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Exploring the concepts of electrochemical blocking for single entity detection

Abstract : This dissertation is dedicated to exploring the concepts of electrochemical blocking for single entity detection. Electrochemical blocking is a type of single-entity electrochemical measurement particularly well adapted to the detection of insulating entities, including artificial entities like polymer particles or bioparticles like proteins and bacteria. The size of these entities spans between few nm to several microns and their electronic structure covers the entire spectrum from insulator to single accepting donating electronic state to semiconducting and metallic behavior. Currently, the accurate determination of the size of a particle by electrochemical blocking remains an analytical challenge, owing to the uneven current distribution on disk ultra-microelectrodes UMEs (so-called edge effect). The goal of this dissertation is to develop this elegant and straightforward methodology into a versatile and quantitative analytical tool.First, we describe the use of hemispherical Hg UME to detect individual insulating particles in order to remove the edge effects on disk UMEs. The use of hemispherical Hg UME enables simultaneous measurements of the size distribution and concentration of particles in suspension. Using numerical simulations, we deduce the quantitative relation between the magnitude of the current step and the size of the bead. The frequency of collision measured for a given size of bead is then converted into a concentration (in mol/L) by quantification of the relative contributions of migration and diffusion for each size of the bead. Under our experimental conditions (low concentration of supporting electrolyte), migration dominates the flux of bead. The average size of polystyrene beads of 0.5 and 1 μm radius obtained by electrochemistry and scanning electron microscopy (SEM) differs by only -8% and -9%, respectively. The total concentration of polystyrene beads of 0.5 and 1 μm radius obtained by electrochemistry is found in close agreement (<10% of error) with their nominal concentrations (25 and 100 fM).Second, we extend the strategy of electrochemical blocking to the detection of electrically conducting particles. This strategy, electro-catalytic depression, is based on the intrinsic difference in electron transfer kinetics between materials to detect poorly catalytic particles such as graphene nanoplatelets (GNPs). Under the potential of 0.1 V vs. Ag/AgCl, GNPs block the oxidation of hydrazine on a 5 µm radius Pt UME, producing staircase-shaped drops of current (negative steps) similar to the signal obtained with insulating particles like polystyrene beads At high potentials (> 0.1 V), where hydrazine oxidation occurs on the GNP, the kinetic difference between GNP and Pt decreases, leading to the decrease of both average and median current step size and the appearance of positive steps.Finally, we couple electrochemistry and bright-field microscopy to elucidate how the translation and rotation of GNPs affect the current response. Once the GNP touches the surface of Pt, the transient current responses come from the instantaneous increase in the electroactive surface area of GNP. Importantly, the rotation of GNP will cause changes in current transients.
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https://tel.archives-ouvertes.fr/tel-02983049
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Submitted on : Thursday, October 29, 2020 - 11:34:11 AM
Last modification on : Friday, October 30, 2020 - 3:29:49 AM

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  • HAL Id : tel-02983049, version 1

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Zejun Deng. Exploring the concepts of electrochemical blocking for single entity detection. Analytical chemistry. Institut Polytechnique de Paris, 2020. English. ⟨NNT : 2020IPPAX059⟩. ⟨tel-02983049⟩

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