Bimetallic silver-gold alloy nanoparticles on zirconia with varying Ag/Au ratios were designed by a rational approach and tested as catalysts for the selective oxidation of the promising biomass platform molecule 5-(hydroxymethyl)furfural (HMF). For this purpose, colloidal Ag x Au 10-x particles with molar compositions x = 1/3/5/7/9 were prepared by laser ablation in liquids, a surfactant-free method for the preparation of highly pure nanoparticles, before adsorption on zirconia. In-depth characterization of the supported catalysts evidenced alloyed nanoparticles with distinct trends of the surface and bulk composition depending on the overall Ag/Au molar ratio as determined by X-ray photoelectron spectroscopy (XPS) and Xray absorption spectroscopy (XAS), respectively. To uncover the synergistic effect of the Ag/Au ratio, the catalysts were further studied in terms of the catalytic activity and selectivity in HMF oxidation. Either the aldehyde moiety or both functional groups of HMF were selectively oxidized depending on the Ag/Au composition resulting in 5-hydroxymethyl-2-furan-carboxylic acid (HFCA) or 2,5-furandicarboxylic acid (FDCA), respectively. Optimization of the reaction conditions allowed the quantitative production of HFCA over most catalysts, also after re-use. Only gold rich catalysts Ag 1 Au 9 /ZrO 2 and particularly Ag 3 Au 7 /ZrO 2 were highly active in FDCA synthesis. While Ag 3 Au 7 /ZrO 2 deactivated upon re-use due to sintering, no structural changes were observed for the other catalysts and all catalysts were stable against metal leaching. The present work thus provides fundamental insights into the synergistic effect of Ag and Au in alloyed nanoparticles as active and stable catalysts for the oxidation of HMF.

Gold−silver alloy nanoparticles are interesting for multiple applications, including heterogeneous catalysis, optical sensing, and antimicrobial properties. The inert element gold acts as a stabilizer for silver to prevent particle corrosion, or conversely, to control the release kinetics of antimicrobial silver ions for long-term efficiency at minimum cytotoxicity. However, little is known about the kinetics of silver ion leaching from bimetallic nanoparticles and how it is correlated with silver content, especially not on a single-particle level. To characterize the kinetics of silver ion release from gold−silver alloy nanoparticles, we employed a combination of electron microscopy and single-particle hyperspectral imaging with an acquisition speed fast enough to capture the irreversible silver ion leaching. Single-particle leaching profiles revealed a reduction in silver ion leaching rate due to the alloying with gold as well as two leaching stages, with a large heterogeneity in rate constants. We modeled the initial leaching stage as a shrinking-particle with a rate constant that exponentially depends on the silver content. The second, slower leaching stage is controlled by the electrochemical oxidation potential of the alloy being steadily increased by the change in relative gold content and diffusion of silver atoms through the lattice. Interestingly, individual nanoparticles with similar sizes and compositions exhibited completely different silver ion leaching yields. Most nanoparticles released silver completely, but 25% of them appeared to arrest leaching. Additionally, nanoparticles became slightly porous. Alloy nanoparticles, produced by scalable laser ablation in liquid, together with kinetic studies of silver ion leaching, provide an approach to design the durability or bioactivity of alloy nanoparticles.

Catalytic activity and toxicity of mixed-metal nanoparticles have been shown to correlate often and are known to be ruled by surface composition. The surface chemistry of fully inorganic, ligand-free silver-gold...

Nano-bio-conjugates, featuring noble metal gold− silver alloy nanoparticles, represent a versatile tool in diagnostics and therapeutics due to their plasmonic and antimicrobial properties tunable by the particle's gold molar fraction. However, little is known about how the binding of thiolated biomolecules to noble metal nanoparticles is influenced by the fraction of gold and silver atoms on the nanoparticle's surface and to which extend this would affect the functionality of the conjugated biomolecules. In this work, we generated gold−silver alloy nanoparticles with average diameters of 7−8 nm using the modern, surfactant-free laser ablation in liquids (LAL) synthesis approach. We conjugated them with thiolated miniStrep aptamer ligands at well-controlled aptamer-to-nanoparticle surface area ratios with maxima between 12 and 27 pmol aptamer/cm 2 particle surface area. The results revealed a clear correlation between surface coverage and the nanoparticles' nominal gold/silver ratio, with maximum coverage reached for gold-rich alloys and a pronounced maximum for silverrich alloys. However, the conjugates' functionality, evaluated by binding of streptavidin, was surprisingly robust and hardly affected by the nominal composition. However, 1.5 times higher surface coverage was needed to obtain maximum functionality in the silverrich conjugates. Based on these results, it may be concluded that the nominal composition of gold−silver alloy nano-bioconjugates is freely tunable without a pronounced impact on the attached ligands' functionality, a finding highly relevant for the flexible design of nano-bio-conjugates for future biomedical applications. This study's results may facilitate the design of alloy nano-bio-conjugates for future applications in therapeutics and diagnostics.

Modification of the size and phase composition of magnetic oxide nanomaterials dispersed in liquids by laser synthesis and processing of colloids has high implications for applications in biomedicine, catalysis and for nanoparticle-polymer composites. Controlling these properties for ternary oxides, however, is challenging with typical additives like salts and ligands and can lead to unwanted byproducts and various phases. In our study, we demonstrate how additive-free pulsed laser post-processing (LPP) of colloidal yttrium iron oxide nanoparticles using high repetition rates and power at 355 nm laser wavelength can be used for phase transformation and phase purification of the garnet structure by variation of the laser fluence as well as the applied energy dose. Furthermore, LPP allows particle size modification between 5 nm (ps laser) and 20 nm (ns laser) and significant increase of the monodispersity. Resulting colloidal nanoparticles are investigated regarding their size, structure and temperature-dependent magnetic properties.