Bimetallic PdPt dendrimer-encapsulated nanoparticles (DENs) having sizes of about 2 nm were synthesized by a homogeneous route that involved (1) formation of a Pd core, (2) deposition of a Cu shell onto the Pd core in the presence of H2 gas, and (3) galvanic exchange of Pt for the Cu shell. Under these conditions, a Pd@Pt core@shell DEN is anticipated, but detailed characterization by in-situ extended X-ray absorption fine structure (EXAFS) spectroscopy and other analytical methods indicate that the metals invert to yield a Pt-rich core with primarily Pd in the shell. The experimental findings correlate well with density functional theoretical (DFT) calculations. Theory suggests that the increased disorder associated with <~2 nm diameter nanoparticles, along with the relatively large number of edge and corner sites, drives the structural rearrangement. This type of rearrangement is not observed on larger nanoparticles or in bulk metals.

Here, we report the use of microwire and mesh working electrodes in paper analytical devices fabricated by origami paper folding (oPADs). The important new result is that Au wires and carbon fibers having diameters ranging from micrometers to tens of micrometers can be incorporated into oPADs and that their electrochemical characteristics are consistent with the results of finite element simulations. These electrodes are fully compatible with both hollow channels and paper channels filled with cellulose fibers, and they are easier to incorporate than typical screen-printed carbon electrodes. The results also demonstrate that the Au electrodes can be cleaned prior to device fabrication using aggressive treatments and that they can be easily surface modified using standard thiol-based chemistry.

We report electrochemical catalytic amplification of individual collisions between ∼57 nm diameter Pt nanoparticles (Pt NPs) and 12.5 μm diameter Au ultramicroelectrodes modified with passivating, electrostatically assembled polyelectrolyte multilayer (PEM) films prepared by the layer-by-layer deposition method. Two key findings are reported. First, despite the thicknesses of the insulating PEM films, which range up to 5 nm, electrons are able to tunnel from the Pt NPs to the electrode resulting in electrocatalytic N2H4 oxidation at the PEM film-solution interface. These single-particle measurements are in accord with prior reports showing that the electrochemical activity of passive PEM films can be reactivated by adsorption of metallic NPs. Second, it is possible to control the frequency of the collisions by manipulating the net electrostatic charge present on the outer surface of the PEM thin film. These results, which demonstrate that chemistry can be used to control electrocatalytic amplification, set the stage for future sensing applications.

Here we outline a new method for synthesizing fully reduced Pt dendrimer-encapsulated nanoparticles (DENs). This is achieved by first synthesizing Cu DENs of the appropriate size through sequential dendrimer loading and reduction steps, and then galvanically exchanging the zerovalent Cu DENs for Pt. The properties of Pt DENs having an average of 55, 140, and 225 atoms prepared by direct chemical reduction and by galvanic exchange are compared. Data obtained by UV–vis spectroscopy, X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, and high-resolution electron microscopy confirm only the presence of fully reduced Pt DENs when synthesized by galvanic exchange, while chemical reduction leads to a mixture of reduced DENs and unreduced precursor. These results are significant because Pt DENs are good models for developing a better understanding of the effects of finite size on catalytic reactions. Until now, however, the results of such studies have been complicated by a heterogeneous mixture of Pt catalysts.

Here we report on the electrochemical properties of carbon electrodes coated with thin layers of Al2O3 and SnO2. These oxide films were deposited using atomic layer deposition (ALD) and range in thickness from 1 to 6 nm. Electrochemical experiments show that the thinnest oxide layers contain defects that penetrate to the underlying carbon electrode. However, oxygenation of the carbon surface prior to ALD increases the surface concentration of nucleation sites for oxide growth and suppresses the defect density. Films of Al2O3 just ∼3-4 nm in thickness are free of pinholes. Slightly thicker coatings of SnO2 are required for equivalent passivation. Both Al2O3 and SnO2 films are stable in both neutral and acidic electrolytes even after repeated voltammetric scanning. The results reported here open up the possibility of studying the effect of oxide supports on electrocatalytic reactions.