Synthesis, characterization and SERS activity of Au-Ag nanorods

The formation mechanism and morphology of Au-Ag bimetallic colloidal nanoparticles depend on the composition. Ag coated Au colloidal nanoparticles have been prepared by deposition of Ag through chemical reduction on performed Au colloid. The composition of the Au(100-x)-Ag(x) particles was varied from x=0 to 50. The obtained colloids were characterized by UV-vis spectroscopy and transmission electron microscopy (TEM). The Au(80)-Ag(20) colloid consists of alloy nanorods with dimension of 25nmx100nm. The activity of these nanorods in surface enhanced Raman spectroscopy (SERS) was checked by using sodium salicylate as an adsorbate probe. Intense SERS bands are observed indicating its usefulness as a SERS substrate in near infrared (NIR) laser excitation.     

Size and morphology of assemblies formed by DNA and lysozyme in dilute aqueous mixtures

Assemblies formed by a well-defined quality of DNA (4331 bp T7 DNA) and the small net-cationic protein lysozyme in dilute aqueous solutions have been characterized using cryo-transmission electron microscopy (cryo-TEM) and dynamic light scattering (DLS) as the main techniques. In a wide range of different DNA to lysozyme ratios in solutions of low ionic strength, dispersions of aggregates with the same general morphology and a practically constant hydrodynamic size are formed. The basic structure formed in the dispersions is that of rather flexible worm-like assemblies with a diameter of 10-20 nm, which are suggested to be made up by bundles of on the order of 10 DNA chains with an intervening matrix of lysozyme. With increased ionic strength, the worm-like appearance of the assemblies is lost and they adopt a less well-defined shape. The results suggest that the formation of the DNA-lysozyme aggregates is strongly influenced by cooperative assembly of the components and that, in addition to the electrostatic attraction between DNA and lysozyme, attractive interactions between the protein units are important in governing the behavior of the system.     

Clay-mediated synthesis of silver nanoparticles exhibiting low-temperature melting

Nanohybrids of silver nanoparticles (AgNPs) supported on mica clay were synthesized by in situ reduction of silver nitrate in an aqueous solution. The required mica platelets of high aspect ratio were previously prepared by the exfoliation of mica clay stacks in a multilayered structure through an ionic exchange reaction with poly(oxypropylene)-amine-salt. The exfoliated nanoscale mica platelets (Mica) are polydispersed such that each platelet is 300-1000 nm in width and 1 nm in thickness. These platelets possess ionic charges in the form of ≡SiO(-)Na(+) at 120 mequiv/100 g and are suitable for supporting AgNPs in the process of in situ reduction of silver nitrate. Transmission electronic microscopy revealed the formation of AgNPs with a narrow size distribution of ca. 8 nm in diameter on the rim of individual Mica platelets. However, the pristine layered Mica structure without exfoliation failed to produce a fine AgNP distribution but instead generated particles larger than 30 nm and some precipitates. Characterization by differential scanning calorimetry and field emission scanning electron microscopy revealed that the fine AgNPs on Mica platelets exhibited a low melting temperature of 110 °C. The AgNP/Mica nanohybrid not containing an organic dispersant is considered to be a "naked" silver particle.     

Adsorption and aggregation of cationic amphiphilic polyelectrolytes on silica

The adsorption of two cationic amphiphilic polyelectrolytes, which are copolymers of two charged monomers, triethyl(vinylbenzyl)ammonium chloride and dimethyldodecyl(vinylbenzyl)ammonium chloride (which is the amphiphilic one) with different contents of amphiphilic groups (40% (40DT) and 80% (80DT)), onto the hydrophilic silica-aqueous solution interface has been studied by in situ null ellipsometry and tapping mode atomic force microscopy (AFM). Adsorption isotherms for both polyelectrolytes were obtained at 25 degrees C and at different ionic strengths, and the adsorption kinetics was also investigated. At low ionic strength, thin adsorbed layers were observed for both polyelectrolytes. The adsorption increases with polymer concentration and reaches, in most cases, a plateau at a concentration below 50 ppm. For the 80DT polymer, at higher ionic strength, an association into aggregates occurs at concentrations at and above 50 ppm. The aggregates were observed directly by AFM at the surface, and by dynamic light scattering in the solution. The adsorption data for this case demonstrated multilayer formation, which correlates well with the increase in viscosity with the ionic strength observed for 80DT.     

Formation of nematic liquid crystals of sterically stabilized layered double hydroxide platelets

Colloidal platelets of hydrotalcite, a layered double hydroxide, have been prepared by coprecipitation at pH 11-12 of magnesium nitrate and aluminum nitrate at two different magnesium to aluminum ratios. Changing the temperature and ionic strength during hydrothermal treatment, the platelets were tailored to different sizes and aspect ratios. Amino-modified polyisobutylene molecules were grafted onto the platelets following a convenient new route involving freeze-drying. Organic dispersions in toluene were prepared of the particles with the largest size and highest aspect ratio. The colloidal dispersions prepared in this way showed isotropic-nematic phase transitions above a limiting concentration in a matter of days. The number density at the transition and the width of the biphasic region were determined and compared to theory. The orientation of the platelets in nematic droplets (tactoids) and at the isotropic-nematic interface were analyzed by polarization microscopy. It was observed that sedimentation induces a nematic layer in samples that are below the limiting concentration for isotropic-nematic phase separation. No nematic phase was observed in the initial aqueous suspensions of the ungrafted particles.     

Formation of hierarchical molecular assemblies from poly(oxypropylene)-segmented amido acids under AFM tapping

Molecular self-aligning of amphiphilic molecules into bundles with a constant width of 7-13 nm was observed under tapping-mode atomic force microscopy (TM-AFM). The requisite amphiphile, a poly(oxypropylene)-trimellitic amido acid sodium salt, is constituted of a symmetric amido acid structure with potential noncovalent forces of ionic charges, hydrogen bonds, pi-pi aromatic stacking, and hydrophobic interactions for intermolecular interaction. The amphiphiles are able to self-align into orderly hierarchical assemblies after simply being dissolved in water and dried under spin-coated evaporation. Under the TM-AFM tapping process, the bundles increased their length from an initial 20 to 600 nm. A sequential TM-AFM scanning and interval heating process was designed to probe the morphological transformations from the molecular bundles to lengthy strips (nearly micrometer scale) and to columns (with 5-7 nm spacing between the parallel strips). The formation of hierarchical arrays via molecular stretching, aligning, and connecting to each other was simultaneously observed and accelerated under the TM-AFM vibration energy. The molecular self-alignment caused by vibrations is envisioned to be a potential methodology for manipulating molecules into assembled templates, sensors, and optoelectronic devices.     

Electrodeposition of Ge, Si and Si x Ge 1-x from an air- and water-stable ionic liquid

The electrodeposition of Ge, Si and, for the first time, of Si(x)Ge(1-x) from the air- and water-stable ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([Py(1,4)]Tf(2)N) containing GeCl(4) and/or SiCl(4) as precursors is investigated by cyclic voltammetry and high-resolution scanning electron microscopy. GeCl(2) in [Py(1,4)]Tf(2)N is electrochemically prepared in a two-compartment cell to be used as Ge precursor instead of GeCl(4) in order to avoid the chemical attack of Ge(iv) on deposited Ge. Silicon, germanium and Si(x)Ge(1-x) can be deposited reproducibly and easily in this ionic liquid. Interestingly, the Si(x)Ge(1-x) deposit showed a strong colour change (from red to blue) at room temperature during electrodeposition, which is likely to be due to a quantum size effect. The observed colours are indicative of band gaps between at least 1.5 and 3.2 eV. The potential of ionic liquids in Si(x)Ge(1-x) electrodeposition is demonstrated.     

Dynamics or Stochastic Conductance Switching of Phenylene–Ethynylene Oligomers?

Scanning tunneling microscopy (STM) studies of phenylene-ethynylene oligomers inserted in alkanethiolate self assembled monolayers (SAMs) are presented. Spontaneous changes in appearance of bundles of inserted molecules during imaging are observed. The results indicate that the appearance changes are caused by fluctuations of the number of molecules in the bundles, by diffusion and exchange of molecules, in contrast to previous reports which attribute the changes to stochastic conductance switching. The packing density of the SAM around the bundles of inserted molecules influence the fluctuations, as the fluctuations observed at 77 K all take place in bundles inserted at locally less-densely packed SAM areas. At room temperature fluctuations of bundles inserted in well-ordered areas are also observed.     

Influence of plasticizer type on the properties of polymer electrolytes based on chitosan

Polymer electrolytes were obtained by the casting technique from a solution containing chitosan, hydrochloric acid, and plasticizer such as glycerol, ethylene glycol, and sorbitol. The transparent membranes with good ionic conductivity properties were characterized by impedance and UV-vis spectroscopies, thermal analysis (DSC), and X-ray diffraction. The best ionic conductivity values of 9.5 x 10(-4) S cm(-1) at room temperature and 2.5 x 10(-3) S cm(-1) at 80 degrees C were obtained for the sample containing 59 wt% of glycerol and an equimolar amount of HCl with respect to NH2 groups in chitosan. The temperature dependence of the ionic conductivity exhibits an Arrhenius behavior with activation energy of 16.6 kJ mol(-1). The thermal analysis indicates that both glass transition temperature (-87 degrees C) and crystallinity are low for this electrolyte. The samples with 13 wt% of LiCF3SO3 showed that the ionic conductivity values of 2.2 x 10(-5) S cm(-1) at room temperature and 4 x 10(-4) S cm(-1) at 80 degrees C are predominantly amorphous and showed a low glass transition temperature of about -73 degrees C.     

Ni-Mo双金属氧化物催化剂CVD法大量制备成束多壁纳米碳管

采用溶胶凝胶法合成的Ni-Mo双金属氧化物催化剂,用CVD法催化裂解甲烷从而大量制备高质量高纯度的成束多壁纳米碳管.实验结果表明,该催化剂具有很高的活性和催化效率.反应2 h后,制备的多壁纳米碳管的量可达到初始催化剂量的80倍以上.碳管的直径较均匀,在10～20 nm之间.随着反应时间的延长,制备的纳米碳管石墨化程度增加,反应1 h后,粗产品中纳米碳管的含量就超过了97％. 简单放大后,单炉每克催化剂可以在0.5 h内制得40 g以上多壁纳米碳管.     

Selective-precursor reducing route to cobalt nanocrystals and ferromagnetic property

Cobalt nanocrystals were prepared by controlled chemical route at mild condition through selective-precursor reducing synthesis. Nanorod bundles and three-dimensional (3D) dendritic nanocrystal networks of Co were prepared by selecting different precursors. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) technologies. Field-emission scanning electron microscopy (FE-SEM), SEM, and TEM images indicate the nanorod bundles mainly consist of nanorods with the diameter of 70 nm. In 3D dendritic nanocrystal networks there are numerous secondary and sub-secondary branches were grown at right angles on each main stem. Room temperature magnetic measure of the Co samples demonstrates much enhanced ferromagnetic property, which might be attributed to their organization of specific shape. The possible formation mechanism of the cobalt nanocrystals with different morphologies was also discussed.     

Topological derivation of shape exponents for stretched exponential relaxation

In homogeneous (ideal) glasses, the important dimensionless stretched-exponential shape parameter beta is described by magic (not adjusted) simple fractions derived from fractal configuration spaces of effective dimension d* determined by different topological axioms (rules) in the presence (absence) of a forcing electric field. The rules are based on a new central principle for defining glassy states: equal a priori distributions of fractal residual configurational entropy. Our approach and its beta estimates are fully supported by the results of relaxation measurements involving many different glassy materials and probe methods. The present unique topological predictions for beta typically agree with observed values to approximately 1% and indicate that for field-forced conditions beta should be constant for appreciable ranges of such exogenous variables as temperature and ionic concentration, as indeed observed using appropriate frequency-domain data analysis. The present approach can also be inverted and used to test sample homogeneity and quality.     

Heterogeneous electron transfer kinetics at the ionic liquid/metal interface studied using cyclic voltammetry and scanning electrochemical microscopy

The electrochemical behavior of a redox-active, ferrocene-modified ionic liquid (1-ferrocenylmethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) in acetonitrile and in an ionic liquid electrolyte (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) is reported. Reversible electrochemical behavior was observed in each electrolyte with responses typical of those for unmodified ferrocene observed in each medium. In the ionic liquid electrolyte, the diffusion coefficient of the redox-active ionic liquid increased by a factor of 5 upon increasing the temperature from 27 to 90 degrees C. The kinetics of electron transfer across the ionic liquid/electrode interface were studied using cyclic voltammetry, and the standard heterogeneous electron transfer rate constant, k (0) was determined to be 4.25 x 10 (-3) cm s (-1). Scanning electrochemical microscopy was then also used to probe the heterogeneous kinetics at the interface between the ionic liquid and the solid electrode and conventional kinetic SECM theory was used to determine k (0). The k (0) value obtained using SECM was higher than that determined using cyclic voltammetry. These results indicate that SECM is a very useful technique for studying electron transfer dynamics in ionic liquids.     

Effects of injector geometry and sample matrix on injection and sample loading in integrated capillary electrophoresis devices

The double-T injector design employed in many microchip capillary electrophoresis devices allows for the formation of very small (50-500 pL) sample plugs for subsequent analysis on-chip. In this study, we show that sample plugs formed at the channel junction can be geometrically defined. The channel width and injector symmetry prove to be of great importance to good performance. A unique pushback of solvent into the side channels can be induced when the side channels have a very low resistance to flow, and this helps to better define the injected sample plug. Samples and running buffers of differing ionic strength (e.g., 10 mM KCl buffer and 20 mM KCl sample) can yield widely variable results in terms of plug shape and amount injected (variations of 1.5 to 10x). Applying bias voltages to all the intersecting channels aids in controlling the plug shape. However, when the ionic strengths of buffer and sample are not matched, the actual amount injected (up to 10x variations) can be inconsistent with the appearance of the plug formed in the injector (up to only 30 % variations). Operating at constant pH and ionic strength produced the most consistent results. This report examines the effects of altering the injector geometry and solution ionic strengths, and presents the results of using bias voltages to control plug formation. The observed results should provide a benchmark for modeling of the fluid dynamics in channel intersections.     

The synthesis of the near-spherical AgCl crystal for visible light photocatalytic applications

The near-spherical AgCl micro-crystals were prepared by an ionic liquids-assisted hydrothermal method. The influence of ionic liquids (C(x)MimCl x = 4, 8, 12, 16) on (001) facet growth of AgCl was studied systemically. The composition of the as-prepared samples was characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA) and FTIR spectra. The morphological structures were characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Based on experimental data and the classical crystal growth theory, a plausible growth mechanism of the near-spherical AgCl was proposed. The plasmonic photocatalyst Ag@AgCl prepared from the near-spherical AgCl showed higher activity than that of cubic morphologies.     

Guest-free self-assembly of alpha-cyclodextrins leading to channel-type nanofibrils as mesoporous framework

Guest-free alpha-cyclodextrin self-assembly (alpha-CD-SA) was successfully obtained through a simple treatment such as sonication of alpha-CD in a specific solvent. From wide-angle X-ray diffraction (WAXD), it was found that the crystalline structure of alpha-CD changed upon increasing the treatment time, resulting in alpha-CD-SA in which the alpha-CDs were closely packed in the vertical direction and hexagonally aligned in the horizontal direction (what is called as "channel structure"). In particular, these structures were developed only in tetrahydrofuran (THF) as a specific solvent. In addition, it was found by inclusion experiment and field-emission scanning electron microscopy (FE-SEM) that propionic acid was able to be included into the channel of alpha-CD-SA and that alpha-CD-SA had alpha-CD bundles with a fibril-like shape, respectively. These results demonstrate that the alpha-CD-SA consists of nanofibril-like alpha-CD bundles with cylindrical nanopores open at least at one end, resulting from the dispersion of alpha-CD molecules by sonication in THF and the subsequent re-formation of strong hydrogen bonding between the alpha-CDs with the aid of THF (so-called "slow recrystallization"). Interestingly, it was observed from FE-SEM and nitrogen adsorption-desorption measurement that the alpha-CD-SA had a wormhole-like mesopore with inkbottle shape (average desorption pore size = ca. 25 nm). This mesoporous structure was considered to be attributed to the formation of a mesoporous framework by the disordered aggregation of the nanofibril-like alpha-CD bundles.     

Development and optimization of a system for comprehensive two-dimensional liquid chromatography with UV and mass spectrometric detection for the separation of complex samples by multi-step gradient elution

Comprehensive two-dimensional liquid chromatography (LC x LC) is a powerful tool for the separation of complex biological samples. This technique offers the advantage of simplified automation and greater reproducibility in a shorter analysis time than off-line two-dimensional separation systems. In the present study, an LC x LC system is developed enabling simultaneous UV and MS detection, and which can be easily converted to a conventional reversed-phase LC-UV/MS system. In LC x LC, a 60-min reversed-phase LC separation with a linear solvent gradient in the first dimension is coupled to a second-dimension separation on a mixed-mode cation-exchange/reversed-phase column with a modulation time of 60s. The isocratic separation in the second-dimension column is optimized by the use of a multi-step gradient where the organic and the ionic modifier are varied independently. Intraday (n=3) and interday (n=4) variability of the retention times were evaluated with the complete system and found to be 0.5% and 0.7%, respectively. Good linearity was observed in calibration curves for three different compounds varying in polarity.     

Thermodynamics study of the dimerization equilibria of rhodamine B and 6G in different ionic strengths by photometric titration and chemometrics method

The dimerization constants of rhodamine B and 6G have been determined by studying the dependence of their absorption spectra on the temperature in the range 20-80 degrees C at different total concentrations of rhodamine B (5.89 x 10(-6) to 2.36 x 10(-4)M) and rhodamine 6G (2.34 x 10(-5) to 5.89 x 10(-4)M) and in different concentrations of LiCl, NaCl and KCl salts as supporting electrolytes. The monomer-dimer equilibrium of rhodamine B and 6G have been determined by chemometrics refinement of the absorption spectra obtained by thermometric titrations performed at different ionic strengths. The quantitative analysis of the data of undefined mixtures, was carried out by simultaneous resolution of the overlapping spectral bands in the whole set of absorption spectra. The dimerization constants are varied by changing the ionic strength and the degree of dimerization are decreased by increasing of the ionic strength of the medium. The enthalpy and entropy of the dimerization reactions were determined from the dependence of the equilibrium constants on the temperature (van't Hoff equation). From the thermodynamic results the TDeltaS degrees -DeltaH degrees plot was sketched. It shows a fairly good positive correlation which indicates the enthalpy-entropy compensation in the dimerization reactions (compensation effect).     

Communication: Bulkiness versus anisotropy: the optimal shape of polarizable Brownian nanoparticles for alignment in electric fields

Self-assembly and alignment of anisotropic colloidal particles are important processes that can be influenced by external electric fields. However, dielectric nanoparticles are generally hard to align this way because of their small size and low polarizability. In this work, we employ the coupled dipole method to show that the minimum size parameter for which a particle may be aligned using an external electric field depends on the dimension ratio that defines the exact shape of the particle. We show, for rods, platelets, bowls, and dumbbells, that the optimal dimension ratio (the dimension ratio for which the size parameter that first allows alignment is minimal) depends on a nontrivial competition between particle bulkiness and anisotropy because more bulkiness implies more polarizable substance and thus higher polarizability, while more anisotropy implies a larger (relative) difference in polarizability.     

Magnetic nanoparticles with hybrid shape

Co₅₀Ni₅₀ particles with very unusual shapes, resembling dumbbells or diabolos, are obtained by reducing mixtures of cobalt and nickel acetates in sodium hydroxide solution in 1,2-propane diol. These particles consist of a central column richer in cobalt than the overall composition, capped with two terminal platelets that are richer in nickel. These hybrid shapes are the result of a two-step growth mechanism due to a difference in reactivity of the two metal ions. The sodium hydroxide concentration controls the length and diameter of the column, in the ranges 50–250 nm and 5–15 nm, respectively, and the diameter of the platelets in the range 25–50 nm. The X-ray diffraction patterns show a mixture of hcp and fcc phases in various proportions depending on the particle shape. High-resolution electron microscopy shows that the hcp phase is located mainly in the central column and the fcc phase mainly in the terminal platelets. The particles are ferromagnetic at room temperature. When the volume fraction of the central column is high enough and the hcp phase is predominant and not much faulted, high coercivity (up to 1900 Oe) is observed.