Observing iridium oxide (IrO(x)) single nanoparticle collisions at ultramicroelectrodes.

We describe the electrochemical detection of single iridium oxide nanoparticle (IrO(x) NP) collisions on a NaBH(4)-treated Pt ultramicroelectrode (UME). We observe single NP events through the enhanced current by electrocatalytic water oxidation, when IrO(x) contacts the electrode and transiently sticks to it. The overall current transient consists of repeated current spikes that return to the background level, superimposed on a current decay, rather than the staircase response seen where an NP sticks on the UME. Here each event produces a unique current spike (or "blip"). The frequency of the spikes was directly proportional to the particle concentration, and the peak current increased with the applied potential. The observed current is very sensitive to the material and surface state of the measuring electrode; a NaBH(4)-treated Pt UME was important in obtaining reproducible results.