Showing papers in "Angewandte Chemie in 2010"
TL;DR: This Review summarize recent advances in the synthesis and characterization of C-dots and speculate on their future and discuss potential developments for their use in energy conversion/storage, bioimaging, drug delivery, sensors, diagnostics, and composites.
Abstract: Similar to its popular older cousins the fullerene, the carbon nanotube, and graphene, the latest form of nanocarbon, the carbon nanodot, is inspiring intensive research efforts in its own right. These surface-passivated carbonaceous quantum dots, so-called C-dots, combine several favorable attributes of traditional semiconductor-based quantum dots (namely, size- and wavelength-dependent luminescence emission, resistance to photobleaching, ease of bioconjugation) without incurring the burden of intrinsic toxicity or elemental scarcity and without the need for stringent, intricate, tedious, costly, or inefficient preparation steps. C-dots can be produced inexpensively and on a large scale (frequently using a one-step pathway and potentially from biomass waste-derived sources) by many approaches, ranging from simple candle burning to in situ dehydration reactions to laser ablation methods. In this Review, we summarize recent advances in the synthesis and characterization of C-dots. We also speculate on their future and discuss potential developments for their use in energy conversion/storage, bioimaging, drug delivery, sensors, diagnostics, and composites.
3,991 citations
TL;DR: The most recent developments and emerging concepts in CO(2) separations by solvent absorption, chemical and physical adsorption, and membranes, amongst others, will be discussed, with particular attention on progress in the burgeoning field of metal-organic frameworks.
Abstract: The escalating level of atmospheric carbon dioxide is one of the most pressing environmental concerns of our age. Carbon capture and storage (CCS) from large point sources such as power plants is one option for reducing anthropogenic CO(2) emissions; however, currently the capture alone will increase the energy requirements of a plant by 25-40%. This Review highlights the challenges for capture technologies which have the greatest likelihood of reducing CO(2) emissions to the atmosphere, namely postcombustion (predominantly CO(2)/N(2) separation), precombustion (CO(2)/H(2)) capture, and natural gas sweetening (CO(2)/CH(4)). The key factor which underlies significant advancements lies in improved materials that perform the separations. In this regard, the most recent developments and emerging concepts in CO(2) separations by solvent absorption, chemical and physical adsorption, and membranes, amongst others, will be discussed, with particular attention on progress in the burgeoning field of metal-organic frameworks.
3,388 citations
TL;DR: The radical-mediated thiol-ene reaction has all the desirable features of a click reaction, being highly efficient, simple to execute with no side products and proceeding rapidly to high yield.
Abstract: Following Sharpless' visionary characterization of several idealized reactions as click reactions, the materials science and synthetic chemistry communities have pursued numerous routes toward the identification and implementation of these click reactions. Herein, we review the radical-mediated thiol-ene reaction as one such click reaction. This reaction has all the desirable features of a click reaction, being highly efficient, simple to execute with no side products and proceeding rapidly to high yield. Further, the thiol-ene reaction is most frequently photoinitiated, particularly for photopolymerizations resulting in highly uniform polymer networks, promoting unique capabilities related to spatial and temporal control of the click reaction. The reaction mechanism and its implementation in various synthetic methodologies, biofunctionalization, surface and polymer modification, and polymerization are all reviewed.
3,229 citations
TL;DR: PEG is the most used polymer and also the gold standard for stealth polymers in the emerging field of polymer-based drug delivery and alternative polymers will be evaluated.
Abstract: Poly(ethylene glycol) (PEG) is the most used polymer and also the gold standard for stealth polymers in the emerging field of polymer-based drug delivery. The properties that account for the overwhelming use of PEG in biomedical applications are outlined in this Review. The first approved PEGylated products have already been on the market for 20 years. A vast amount of clinical experience has since been gained with this polymer--not only benefits, but possible side effects and complications have also been found. The areas that might need consideration and more intensive and careful examination can be divided into the following categories: hypersensitivity, unexpected changes in pharmacokinetic behavior, toxic side products, and an antagonism arising from the easy degradation of the polymer under mechanical stress as a result of its ether structure and its non-biodegradability, as well as the resulting possible accumulation in the body. These possible side effects will be discussed in this Review and alternative polymers will be evaluated.
2,815 citations
TL;DR: The facile one-step alkali-assisted electrochemical fabrication of CQDs with sizes of 1.2– 3.8 nm which possess size-dependent photoluminescence (PL) and excellent upconversion luminescence properties are reported and the design of photocatalysts is demonstrated to harness the use of the full spectrum of sunlight.
Abstract: Carbon nanostructures are attracting intense interest because of their many unique and novel properties. The strong and tunable luminescence of carbon materials further enhances their versatile properties; in particular, the quantum effect in carbon is extremely important both fundamentally and technologically. Recently, photoluminescent carbonbased nanoparticles have received much attention. They are usually prepared by laser ablation of graphite, electrochemical oxidation of graphite, electrochemical soaking of carbon nanotubes, thermal oxidation of suitable molecular precursors, vapor deposition of soot, proton-beam irradiation of nanodiamonds, microwave synthesis, and bottom-up methods. Although small (ca. 2 nm) graphite nanoparticles show strong blue photoluminescence (PL), definitive experimental evidence for luminescence of carbon structure arising from quantum-confinement effects and size-dependent optical properties of carbon quantum dots (CQDs) remains scarce. Herein, we report the facile one-step alkali-assisted electrochemical fabrication of CQDs with sizes of 1.2– 3.8 nm which possess size-dependent photoluminescence (PL) and excellent upconversion luminescence properties. Significantly, we demonstrate the design of photocatalysts (TiO2/CQDs and SiO2/CQDs complex system) to harness the use of the full spectrum of sunlight (based on the upconversion luminescence properties of CQDs). It can be imagined that judicious cutting of a graphite honeycomb layer into ultrasmall particles can lead to tiny fragments of graphite, yielding CQDs, which may offer a straightforward and facile strategy to prepare high-quality CQDs. Using graphite rods as both anode and cathode, and NaOH/EtOH as electrolyte, we synthesized CQDs with a current intensity of 10–200 mAcm . As a reference, a series of control experiments using acids (e.g. H2SO4/EtOH) as electrolyte yielded no formation of CQDs. This result indicates that alkaline environment is the key factor, and OH group is essential for the formation of CQDs by the electrochemical oxidation process. Figure 1a shows a trans-
2,266 citations
TL;DR: This Review highlights recent advances in the synthesis, bioconjugation, and cellular uses of gold nanoconjugates.
Abstract: Gold colloids have fascinated scientists for over a century and are now heavily utilized in chemistry, biology, engineering, and medicine. Today these materials can be synthesized reproducibly, modified with seemingly limitless chemical functional groups, and, in certain cases, characterized with atomic-level precision. This Review highlights recent advances in the synthesis, bioconjugation, and cellular uses of gold nanoconjugates. There are now many examples of highly sensitive and selective assays based upon gold nanoconjugates. In recent years, focus has turned to therapeutic possibilities for such materials. Structures which behave as gene-regulating agents, drug carriers, imaging agents, and photoresponsive therapeutics have been developed and studied in the context of cells and many debilitating diseases. These structures are not simply chosen as alternatives to molecule-based systems, but rather for their new physical and chemical properties, which confer substantive advantages in cellular and medical applications.
2,063 citations
TL;DR: The possibilities of creating highly sophisticated functional hierarchical systems with multiple, interdependent, functionalities along with a critical analysis that allows the non-specialist to learn the salient features of POMs are discussed.
Abstract: Polyoxometalates (POMs) are a subset of metal oxides that represent a diverse range of molecular clusters with an almost unmatched range of physical properties and the ability to form dynamic structures that can range in size from the nano- to the micrometer scale Herein we present the very latest developments from synthesis to structure and function of POMs We discuss the possibilities of creating highly sophisticated functional hierarchical systems with multiple, interdependent, functionalities along with a critical analysis that allows the non-specialist to learn the salient features We propose and present a "periodic table of polyoxometalate building blocks" We also highlight some of the current issues and challenges that need to be addressed to work towards the design of functional systems based upon POM building blocks and look ahead to possible emerging application areas
1,884 citations
TL;DR: Sterically encumbered Lewis acid and Lewis base combinations do not undergo the ubiquitous neutralization reaction to form "classical" Lewis acid/Lewis base adducts, but both the unquenched Lewis acidity and basicity of such sterically "frustrated Lewis pairs (FLPs)" is available to carry out unusual reactions.
Abstract: Sterically encumbered Lewis acid and Lewis base combinations do not undergo the ubiquitous neutralization reaction to form "classical" Lewis acid/Lewis base adducts. Rather, both the unquenched Lewis acidity and basicity of such sterically "frustrated Lewis pairs (FLPs)" is available to carry out unusual reactions. Typical examples of frustrated Lewis pairs are inter- or intramolecular combinations of bulky phosphines or amines with strongly electrophilic RB(C(6)F(5))(2) components. Many examples of such frustrated Lewis pairs are able to cleave dihydrogen heterolytically. The resulting H(+)/H(-) pairs (stabilized for example, in the form of the respective phosphonium cation/hydridoborate anion salts) serve as active metal-free catalysts for the hydrogenation of, for example, bulky imines, enamines, or enol ethers. Frustrated Lewis pairs also react with alkenes, aldehydes, and a variety of other small molecules, including carbon dioxide, in cooperative three-component reactions, offering new strategies for synthetic chemistry.
1,621 citations
TL;DR: This work aims to prepare graphene-like MoS2 and WS2, which are quasi-two-dimensional compounds in which the atoms within the layer are held together by strong covalent forces while van der Waals interaction enables stacking of the layers.
Abstract: Inorganic sheets: Graphene-like MoS2 and WS2 were prepared by three different chemical methods. Examination by microscopic techniques revealed that they consist of one or a few layers (see depicted TEM image of WS2 layers), and an atomic-resolution TEM image showed that layered MoS2 has a hexagonal arrangement of Mo and S atoms (see inset).
1,541 citations
TL;DR: Nonprecious-metal and metal-free catalysts for ORR have attracted enormous interest as an alternative to platinum-based catalysts and would aid attempts to elucidate the correlation between the structure, composition, and electrochemical activity of nitrogen-doped carbon materials.
Abstract: The cathodic oxygen-reduction reaction (ORR) is one of the most crucial factors in the performance of a fuel cell. The development of efficient ORR electrocatalysts is thus of great significance for the commercialization of fuel cells. Platinum-based materials have long been investigated as active catalysts for ORR; however, the large-scale application of fuel cells has been hampered by the high cost and inadequacy of this metal. Recently, nonprecious-metal and metal-free catalysts for ORR have attracted enormous interest as an alternative to platinum-based catalysts. In particular, nitrogen-doped carbon materials, which are typical metal-free catalysts, exhibit excellent electrocatalytic activity for ORR as a result of their unique electronic properties derived from the conjugation between the nitrogen lone-pair electrons and the graphene p system. Generally, nitrogen-doped carbon materials can be prepared by the pyrolysis of transition-metal macrocyclic compounds or mixtures of metal salts and nitrogen-containing precursors. In these processes, the transition metals play an important role not only in the formation of graphitic frameworks, but also in the introduction of nitrogen active sites. Drawbacks are the use of expensive precursors and the need for extra steps to remove metal species. Furthermore, metal nanoparticles encapsulated in the graphite framework still remain even after a tedious removal process. The nature of the nitrogen atoms in nitrogen-doped carbon materials and whether they are really the active catalytic sites is still controversial. Thus, the development of nitrogen-doped carbon materials with excellent electrochemical performance but without any metal components is an urgent issue. Such materials would not only be promising candidates for ORR catalysts but would aid attempts to elucidate the correlation between the structure, composition, and electrochemical activity of nitrogen-doped carbon materials.
1,320 citations
TL;DR: The introduction of magnetic nanoparticles in a variety of solid matrices allows the combination of well-known procedures for catalyst heterogenization with techniques for magnetic separation.
Abstract: Recovery and reuse of expensive catalysts after catalytic reactions are important factors for sustainable process management. The aim of this Review is to highlight the progress in the formation and catalytic applications of magnetic nanoparticles and magnetic nanocomposites. Directed functionalization of the surfaces of nanosized magnetic materials is an elegant way to bridge the gap between heterogeneous and homogeneous catalysis. The introduction of magnetic nanoparticles in a variety of solid matrices allows the combination of well-known procedures for catalyst heterogenization with techniques for magnetic separation.
TL;DR: This Review explores issues by presenting the latest advances in electrochemical, electrical, and optical biosensors that use carbon nanotubes and graphene, and critically compares the performance of the two carbon allotropes in this application.
Abstract: From diagnosis of life-threatening diseases to detection of biological agents in warfare or terrorist attacks, biosensors are becoming a critical part of modern life. Many recent biosensors have incorporated carbon nanotubes as sensing elements, while a growing body of work has begun to do the same with the emergent nanomaterial graphene, which is effectively an unrolled nanotube. With this widespread use of carbon nanomaterials in biosensors, it is timely to assess how this trend is contributing to the science and applications of biosensors. This Review explores these issues by presenting the latest advances in electrochemical, electrical, and optical biosensors that use carbon nanotubes and graphene, and critically compares the performance of the two carbon allotropes in this application. Ultimately, carbon nanomaterials, although still to meet key challenges in fabrication and handling, have a bright future as biosensors.
TL;DR: It is demonstrated that the optical absorption of carbon nitride semiconductor materials is extendable into the visible region up to about 750 nm by simple copolymerization with organic monomers like barbituric acid (BA).
Abstract: and nonmetallic elements (N, C, B) creates localized/ delocalized states in the band gap and thus extends its optical absorption to the visible region, but doping usually comes with accelerated charge recombination and lower stability of the doped materials. Meanwhile, various other inorganic, non-TiO2-based, visible-light catalysts have been developed (e.g., metal oxides, nitrides, sulfides, phosphides, and their mixed solid solutions), whereby Ga, Ge, In, Ta, Nb, and W are the main metal constituents. However, sustained utilization of solar energy calls for the development of more abundant and stable catalysts working with visible light, and this has remained challenging so far. Recently, a polymeric semiconductor on the basis of a defecteous graphitic carbon nitride (g-C3N4), was introduced as a metal-free photocatalyst which fulfills the basic requirements for a water-splitting catalyst, including being abundant, stable, and responsive to visible light. In the following, we use the notation “g-C3N4” to describe this class of materials rather than the idealized structure. The most active system is in fact presumably an N-bridged “poly(tri-s-triazine)”, already described by Liebig as “melon”. A semiconductor structure with band edges straddling the water redox potential was revealed for melon by DFT calculations, albeit electrochemical analysis is still awaited. g-C3N4 is considered to be the most stable phase of covalent carbon nitride, and facile synthesis of the melon substructure from simple liquid precursors and monomers allows easy engineering of carbon nitride materials to achieve the desired nanostructures via soft-chemical processing routes and methods. For instance, a high surface area (67–400 mg ) can be imparted on g-C3N4 materials by polymerization of cyanamide on a silica template, which results in photocatalytically more active g-C3N4 nanostructures. [8] Metal-doped gC3N4 can also be conveniently obtained by polymerization of dicyandiamine in the presence of metal salts, and thus multifunctionalization of such materials for a variety of applications can be achieved. Most importantly, the electronic and optical properties of carbon nitride, regarded as a polymer semiconductor, are in principle adjustable by organic protocols. Such organic protocols have been widely used to control the performance of traditional p-conjugated polymers, for example, to improve solar-cell efficiencies by constructing copolymerized donor–acceptor structures, or to modify electronic properties by co-blending with p/n-type organic dopants. Our aim was to use such organic modifications to extend the insufficient light absorption of g-C3N4 (a result of its large band gap of 2.7 eV, which corresponds to wavelengths shorter than 460 nm) towards the maximum of the solar spectrum. Here we demonstrate that the optical absorption of carbon nitride semiconductor materials is extendable into the visible region up to about 750 nm by simple copolymerization with organic monomers like barbituric acid (BA). The electronic and photoelectric properties of the modified carbon nitrides were then investigated to elucidate their enhanced activity for hydrogen production from water containing an appropriate sacrificial reagent with visible light. In principle, BA can be directly incorporated into the classical carbon nitride condensation scheme (Scheme 1). New carbon nitride structures were therefore synthesized by dissolving dicyandiamide with different amounts of BA in water, followed by thermally induced copolymerization at 823 K. For simplicity, the resulting samples are denoted CNBx, where x (0.05, 0.1, 0.2, 0.5, 1, 2) refers to the weighedin amount of BA. The structure, texture, and electrochemical properties of these materials were characterized, and their photochemical performance analyzed. Their XRD patterns (Figure S1, Supporting Information) are dominated by the characteristic (002) peak at 27.48 of a graphitic, layered structure with an interlayer distance of d = [*] J. Zhang, X. Chen , Prof. X. Fu, Prof. X. Wang State Key Laboratory Breeding Base of Photocatalysis Fuzhou University, Fuzhou 350002 (China) E-mail: xcwang@fzu.edu.cn
TL;DR: Inorganic enzyme?
Abstract: Methods, systems, compositions include biocompatible polymer coated nanoceria that function as aqueous redox catalyst with enhanced activity at an acidic to moderately alkaline pH value between 1 and 8. The compositions are used as oxidizing agents for decomposition, decontamination or inactivation of organic contaminants, such as, pesticides and chemical warfare agents. Another use includes nanoceria as targetable nanocatalyst prepared by conjugating various targeting ligands to the nanoparticle coating to form a colorimetric or fluorescent probe in immunoassays and other molecule binding assays that involve the use of a molecule in solution that changes the color of the solution or emits a fluorescent signal, where localization of nanoceria to organs or tissue is assessed by treatment with an oxidation sensitive dye or other detection devices. Versatility and uses of the nanoceria compositions are controlled by pH value, choice of dye substrate and thickness of the polymer coating on the ceria nanoparticles.
TL;DR: Physicochemical data (electronics, sterics, bond strength) of NHCs that are essential for the design, application, and mechanistic understanding of N-Heterocyclic carbenes in catalysis are provided.
Abstract: Quantification and variation of characteristic properties of different ligand classes is an exciting and rewarding research field. N-Heterocyclic carbenes (NHCs) are of special interest since their electron richness and structure provide a unique class of ligands and organocatalysts. Consequently, they have found widespread application as ligands in transition-metal catalysis and organometallic chemistry, and as organocatalysts in their own right. Herein we provide an overview on physicochemical data (electronics, sterics, bond strength) of NHCs that are essential for the design, application, and mechanistic understanding of NHCs in catalysis.
TL;DR: The challenges in the metal-organic frameworks area encompass not only the development of new solids but also improvements in the formulation and processing of the materials, including tailoring the morphology and surface chemistry of the frameworks to fit the proposed applications.
Abstract: The class of highly porous materials called metal-organic frameworks offer many opportunities for applications across biology and medicine. Their wide range of chemical composition makes toxicologically acceptable formulation possible, and their high level of functionality enables possible applications as imaging agents and as delivery vehicles for therapeutic agents. The challenges in the area encompass not only the development of new solids but also improvements in the formulation and processing of the materials, including tailoring the morphology and surface chemistry of the frameworks to fit the proposed applications.
TL;DR: Examples are presented to show how compartmentalization, monodispersity, single-molecule sensitivity, and high throughput have been exploited in experiments that would have been extremely difficult outside the microfluidics platform.
Abstract: Microdroplets in microfluidics offer a great number of opportunities in chemical and biological research. They provide a compartment in which species or reactions can be isolated, they are monodisperse and therefore suitable for quantitative studies, they offer the possibility to work with extremely small volumes, single cells, or single molecules, and are suitable for high-throughput experiments. The aim of this Review is to show the importance of these features in enabling new experiments in biology and chemistry. The recent advances in device fabrication are highlighted as are the remaining technological challenges. Examples are presented to show how compartmentalization, monodispersity, single-molecule sensitivity, and high throughput have been exploited in experiments that would have been extremely difficult outside the microfluidics platform.
TL;DR: Gold catalysis is a very active area in the field of catalysis research, new reactions are published every week, amazing changes in the connectivity are often observed, the number of applications in total synthesis is increasing...--but what are the mechanisms of these reactions?
Abstract: Gold catalysis is a very active area in the field of catalysis research. New reactions are published every week, amazing changes in the connectivity are often observed, the number of applications in total synthesis is increasing...--but what are the mechanisms of these reactions? Sound information can be provided by knowledge about the intermediates of these reactions.
TL;DR: Light-induced water splitting over iron oxide (hematite) has been achieved by using a particle-assisted deposition technique and IrO2-based surface catalysis and these photocurrents are unmatched by any other oxide-based photoanode.
Abstract: Revved-up rust! Light-induced water splitting over iron oxide (hematite) has been achieved by using a particle-assisted deposition technique and IrO2-based surface catalysis. Photocurrents in excess of 3 mA cm-2 were obtained at +1.23 V versus the reversible hydrogen electrode under AM 1.5 G 100 mW cm-2 simulated sunlight. These photocurrents are unmatched by any other oxide-based photoanode. FTO=fluorine-doped tin oxide. Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
TL;DR: Flexible and lightweight chemiresistormade of a thin film composed of overlapped and reduced graphene oxide platelets (RGO film) that can reversibly and selectively detect chemicallyaggressive vapors such asNO.
Abstract: Described herein is a flexible and lightweight chemiresistormade of a thin film composed of overlapped and reducedgraphene oxide platelets (RGO film), which were printedonto flexible plastic surfaces by using inkjet techniques. TheRGO films can reversibly and selectively detect chemicallyaggressivevapors suchasNO
TL;DR: During the last five years, new types of stable cyclic carbenes, as well as related carbon-based ligands (which are not NHCs), and which feature even stronger σ-donor properties have been developed.
Abstract: The success of homogeneous catalysis can be attributed largely to the development of a diverse range of ligand frameworks that have been used to tune the behavior of various systems. Spectacular results in this area have been achieved using cyclic diaminocarbenes (NHCs) as a result of their strong σ-donor properties. Although it is possible to cursorily tune the structure of NHCs, any diversity is still far from matching their phosphorus-based counterparts, which is one of the great strengths of the latter. A variety of stable acyclic carbenes are known, but they are either reluctant to bind metals or they give rise to fragile metal complexes. During the last five years, new types of stable cyclic carbenes, as well as related carbon-based ligands (which are not NHCs), and which feature even stronger σ-donor properties have been developed. Their synthesis and characterization as well as the stability, electronic properties, coordination behavior, and catalytic activity of the ensuing complexes are discussed, and comparisons with their NHC cousins are made.
TL;DR: A synthetic method using reverse micelles at high pressure and temperature in a bomb that produces Si nanoparticles (n-Si) with various particle sizes without aggregation and thus enables the optimal nanoparticle size for use in anode materials to be chosen is reported.
Abstract: Due to the high theoretical capacity (ca. 4200 mAhg ) of Si when Li4.4Si is formed, it has been extensively investigated for use as a high-capacity anode material that can replace graphite, which is currently used (372 mAhg ). However, Si exhibits significant volume changes (> 360%) during Li alloying and dealloying. These changes cause cracking and crumbling of the electrode material and a consequent loss of electrical contact between individual particles and hence severe capacity drop. However, such mechanical strain induced by volume change can be reduced by employing smaller particles. To this end, synthetic methods such as spark ablation, aerogel techniques, and sputtering have been employed. Formation of crystalline Si nanoparticles requires higher temperatures due to the more covalent nature of these particles compared to Ge particles, and at low temperature amorphous phases become more common. The first commonly recognized successful production of Si nanoclusters was reported byHeath et al. They showed that Si nanocrystals capped with alkyl groups can be produced by reduction of SiCl4 and RSiCl3 (R=H, C8H17) according to the reaction SiCl4+RSiCl3+Na!Si+NaCl. This process was carried out at high temperature (385 8C) and high pressure (>100 atm) in a steel bomb fitted into a heating mantle. A process that utilizes SiCl4 reduction at room temperature under an inert atmosphere was initially reported by Kauzlarich et al. However, the drawback of their method was that the product obtained at room temperature was not fully crystallized and was severely capped with alkyl terminators. Moreover, an annealing process above 900 8C is required to obtain the crystalline phase. Similar solution syntheses have been reported at low or high temperature after reducing Si salts with LiAlH4 [13,14] or alkyl silanes. However, all of these methods produce a broad particle size distribution or involve aggregation of the nanoparticles. Furthermore, they all yield amounts of material too small for use in anode production for lithium secondary batteries. We now report a synthetic method using reverse micelles at high pressure and temperature in a bomb that produces Si nanoparticles (n-Si) with various particle sizes without aggregation and thus enables the optimal nanoparticle size for use in anode materials to be chosen. Figure 1 shows the XRD pattern and TEM images of n-Si prepared with trimethyloctadecylammonium bromide (OTAB) surfactant. The XRD pattern clearly shows forma-
TL;DR: A comparative spectroscopic investigation of a series of Yb/Tm co-doped hexagonal-phase NaGdF4 nanoparticles with or without a thin surface protection layer shows that, through the thin layer coating, the characteristic optical features of these nanoparticles can be retained, thereby providing direct evidence to support the surface quenching effect responsible for the size-dependent UC luminescence.
Abstract: Lanthanide-doped upconversion (UC) nanoparticles have shown considerable promise in biological labeling, imaging, and therapeutics. However, although current synthetic approaches allow for preparation of ultrasmall UC nanoparticles with precise control over particle morphology and emission color, smaller nanoparticles come at the expense of weaker emissions, which is a constraint that is practically impossible to surpass. Many fundamental aspects of the UC luminescence in these nanomaterials still lack sufficient understanding. In particular, several groups have observed varied relative intensity of the multi-peak UC emissions with varying particle size. The UC luminescence primarily originates from intra-configurational 4f electron transitions within the localized lanthanide dopant ions. Due to a small Bohr radius of the exciton in UC hosts and weak interactions between 4f electrons of the lanthanide dopant ions and the host matrix, the size-dependent UC luminescence can hardly be explained by classic theories, such as quantum confinement and surface plasmon resonance related to optical properties of semiconductor and metal nanoparticles. Although phonon confinement has been used to account for the size-dependent UC luminescence, it has been a matter of much debate, owing to the constraints typically associated with solid-state sample measurements at extreme conditions (for example, low temperatures of ca. 10 K) and exclusion of vibrational energies and optical traps arising from particle surface. To this end, a surface quenching effect is proposed and correlated with size-dependent UC luminescence. However, the surface quenching effect has not been conclusively established, largely because of the lack of direct evidence on surface-quenching-induced luminescence modulation of different-sized particles. Herein, we present a comparative spectroscopic investigation of a series of Yb/Tm co-doped hexagonal-phase NaGdF4 nanoparticles (10, 15, and 25 nm) with or without a thin (ca. 2.5 nm) surface protection layer. We show that, through the thin layer coating, the characteristic optical features (such as relative emission intensities) of these nanoparticles can be retained, thereby providing direct evidence to support the surface quenching effect responsible for the size-dependent UC luminescence. Hexagonal-phase NaGdF4 was chosen as the model host system owing to its ability to render high UC efficiency and the benefits of producing relatively small (< 20 nm) and uniform nanoparticles. Furthermore, the Gd host ion that features half-filled 4f orbitals is relatively inert in the luminescence process and thus has negligible interaction with the dopant ions. To provide a direct comparison over a broad wavelength range between the relative emission intensity of the particles, the Tm ion with a ladder-like arrangement of energy levels was selected as the activator capable of generating upconverted emission peaks that span from ultraviolet (UV) to near-infrared (NIR) spectral regions (Figure 1a).
TL;DR: A greatly enhanced photovoltaic performance is observed when the porphyrin dye is cosensitized with a metal-free dye that has a complementary spectral response.
Abstract: To dye for: A porphyrin chromophore, which is integrated into a donor-acceptor dye as a π-conjugated bridge (see picture), exhibits an unprecedented efficiency of 11 □ % when used as a photosensitizer in a double-layer TiO2 film. A greatly enhanced photovoltaic performance is observed when the porphyrin dye is cosensitized with a metal-free dye that has a complementary spectral response. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
TL;DR: This detailed account of the fascinating development of the synthesis and characterization of graphene is hoped to demonstrate that the rich history of graphene chemistry laid the foundation for the exciting research that continues to this day.
Abstract: There has been an intense surge in interest in graphene during recent years. However, graphene-like materials derived from graphite oxide were reported in 1962, and related chemical modifications of graphite were described as early as 1840. In this detailed account of the fascinating development of the synthesis and characterization of graphene, we hope to demonstrate that the rich history of graphene chemistry laid the foundation for the exciting research that continues to this day. Important challenges remain, however; many with great technological relevance.
TL;DR: The first generation of biofuels is presently produced from Sugars, starches, and vegetable oil, but a morepromising feedstock is lignocellulosic material, which is more abundant, has a lower cost, and is potentially more sustain-able.
Abstract: The first generation of biofuels is presently produced fromsugars, starches, and vegetable oil. Although instrumental indeveloping the market, these biofuels are not likely to deliverthe large volumes needed for the transport sector becausethey directly compete with food for their feedstock. A morepromising feedstock is lignocellulosic material, which is moreabundant, has a lower cost, and is potentially more sustain-able.
TL;DR: The synthesis of cyclic carbonates by the coupling reaction of epoxides with CO(2) has received increased attention due to the importance of using a greenhouse gas as a feedstock for the synthesis of useful molecules.
Abstract: Metal complexes of salen ligands are an important class of compounds, and they have been widely studied in the past. Among their successful catalytic applications, the synthesis of cyclic carbonates by the coupling reaction of epoxides with CO(2) has received increased attention; this is mostly due to the importance of using a greenhouse gas as a feedstock for the synthesis of useful molecules. Herein the most relevant past and present research surrounding this topic is presented.
TL;DR: This Review focuses on recent strategies used to create semiconductor-metal hybrid nanoparticles, presents the emergent properties of these multicomponent materials, and discusses their potential applicability in different technologies.
Abstract: One key goal of nanocrystal research is the development of experimental methods to selectively control the composition and shape of nanocrystals over a wide range of material combinations. The ability to selectively arrange nanosized domains of metallic, semiconducting, and magnetic materials into a single hybrid nanoparticle offers an intriguing route to engineer nanomaterials with multiple functionalities or the enhanced properties of one domain. In this Review, we focus on recent strategies used to create semiconductor-metal hybrid nanoparticles, present the emergent properties of these multicomponent materials, and discuss their potential applicability in different technologies.
TL;DR: Possibility for introducing organic groups that exhibit chemical and/or physical functions into porous materials will be described, with a focus on the incorporation of such functional groups as a supporting part of the pore walls.
Abstract: This Review aims to give an overview of recent research in the area of porous, organic-inorganic and purely organic, functional materials. Possibilities for introducing organic groups that exhibit chemical and/or physical functions into porous materials will be described, with a focus on the incorporation of such functional groups as a supporting part of the pore walls. The number of organic groups in the network can be increased such that porous, purely organic materials are obtained.