Facile Amplification of Solution-State Surface-Enhanced Raman Scattering of Small Molecules Using Spontaneously Formed 3D Nanoplasmonic Wells.

Surface-enhanced Raman scattering (SERS) has recently been considered as one of the most promising tools to directly analyze small molecules without labels, owing to advantages in sensitivity, specificity, and speed. However, collecting reproducible SERS signals from small molecules on substrates or in solutions is challenging because of random molecular adsorption on surfaces and laser-induced molecular convection in solutions. Herein, we report a novel and efficient way to collect SERS signals from solution samples using three-dimensional nanoplasmonic wells spontaneously formed by interfacial reactions between liquid polydimethylsiloxane (PDMS) and small droplets of metal ion solutions (e.g., HAuCl4 and AgNO3). A SERS signal is easily maximized at the center near the bottom of the well due to spherical feature of the fabricated wells and electromagnetic field enhancement by the metallic nanoparticles (e.g., Au and Ag) integrated on their surfaces. Through the systematic control over the volume, concentration, and composition of the metal ion solution, optical functions of the nanoplasmonic wells were optimized for SERS, which was further amplified by exploiting the plasmonic couplings with colloidal nanoparticles. By using the optimized nanoplasmonic wells and the detection protocol, we successfully obtained intrinsic spectra of biomolecules (e.g., adenine, glucose, amyloid ß) and toxic environmental molecules (e.g., 1,1'-diethyl-2,2'-cyanine iodide and chloromethyliothiazolinone/methylisothiazolinone) as well as Raman active molecules, such as rhodamine 6G and 1,2-bis(4-pyridyl)ethylene at a low concentrations down to the picomolar level. Our detection platform provides a powerful way to develop highly sensitive sensors and high-throughput analyzing protocols for fieldwork applications as well as diagnosing diseases.

Dual-Pore Carbon Shells for Efficient Removal of Humic Acid from Water.

A template-mediated process for the preparation of mesoporous carbon shells with high surface area, dual-pore structure, and excellent performance in the adsorption of humic acid is reported. Their synthesis involves templating phenolic resin against wrinkled silica nanospheres, subsequent carbonization under Ar atmosphere, and final release of dual-pore mesoporous carbon shells by etching the silica templates. An additional silica layer was used to protect the phenolic resin from aggregation during carbonization, and its subsequent removal gives the carbon shells a hydrophilic surface, which significantly improves their dispersity in aqueous media. When used as adsorbents for humic acid removal, the as-prepared dual-pore mesoporous carbon shells show superior adsorption performance to activated carbon.

Promotion of atomic hydrogen recombination as an alternative to electron trapping for the role of metals in the photocatalytic production of H2.

The production of hydrogen from water with semiconductor photocatalysts can be promoted by adding small amounts of metals to their surfaces. The resulting enhancement in photocatalytic activity is commonly attributed to a fast transfer of the excited electrons generated by photon absorption from the semiconductor to the metal, a step that prevents deexcitation back to the ground electronic state. Here we provide experimental evidence that suggests an alternative pathway that does not involve electron transfer to the metal but requires it to act as a catalyst for the recombination of the hydrogen atoms made via the reduction of protons on the surface of the semiconductor instead.

Core-shell nanostructured catalysts.

Novel nanotechnologies have allowed great improvements in the syn-thesis of catalysts with well-controlled size, shape, and surface properties. Transition metal nanostructures with specific sizes and shapes, for instance, have shown great promise as catalysts with high selectivities and relative ease of recycling. Researchers have already demonstrated new selective catalysis with solution-dispersed or supported-metal nanocatalysts, in some cases applied to new types of reactions. Several challenges remain, however, particularly in improving the structural stability of the catalytic active phase. Core-shell nanostructures are nanoparticles encapsulated and protected by an outer shell that isolates the nanoparticles and prevents their migration and coalescence during the catalytic reactions. The synthesis and characterization of effective core-shell catalysts has been at the center of our research efforts and is the focus of this Account. Efficient core-shell catalysts require porous shells that allow free access of chemical species from the outside to the surface of nanocatalysts. For this purpose, we have developed a surface-protected etching process to prepare mesoporous silica and titania shells with controllable porosity. In certain cases, we can tune catalytic reaction rates by adjusting the porosity of the outer shell. We also designed and successfully applied a silica-protected calcination method to prepare crystalline shells with high surface area, using anatase titania as a model system. We achieved a high degree of control over the crystallinity and porosity of the anatase shells, allowing for the systematic optimization of their photocatalytic activity. Core-shell nanostructures also provide a great opportunity for controlling the interaction among the different components in ways that might boost structural stability or catalytic activity. For example, we fabricated a SiO2/Au/N-doped TiO2 core-shell photocatalyst with a sandwich structure that showed excellent catalytic activity for the oxidation of organic compounds under UV, visible, and direct sunlight. The enhanced photocatalytic efficiency of this nanostructure resulted from an added interfacial nonmetal doping, which improved visible light absorption, and from plasmonic metal decoration that enhanced light harvesting and charge separation. In addition to our synthetic efforts, we have developed ways to evaluate the accessibility of reactants to the metal cores and to characterize the catalytic properties of the core-shell samples we have synthesized. We have adapted infrared absorption spectroscopy and titration experiments using carbon monoxide and other molecules as probes to study adsorption on the surface of metal cores in metal oxide-shell structures in situ in both gas and liquid phases. In particular, the experiments in solution have provided insights into the ease of diffusion of molecules of different sizes in and out of the shells in these catalysts.

Investigation on structure and photoluminescence properties of Ho3+ doped Ca3(VO4)2 phosphors for luminescent devices.

This study focuses on the synthesis and characterization of Ho3+ doped Ca3(VO4)2 phosphor for potential application in solid-state lighting technology. A citrate-based sol-gel process is optimized to achieve sheet-like morphologies in the phosphor material. The investigation reveals UV absorption at 371 nm, indicating a band gap of 3.28 eV. Emission transitions at (506, 541, and 651) nm are observed when excited at 451 nm, with an optimal Ho3+ concentration of 0.05 mol resulting in robust green emission at 541 nm. The concentration quenching in Ca3(VO4)2:xHo3+ phosphors is discussed in detail with Blesse's and Dexter's models. The concentration quenching effect found in the studied samples is due to the dipole-dipole interactions. Judd-Ofelt intensity parameters were calculated from the excitation bands, and for Ω 2, Ω 4, and Ω 6 are (0.16, 0.17, and 0.36) × 10-20 cm2, respectively. The emission properties for the (5S2 + 5F4) → 5I8 and 5F5 → 5I8 transitions are also estimated with J-O parameters. The higher magnitude of branching ratios (83%) and emission cross-sections (1.6 × 10-21 cm2) suggest that the Ca3(VO4)2:0.05Ho3+ phosphor materials may be suitable for efficient green-emitting device applications. The CIE coordinates confirm the potential of Ho3+-doped phosphors for green emissions, making them suitable for solid-state lighting and display technology.

Correlating the excited state relaxation dynamics as measured by photoluminescence and transient absorption with the photocatalytic activity of Au@TiO2 core-shell nanostructures.

The spectroscopic and photocatalytic properties of a series of Au@TiO(2) core-shell nanostructures are characterized. The crystallinity of the TiO(2) shells was varied by changing the etching and calcination conditions. Measurements of the photoluminescence, transient absorption, and H(2) production rate permit us to look for correlations between the spectroscopic and catalytic behaviors. We found that there is a strong effect of crystallinity on the H(2) production rate and also the stretched exponential lifetime of the photoluminescence created by short-wavelength (266 and 300 nm) photoexcitation. As the TiO(2) crystallinity is increased, the photoluminescence lifetime increases from 22 to 140 ps in a 1 ns detection window, while the H(2) production rate increases by a factor of ~4. There is no discernible effect of crystallinity on the photoluminescence dynamics excited at 350 or 430 nm, or on the electronic dynamics measured by femtosecond transient absorption after excitation at 300 nm. We hypothesize that high-energy photons create reactive and emissive charge-separated states in parallel, and that both species are subject to similar electron-hole recombination processes that depend on sample crystallinity. Based on our observations, it can be concluded that the photoluminescence dynamics may be used to evaluate the potential performance of this class of photocatalysts.

Water Orientation at the Anatase TiO2 Nanoparticle Interface: A Probe of Surface pKa Values.

The acid-base properties of surfaces significantly influence catalytic and (photo)electrochemical processes. Estimation of acid dissociation constants (pKa values) for colloids is commonly performed through electroanalytical techniques or spectroscopic methods employing label molecules. Here, we show that polarimetric angle-resolved second harmonic scattering (AR-SHS) can be used as an all-optical, label-free probe of colloid surface pKa values. We apply AR-SHS to dispersions of 100 nm anatase TiO2 particles to extract surface potential and surface susceptibility, a measure of interfacial water orientation, as a function of pH. The surface potential follows changes in surface charge density, while the interfacial water orientation inverts at pH ∼4.8, ∼6, and ∼7.6. As the variation in bulk pH modifies the populations of Ti-OH2+, Ti-OH, and Ti-O- interfacial groups, a change in water orientation reports on the ratio of protonated/deprotonated species. Such observation allows for pKa evaluation from plots of surface susceptibility versus pH. A Nerstian trend in the surface potential is additionally demonstrated.

Effects of the Crystalline Properties of Hollow Ceria Nanostructures on a CuO-CeO2 Catalyst in CO Oxidation.

The development of an efficient and economic catalyst with high catalytic performance is always challenging. In this study, we report the synthesis of hollow CeO2 nanostructures and the crystallinity control of a CeO2 layer used as a support material for a CuO-CeO2 catalyst in CO oxidation. The hollow CeO2 nanostructures were synthesized using a simple hydrothermal method. The crystallinity of the hollow CeO2 shell layer was controlled through thermal treatment at various temperatures. The crystallinity of hollow CeO2 was enhanced by increasing the calcination temperature, but both porosity and surface area decreased, showing an opposite trend to that of crystallinity. The crystallinity of hollow CeO2 significantly influenced both the characteristics and the catalytic performance of the corresponding hollow CuO-CeO2 (H-Cu-CeO2) catalysts. The degree of oxygen vacancy significantly decreased with the calcination temperature. H-Cu-CeO2 (HT), which presented the lowest CeO2 crystallinity, not only had a high degree of oxygen vacancy but also showed well-dispersed CuO species, while H-Cu-CeO2 (800), with well-developed crystallinity, showed low CuO dispersion. The H-Cu-CeO2 (HT) catalyst exhibited significantly enhanced catalytic activity and stability. In this study, we systemically analyzed the characteristics and catalyst performance of hollow CeO2 samples and the corresponding hollow CuO-CeO2 catalysts.

Double-Layered Polymer Microcapsule Containing Non-Flammable Agent for Initial Fire Suppression.

Fire in energy storage systems, such as lithium-ion batteries, has been raised as a serious concern due to the difficulty of suppressing it. Fluorine-based non-flammable agents used as internal substances leaked through the fine pores of the polymer outer shell, leading to a degradation of fire extinguishing performance. To improve the durability of the fire suppression microcapsules and the stability of the ouster shell, a complex coacervation was used, which could be microencapsulated at a lower temperature, and the polymer shell was coated with urea-formaldehyde (UF) resin. The outermost UF resin formed elaborate bonds with the gelatin-based shell, and thus, the structure of the outer shell became denser, thereby improving the loss resistance of the inner substance and thermal stability. The double-layered microcapsules had an average particle diameter of about 309 µm, and a stable outer shell formed with a mass loss of 0.005% during long-term storage for 100 days. This study confirmed that the double-layered microcapsules significantly improved thermal stability, resistance to core material loss, core material content and fire suppression performance compared to single wall microcapsules. These results indicated that the double-layered structure was suitable for the production of microcapsules for initial fire suppression, including highly volatile non-flammable agents with a low boiling point.

Preparation of nanoporous carbons via a grafting method for the application to electrochemical double layer capacitors.

Nanoporous carbon materials with a controlled pore size and surface area were prepared using grafting method. The use of 3-mercaptopropyltrimethoxysilane (MPTMS) as a grafting material played an important role in producing a porous structure by linking the silica to the polymer, with the subsequent formation of a silica-polymer composite. Importantly, the use of an organic solvent, compared to an aqueous solvent, has a positive effect in forming uniform and well-developed carbon structures, due to the high degree of dispersion with well-mixing of the carbon and silica precursors. The amounts of MPTMS and carbon precursor used determined the pore size and surface area of resulting carbon materials. The optimum ratio of MPTMS and carbon precursor for achieving a high surface area in excess of 2000 m2/g was determined. The use of a large amount of carbon precursor resulted in carbons with a relatively small surface area and an increase in MPTMS content led to an increase in the microporous structures. The capacitance value of the porous carbon prepared using the optimum ratio was determined to be 150 F/g.

Molybdenum and tungsten promoted carbons for use as catalyst supports in methanol electro-oxidation.

Two types of the molybdenum and tungsten promoted carbons (MWCs) were prepared by the successive impregnation and co-impregnation of molybdenum and tungsten precursors on carbon black with a heat treatment. The resulting MWCs were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and electrochemical techniques and evaluated as a catalyst support for a Pt catalyst in methanol electro-oxidation. MWC-S, prepared by successive impregnation, showed a well developed carbidic phase with a large portion of tungsten carbide phase. In contrast, MWC-C, prepared by co-impregnation, had a lower amount of carbidic carbon and larger fraction of molybdenum carbide phase. MWCs-supported Pt catalysts showed higher catalytic activities than the Pt catalyst on pure carbon black in both CO oxidation and methanol electro-oxidation. In particular, Pt/MWC-S exhibited the highest performance among the Pt catalysts employed in this work. The high activity of Pt/MWC-S can be attributed to the positive effect of molybdenum-tungsten carbide phase, which promotes the activation of water and the removal of intermediate CO species.

Preparation and characterization of Rh catalyst supported on nanoporous alumina for the ethylene hydroformylation.

Nanoporous gamma-aluminas were prepared by a sol-gel method with and without surfactant, and characterized by nitrogen adsorption-desorption, transmission electron microscopy (TEM), X-ray diffraction (XRD) and temperature programmed reduction (TPR). The resulting materials were applied to Rh catalyst supports for the ethylene hydroformylation. The ordered nanoporous alumina (A-1) which was prepared using surfactant, showed well-developed pore structures with high surface area. Rh catalyst supported on A-1 alumina (Rh/A-1) exhibited higher catalytic activity in the ethylene hydroformylation than other Rh catalysts. It is believed that the high catalytic performance of Rh/A-1 resulted from the well-developed pore structure with high surface area of ordered nanoporous A-1 and consequently finely dispersed Rh particle on the surface of gamma-alumina support.

Biotransformation of methane into methanol by methanotrophs immobilized on coconut coir.

This study aimed to establish a unique approach for the production of methanol from methane (CH4) in the presence of paraffin oil mediated by methanotrophs immobilized on coconut coir (CC). Immobilization of different methanotrophs through covalent method increased the immobilization yield and relative efficiency for methanol production to 48.6% and 96.8%, respectively. In the presence of paraffin oil, methanol production was 1.6-fold higher by Methylocystis bryophila than by control. Compared to free cells, whole cells immobilized on CC showed higher stability for methanol production. Under repeated batch conditions, cumulative methanol production by immobilized cells and free cells, after eight cycles of reuse, was 52.9 and 30.9 mmol/L, respectively. This study effectively demonstrated the beneficial influence of lignocellulosic biowaste CC as support for immobilization of methanotrophs and paraffin oil on bioconversion of CH4 to methanol.

Bead-Shaped Mesoporous Alumina Adsorbents for Adsorption of Ammonia.

It is of great importance to remove toxic gases by efficient methods for recovering the atmosphere to safe levels. The adsorption of the toxic gas molecules on solid adsorbents is one of the most useful techniques because of its simple operation and economic feasibility. Here, we report the uniform Bead-Shaped Mesoporous Alumina (BSMA) with tunable particle size for use as an adsorbent for removal of toxic ammonia. The BSMA particles with tunable diameters were synthesized by means of a sol-gel reaction of Al(NO3)3∙9H2O as an alumina precursor in the presence of chitosan as a template. When the ammonia solution is added dropwise to the prepared viscose mixture containing chitosan, acetic acid, and the alumina precursor solution, the sol-gel condensation reaction of the alumina precursor occurs in the chitosan polymer metrics, resulting in bead-shaped chitosan-aluminum hydroxide particles. Then, final Bead-Shaped Mesoporous Alumina (BSMA) particles are obtained by calcination at a high temperature. During the synthesis, changing the mole ratio of the chitosan template to the alumina precursor allowed the particle diameter of the final bead sample to be finely controlled. In addition, the prepared BSMA particles have well-developed mesoporous characteristics with relatively large surface areas, which are beneficial for adsorption of gas molecules. In an ammonia adsorption experiment, the BSMA-1.5 sample, which has the smallest particle diameter among the bead samples, was the best in terms of adsorption capacity. In this manuscript, we systemically discuss the relationship between the characteristics of BSMA samples and their adsorption of ammonia.

Improving the Understanding of the Redox Properties of Fluoranil Derivatives for Cathodes in Sodium-Ion Batteries.

Despite the potential of organic cathodes in sodium-ion batteries, their redox properties still need to be explored. In this study, a density functional theory modeling approach is employed to comprehensively investigate the redox properties and theoretical performance parameters for a selected set of fluoranil derivatives as cathode materials. The redox properties are further correlated with various characteristics including structural variations, electronic properties, and solvation. Three primary conclusions are drawn. First, the incorporation of bulky trifluoromethyl functional group(s) into fluoranil increases its redox potential but significantly decreases its gravimetric charge capacity. This suggests that the trifluoromethyl functional group(s) would be detrimental to the design of high-performance batteries. Second, fluoranil exhibits significant enhancements in terms of redox properties and theoretical performance compared with its hydrogenated form, benzoquinone, suggesting a desired strategy for designing high-performance batteries. Third, the redox properties of fluoranil derivatives would strongly rely not only on structural variations (e.g., bulkiness) and electronic properties (e.g., functionality) but also on solvation energy. It is further verified that cathodic deactivation could be completed by solvation energy. The new understanding will provide us with guidelines for an efficient design of promising organic cathode materials.

Preparation and characterization of a PtSn nanocatalyst for use in ethanol electro-oxidation.

A PtSn nanocatalyst supported on carbon (PtSn) was prepared with the enhancement of the extent of alloy degree of Pt with Sn by the heat treatment at 300 degrees C in a nitrogen atmosphere (PtSn/C-HT). The PtSn and PtSn/C-HT catalysts prepared were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and electrochemical methods, and were tested as an electro-catalyst in ethanol electro-oxidation. Results showed that the average particle size of the PtSn/C-HT was increased by the heat treatment, compared to that of PtSn. The catalytic activity of PtSn/C-HT, however, was superior in ethanol electro-oxidation. The enhanced catalytic activity of PtSn/C-HT was attributed to both the high degree of alloying in the PtSn structure and the extended Pt lattice, which promote catalytic activities in ethanol electro-oxidation and removal of intermediate CO species.

Controllable one-pot synthesis of uniform colloidal TiO2 particles in a mixed solvent solution for photocatalysis.

This study reports on the controllable synthesis of uniform colloidal titanium dioxide (TiO2) particles and their photocatalytic applications toward rhodamine B (RhB) degradation. The monodispersed TiO2 particles were synthesized under mixed solvent conditions by sol-gel chemistry in a one-pot process. Varying the ratio of solvent composition, the concentration of surfactant and TiO2 precursor was used to control the particle diameter, degree of monodispersity and morphology. The modification of the calcination temperature affected the crystallinity and crystalline phase of the colloidal TiO2 particles. When uniform, amorphous TiO2 particles were calcined at an optimal temperature (500 °C), the final sample exhibited beneficial characteristics such as high anatase crystallinity with a mixed phase of anatase and rutile and relatively high surface area. The photocatalytic efficiency of the uniform TiO2 sample with high anatase crystallinity with mixed phase and high surface area was dramatically enhanced towards RhB degradation under UV-vis irradiation. We systemically discuss the relationship between the synthetic parameters in our synthesis and the properties of the final TiO2 products, as well as the crystalline properties and performance enhancement of TiO2 photocatalysts calcined at different temperatures.

Controlled drug release with surface-capped mesoporous silica nanoparticles and its label-free in situ Raman monitoring.

Mesoporous silica nanoparticles (MSNs) have drawn attention as efficient nanocarriers for drug delivery systems owing to their unique physiochemical properties. However, systemically controlling the kinetics of drug release from the nanocarriers and in situ monitoring of the drug release are still challenging. Here, we report surface-capped MSNs used for controlled drug release and demonstrate label-free in situ Raman monitoring of released drugs based on the molecule-specific spectral fingerprints. By capping the surface of MSNs with amine moieties, gold nanoparticles, and albumin, we achieved high loading efficiencies (up to 97%) of doxorubicin and precisely controlled drug release stimulated by changing pH value. Moreover, we monitored in real-time drug release profile and visualized cellular distribution of the delivered drug at nanoscale based on its intrinsic Raman peak. Finally, we evaluated drug responses in cancer cells and normal cells to investigate whether capped-dMSNs exhibit selective drug release. Our findings would be beneficial for designing smart drug carriers and directly monitoring the release behavior of drugs in actual cellular environments.

Reductive dechlorination of carbon tetrachloride by bioreduction of nontronite.

Reductive dechlorination of carbon tetrachloride (CT) was investigated during bioreduction of iron-containing clay mineral (i.e., nontronite) by iron-reducing bacteria (Shewanella putrefaciens CN32 (CN32)). In the absence of CT, the production of Fe(II) significantly increased in nontronite suspension with CN32 in 124 d (11.1% of Fe(III) reduction), resulting in formation of new secondary Fe(II) mineral phase (i.e., vivianite (FeII3(PO4)2·8H2O)). In the presence of CT, an acceleration of CT dechlorination was observed after 13 d and it reached almost 68% of removal efficiency at 32 d in nontronite suspension with CN32, which was 1.8 times higher than that by CN32 alone (37%). Significant amounts of formate (30.1%) and CO (2.4%) were measured during the CT dechlorination in the nontronite suspension with CN32. Results obtained from Fe(II) measurement and X-ray diffraction (XRD) showed the acceleration of Fe(II) production after 13 d and the formation of vivianite in the range of 13-25 d, suggesting that the biogenic vivianite enhanced the CT dechlorination in this study. Experimental results from batch kinetic tests, Fe(II) measurements, XRD analysis, and by-product study suggested that the formation of vivianite can play a crucial role for the enhanced reductive dechlorination of CT in phosphorous enriched subsurface environments with iron-containing clay minerals.