Reflective dark field microscopy is used to observe the decrease in the light scattered from Ag nanoparticles immobilised on differing solid substrates. The nanoparticles are exposed to solutions containing halide ions, both at open circuit and under potentiostatic control, leading to the loss of the nanomaterial. By coupling optical and electrochemical techniques the physical origin of this transformation is demonstrated to be the electrochemical dissolution of the metal nanoparticles driven by electron transfer to ultra-trace dissolved oxygen. The dissolution kinetics of the surface-supported metal nanoparticles is compared on four substrate materials (i.e., glass, indium titanium oxide, glassy carbon and platinum) with different electrical conductivity. The three conductive substrates catalyse the redox-driven dissolution of Ag nanoparticles with the electrons transferred from the nanoparticles, via the macroscopic electrode to the dioxygen electron acceptor.

Enhancing mass transport to electrodes is desired in almost all types of electrochemical sensing, electrocatalysis, and energy storage or conversion. Here, a method of doing so by means of the magnetic gradient force generated at magnetic-nanoparticle-modified electrodes is presented. It is shown using Fe₃O₄-nanoparticle-modified electrodes that the ultrahigh magnetic gradients (> 10⁸ T·m^–1) established at the magnetized Fe₃O₄ nanoparticles speed up the transport of reactants and products at the electrode surface. Using the Fe(Ⅲ)/Fe(Ⅱ)-hexacyanoferrate redox couple, it is demonstrated that this mass transport enhancement can conveniently and repeatedly be switched on and off by applying and removing an external magnetic field, owing to the superparamagnetic properties of magnetite nanoparticles. Thus, it is shown for the first time that magnetic nanoparticles can be used to control mass transport in electrochemical systems. Importantly, this approach does not require any means of mechanical agitation and is therefore particularly interesting for application in micro- and nanofluidic systems and devices.

The application of naive Koutecky–Levich analysis to micro-and nano-particle modified rotating disk electrodes of partially covered and non-planar geometry is critically analysed. Assuming strong overlap of the diffusion fields of the particles such that transport to the entire surface is time-independent and one-dimensional, the observed voltammetric response reflects an apparent electrochemical rate constant k°_app, equal to the true rate constant k° describing the redox reaction of interest on the surface of the nanoparticles and the ratio, ψ, of the total electroactive surface area to the geometric area of the rotating disk surface. It is demonstrated that Koutecky–Levich analysis is applicable and yields the expected plots of I^–1 versus ω^–1 where I is the current and ω is the rotation speed but that the values of the electrochemical rate constants inferred are thereof k°_app, not k°. Thus, for ψ > 1 apparent electrocatalysis might be naively but wrongly inferred whereas for ψ < 1 the deduced electrochemical rate constant will be less than k°. Moreover, the effect of ψ on the observed rotating disk electrode voltammograms is significant, signalling the need for care in the overly simplistic application of Koutecky–Levich analysis to modified rotating electrodes, as is commonly applied for example in the analysis of possible oxygen reduction catalysts.

Anodic particle coulometry (APC) is a recently established method of sizing individual metal nanoparticles by oxidising them during their impact on a micro electrode. Here it is demonstrated that the application of APC can be extended to sizing of metal oxide nanoparticles, such as Fe₃O₄ magnetite nanoparticles. Additionally, a new route to electrochemical nanoparticle sizing is introduced—cathodic particle coulometry (CPC). This method uses the reduction of impacting nanoparticles, e.g., metal oxide nanoparticles, and is demonstrated to yield correct size information for Fe₃O₄ nanoparticles. The combination of these two independent electrochemical methods of nanoparticle sizing, allows for purely electrochemical sizing of single nanoparticles and simultaneous verification of the obtained results.

The electrocatalytic activity for oxygen reduction reaction (ORR) at neutral pH of citrate-capped silver nanoparticles (diameter = 18 nm) supported on glassy carbon (GC) is investigated voltammetrically. Novelly, the modification of the substrate by nanoparticles sticking to form a random nanoparticle array and the voltammetric experiments are carried out simultaneously by immersion of the GC electrode in an air-saturated 0.1 M NaClO₄ solution (pH = 5.8) containing chemically-synthesized nanoparticles.

The experimental voltammograms of the resulting nanoparticle array are simulated with homemade programs according to the two-proton, two-electron reduction of oxygen to hydrogen peroxide where the first electron transfer is rate determining. In the case of silver electrodes, the hydrogen peroxide generated is partially further reduced to water via heterogeneous decomposition.

Comparison of the results obtained on a silver macroelectrode and silver nanoparticles indicates that, for the silver nanoparticles and particle coverages (0.035%–0.457%) employed in this study, the ORR electrode kinetics is slower and the production of hydrogen peroxide larger on the glassy carbon-supported nanoparticles than on bulk silver.