Structural Study of Pt–Fe Nanoparticles: New Insights into Pt Bimetallic Nanoparticle Formation with Oxidized Fe Species

A combination of physicochemical methods allowed us to assess a structure of comparatively monodisperse 3–4 nm Pt–Fe_<i>x</i>O_<i>y</i> nanoparticles (NPs) synthesized by thermal decomposition of platinum acetylacetonate in the presence of iron oxide NPs as an iron source. Unlike traditional PtFe alloys composed of zerovalent Pt and Fe species with the surface enriched by Pt atoms, the NPs discussed in this work contain Pt(0) and oxidized Fe species (most probably Fe³⁺ or Fe²⁺) as is proven by X-ray photoelectron spectroscopy (XPS). Angular dependence XPS measurements demonstrated the absence of the core–shell structure, although a minor enrichment of the NP surface with Fe species was observed. High-resolution transmission electron microscopy and X-ray powder diffraction (XRD) reveal that these Pt–Fe_<i>x</i>O_<i>y</i> NPs are not alloys, but consist of different domains; i.e., they have a “cluster-in-cluster” morphology. A comparison of the XPS and XRD data allowed us to conclude that the NPs also include amorphous iron oxide. These results allow better understanding of the mechanism of such NP formation and possible predictions of their catalytic performance

Graphene Derivative in Magnetically Recoverable Catalyst Determines Catalytic Properties in Transfer Hydrogenation of Nitroarenes to Anilines with 2‑Propanol

Here, we report transfer hydrogenation of nitroarenes to aminoarenes using 2-propanol as a hydrogen source and Ag-containing magnetically recoverable catalysts based on partially reduced graphene oxide (pRGO) sheets. X-ray diffraction and X-ray photoelectron spectroscopy data demonstrated that, during the one-pot catalyst synthesis, formation of magnetite nanoparticles (NPs) is accompanied by the reduction of graphene oxide (GO) to pRGO. The formation of Ag⁰ NPs on top of magnetite nanoparticles does not change the pRGO structure. At the same time, the catalyst structure is further modified during the transfer hydrogenation, leading to a noticeable increase of sp² carbons. These carbons are responsible for the adsorption of substrate and intermediates, facilitating a hydrogen transfer from Ag NPs and creating synergy between the components of the catalyst. The nitroarenes with electron withdrawing and electron donating substituents allow for excellent yields of aniline derivatives with high regio and chemoselectivity, indicating that the reaction is not disfavored by these functionalities. The versatility of the catalyst synthetic protocol was demonstrated by a synthesis of an Ru-containing graphene derivative based catalyst, also allowing for efficient transfer hydrogenation. Easy magnetic separation and stable catalyst performance in the transfer hydrogenation make this catalyst promising for future applications

Coat Protein-Dependent Behavior of Poly(ethylene glycol) Tails in Iron Oxide Core Virus-like Nanoparticles

Here we explore the formation of virus-like nanoparticles (VNPs) utilizing 22–24 nm iron oxide nanoparticles (NPs) as cores and proteins derived from viral capsids of brome mosaic virus (BMV) or hepatitis B virus (HBV) as shells. To accomplish that, hydrophobic FeO/Fe₃O₄ NPs prepared by thermal decomposition of iron oleate were coated with poly­(maleic acid-<i>alt</i>-octadecene) modified with poly­(ethylene glycol) (PEG) tails of different lengths and grafting densities. MRI studies show high <i>r</i>₂/<i>r</i>₁ relaxivity ratios of these NPs that are practically independent of the polymer coating type. The versatility and flexibility of the viral capsid protein are on display as they readily form shells that exceed their native size. The location of the long PEG tails upon shell formation was investigated by electron microscopy and small-angle X-ray scattering. PEG tails were located differently in the BMV and HBV VNPs, with the BMV VNPs preferentially entrapping the tails in the interior and the HBV VNPs allowing the tails to extend through the capsid, which highlights the differences between intersubunit interactions in these two icosahedral viruses. The robustness of the assembly reaction and the protruding PEG tails, potentially useful in modulating the immune response, make the systems introduced here a promising platform for biomedical applications

Viruslike Nanoparticles with Maghemite Cores Allow for Enhanced MRI Contrast Agents

Here, for the first time, we demonstrate formation of virus-like nanoparticles (VNPs) utilizing gold-coated iron oxide nanoparticles as cores and capsid protein of brome mosaic virus (BMV) or hepatitis B virus (HBV) as shells. Further, utilizing cryo-electron microscopy and single particle methods, we are able to show that the BMV coat on VNPs assembles into a structure very close to that of a native virion. This is a consequence of an optimal iron oxide NP size (∼11 nm) fitting the virus cavity and an ultrathin gold layer on the maghemite cores, which allows for utilization of SH-(CH₂)₁₁-(CH₂-CH₂-O)₄-OCH₂-COOH as capping molecules to provide sufficient stability, charge density, and small form factor. MRI studies show unique relaxivity ratios that diminish only slightly with gold coating. A virus protein coating of a magnetic core mimicking the wild-type virus makes these VNPs a versatile platform for biomedical applications

Functionalization of Monodisperse Iron Oxide NPs and Their Properties as Magnetically Recoverable Catalysts

Here we report the functionalization of monodisperse iron oxide nanoparticles (NPs) with commercially available functional acids containing multiple double bonds such as linolenic (LLA) and linoleic (LEA) acids or pyridine moieties such as 6-methylpyridine-2-carboxylic acid, isonicotinic acid, 3-hydroxypicolinic acid, and 6-(1-piperidinyl)­pyridine-3-carboxlic acid (PPCA). Both double bonds and pyridine groups can be reacted with noble metal compounds to form catalytically active species in the exterior of magnetic NPs, thus making them promising magnetically recoverable catalysts. We determined that both LLA and LEA stabilize magnetic iron oxide NPs, allowing the formation of π-complexes with bis­(acetonitrile)­dichloropalladium­(II) in the NP shells. In both cases, this leads to the formation of NP aggregates because of interparticle complexation. In the case of pyridine-containing ligands, only PPCA with two N-containing rings is able to provide NP stabilization and functionalization whereas other pyridine-containing acids did now allow sufficient steric stabilization. The interaction of PPCA-based particles with Pd acetate also leads to aggregation because of interparticle interactions, but the aggregates that are formed are much smaller. Nevertheless, the catalytic properties in the selective hydrogenation of dimethylethynylcarbinol (DMEC) to dimethylvinylcarbinol were the best for the catalyst based on LLA, demonstrating that the NP aggregates in all cases are penetrable for DMEC. Easy magnetic separation of this catalyst from the reaction solution makes it promising as a magnetically recoverable catalyst

Insights into Sustainable Glucose Oxidation Using Magnetically Recoverable Biocatalysts

Here, we developed magnetically recoverable biocatalysts for enzymatic oxidation of d-glucose to d-gluconic acid with high product yields. The catalyst support is based on nanoparticle clusters (NPCs) composed of magnetite particles and coated with the amino terminated silica layer to facilitate further functionalization. It involves the attachment of the glutaraldehyde linker followed by the covalent attachment of glucose oxidase (GOx) via its amino groups. It was established that the NPCs with a diameter of ∼430 nm attach 33% more GOx molecules than NPCs with a diameter of ∼285 nm, although the surface area of the former is lower than that of the latter. At the same time, the biocatalyst based on the smaller NPCs shows higher relative activity of 94% than that (87%) of the biocatalyst based on the larger NPCs, both at 50 °C and pH 7 (optimal reaction conditions). This surprising result has been explained by a combination of two major factors such as GOx crowding on the support surface which should prevent denaturation (similar to the enzyme behavior in cells) and the enzyme mobility which should be preserved upon immobilization. Apparently, for the biocatalyst based on 285 nm NPCs, the lower GOx crowding is compensated by its higher mobility. The high stability of these GOx based biocatalysts in 10 consecutive reactions as well as facile magnetic recovery combined with excellent catalytic activity in “tolerant” pH range make this biocatalyst design promising for other types of enzymatic catalysts

Multifunctional Nanohybrids by Self-Assembly of Monodisperse Iron Oxide Nanoparticles and Nanolamellar MoS₂ Plates

Here, we report the synthesis, characterization, and properties of novel nanohybrids formed by self-assembly of negatively charged MoS₂ nanoplates and positively charged iron oxide nanoparticles (NPs) of two different sizes, 5.1 and 11.6 nm. Iron oxide NPs were functionalized with an amphiphilic random copolymer, quaternized poly­(2-(di­methyl­amino)­ethyl metacrylate-<i>co</i>-stearyl meta­crylate), synthesized for the first time using atom transfer radical polymerization. The influence of the MoS₂ fraction and the iron oxide NP size on the structure of the nanohybrids has been studied. Surprisingly, larger NPs retained a larger fraction of the copolymer, thus requiring more MoS₂ nanoplates for charge compensation. The nanohybrid based on 11.6 nm NPs was studied in oxidation of sulfide ions. This reaction could be used for removing the dangerous pollutant from wastewater and in the production of hydrogen from water using solar energy. We demonstrated a higher catalytic activity of the NP/MoS₂ nanohybrid than that of merely dispersed MoS₂ in catalytic oxidation of sulfide ions and facile magnetic recovery of the catalyst after the reaction

Multicore Iron Oxide Mesocrystals Stabilized by a Poly(phenylenepyridyl) Dendron and Dendrimer: Role of the Dendron/Dendrimer Self-Assembly

We report the formation of multicore iron oxide mesocrystals using the thermal decomposition of iron acetyl acetonate in the presence of the multifunctional and rigid poly­(phenylenepyridyl) dendron and dendrimer. We thoroughly analyze the influence of capping molecules of two different architectures and demonstrate for the first time that dendron/dendrimer self-assembly leads to multicore morphologies. Single-crystalline ordering in multicore NPs leads to cooperative magnetic behavior: mesocrystals exhibit ambient blocking temperatures, allowing subtle control over magnetic properties using a minor temperature change

Multicore Iron Oxide Mesocrystals Stabilized by a Poly(phenylenepyridyl) Dendron and Dendrimer: Role of the Dendron/Dendrimer Self-Assembly

We report the formation of multicore iron oxide mesocrystals using the thermal decomposition of iron acetyl acetonate in the presence of the multifunctional and rigid poly­(phenylenepyridyl) dendron and dendrimer. We thoroughly analyze the influence of capping molecules of two different architectures and demonstrate for the first time that dendron/dendrimer self-assembly leads to multicore morphologies. Single-crystalline ordering in multicore NPs leads to cooperative magnetic behavior: mesocrystals exhibit ambient blocking temperatures, allowing subtle control over magnetic properties using a minor temperature change

Metal-Ion Distribution and Oxygen Vacancies That Determine the Activity of Magnetically Recoverable Catalysts in Methanol Synthesis

Here, we report on the development of novel Zn-, Zn–Cr-, and Zn–Cu-containing catalysts using magnetic silica (Fe₃O₄–SiO₂) as the support. Transmission electron microscopy, powder X-ray diffraction, and X-ray photoelectron spectroscopy (XPS) showed that the iron oxide nanoparticles are located in mesoporous silica pores and the magnetite (spinel) structure remains virtually unchanged despite the incorporation of Zn and Cr. According to XPS data, the Zn and Cr species are intermixed within the magnetite structure. In the case of the Zn–Cu-containing catalysts, a separate Cu₂O phase was also observed along with the spinel structure. The catalytic activity of these catalysts was tested in methanol synthesis from syngas (CO + H₂). The catalytic experiments showed an improved catalytic performance of Zn- and Zn–Cr-containing magnetic silicas compared to that of the ZnO–SiO₂ catalyst. The best catalytic activity was obtained for the Zn–Cr-containing magnetic catalyst prepared with 1 wt % Zn and Cr each. X-ray absorption spectroscopy demonstrated the presence of oxygen vacancies near Fe and Zn in Zn-containing, and even more in Zn–Cr-containing, magnetic silica (including oxygen vacancies near Cr ions), revealing a correlation between the catalytic properties and oxygen vacancies. The easy magnetic recovery, robust synthetic procedure, and high catalytic activity make these catalysts promising for practical applications