Showing papers in "Angewandte Chemie in 2006"

Silica-based mesoporous organic-inorganic hybrid materials.

Abstract: Mesoporous organic-inorganic hybrid materials, a new class of materials characterized by large specific surface areas and pore sizes between 2 and 15 nm, have been obtained through the coupling of inorganic and organic components by template synthesis. The incorporation of functionalities can be achieved in three ways: by subsequent attachment of organic components onto a pure silica matrix (grafting), by simultaneous reaction of condensable inorganic silica species and silylated organic compounds (co-condensation, one-pot synthesis), and by the use of bissilylated organic precursors that lead to periodic mesoporous organosilicas (PMOs). This Review gives an overview of the preparation, properties, and potential applications of these materials in the areas of catalysis, sorption, chromatography, and the construction of systems for controlled release of active compounds, as well as molecular switches, with the main focus being on PMOs.

Changing the Activity of Electrocatalysts for Oxygen Reduction by Tuning the Surface Electronic Structure

Abstract: The fuel cell is a promising alternative to environmentally unfriendly devices that are currently powered by fossil fuels. In the polymer electrolyte membrane fuel cell (PEMFC),the main fuel is hydrogen,which through its reaction with oxygen produces electricity with water as the only by-product. To make PEMFCs economically viable,there are several problems that should be solved; the main one is to find more effective catalysts than Pt for the oxygen reduction reaction (ORR),1/2 O 2 + 2H + + 2e = H2O. The design of inexpensive,stable,and catalytically active materials for the ORR will require fundamental breakthroughs,and to this end it is important to develop a fundamental understanding of the catalytic process on different materials. Herein,we describe how variations in the electronic structure determine trends in the catalytic activity of the ORR across the periodic table. We show that Pt alloys involving 3d metals are better catalysts than Pt because the electronic structure of the Pt atoms in the surface of these alloys has been modified slightly. With this understanding,we hope that electrocatalysts can begin to be designed on the basis of fundamental insight.

Hydrogen Peroxide Synthesis: An Outlook beyond the Anthraquinone Process

Abstract: Hydrogen peroxide (H2O2) is widely used in almost all industrial areas, particularly in the chemical industry and environmental protection. The only degradation product of its use is water, and thus it has played a large role in environmentally friendly methods in the chemical industry. Hydrogen peroxide is produced on an industrial scale by the anthraquinone oxidation (AO) process. However, this process can hardly be considered a green method. It involves the sequential hydrogenation and oxidation of an alkylanthraquinone precursor dissolved in a mixture of organic solvents followed by liquid–liquid extraction to recover H2O2. The AO process is a multistep method that requires significant energy input and generates waste, which has a negative effect on its sustainability and production costs. The transport, storage, and handling of bulk H2O2 involve hazards and escalating expenses. Thus, novel, cleaner methods for the production of H2O2 are being explored. The direct synthesis of H2O2 from O2 and H2 using a variety of catalysts, and the factors influencing the formation and decomposition of H2O2 are examined in detail in this Review.

Cascade reactions in total synthesis.

Abstract: The design and implementation of cascade reactions is a challenging facet of organic chemistry, yet one that can impart striking novelty, elegance, and efficiency to synthetic strategies. The application of cascade reactions to natural products synthesis represents a particularly demanding task, but the results can be both stunning and instructive. This Review highlights selected examples of cascade reactions in total synthesis, with particular emphasis on recent applications therein. The examples discussed herein illustrate the power of these processes in the construction of complex molecules and underscore their future potential in chemical synthesis.

Reactions in Droplets in Microfluidic Channels

Abstract: Fundamental and applied research in chemistry and biology benefits from opportunities provided by droplet-based microfluidic systems. These systems enable the miniaturization of reactions by compartmentalizing reactions in droplets of femoliter to microliter volumes. Compartmentalization in droplets provides rapid mixing of reagents, control of the timing of reactions on timescales from milliseconds to months, control of interfacial properties, and the ability to synthesize and transport solid reagents and products. Droplet-based microfluidics can help to enhance and accelerate chemical and biochemical screening, protein crystallization, enzymatic kinetics, and assays. Moreover, the control provided by droplets in microfluidic devices can lead to new scientific methods and insights.

Asymmetric catalysis by chiral hydrogen-bond donors.

Abstract: Hydrogen bonding is responsible for the structure of much of the world around us. The unusual and complex properties of bulk water, the ability of proteins to fold into stable three-dimensional structures, the fidelity of DNA base pairing, and the binding of ligands to receptors are among the manifestations of this ubiquitous noncovalent interaction. In addition to its primacy as a structural determinant, hydrogen bonding plays a crucial functional role in catalysis. Hydrogen bonding to an electrophile serves to decrease the electron density of this species, activating it toward nucleophilic attack. This principle is employed frequently by Nature's catalysts, enzymes, for the acceleration of a wide range of chemical processes. Recently, organic chemists have begun to appreciate the tremendous potential offered by hydrogen bonding as a mechanism for electrophile activation in small-molecule, synthetic catalyst systems. In particular, chiral hydrogen-bond donors have emerged as a broadly applicable class of catalysts for enantioselective synthesis. This review documents these advances, emphasizing the structural and mechanistic features that contribute to high enantioselectivity in hydrogen-bond-mediated catalytic processes.

Functional liquid-crystalline assemblies: self-organized soft materials.

Abstract: In the 21st century, soft materials will become more important as functional materials because of their dynamic nature. Although soft materials are not as highly durable as hard materials, such as metals, ceramics, and engineering plastics, they can respond well to stimuli and the environment. The introduction of order into soft materials induces new dynamic functions. Liquid crystals are ordered soft materials consisting of self-organized molecules and can potentially be used as new functional materials for electron, ion, or molecular transporting, sensory, catalytic, optical, and bio-active materials. For this functionalization, unconventional materials design is required. Herein, we describe new approaches to the functionalization of liquid crystals and show how the design of liquid crystals formed by supramolecular assembly and nano-segregation leads to the formation of a variety of new self-organized functional materials.

Materials for Fluorescence Resonance Energy Transfer Analysis: Beyond Traditional Donor-Acceptor Combinations

Abstract: The use of Forster or fluorescence resonance energy transfer (FRET) as a spectroscopic technique has been in practice for over 50 years. A search of ISI Web of Science with just the acronym "FRET" returns more than 2300 citations from various areas such as structural elucidation of biological molecules and their interactions, in vitro assays, in vivo monitoring in cellular research, nucleic acid analysis, signal transduction, light harvesting and metallic nanomaterials. The advent of new classes of fluorophores including nanocrystals, nanoparticles, polymers, and genetically encoded proteins, in conjunction with ever more sophisticated equipment, has been vital in this development. This review gives a critical overview of the major classes of fluorophore materials that may act as donor, acceptor, or both in a FRET configuration. We focus in particular on the benefits and limitations of these materials and their combinations, as well as the available methods of bioconjugation.

A Microporous Metal–Organic Framework for Gas-Chromatographic Separation of Alkanes†

Abstract: A zinc-based metal-organic framework (MOF) can be transformed reversibly from an open (a) to a dense (b) configuration. The microporous solid is the first example of a MOF that is highly selective in the gas-chromatographic separation of alkanes.

Shape Control of Semiconductor and Metal Oxide Nanocrystals through Nonhydrolytic Colloidal Routes

Abstract: Inorganic nanocrystals with tailored geometries exhibit unique shape-dependent phenomena and subsequent utilization of them as building blocks for the fabrication of nanodevices is of significant interest. Herein, we review the recent developments in the shape control of colloidal nanocrystals with a focus on the scientifically and technologically important semiconductor and metal oxide nanocrystals obtained by nonhydrolytic synthetic methods. Many structurally unprecedented motifs have been discovered including polyhedrons, rods and wires, plates and prisms, and other advanced shapes such as branched rods, stars, inorganic dendrites, and dumbbells. The currently proposed shape-guiding mechanisms are presented and the important pioneering studies on the assembly of shape-controlled nanocrystals into ordered superlattices and the fabrication of prototype advanced nanodevices are discussed.

High gas adsorption in a microporous metal-organic framework with open-metal sites

Abstract: A gas storage material contains a metal-organic framework that includes a plurality of metal clusters and a plurality of charged multidentate linking ligands that connect adjacent metal clusters. Each metal cluster includes one or more metal ions and at least one open metal site. The metal-organic framework includes one or more sites for storing molecular hydrogen. A hydrogen storage system using the hydrogen storage material is provided.

Polymer Therapeutics: Concepts and Applications

Abstract: Polymer therapeutics encompass polymer-protein conjugates, drug-polymer conjugates, and supramolecular drug-delivery systems. Numerous polymer-protein conjugates with improved stability and pharmacokinetic properties have been developed, for example, by anchoring enzymes or biologically relevant proteins to polyethylene glycol components (PEGylation). Several polymer-protein conjugates have received market approval, for example the PEGylated form of adenosine deaminase. Coupling low-molecular-weight anticancer drugs to high-molecular-weight polymers through a cleavable linker is an effective method for improving the therapeutic index of clinically established agents, and the first candidates have been evaluated in clinical trials, including, N-(2-hydroxypropyl)methacrylamide conjugates of doxorubicin, camptothecin, paclitaxel, and platinum(II) complexes. Another class of polymer therapeutics are drug-delivery systems based on well-defined multivalent and dendritic polymers. These include polyanionic polymers for the inhibition of virus attachment, polycationic complexes with DNA or RNA (polyplexes), and dendritic core-shell architectures for the encapsulation of drugs. In this Review an overview of polymer therapeutics is presented with a focus on concepts and examples that characterize the salient features of the drug-delivery systems.

Biologically active molecules with a "light switch".

Abstract: Biologically active compounds which are light-responsive offer experimental possibilities which are otherwise very difficult to achieve. Since light can be manipulated very precisely, for example, with lasers and microscopes rapid jumps in concentration of the active form of molecules are possible with exact control of the area, time, and dosage. The development of such strategies started in the 1970s. This review summarizes new developments of the last five years and deals with "small molecules", proteins, and nucleic acids which can either be irreversibly activated with light (these compounds are referred to as "caged compounds") or reversibly switched between an active and an inactive state.

Olefin-metathesis catalysts for the preparation of molecules and materials (Nobel Lecture)

Abstract: This is a story of our exploration of the olefin-metathesis reaction, a reaction that has been the major emphasis of my independent research. As with all stories of scientific discovery, there are three components: the discoveries, the resulting applications, and, perhaps the most important of all, the people involved. Starting from observations made from seemingly unrelated work, our investigations into the fundamental chemistry of this transformation have been an exciting journey, with major advances often resulting from complete surprises, mistakes, and simple intuition. Ultimately, these efforts have contributed to olefin metathesis becoming the indispensable synthetic tool that it is today.

Fast Colorimetric Sensing of Adenosine and Cocaine Based on a General Sensor Design Involving Aptamers and Nanoparticles

Abstract: Aptamers are nucleic acid based binding molecules that are obtained through a combinatorial selection process known as systematic evolution of ligands by exponential enrichment (SELEX). 2] They are emerging as a new class of molecules that can rival antibodies in terms of the broad range of molecules they can selectively bind. In comparison with antibodies, aptamers, particularly DNA aptamers, are relatively easy to obtain, more stable to biodegradation, and less vulnerable to denaturation. Therefore they are prime candidates as sensors in a number of applications, such as environmental monitoring and medical diagnostics. The key challenge to their successful application is transforming the aptamer-binding events into physically detectable signals. To meet the challenge, a number of methods have been developed, most of which involve fluorescence-based detection. Simple colorimetric sensors can eliminate the use of analytical instruments and have attracted much attention recently. For example, organic-dye replacement was employed to design a colorimetric cocaine sensor. However, an appropriate dye has to be found for a designated aptamer, and a waiting time of 12 hours is needed to observe a color change. Cationic conjugated polymers form complexes of different color with aptamers in the presence or absence of a target analyte. A number of colorimetric sensors were made with this method. The high extinction coefficients and distance-dependent optical properties have made metallic nanoparticles very attractive in DNA-related colorimetric assays, such as the detection of DNAwith high sequence selectivity, 22] and the detection of metal ions 24] and other analytes. 17] Recently, aptamer-functionalized gold nanoparticles were employed to detect thrombin. This system took advantage of the fact that each thrombin molecule binds two aptamers, so nanoparticles

Optimizing Dyes for Dye-Sensitized Solar Cells

Abstract: Dye-sensitized solar cells (DSSCs) have emerged as an important cheap photovoltaic technology. Charge separation is initiated at the dye, bound at the interface of an inorganic semiconductor and a hole-transport material. Careful design of the dye can minimize loss mechanisms and improve light harvesting. Mass application of DSSCs is currently limited by manufacturing complexity and long-term stability associated with the liquid redox electrolyte used in the most-efficient cells. In this Minireview, dye design is discussed in the context of novel alternatives to the standard liquid electrolyte. Rapid progress is being made in improving the efficiencies of such solid and quasi-solid DSSCs which promises cheap, efficient, and robust photovoltaic systems.

Carbon Nanotubes as Intracellular Transporters for Proteins and DNA: An Investigation of the Uptake Mechanism and Pathway

Abstract: New materials for the intracellular transport of biological cargos such as DNA, proteins, and drug molecules have been actively sought to effectively breach the cell-membrane barriers for delivery and enabling functionality of extracellular agents. Single-walled carbon nanotubes (SWNT) have been recently shown to shuttle various molecular cargos inside living cells including proteins, short peptides, and nucleic acids. The internalized nanotubes were found to be biocompatible and nontoxic at the cellular level. Furthermore, a very recent study has established targeted internalization of SWNTs into cancer cells that express specific cellsurface receptors and subsequent use as high near-infrared (NIR) absorbing agents for cancer-cell destruction without harming normal cells. Release of oligodeoxynucleotides from SWNT transporters in vitro has also been demonstrated by NIR excitation of nanotubes that are located inside living cells. The utilization of the intrinsic physical properties of SWNTs allows the realization of a new class of biotransporters and opens up new possibilities in drug delivery and NIR radiation therapy. At the present time, several fundamental issues remain to be addressed for the use of carbon nanotubes as potential biological transporters. One such issue is the entry mechanism that regulates the cellular internalization of SWNTs and their cargos. We have suggested that SWNTs traverse the cellular membrane through endocytosis, whereas Pantarotto et al. have suggested an energy independent nonendocytotic mechanism that involves insertion and diffusion of nanotubes through the lipid bilayer of the cell membrane. Detailed work to establish the cellular uptake mechanism and pathway for SWNTs is currently lacking. Herein, we present the first systematic investigation of the cellular uptake mechanism and pathway for carbon nanotubes. We first show that intracellular transportation of proteins and DNA by SWNTs is indeed general, thus further confirming the transporter ability of these materials. We then present evidence that shows clathrin-dependent endocytosis as the pathway for the uptake of various SWNT conjugates with proteins and DNA. We also discuss the differences between the nanotube materials and the experimental procedures used in our work and by Pantarotto et al. who suggested an energy-independent nonendocytotic uptake of nanotubes. In our current work, we aim to clearly establish the intracellular uptake mechanism of SWNTs in the form of individual and small bundles with lengths of < 1 micron and to avoid any confusion and controversy over the cellularuptake mechanism for these materials. To investigate the generality of carbon nanotubes for the transportation of proteins and DNA inside mammalian cells, we used, in our current work, two different mammalian cells lines, adherent HeLa cells and non-adherent HL60 cells. Control experiments were carried out through the incubation of cells in protein and DNA solutions in the absence of nanotubes. In parallel experiments, HeLa and HL60 cells were incubated with various noncovalent protein–SWNTand DNA–SWNT conjugates (Figure 1a, b) and analyzed by both

Multiple Metal–Carbon Bonds for Catalytic Metathesis Reactions (Nobel Lecture)

Abstract: It’s my great priviledge to be here today, in a position I never thought possible. I hope the story that I will tell you will give you some idea what I have contributed to the area for which the Nobel Prize in Chemistry was awarded this year. The story begins thirty two years ago in 1973, the year the Nobel Prize was shared by G. Wilkinson and E. O. Fischer. Wilkinson’s Nobel Lecture1 concerned the nature of a single bond between a transition metal and a carbon atom in an alkyl group, and emphasized the fact that the metal-carbon bond is not inherently weak. E. O. Fischer in his Nobel Lecture2 summarized the extensive chemistry of transition metal “carbene” complexes3,4 that contain a metal-carbon double bond discovered by him and his group in 1964 (Fig 1).5 He also reported new “carbyne” complexes that contain a metal-carbon triple bond.6 It was clear that metal-carbon single bonds were of great importance in the emerging area of homogeneous catalysis. However, no catalytic reactions involving species that contain metal-carbon double or triple bonds were known. When I went to the Central Research Department of E. I. DuPont de Nemours and Company in 1972, transition metal organometallic chemistry and homogeneous catalysis were of great interest as a consequence of their huge potential in organic chemistry and therefore in industry. In the early 1970’s inorganic chemists knew that many transition metal species containing a metal-carbon bond are subject to various modes of decomposition that are much more rapid than in a non-transition metal species such as Zn(CH2CH3)2 or Al(CH2CH3)3. The most common of these involves transfer of a hydrogen, from an ethyl group (MCH2CH3) for example, to the metal to

Dysprosium triangles showing single-molecule magnet behavior of thermally excited spin states

Abstract: The study of paramagnetic metal-ion aggregates has been of increasing interest since the observation that such molecules can exhibit magnetic memory effects. Termed singlemolecule magnets or SMMs, the important factors leading to such properties derive from the combination of a large ground-state spin and a large magnetic anisotropy of the Ising (easy-axis) type. Studies have largely been based on transition-metal compounds since they typically exhibit both of the aforementioned features. The incorporation of lanthanides into these complexes has been investigated to take advantage of the potentially large number of unpaired f-electrons available. However, very little work has been done to date on purely lanthanide-based systems. The origin of SMM behavior in lanthanide-containing compounds is more complicated than that of d-block transition-metal ions since there is likely to be a significant orbital component. In the lanthanide-containing phthalocyanine complexes reported in the literature the ligand environment induces a large splitting of the ground Jmanifold, whereas in SMMs large-spin ground states arising from magnetic interactions between the metal centers of the cluster can enhance the weaker single-ion

Electric Control of Droplets in Microfluidic Devices

Abstract: The precision manipulation of streams of fluids with microfluidic devices is revolutionizing many fluid-based technologies and enabling the development of high-throughput reactors that use minute quantities of reagents. However, as the scale of these reactors shrinks, contamination effects due to surface adsorption and diffusion limit the smallest quantities that can be used. The confinement of reagents in droplets in an immiscible carrier fluid overcomes these limitations, but demands new fluid-handling technology. We present a platform technology based on charged droplets and electric fields that enables electrically addressable droplet generation, highly efficient droplet coalescence, precision droplet breaking and recharging, and controllable droplet sorting. This is an essential enabling technology for a high-throughput droplet microfluidic reactor. Networks of small channels are a flexible platform for the precision manipulation of small amounts of fluids. 2] The utility of such microfluidic devices depends critically on enabling technologies such as the microfluidic peristaltic pump, electrokinetic pumping, 5] and dielectrophoreticpump or electrowetting-driven flow; these technologies can form the essential building blocks for the assembly of fluidhandling modules. These modules can be used to perform a variety of key tasks including the measurement of precise aliquots of fluids, the combination of fluid streams, and the mixing of multiple fluid components. The assembly of such modules into complete systems provides a convenient and robust way to construct microfluidic devices. These have myriad uses; for example, high-throughput screening, the exploration of chemical phase diagrams, assays of biological molecules, single-cell analysis, and combinatorial approaches to protein crystallization can all be performed with only minimal consumption of reagents. However, virtually all microfluidic devices are based on flows of streams of fluids; this sets a limit on the smallest volume of reagent that can be used effectively because of the contaminating effects of diffusion and surface adsorption. As the dimensions of small volumes are decreased, diffusion becomes the dominant mechanism for mixing leading to dispersion of reactants. Moreover, surface adsorption of reactants, although small, can be highly detrimental at low concentrations and small volumes. As a result current microfluidic technologies cannot be reliably used for applications involving minute quantities of reagent—for example, bioassays at levels down to the single molecule are not easily performed. An approach that overcomes these limitations is the use of aqueous droplets in an immiscible carrier fluid; these droplets provide a well-defined, encapsulated microenvironment that eliminates cross-contamination or changes in concentration caused by diffusion or surface interactions. Droplets provide the ideal microcapsule that can isolate reactive materials, cells, or small particles for further manipulation and study. Moreover, by making droplets as small as one femtoliter, reactions of single biomolecules can be investigated. However, essentially all enabling technology for microfluidic systems developed thus far has focused on single-phase fluid flow, and there are few corresponding, active means to manipulate droplets. In particular, manipulating, mixing, and combining reagents in microfluidic geometries is much more difficult for droplets than for single streams, 20] especially when the droplets are stabilized with [*] Dr. D. R. Link, E. Grasland-Mongrain, A. Duri, F. Sarrazin, Prof. Z. Cheng, G. Cristobal, Prof. D. A. Weitz Department of Physics and DEAS Harvard University Cambridge, MA 02138 (USA) Fax: (+1)203-458-2514 E-mail: dlink@raindancetechnologies.com M. Marquez Los Alamos National Laboratory Chemistry Division Los Alamos, NM 87545 (USA) M. Marquez INEST Group Philip Morris USA Research Center Richmond, VA 23234 (USA) [**] This work was supported by Kraft Foods, Rhodia Corporation, and the NSF through the Harvard MRSEC (DMR-0213805 and DMR0243715). The authors thank H. A. Stone and M. P. Brenner for valuable discussions. Supporting information for this article is available on the WWW under http://www.angewandte.org or from the author. Communications

Synthetic Multivalent Ligands as Probes of Signal Transduction

Abstract: Cell-surface receptors acquire information from the extracellular environment and coordinate intracellular responses Many receptors do not operate as individual entities, but rather as part of dimeric or oligomeric complexes Coupling the functions of multiple receptors may endow signaling pathways with the sensitivity and malleability required to govern cellular responses Moreover, multireceptor signaling complexes may provide a means of spatially segregating otherwise degenerate signaling cascades Understanding the mechanisms, extent, and consequences of receptor co-localization and interreceptor communication is critical; chemical synthesis can provide compounds to address the role of receptor assembly in signal transduction Multivalent ligands can be generated that possess a variety of sizes, shapes, valencies, orientations, and densities of binding elements This Review focuses on the use of synthetic multivalent ligands to characterize receptor function

Energetic nitrogen-rich salts and ionic liquids.

Abstract: Energetic salts offer many advantages over conventional energetic molecular compounds. The use of nitrogen containing anions and cations contributes to high heats of formations and high densities. Their low carbon and hydrogen content gives rise to a good oxygen balance. The decomposition of these compounds is predominantly through the generation of dinitrogen which makes them very promising candidates for highly energetic materials for industrial or military applications.

Thermosensitive core--shell particles as carriers for Ag nanoparticles : modulating the catalytic activity by a phase transition in networks

Abstract: Metal nanoparticles have properties that are significantly different from the bulk properties of the metals. Moreover, their high surface-to-volume ratio renders them ideal candidates for application as catalysts. However, the pronounced tendency of nanoparticles to aggregate must be overcome by using suitable carrier systems. Recently, a number of systems have been discussed that are suitable for applications in aqueous environments. These include polymers, dendrimers, microgels, 18] and other colloidal systems. 20] In all the cases studied so far, these carrier systems only provide a suitable support for the nanoparticles and prevent them from aggregating. In this way the carrier system of, for example, dendrimers or microgels acts much in the same way as a “nanoreactor” that immobilizes the particles and leads to their more convenient handling. Here we report on the first system that allows us to modulate the activity of nanoparticles through a thermodynamic transition that takes place within the carrier system. Figure 1 displays the principle. Metallic nanoparticles are embedded in a polymeric network attached to a colloidal core particle. In all the cases discussed here the core consists of poly(styrene) (PS) while the network consists of poly(Nisopropylacrylamide) (PNIPA) cross-linked with N,N’-methylenebisacrylamide (BIS). The particles are suspended in water, which swells the PNIPA at room temperature. The PNIPA network, however, undergoes a phase transition around 30 8C, during which most of the water is expelled. Previous experiments have demonstrated that this transition is perfectly reversible and the process of shrinking and reswelling can be repeated without degradation or coagulation of the particles. Metallic nanoparticles embedded in such a network are fully accessible to reactants at low temperature. Above the phase transition, however, the marked shrinkage of the network should be followed by a concomitant slowing down of the diffusion of the reactants within the network. The rate of reactions catalyzed by the nanoparticles should thus be slowed down considerably. In this way, the network could act as a “nanoreactor” that can be opened or closed to a certain extent. Herein we demonstrate that thermosensitive core–shell networks may indeed be used as such a nanoreactor. The activity of the catalyst can be modulated by temperature over a wide range. As the model reaction we chose the reduction of 4-nitrophenol to 4-aminophenol by sodium borohydride. The reaction was repeatedly performed to check the catalytic activity of the metal nanoparticles, and the results obtained in the present study can be directly compared to literature data. The carrier particles having a PS core and a PNIPA shell were prepared as described recently. 24] Figure 2 shows a schematic representation of the silver nanoparticles being

Polyhedral silver nanocrystals with distinct scattering signatures

Abstract: Silver nanoparticles are ideal building blocks for opticalmaterials that seek to manipulate, transport, or amplify light becausethey support surface plasmons with frequencies in the visible and near-IRregime. We demonstrate that polyhedral silvernanocrystals displaycomplex and distinct scattering signatures dictated by shape and size.The ability to engineer specific plasmon modes should have profoundconsequences for surface-enhanced Raman spectroscopy, sub-wavelengthoptics, and plasmonic transport.