Electrophoretic migration and particle collisions in scanning electrochemical microscopy.

We report for the first time how electrophoretic migration of ions and charged nanoparticles (NPs) in low electrolyte concentration solutions affects positive feedback in scanning electrochemical microscopy (SECM). The strength of the electric field in the gap between either the tip and the substrate, or the tip and counter electrodes, is shown to increase proportionally to the decrease in gap size. This field affects the flux of the charged redox species as expected for dilute electrolyte solutions. However, the shape of the normalized approach curve is unaffected by the electrophoretic migration. We also report that the rate of collisions of charged insulating NPs with the tip electrode decreases as the tip is brought closer to the substrate electrode. This rather unexpected result (negative feedback) can be explained by the blocking of the particle flux with the glass insulating layer around the metal microwires. Observation of simultaneous changes in the faradaic current at the tip and substrate electrodes due to particle collisions with the tip confirms a high rate of mass transport between the two electrodes under the conditions of positive feedback SECM.