Metal nanoparticles (NPs) find widespread application as a result of their unique physical and chemical properties. NPs have generated considerable interest in catalysis and electrocatalysis, where they provide a high surface area to mass ratio and can be tailored to promote particular reaction pathways. The activity of NPs can be analyzed especially well using electrochemistry, which probes interfacial chemistry directly. In this Review, we discuss key issues related to the electrochemistry of NPs. We highlight model studies that demonstrate exceptional control over the NP shape and size, or mass-transport conditions, which can provide key insights into the behavior of ensembles of NPs. Particular focus is on the challenge of ultimately measuring reactions at individual NPs, and relating the response to their structure, which is leading to imaginative experiments that have an impact on electrochemistry in general as well as broader surface and colloid science.

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We demonstrate a novel and versatile pipetbased approach to study the landing of individual nanoparticles (NPs) on various electrode materials without any need for encapsulation or fabrication of complex substrate electrode structures, providing great flexibility with respect to electrode materials. Because of the small electrode area defined by the pipet dimensions, the background current is low, allowing for the detection of minute current signals with good time resolution. This approach was used to characterize the potential-dependent activity of Au NPs and to measure the catalytic activity of a single NP on a TEM grid, combining electrochemical and physical characterization at the single NP level for the first time. Such measurements open up the possibility of studying the relation between the size, structure and activity of catalyst particles unambiguously.

a b s t r a c tArrhenius plots, which are used to represent the effects of temperature on the rates of chemical and biophysical processes and on various transport phenomena in materials science, may exhibit deviations from linearity. Account of curvature is provided here by a formula which involves a deformation of the exponential function, of the kind recently encountered in treatments of non-extensivity in statistical mechanics. We present here examples on diverse topics -respiration rates of plants, speed of gliding of bacteria, quantum mechanical tunneling in a chemical reaction -illustrating the variety of possible applications and the additional insight that can be gained.

To study the catalytic activity of single nanoparticles (NPs) electrochemically, we investigated the applicability of a novel method for nanoparticle detection as a means to immobilize individual NPs. This method consists of analyzing the current steps that can be measured at an ultramicroelectrode (UME) when a colloid of NPs is injected into an electrolyte containing an electroactive species, that is turned over at the NP but not the UME surface. We have measured these current steps for the hydrazine oxidation at Pt NPs landing on a lithographically fabricated Au UME, showing a mean step size comparable to theory and prior measurements. We found a reduced landing frequency with respect to values reported in the literature and those predicted from theory, while the current step distribution showed a long tail of large current steps. This could be explained by the particle aggregation, which would lower the effective NP concentration and therefore lower the landing frequency and would result in higher current steps when aggregates reach the electrode. Cyclic voltammetry (CV) measurements of the Pt-modified Au UME showed a signal characteristic of the presence of Pt, while electron microscopy revealed aggregated NPs, after landings were performed in the presence of hydrazine or hydrogen gas. Conversely, no aggregates were found after particles were injected in absence of such reducing agents, while CV still suggested the presence of Pt, indicating individual particles. The finding, that landing nanoparticles in the presence of hydrazine yields NP aggregates on the surface, means that this particular method is currently not suited for the preparation of individually immobilized particles to facilitate catalysis studies at individual nanoparticles.