Gold@silica catalyst: Porosity of silica shells switches catalytic reactions

Role of Solvent in Electron-Phonon Relaxation Dynamics in Core-Shell Au-SiO2 Nanoparticles

Relaxation dynamics of plasmons in Au-SiO₂ core-shell nanoparticles have been followed by femtosecond pump-probe technique. The effect of excitation pump energy and surrounding medium on the time constants associated with the hot electron relaxation has been elucidated. A gradual increase in the electron-phonon relaxation time with pump energy is observed and can be attributed to the higher perturbation of the electron distribution in AuNPs at higher pump energy. Variation in time constants for the electron-phonon relaxation in different solvents is rationalized on the basis of their thermal conductivities, which govern the rate of dissipation of heat of photoexcited electrons in the nanoparticles. On the other hand, phonon-phonon relaxation is found to be much less effective than electron-phonon relaxation for the dissipation of energy of the excited electron and the time constants associated with it remain unaffected by thermal conductivity of the solvent.

Palladium oxide-decorated mesoporous silica on graphene oxide nanosheets as a heterogeneous catalyst for the synthesis of β-substituted indole derivatives

In this work, an efficient and facile strategy has been adopted for the stepwise synthesis of the RGO-MSiO2/PdO hybrid nanomaterial (HY-NM). Herein, a hybrid nanostructure of mesoporous silica over graphene oxide (GO) sheets has been developed followed by immobilizing palladium oxide nanoparticles (PdO NPs), and then it has been utilized for catalyzing a multicomponent reaction (MCR). To authenticate the successful synthesis of the HY-NM and successive immobilization of PdO NPs, various physicochemical characterization techniques were utilized such as SEM, EDAX, HR-TEM, HR-XRD, TGA, BET, FT-IR, and XPS analysis. The activity of the HY-NM has been determined by performing the catalyst-mediated synthesis of β-substituted indole derivatives (yield 90-98%). The excellent catalytic activity of the prepared HY-NM could be observed due to its high surface area and large porosity, which facilitates the penetration and interaction of reactant molecules with the catalytic active species. This protocol eliminates the requirement of further purification after the isolation of the product from the reaction mixture. The ease of handling, recyclability of the catalyst, and simple work-up procedure are the main features of this protocol. The synthesized HY-NM could be recycled for multiple catalytic cycles making it a very effective heterogeneous catalyst.

Plant-derived silica nanoparticles and composites for biosensors, bioimaging, drug delivery and supercapacitors: a review

Silica nanoparticles have rapidly found applications in medicine, supercapacitors, batteries, optical fibers and concrete materials, because silica nanoparticles have tunable physical, chemical, optical and mechanical properties. In most applications, high-purity silica comes from synthetic organic precursors, yet this approach could be costly, polluting and non-biocompatible. Alternatively, natural silica sources from biomass are often cheap and abundant, yet they contain impurities. Silica can be extracted from corn cob, coffee husk, rice husk, sugarcane bagasse and wheat husk wastes, which are often disposed of in rivers, lands and ponds. These wastes can be used to prepare homogenous silica nanoparticles. Here we review properties, preparation and applications of silica nanoparticles. Preparation includes chemical and biomass methods. Applications include biosensors, bioimaging, drug delivery and supercapacitors. In particular, to fight the COVID-19 pandemic, recent research has shown that silver nanocluster/silica deposited on a mask reduces SARS-Cov-2 infectivity to zero.

Activation of atom-precise clusters for catalysis

The use of atom-precise, ligand-protected metal clusters has exceptional promise towards the fabrication of model supported-nanoparticle heterogeneous catalysts which have controlled sizes and compositions. One major challenge in the field involves the ease at which metallic clusters sinter upon removal of protected ligands, thus destroying the structural integrity of the model system. This review focuses on methods used to activate atom-precise thiolate-stabilized clusters for heterogeneous catalysis, and strategies that can be used to mitigate sintering. Thermal activation is the most commonly employed approach to activate atom-precise metal clusters, though a variety of chemical and photochemical activation strategies have also been reported. Material chemistry methods that can mitigate sintering are also explored, which include overcoating of clusters with metal oxide supports fabricated by sol-gel chemistry or atomic layer deposition of thin oxide films or encapsulating clusters within porous supports. In addition to focusing on the preservation of the size and morphology of deprotected metal clusters, the fate of the removed ligands is also explored, because detached and/or oxidized ligands can also greatly influence the overall properties of the catalyst systems. We also show that modern characterization techniques such as X-ray absorption spectroscopy and high-resolution electron microscopy have the capacity to enable careful monitoring of particle sintering upon activation of metal clusters.