Single-Entity Electrochemistry Reveals Solvent-Dependent Polarity Inversion in Current Steps Resulting from Polystyrene Microsphere Collisions

Single-entity electrochemistry detects encounters between individual particles and electrodes as discrete current steps. Electrochemically inert polymer beads are generally interpreted as obstructions that reduce redox flux to ultramicroelectrodes. This study demonstrates that polystyrene beads produce polarity-inverted collision responses depending on the solvent environment, resulting in current-decrease steps in aqueous media and current-increase steps in organic media. Conventional blocking behavior is observed for ferrocyanide and catechol in aqueous solution, whereas anthracene and catechol in organic solution generate positive collision steps. In both media, when catechol is used, signal reversal is not governed solely by redox identity but is influenced by the solvent environment and interactions between the bead and redox species. COMSOL simulations comparing a purely blocking bead and flux-active bead boundary reveal that a localized bead-associated flux contribution can reproduce the current-increase response. These findings challenge the conventional assumption within current-blockade frameworks that inert polymer particles act only as passive flux-blocking objects and highlight solvent-controlled interfacial flux modulation as an additional factor influencing particle-impact electrochemistry.