Influence of the Redox Indicator Reaction on Single-Nanoparticle Collisions at Mercury- and Bismuth-Modified Pt Ultramicroelectrodes

Single-Pt nanoparticles (NPs) can be detected electrochemically by measuring the current-time (i-t) response associated with both hydrazine oxidation and proton reduction during individual Pt NP collisions with noncatalytic Hg- and Bi-modified Pt ultramicroelectrodes (Hg/Pt and Bi/Pt UMEs, respectively). At Hg/Pt UMEs, the i-t response for both hydrazine oxidation and proton reduction consists of repeated current "spikes" that return to the background level as Hg poisons the Pt NP after collision with the Hg/Pt UME due to amalgamation and deactivation of the redox reaction. Furthermore, at a Hg/Pt UME, the applied potential directly influences the interfacial surface tension (electrocapillarity) that also impacts the observed i-t response for single-Pt NP collisions for proton reduction that exhibits a faster decay of current (0.7-4 ms) to background levels than hydrazine oxidation (2-5 s). Because the surface tension of Hg is lower (-0.9 V), Pt NPs possibly react faster with Hg (amalgamate at a faster rate), resulting in sharp current spikes for proton reduction compared to hydrazine oxidation. In contrast, a stepwise "staircase" i-t response is observed for proton reduction for single-Pt NP collisions at a Bi/Pt UME. This different response suggests that electrostatic forces of negatively charged citrate-capped Pt NPs also influence the i-t response at more negative applied potentials, but the Pt NPs do not poison the electrochemical activity at Bi/Pt UMEs.