Creative approaches to the design of catalytic nanomaterials are necessary in achieving environmentally sustainable energy sources. Integrating dissimilar metals into a single nanoparticle (NP) offers a unique avenue for customizing catalytic activity and maximizing surface area. Alloys containing five or more equimolar components with a disordered, amorphous microstructure, referred to as High-Entropy Metallic Glasses (HEMGs), provide tunable catalytic performance based on the individual properties of incorporated metals. Here, we present a generalized strategy to electrosynthesize HEMG-NPs with up to eight equimolar components by confining multiple metal salt precursors to water nanodroplets emulsified in dichloroethane. Upon collision with an electrode, alloy NPs are electrodeposited into a disordered microstructure, where dissimilar metal atoms are proximally arranged. We also demonstrate precise control over metal stoichiometry by tuning the concentration of metal salt dissolved in the nanodroplet. The application of HEMG-NPs to energy conversion is highlighted with electrocatalytic water splitting on CoFeLaNiPt HEMG-NPs.

Single-molecule detection is the ultimate sensitivity in analytical chemistry and has been largely unavailable in electrochemical analysis. Here, we demonstrate the feasibility of detecting electrochemically inactive single biomacromolecules, such as enzymes, antibodies, and DNA, by blocking a solution redox reaction when molecules adsorb and block electrode sites. By oxidizing a large concentration of potassium ferrocyanide on an ultramicroelectrode (UME, radius ≤150 nm), time-resolved, discrete adsorption events of antibodies, enzymes, DNA, and polystyrene nanospheres can be differentiated from the background by their "footprint". Further, by assuming that the mass transport of proteins to the electrode surface is controlled mainly by diffusion, a size estimate using the Stokes-Einstein relationship shows good agreement of electrochemical data with known protein sizes.

Understanding the relationship between the structure and the reactivity of catalytic metal nanoparticles (NPs) is important to achieve higher efficiencies in electrocatalytic devices. A big challenge remains, however, in studying these relations at the individual NP level. To address this challenge, we developed an approach using nanometer-scale scanning electrochemical microscopy (SECM) for the study of the geometric property and catalytic activity of individual Pt NPs in the hydrogen oxidation reaction (HOR). Herein, Pt NPs with a few tens to a hundred nm radius were directly electrodeposited on a highly oriented pyrolitic graphite (HOPG) surface via nucleation and growth without the necessity of capping agents or anchoring molecules. A well-defined nanometer-sized tip comparable to the dimensions of the NPs and a stable nanogap between the tip and NPs enabled us to achieve lateral and vertical spatial resolutions at a nanometer-scale and study fast electron-transfer kinetics. Specifically, the use of two different types of redox mediators: (1) outer-sphere mediator and (2) inner-sphere mediators could differentiate between the topography and the catalytic activity of individual Pt NPs and measure a large effective rate constant of HOR, k eff 0 of ≥2 cm/s as a lower limit at each Pt NP. Consequently, the size, shape, spatial orientation and the catalytic activity of Pt NPs could be determined at an individual level in nanoscale SECM where imaging accompanied by theoretical modeling and analysis. This approach can be easily extended to quantitatively probe the effects of the surface property, such as capping agent effects on the catalytic activity of a variety of metal NPs for the design and assessment of NP catalysts.