'From the mole to the molecule': ruthenium catalyzed nitroarene reduction studied with 'bench', high-throughput and single molecule fluorescence techniques

Single molecule fluorescence microscopy techniques are used to complement conventional catalysis and high-throughput experiments in order to gain a complete picture of a model reaction. In these experiments a model nitroarene is reduced to an amine where, upon reduction, a red shift in absorption/emission, as well as an increase in emission, is observed. The reaction is studied under bulk reaction conditions by NMR spectroscopy and the fluorescence activation makes it possible to also study this reaction at the single molecule level. Fluorescence correlation spectroscopy is a valuable technique in supporting the proposed reaction mechanism and understanding the nature and duration of molecular 'visits' to catalytic sites, where both the starting material, nitroarene, and the amine product have an affinity for the catalyst.Thanks are due to the Natural Sciences and Engineering Council of Canada and the Canadian Foundation for Innovation for generous support. M. L. 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Synergistic Effects in the Coupling of Plasmon Resonance of Metal Nanoparticles with Excited Gold Clusters

When molecules or clusters are within the proximity of metal particles, their electronic transitions can be drastically enhanced. We have now probed the off-resonance excitation of molecule-like, glutathione-capped gold clusters (Au-GSH) in the close proximity of larger (plasmonic) Au and Ag nanoparticles. The excited state absorption spectrum of Au-GSH* is obtained with monophotonic excitation. The characteristic absorption of Au-GSH* allows us to probe the influence of excited plasmonic nanoparticles coupled with the clusters. Although infrared (775 nm) lasers pulses do not produce Au-GSH*, the excited states of these clusters are formed when coupled with metal (Au, Ag) nanoparticles. Interestingly, the coupled excitation of Au-GSH/AgNP with 775 nm laser pulses also results in an enhanced field effect, as seen from increased plasmon response of the metal nanoparticles. Transient absorption measurements confirm the synergy between these two inherently different nanomaterials, causing them to display greater excitation features. Better understanding of metal cluster–metal nanoparticle interactions will have important implications in designing light harvesting systems, and optoelectronic devices

Facile SILAR Approach to Air-Stable Naked Silver and Gold Nanoparticles Supported by Alumina

A synthetically convenient and scalable SILAR (successive ion layer adsorption and reaction) method is used to make air-stable films of silver and gold nanoparticles supported on alumina scaffolds. This solution-based deposition technique yields particles devoid of insulating capping agents or ligands. The optical properties of the nanoparticle films were investigated using femtosecond transient absorption spectroscopy. A linear absorption arising from intraband excitation (775 nm laser pulse) is seen only for Au nanoparticles at low intensity. However, both Au and Ag particles exhibit plasmon resonance responses at high excitation intensity via two photon absorption of the 775 nm pump pulse. The difference in optical response to near-IR laser excitation is rationalized based on the known density of states for each metal. To demonstrate the potential applications of these films, alumina-supported Ag nanoparticles were utilized as substrates for surface enhanced Raman spectroscopy, resulting in a 65-fold enhancement in the Raman signal of the probe molecule rhodamine 6G. The exceptional stability and scalability of these SILAR films opens the door for further optical and photocatalytic studies and applications, particularly with ligand-free Ag nanoparticles that typically oxidize under ambient conditions. Additionally, isolating plasmonic and interband electronic excitations in stable AgNP under visible light irradiation could enable elucidation of the mechanisms that drive noble metal-assisted photocatalytic processes

Size-Dependent Excited State Behavior of Glutathione-Capped Gold Clusters and Their Light-Harvesting Capacity

Glutathione-protected gold clusters exhibit size-dependent excited state and electron transfer properties. Larger-size clusters (e.g., Au₂₅GSH₁₈) with core-metal atoms display rapid (<1 ps) as well as slower relaxation (∼200 ns) while homoleptic clusters (e.g., Au_10–12GSH_10–12) exhibit only slower relaxation. These decay components have been identified as metal–metal transition and ligand-to-metal charge transfer, respectively. The short lifetime relaxation component becomes less dominant as the size of the gold cluster decreases. The long-lived excited state and ability to participate in electron transfer are integral for these clusters to serve as light-harvesting antennae. A strong correlation between the ligand-to-metal charge-transfer excited state lifetime and photocatalytic activity was evidenced from the electron transfer to methyl viologen. The photoactivity of these metal clusters shows increasing photocatalytic reduction yield (0.05–0.14) with decreasing cluster size, Au₂₅ < Au₁₈ < Au₁₅ < Au_10–12. Gold clusters, Au₁₈GSH₁₄, were found to have the highest potential as a photosensitizer on the basis of the quantum yield of electron transfer and good visible light absorption properties

Plasmon-mediated photopolymerization maps plasmon fields for silver nanoparticles

Visible light exposure of films containing silver nanoparticles (AgNPs) shows that the enhanced field around AgNPs in a thin film containing an azo free radical initiator (AIBN) and a triacrylate selectively cross-links the triacrylate within the plasmonic region around the particles. The cross-linked polymer is less soluble than its precursor and behaves as a solubility switch. After the film is developed with ethanol, polymer-encapsulated nanoparticles are preserved on the surface. The 8-10 nm polymer structure that encapsulates the particles effectively maps and preserves the morphology of the plasmon field in AgNP-controlled nanostructures. © 2011 American Chemical Society.Fil: Stamplecoskie, Kevin G.. University of Ottawa; CanadáFil: Pacioni, Natalia Lorena. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. University of Ottawa; CanadáFil: Larson, Dayle. University of Ottawa; CanadáFil: Scaiano, Juan C.. University of Ottawa; Canad

Self-Assembled Dipole Nanolasers

Visible light excitation of silver nanoparticles in the presence of polymerizable monomers and selected dyes triggers the self-assembly of nanolasers in a synthetically simple process. The new nanolasers incorporate a thin, fully organic gain medium that allows the tuning of the core absorption to a selected dye, or of the dye to a preselected core material. This versatile synthesis of surface plasmon lasers, or “spasers”, has unique simplicity and enables spatial and temporal control of the nanolaser fabrication process

Two Distinct Transitions in Cu_<i>x</i>InS₂ Quantum Dots. Bandgap versus Sub-Bandgap Excitations in Copper-Deficient Structures

Cu-deficient CuInS₂ quantum dots (QDs) synthesized by varying the [Cu]:[In] ratio allow modulation of optical properties as well as identification of the radiative emission pathways. Absorption and emission spectral features showed a strong dependence on the [Cu]:[In] ratio of Cu_<i>x</i>InS₂ QDs, indicating two independent optical transitions. These effects are pronounced in transient absorption spectra. The bleaching of band edge absorption and broad tail absorption bands in the subpicosecond–nanosecond time scale provide further evidence for the dual optical transitions. The recombination process as monitored by photoemission decay indicated the involvement of surface traps in addition to the bandgap and sub-bandgap transitions. Better understanding of the origin of the optical transitions and their influence on the photodynamics will enable utilization of ternary semiconductor quantum dots in display and photovoltaic devices

Excited-State Behavior of Luminescent Glutathione-Protected Gold Clusters

The excited-state behavior of luminescent gold clusters provides new insights in understanding their photocatalytic activity in the visible region. The excited state of glutathione-protected gold nanoclusters (AuGSH), which is characterized by the long-lived excited state (τ = 780 ns), arises from the ligand-to-metal type transition. These AuGSH clusters are in a partially oxidized state (Au­(I)) and are readily reduced by chemical or electrochemical methods. Interestingly, a metal core transition with short-lived lifetime (τ < 3 ps) appears along with a longer lifetime in reduced AuGSH clusters. The role of the oxidation state of gold clusters in dictating the photocatalytic reduction of methyl viologen is discussed

Dual-Stage Lithography from a Light-Driven, Plasmon-Assisted Process: A Hierarchical Approach to Subwavelength Features

A hierarchy of lithographic-type imaging generating 3 μm lines incorporating subdiffraction limit features was obtained through a novel two-step reaction process. Photochemically generated ketyl radicals were used to make defined lines of silver nanoparticles. The excitation of nanoparticle surface plasmons was then used to generate highly localized heat that causes polymerization selectively on the surfaces of excited particles. The nylon-6 polymer that is generated serves as a solubility switch used to retain the features on the substrate selectively; various imaging techniques were used to establish the nature of the nylon shells. This work shows that the heat generated by plasmon excitation can be exploited to generate negative-type lithographic features with dimensions well below the diffraction limit