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International Collaboration in Chemistry: Mechanistic studies of oxygen electrocatalysis by nanoelectrochemical techniques – ChemCatNanoTech

Oxygen reduction and nano-electrochemistry: from electrochemical catalysts to the Living World

Oxygen reduction (ORR) is an essential process both because of its socioeconomic importance linked to electrocatalytic storage technologies and to the production of energy (fuel cells, lithium-air batteries, etc.), and for its involvement in the living world through its medical consequences (oxidative stress, alterations due to free radicals, cancers, Alzheimer's disease, etc.).

Experimental and theoretical studies of the oxygen reduction on metallic electrochemical catalysts or at the intracellular single cell level in the Living World.

The main objective of the project concerned the quantitative characterization (existence? Importance?) of superoxide ion production during the electrochemical reduction of oxygen at noble metals or, in the Living World and at the level of the single cell, in relation with cellular inflammation and cancer.

Our original method, called nano-ITIES, is based on the use of nanopipettes containing a water-immiscible solvent (benzotrifluoride, BTF), in which oxygen can be dissolved at concentrations 50 to 150 times greater than in water, so that a liquid / liquid BTF / water interface spontaneously forms at the nanopipette tip and O2 can be delivered there through. This nanometric delivery tip may be approached at nanometric distances from the surface of a platinum electrode in unbuffered aqueous electrolytes. This makes it possible to inject extremely high oxygen fluxes at nanoscale distances from the surface of an electrode (platinum in our case) by diffusion. On the other hand, the potential difference applied across the water / BTF nano-interface may be selected so that even very small amounts of superoxide ion released by the reduction of O2 at the Pt electrocatalyst are selectively captured at the tip of the nanopipette via their transfer through the water / BTF interface. This generates a ionic current in the nanopipette circuit. The value of this current flux reflects the yield of superoxide ions formed on the platinum surface. The capture of the superoxide selectivity is ensured by imposing a precise value to the interfacial potential difference which avoids any capture of other anions (eg, OH-, HOO-, etc.) or cations across the interface.
Conversely, platinum-derivatized nanoelectrodes can be introduced through the membranes of inflamed cells in order to quantitatively monitor in real time the dynamic production of reactive oxygen species and relate it to the level of inflammation (normal cells, malignant cells or metastatic ones) while preserving the metabolic integrity of the cells studied.

We have been able to demonstrate for the first time and in contradiction with the classical assertions that under the usual experimental conditions about 10% of the oxygen is reduced to superoxide ions in parallel with the majority processes leading to the formation of water. This may be due to the decomposition of surface
intermediates of the Pt-O2 type or the direct transfer of electrons by external sphere from the Pt surface to the oxygen molecules. The theoretical models developed in our study confirmed the validity of such a duality and show that it is a priori impossible to decide between the two possibilities. The superoxide formed spontaneously
disproportionates into oxygen and hydrogen peroxide which is then reduced to water on platinum electrodes. This leads globally to a 4-electron reduction mechanism of oxygen in water so that this parasitic production could never be detected quantitatively and its importance assessed before our work.
At the biological level, the use of platinized carbon nanoelectrodes of sufficiently small size allows passing across a cell membrane while maintaining its tightness (and its intracellular homeostatic equilibria). This enabled us to quantitatively compare the dynamics of production of activated species of oxygen or nitrogen (ROS/RNS) in breast cancer cells at different stages (normal, malignant and metastatic cells) or quantitatively check for the first time at the single cell level the causal role of DAG -lactone, a protein kinase C (PKC) activator, thus confirming, at the cellular level, the direct link between PKC activation and the evolution of dormant cancer cells to an active status. In the same way, we have been able to demonstrate and measure for the first time the production of superoxide and nitric oxide ions inside of phagolysosomes of activated macrophages.

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From its onset this project had a very fundamental and exploratory character. Consequently, no valuation action other than fundamental scientific ones have been considered. Seven articles have been published in the world's best peer-reviewed journals (including two invited ones in special commemorative issues and one being selected
for the cover of ChemElectroChem). An eighth and final paper is being written and will soon be submitted.
Twenty-one conferences and communications were delivered, including 10 plenary lectures or invited ones in major international conferences of the electrochemical field. Each contribution (article, lecture or communication) properly referred and acknowledged the support of the ANR-NSF program and indicated the number of the ANR
contract.
Finally, 2 series of courses (24 h duration overall) bearing in part on the grant subjects have been delivered abroad (China and Colombia).

This project is aimed at developing a new approach to the detection of short-lived charged intermediates of the oxygen reduction reaction (ORR) based on the combination of recently introduced electron transfer/ion transfer mode of scanning electrochemical microscopy (ET/IT SECM) with ultrafast cyclic voltammetry (CV). The advantages of the ET/IT SECM essential for the proposed experiments are high selectivity and sensitivity and a very short (nanometers) separation distance between the catalytic substrate surface and the nanoelectrochemical probe. They will allow performing mechanistic studies of the ORR under different experimental conditions. Quantitative kinetic interpretation of the SECM and CV data will constitute the basis for mechanistic analysis of the catalytic reaction. The proposed approach will be potentially useful for investigating electrocatalytic reactions other than ORR as well as for studies of reactive oxygen species in biological systems.

Project coordinator

Monsieur Christian Amatore (UMR Processus d’Activation Sélective par Transfert d’Energie Uni-électronique ou Radiatif (PASTEUR))

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

Mirkin Group CUNY Queens College
UMR 8640 CNRS-ENS-UPMC UMR Processus d’Activation Sélective par Transfert d’Energie Uni-électronique ou Radiatif (PASTEUR)

Help of the ANR 289,999 euros
Beginning and duration of the scientific project: September 2014 - 36 Months

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