Fig. 5. (A) NP impact or landing studies involve polarizing an inert ultramicroelectrode (UME) in a solution containing a reactant (ox/red) and catalyst NPs (yellow sphere). As the NPs impact the surface they undergo Fermi level equilibration with the UME, as described in Scheme 6, convert the reactant to product and generate a measurable current. (B) Potentiometric monitoring of the open circuit potential (OCP) vs. time for single Pt NP collisions at the surface of a 5 μm radius Au UME in the presence of 5 mM phosphate buffer (pH 7.0) and 15 mM hydrazine with (solid line) and without (dashed line) 7.5 pM Pt NPs. Inset: magnified image of the staircase potential response. Alternatively, current transients may be detected and exhibited. Adapted with permission from ref. 56; H. Zhou, J. H. Park, F.-R. F. Fan and A. J. Bard, J. Am. Chem. Soc., 2012, 134, 13212–13215. Copyright 2012 American Chemical Society. (C) A “staircase” response, if the NPs “stick” to the UME generating a continuous response, as seen for hydrazine oxidation by Pt NPs impacting a polarized Au microelectrode, or (D) a “spike” or “blip” response, as seen for water oxidation by IrOx NPs impacting a Pt microelectrode. (C and D) reproduced from ref. 71 with permission from the PCCP Owner Societies (E) schematic representation of a scanning electrochemical cell microscopy (SECCM) set-up as described by Unwin et al. and (F) a schematic of the liquid meniscus that constitutes the electrochemical cell. Analogous to conventional NP impact studies, the electrode substrate is held at a potential where a reaction occurs on the catalytic impacting NPs but not on the collector electrode. (G) Landing events of individual Au NPs using the SECCM setup with the same Au NP imaged by TEM afterwards providing exceptional opportunities for structure-electrochemical reactivity analysis. (H) Cyclic voltammogram for the oxidation of 2 mM hydrazine measured at a scan rate of 200 mV s–1 for the individual AuNP shown in (G) on a TEM grid electrode. (E–H) adapted with permission from ref. 70; S. E. F. Kleijn, S. C. S. Lai, T. S. Miller, A. I. Yanson, M. T. M. Koper and P. R. Unwin, J. Am. Chem. Soc., 2012, 134, 18558–18561. Copyright 2012 American Chemical Society.
