case of diffusion limited electron transfer (ET) the actual shapes of the unit cells are more complicated and depend on the local configuration of the neighbouring active sites. This was exemplified on the small patches of the random arrays with band-like and disk-like active sites via simulations and in the case of band-like active sites confirmed by analytical derivations. Importantly, by comparing the total array current obtained by employing Voronoi tessellation and simulation of the system without any approximations we found that they agree well (relative error ca. 5% or less). At the same time, the individual contributions from the active sites are reproduced with a much larger relative error [4]. The latter suggests that in the case of kinetic control or reaction mechanisms that are more complicated than simple ET the diffusion-reaction competition between the active sites may become even stronger eventually leading to significant deviations from the total current predicted on the basis of the Voronoi approximation. This is currently investigated in our team. Fig. 1. Comparison between the Voronoi tessellation boundaries (white dashed lines) and the actual boundaries (coloured surface) of the unit cells. Three disk-type active sites are shown as white disks. References [1] O. Sliusarenko, A. Oleinick, I. Svir, C. Amatore. J. Electrochem. Soc. 167, 2020, 013530. [2] T. J. Davies and R. G. Compton. J. Electroanal. Chem. 585, 2005, 63. [3] O. Sliusarenko, A. Oleinick, I. Svir, C. Amatore. ChemElectroChem 2, 2015, 1279. [4] G. Pireddu, I. Svir, A. Oleinick, C. Amatore, Electrochim. Acta 365, 2020, 137338. Figure 1

The individual electrochemical anodic responses of dopamine (DA), epinephrine (EP), and pyrocatechol (CT) were investigated at arrays of recessed gold disk-microelectrodes arrays (MEAs) covered by a gold plane electrode and compared to those of their binary mixture (CT and EP) when the top-plane electrode was operated as a bipolar electrode or as a collector. The interferent species (EP) displays a chemically irreversible wave over the same potential range as the chemically reversible ones of DA or CT. As expected, in the generator-collector (GC) mode, EP did not contribute to the redox cycling amplification that occurred only for DA or CT. Conversely, in the bipolar mode, the presence of EP drastically increased the bipolar redox cycling efficiency of DA and CT. This evidenced that the chemically irreversible oxidation of EP at the anodic poles of the top plane floating electrode provided additional electron fluxes that were used to more efficiently reduce the oxidized DA or CT species at the cathodic poles. This suggests an easy experimental strategy for enhancing the bipolar efficiency of MEAs up to reach a performance identical to that achieved when the same MEAs are operated in a GC mode.

International audience; In order to assess the precision of simulations of diffusion at arrays based on the Voronoi tessellation approach we investigated two representative types of random arrays involving bands or disk electroactive sites [J. Electroanal. Chem. 147 (1983) 39] and modelled their diffusional patterns when the solution contains only one electroactive species undergoing a simple electron transfer reaction at the active sites under chronoamperometric conditions. On the one hand, the ensuing results establish that in both cases the Voronoi approach produces reasonably good predictions of the total current intensities flowing through the elementary cells of each array. Indeed, the relative error introduced by the Voronoi-based approach, being less than 5% in each case, is acceptable from an experimental point of view owing to many other sources of uncertainties involved for random arrays. On the other hand, this work demonstrates that the current intensities predicted for the individual sites within an elementary cell based on simulations using Voronoi approaches are excessively wrong since they totally neglect the excessive redistributions of flux lines that happen when the diffusion layers expand with time. This hints to possible severe complication when Voronoi approaches are applied to encompass complex kinetics or to predict the outcome of electroanalytical methods that rely on a fine coupling between local concentrations and local fluxes.

The growing demand for analytical methods for the detection of biological analytes has stimulated a renewed interest in electrochemical reaction sequences producing an electronically excited species prone to emit a photon upon return to the ground state, a process called electrogenerated chemiluminescence (ECL). Both known types of ECL mechanisms, the so-called annihilation and coreactant ECL, are of importance in physical chemistry, but it is the fact that many luminophore/coreactant pairs may include biological amines as coreactants that has made ECL the method of choice for bioanalytical purposes owing to its high sensitivity and immunity to noise. In this chapter we present theoretical analyses and approaches for numerical simulation of typical ECL systems of both types that help reveal limiting factors controlling the intensity of ECL emission and ways to quantitatively optimise such systems to enhance their analytical efficiency.

Applications of conducting carbon materials for highly efficient electrochemical energy devices require a greater fundamental understanding of heterogeneous electron-transfer (ET) mechanisms. This task, however, is highly challenging experimentally, because an adsorbing carbon surface may easily conceal its intrinsic reactivity through adventitious contamination. Herein, we employ nanoscale scanning electrochemical microscopy (SECM) and cyclic voltammetry to gain new insights into the interplay between heterogeneous ET and adsorption of a Co(III)/Co(II)-complex redox couple at the contamination-free surface of electron-beam-deposited carbon (eC). Specifically, we investigate the redox couple of tris(1,10-phenanthroline)cobalt(II), Co(phen)32+, as a promising mediator for dye-sensitized solar cells and redox flow batteries. A pristine eC surface overlaid with KCl is prepared in vacuum, protected from contamination in air, and exposed to an ultrapure aqueous solution of Co(phen)32+ by the dissolution of the...

Fast Scan Cyclic Voltammetry (FSCV) at high aspect ratio carbon microelectrodes shows adequate high temporal and spatial resolution for in vivo analysis of catecholamines. Though the presence of their surface heterogeneities has been recognized since their earliest introduction for in vivo measurements in the brain, the kinetic consequences on the measurements have not been investigated and FSCV measurements are treated based on pre- and post-calibrations. We establish here that surface heterogeneities play a consequent dynamic role on the oxidation of dopamine taken as an example of catecholamines. Hence, the FSCV current peak intensities do not scale with the scan rate v or its square root. This is rationalized with a simple model involving a co-existence of at least two types of surface nanodomains with different electrochemical reactivities and different time responses. At low scan rates (

The mechanism of mammalian vesicle rupture onto the surface of a polarized carbon fiber microelectrode during electrochemical vesicle cytometry is investigated. It appears that following adsorption to the surface of the polarized electrode, electroporation leads to the formation of a pore at the interface between a vesicle and the electrode and this is shown to be potential dependent. The chemical cargo is then released through this pore to be oxidized at the electrode surface. This makes it possible to quantify the contents as it restricts diffusion away from the electrode and coulometric oxidation takes place. Using a bottom up approach, lipid-only transmitter-loaded liposomes were used to mimic native vesicles and the rupture events occurred much faster in comparison with native vesicles. Liposomes with added peptide in the membrane result in rupture events with a lower duration than that of liposomes and faster in comparison to native vesicles. Diffusional models have been developed and suggest that the trend in pore size is dependent on soft nanoparticle size and diffusion of the content in the nanometer vesicle. In addition, it appears that proteins form a barrier for the membrane to reach the electrode and need to move out of the way to allow close contact and electroporation. The protein dense core in vesicles matrixes is also important in the dynamics of the events in that it significantly slows diffusion through the vesicle.

Electroanalytical arrays of microwells with increased sensitivity and virtual immunity to interferents have been previously reported by us. Such arrays were designed to be operated in a classical generator–collector mode but exhibited also similar properties when their common planar collector was left floating. This evidenced that the unbiased collector acted as a bipolar electrode. In this work, a theory is elaborated to investigate in great detail the generality of this phenomenon and its limits. This establishes that if the analyte pertains to a redox couple with relatively fast electron rate constants, the bipolar collector results to be almost as efficient as when biased, thus suppressing the need for a bi-potentiostat with obvious gain for analytical applications. Conversely, if the analyte rate constant of electron transfer is small, efficient operation in a bipolar mode remains feasible but requires a drastic decrease of the microwell density in the array.

Quantitative theoretical investigations based on a 3D Brownian motion approach established that our previous “cylindrical” approximation remains valid even for drastically irregular arrays. This result led us to propose a simple analytical equation that can be used to predict the chronoamperometric behavior of such commonly used irregular arrays, taking into account the statistical distributions of the Voronoi cells built around the disk electrodes.

© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.This theoretical work examines some basic and overlooked concepts that sustain the efficiency of electrochemiluminescence (ECL) generated by co-reactant systems such as the, nowadays classical, alkyl amine/transition metal(II) complex systems. Changes in the ECL intensities emitted by these systems are much more dependent on the relative diffusivities of the two co-reactants than on the range of thermodynamic and kinetic rate constants that are possible to explore and vary. In particular, decreasing the diffusion coefficients of the metal complexes species (e.g. by adequate redox or photochemically inert large substituents) relative to that of the amine co-reactant leads to a great enhancement in the ECL intensity of the first ECL wave, namely, that observed at the level of the amine oxidation peak. Though investigated by using simulations based on the thermodynamic and kinetic constants of the most common tri-n-propylamine/Ru(bpy) 3 2+ system (bpy=2,2’-bipyridine), the conclusions of this work are more general.

Thirty years ago one of us introduced a central approximation according to which the regular arrays behaviors could be estimated based on those of arrays of cylindrical unit cells with the same relative surface area of their central disks for the insulating section. This work examines and validates this approximation on the basis of 3D-simulations of Brownian particles motion for inlaid or recessed electroactive disks as well as for protruding cylindrical electrodes. The approximation is perfectly adequate for predicting the behavior of realistic experimental arrays and is valid within a few percent even in extreme situations when adjacent electroactive sections are almost touching.

Recessed microelectrode arrays and plane-recessed microelectrode arrays (MEAs) with different center-to-center distances are designed and fabricated using lithographic technology. By comparing electrochemical behavior of plane-recessed MEAs with that of recessed MEAs, bipolar phenomenon of the metallic plane film is revealed. Redox cycling can occur when the top plane electrode was floating; that is, the bipolar behavior of the unbiased top plane electrode may perform locally as a collector and enlarge the concentration gradient of Ru(NH3)6Cl3 and thus promote an apparent generator/collector electrochemical response of the microdisk electrode in the MEAs configuration. By utilizing the bipolar behavior, the center-to-center distance of MEAs required for achieving steady-state current could decrease without favoring at the same time the overlapping of diffusion layers of microelectrodes, and thus, the electrode density of MEAs can be increased. Therefore, the bipolar behavior of the metallic film can increase both the current response of an individual microdisk and the electrode density of microdisks without losing the characteristics of a microelectrode. By just fabricating a thin layer of metallic film on the plane and leaving it floating without potential control, recessed MEAs used in this work can achieve the increase of detection sensitivity by more than 1 order of magnitude.

A new type of three-electrode generator–collector sensor is described for the selective detection of any target analyte whose CV is chemically reversible in the presence of any interferent whose CV is chemically irreversible. The device consists of a layer of metallic material (Au in this work) poked by an array of cylindrical pores containing a ring-band electrode (Au in this work) placed in their middle and a disk electrode at their bottom (Au in this work). Operating the array of recessed disk electrodes to monitor the product of the stable analyte electronation while the top plane and the ring-band electrodes are poised at a potential located on the analyte and on the interferent waves allows suppressing entirely any contamination of the analyte concentration measurement by direct or indirect (EC′) involvement of the interferent. The efficiency of such devices was successfully demonstrated based on the detection of dopamine in the presence of ascorbic acid in PBS electrolyte. Keywords: Selective detection, EC′ mechanism, Inverted generator–collector mode, Dopamine, Electrode array

Recessed generator-collector assemblies consisting of an array of recessed disks (generator electrodes) with a gold layer (collector electrode) deposited over the top-plane insulator reportedly allow increased selectivity and sensitivity during electrochemical detection of dopamine (DA) in the presence of ascorbic acid (AA), a situation which is frequently encountered. In sensor design, the potential of the disk electrodes is set to the wave plateau of DA, whereas the plane electrode is biased at the irreversible wave plateau of AA before the onset of the DA oxidation wave. Thus, AA is scavenged but DA is allowed to enter the nanocavities to be oxidized at the disk electrodes, and its signal is further amplified by redox cycling between disk and plane electrodes. Several different theoretical approaches are elaborated herein to analyze the behavior of the system, and their conclusions are successfully tested by experiments. This reveals the crucial role of the plane-electrode area which screens access to the recessed disks (i.e. acts as a diffusional Faraday cage) and simultaneously contributes to amplification of the analyte signal through positive feedback, as occurs in interdigitated arrays and scanning electrochemical microscopy. Simulations also allow for the evaluation of the benefits of different geometries inspired by the above design and different operating modes for increasing the sensor performance.

Vesicular exocytosis is an essential and ubiquitous process in neurons and endocrine cells by which neurotransmitters are released in synaptic clefts or extracellular fluids. It involves the fusion of a vesicle loaded with chemical messengers with the cell membrane through a nanometric fusion pore. In endocrine cells, unless it closes after some flickering (‘Kiss-and-Run’ events), this initial pore is supposed to expand exponentially, leading to a full integration of the vesicle membrane into the cell membrane—a stage called ‘full fusion’. We report here a compact analytical formulation that allows precise measurements of the fusion pore expansion extent and rate to be extracted from individual amperometric spike time courses. These data definitively establish that, during release of catecholamines, fusion pores enlarge at most to approximately one-fifth of the radius of their parent vesicle, hence ruling out the ineluctability of ‘full fusion’.

ECL reaction mechanisms involved in all experimental approaches are characterized by a sequence of very fast second-order reactions taking place in the diffusion layer between transient species that, according to the exact process of ECL generation, may be generated at a single electrode, a pair of electrodes (anode-cathode), or through electron transfers between activated reactants and co-reactants. These extremely fast second-order reactions generate the extremely short-lived electronically excited state species, S*, which are deactivated through a rapid first-order emissive decay giving rise to the emission of light. These crucial species are therefore always generated and consumed within a narrow layer of solution of very small size compared to diffusion layers. Moreover, they are present within this layer at vanishingly small concentrations. This creates kinetic situations termed “reaction fronts” exact numerical treatment of which is almost impossible by classical numerical approaches. This chapter presents a series of numerical approaches based on the concepts developed by the authors to circumvent these severe complications and allow a precise and fast simulation of ECL reaction mechanisms. These are illustrated taking advantage of experimental examples featuring each main method of ECL generation.

Exocytosis is the fundamental process by which cells communicate with each other. The events that lead up to the fusion of a vesicle loaded with chemical messenger with the cell membrane were the subject of a Nobel Prize in 2013. However, the processes occurring after the initial formation of a fusion pore are very much still in debate. The release of chemical messenger has traditionally been thought to occur through full distention of the vesicle membrane, hence assuming exocytosis to be all or none. In contrast to the all or none hypothesis, here we discuss the evidence that during exocytosis the vesicle-membrane pore opens to release only a portion of the transmitter content during exocytosis and then close again. This open and closed exocytosis is distinct from kiss-and-run exocytosis, in that it appears to be the main content released during regular exocytosis. The evidence for this partial release via open and closed exocytosis is presented considering primarily the quantitative evidence obtained with amperometry.

The simulation of transport to double microband electrodes in generator-collector mode is reported focusing especially on the 'titration curve' approach to electroanalysis in which a titrant is electrogenerated from a redox active precursor on the generator electrode and reacts homogeneously with the target analyte. The current on the detector electrode reflects the amount of titrant 'surviving' passage between the two electrodes. The form of the titration curve - plots of detector current as a function of generator current - is shown to be highly sensitive to the electrode kinetics of the redox couple driven at the generator electrode. Accordingly the natïve use of such methodology for analysis without accompanying simulation and kinetic analysis is fraught with danger. Use of the conformal mapping approach in combination with the ADI method for investigation of the 'titration' current distributions at the double band system gives fast and precise simulation of this and similar problems. Convergence analysis is described which allows for the automatic selection of the simulation grid size so as to obtain a chosen accuracy (for example 1%) of the current for all experimentally meaningful values of the geometrical and physico-chemical parameters of the system to be investigated. © 2003 Elsevier B.V. All rights reserved.

A new simulation algorithm is presented for describing the dynamics of diffusion reactions at the most common microelectrode 1D (planar, cylindrical, spherical) and 2D geometries (band, disk) for electrochemical mechanisms of any complexity and involving fast homogeneous reactions of any kind. A series of typical electrochemical mechanisms that create the most severe simulation difficulties is used to establish the exceptional performance and accuracy of this algorithm, which stem from the combination of (quasi)conformal transformation of space and a new method for auto-adaptive grid compression.

Individual vesicular exocytosis of adrenaline by dense core vesicles in chromaffin cells is considered here as a paradigm of many situations encountered in biology, nanosciences and drug delivery in which a spherical container releases in the external environment through gradual uncovering of its surface. A procedure for extracting the aperture (opening) function of a biological vesicle fusing with a cell membrane from the released molecular flux of neurotransmitter as monitored by amperometry has been devised based on semi-analytical expressions derived in a former work [C. Amatore, A. I. Oleinick, I. Svir, ChemPhysChem 2009, 10, DOI: 10.1002/cphc.200900646]. This precise analysis shows that in the absence of direct information about the radius of the vesicle or about the concentration of the adrenaline cation stored by the vesicle matrix, current spikes do not contain enough information to determine the maximum aperture angle. Yet, a statistical analysis establishes that this maximum aperture angle is most probably less than a few tens of degrees, which suggests that full fusion is a very improbable event.

This article extends our previous works (Amatore, C.; Oleinick, A.; Svir, I. Anal. Chem. 2008, 80, 7947-7956; 7957-7963.) about the effects of resistive and capacitive distortions in voltammetry at disk microelectrodes. The particular case of voltammetry of a self-assembled monolayer carrying one redox site per molecule is investigated here. In addition, the effect of an uneven distribution of the effective electrochemical potential on the possibility of electron hopping (EH) contributions is examined. An original model of EH has been developed considering both diffusion-type (i.e., related to concentration gradients) and migration-type (i.e., imposed by an uneven distribution of the electrical potential due to an ohmic drop and capacitance charging) contributions. This predicts that as soon as the system performs out of thermodynamic equilibrium and provided that the EH rate constants are not too small the system tends to re-establish its out-of-equilibrium state through EH. Hence, EH somewhat tries to compensate the voltammetric distortions that would be enforced by the uneven distribution of the electrochemical driving force incurred by the system due to an ohmic drop and capacitive charging. However, this rigorous analysis established that, though EH may be effective under specific circumstances particularly near the electrode edge, its overall influence on voltammetric waves remains negligible for any realistic experimental situation.

In this article, the numerical approach for flow profile reconstruction in a microfluidic channel equipped with band microelectrodes introduced previously by the authors, based on transient currents, is extended to the exclusive use of steady-state currents. It is shown that, although the currents obey steady state, the flow velocity profile in the channel may be reconstructed rapidly with a high accuracy, provided a sufficient number of electrodes performing under steady state are considered. The present theory demonstrates how the electrode widths and sizes of gaps separating them can be optimized to achieve better performance of the method. This approach has been evaluated theoretically for band microelectrode arrays embedded into one wall of a rectangular channel consisting of three, four, or five electrodes, all of which are operated in the generator mode. The results prove that the proposed approach is able to accurately recover the shape of the flow profile in a wide range of Peclet numbers and flow types ranging from the classical parabolic Poiseuille flow to constant electro-osmotic-type flow.

In this work, we present the results of a mathematical modelling of NO· release by neurons and its transport in the brain by diffusion. The model is applied to analyze the experimental data on NO· release from a neuron monitored during its patch-clamp stimulation by an ultramicroelectrode introduced into a slice of living rat’s brain. The neuron activity function was obtained by numerical deconvolution of the experimental data using the response function of the electrode to an instantaneous spike of neuronal activity. The Gaussian decomposition of NO· release activity function allows qualitative and quantitative conclusions to be drawn about neuron activity. Since the integral activity function is readily obtained by deconvolution, the decomposition can be performed using other more relevant descriptions of NO· bursts emerging from active neurons.

International audience; The complex problem of diffusion−reaction inside of bundles of nanopores assembled into microspherical particles is investigated theoretically based on the numerical solutions of the physicochemical equations that describe the kinetics and the thermodynamics of the phenomena taking place. These theoretical results enable the delineation of the main factors that control the system reactivity and examination of their thermodynamic and kinetic effects to afford quantitative predictions for the optimization of the particles' dimensional characteristics for a targeted application. The validity and usefulness of the theoretical approach disclosed here are established by the presentation of the complete analysis of the performance of thiol-functionalized microspheres aimed for sequestration of Hg(II) ions from solutions to be remediated. This allows the comparison of the microparticles' performance at two different pH (2 and 4) and the rationalization of the observed changes in terms of the main microscopic parameters that define the system.

We propose a theoretical method for reconstructing the shape of a hydrodynamic flow profile occurring locally within a rectangular microfluidic channel based on experimental currents measured at double microband electrodes embedded in one channel wall and operating in the generator-collector regime. The ranges of geometrical and flow parameters providing best conditions for the flow profile determination are indicated. The solution of convection-diffusion equation (direct problem) is achieved through the application of the specifically designed conformal mapping of spatial coordinates and an exponentially expanding time grid for obtaining accurate concentration and current distributions. The inverse problem (the problem of flow profile determination) is approached using a variational formulation whose solution is obtained by the Ritz’s method. The method may be extended for any number of electrodes in the channel and/or different operating regimes of the system (e.g. generator-generator). © 2007 Elsevier Ltd. All rights reserved.

In this work we report the results of the mathematical modelling of NO◦ -release by neurons considering a series of Gaussian bursts, together with its transport in the brain by diffusion. Our analysis relies on the NO◦ -release from a neuron monitored before, during and after its patch-clamp stimulation as detected by an ultramicroelectrode introduced into a slice of living rat’s brain. The parameters of the neuron activity function have been obtained by numerical fitting of experimental data with simulated theoretical results. Within our initial hypothesis about the Gaussian decomposition of NO◦ -release that allowed drawing qualitative and quantitative conclusions about the considered neuron activity function. It is noted that since the activity function can be readily modified this signal processing may be adapted to the treatment of other and maybe more physiologically relevant hypotheses.

The nerve terminals found in the body wall of Drosophila melanogaster larvae are readily accessible to experimental manipulation. We used the light-activated ion channel, channelrhodopsin-2, which is expressed by genetic manipulation in Type II varicosities to study octopamine release in Drosophila. We report the development of a method to measure neurotransmitter release from exocytosis events at individual varicosities in the Drosophila larval system by amperometry. A microelectrode was placed in a region of the muscle containing a varicosity and held at a potential sufficient to oxidize octopamine and the terminal stimulated by blue light. Optical stimulation of Type II boutons evokes exocytosis of octopamine, which is detected through oxidization at the electrode surface. We observe 22700±4200 molecules of octopamine released per vesicle. This system provides a genetically accessible platform to study the regulation of amine release at an intact synapse.

Chemical synaptic transmission is central to the brain functions. In this regard, real-time monitoring of chemical synaptic transmission during neuronal communication remains a great challenge. In this work, in vivo-like oriented neural networks between superior cervical ganglion (SCG) neurons and their effector smooth muscle cells (SMC) were assembled in a microfluidic device. This allowed amperometric detection of individual neurotransmitter release events inside functional SCG-SMC synapse with carbon fiber nanoelectrodes as well as recording of postsynaptic potential using glass nanopipette electrodes. The high vesicular release activities essentially involved complex events arising from flickering fusion pores as quantitatively established based on simulations. This work allowed for the first time monitoring in situ chemical synaptic transmission under conditions close to those found in vivo, which may yield important and new insights into the nature of neuronal communications.

A new method leading to the definition of the optimal space for the numerical simulation of stationary and quasi-stationary diffusion-reaction problems at a microelectrode of any particular geometry amenable to a 2D formulation is introduced. In this first paper, the method basic concept is introduced and exemplified in full details using the classical example of pure diffusion at the microdisk electrode to demonstrate its performance. The method is primarily rooted on closed analytical formulation, yet its scope may be further expanded by utilizing spline functions for the case when the system geometry is such that the optimal coordinate compression function cannot be obtained in the form of a closed analytical expression. This generalized approach has also been applied to the inlaid microdisk electrode and the comparison between the resulting spline-function and the analytical compression function shows excellent convergence.

In this work, we present a complete theoretical analysis of a new concept of electrochemical detector for application in proteomics upon considering two band microelectrodes performing in generator-collector mode. This concept of an original electrochemical detector is aimed at the detection of proteins following their separation in microfluidic biochips. The theoretical analysis is based on the use of the time-dependent coordinate transformation which allows performing precise modeling for a wide range of the key parameters governing the electrochemical detector performance. This allows defining a precise optimization procedure for its best efficiency upon considering the qualitative and quantitative effects of each of the main operational parameters.

The physicochemical process of nitric oxide (NO degrees ) release from an active neuron is modelled based on the results obtained experimentally in independent series of experiments reported elsewhere in which the NO degrees release elicited by patch-clamping a single neuron (stellate neuron from cerebellum area) is monitored by an ultramicroelectrode introduced into a slice of living rat's brain. This process is believed to be central to brain behaviour by coupling neuronal activity with the blood supply to active areas of the living brain through precise control of NO degrees -mediated dilatation of blood capillary vessels. This work, based on the conformal mapping approach initially proposed in a previous work, aims to model the overall physicochemical and diffusional processes giving rise to the release of NO degrees by a neuron and during its collection at an electrode sensor. Fitting simulated currents to experimental ones published previously yields indeed the gross kinetic information which represents the overall neuron activation and defines the instant value of the concentration of NO degrees at the neuron surface. This allows reconstructing the NO degrees fluxes around the neuron body as they would have been in the absence of the electrode sensor. This permits one to appreciate how far NO degrees is released by the neuron at concentrations which greatly exceed their basal values. The success of this procedure is exemplified using a set of three experimental data reported elsewhere.

A theoretical approach is developed for investigation of diffusion within nanometric or micrometric spherical-type domains limited by two types of active interfaces. Typical representative examples of either system consist in: (a) insoluble droplets (defined by spherical cupolas) of electroactive liquids or redox dendrimers adsorbed onto an electrode surface placed in an electrolyte or (b) biological vesicles during exocytotic release occurring in living cells. In situation (a), the active interface is an infinitely small ring defined along the triple-phase interface at the junction between the droplet surface, the electrode surface, and the electrolyte. In situation (b), the interface is a nanometric pore connecting the inside of the vesicle to the outer medium. Either situation imposes extremely difficult conditions for modeling the diffusional behavior of these two systems because of: (a) the conflicting symmetries between the spherical domains shapes vis-a-vis the pore or ring active interface which acts as a diffusional sink or a source and (b) the near-infinite and heterogeneous concentration gradients which develop at the active interface. Despite these intrinsic severe difficulties, introducing an original conformal mapping transform leads to a great simplification of the numerical solution of diffusional problems. This allows the development of fast and extremely precise simulations of the problem at hand. The efficiency of the simulations is tested in each case by imposing diffusion-controlled conditions so as to examine situations which enhance the intrinsic difficulties of the problems. In this first part, only diffusion within the inner domain defined by the vesicle or the droplet is considered in simulations since this is the domain in which the most difficult diffusional situations occur, but the general theory developed encompasses the most global circumstances, viz., when diffusion in both the inner and outer domains controls the system’s behavior and when generalized Butler–Volmer-type kinetics apply at the active interface. It is also shown that this conformal mapping approach leads to the design of a one-dimensional approximation which provides results accurate within a few percent compared to the full treatment of the system. Such equivalence, which is reminiscent of the hemisphere-disk or hemicylinder-band equivalence at ultramicroelectrodes, provides efficient and accurate simulations of the system at hand and may prove extremely valuable for semi-quantitative predictions.

Two different applications of conformal mapping approaches are proposed for the treatment of double band assemblies operating in a microfluidic channel: (i) a quasi-conformal map for the numerical simulation of diffusion–convection processes occurring when the channel is submitted to a microfluidic flow, and (ii) a conformal map specifically designed for obtaining the steady-state analytical solution for the pure diffusional regime (i.e., without flow) within the same channel. The latter applies also when such assemblies perform within a thin film of electrolyte. The results are presented for a generator–collector operation of the assemblies but the methods apply also to other situations (e.g., generator–generator mode). Keywords: Channel flow system, Double band microelectrode assemblies, Microfluidics, Quasi-conformal and conformal maps, Simulation