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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

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.

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.

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.

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.

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.

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 electron-transfer/ion-transfer (ET/IT) mode of scanning electrochemical microscopy (SECM) was developed recently and applied to studies of heterogeneous reactions at the substrate surface. The charged products or intermediates are detected by measuring the ion-transfer current of the species across the liquid/liquid interface supported at the tip of a nanopipette. In this article, we develop the theory for this technique and explore its potential advantages and limitations. By using the COMSOL Multiphysics package, the approach curves were simulated for three commonly encountered experimental situations, that is, the surface-generated ionic species is either chemically stable or participates in a first- or second-order homogeneous reaction. The simulation results are generalized in the form of analytical approximations derived under limiting conditions.