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18 septembre - 14:00 soutenance de thèse de Zejun Deng

par Anne-Marie - publié le

Exploring the concepts of electrochemical blocking for single entity detection

This dissertation is dedicated to exploring the concepts of electrochemical blocking for single entity detection. 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 ultramicroelectrodes 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 report the use of hemispherical UMEs to detect the collision of individual polystyrene beads by electrochemical blocking. We evidenced that the edge effect encountered on disk-shaped UMEs is significantly reduced on hemispherical electrodes. Hemispherical Hg UME enables simultaneous measurements of the size distribution and concentration of particles in suspension. We determine within less than 10% of error the average diameter of polystyrene bead of 0.5 and 1 μm radius. The total concentration of these polystyrene obtained by electrochemistry is found in close agreement (<10% of error) with their nominal concentrations ( 10-15 mol/L).
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.