Encapsulation of emitting CdTe QDs within silica beads to retain initial photoluminescence efficiency

Highly luminescent silica beads (30 nm-2 mum slashed circle) incorporating CdTe quantum dots (QDs) were prepared via a two-step preparation procedure, namely a modified St?ber synthesis and a subsequent reverse micelle route. In the modified St?ber synthesis, the silica molecules are deposited on the surface of the QDs. After this first step, these coated QDs were incorporated into silica beads via a reverse micelle route. Inductively coupled plasma analysis revealed a red-emitting silica bead of 30 nm in diameter thus prepared encapsulated roughly 14 CdTe QDs. These glass beads (30-40 nm slashed circle) retained the initial photoluminescence (PL) efficiencies of the colloidal QDs (27 and 65% for the green- and red-emitting beads, respectively). The protection of QDs by a silica layer at the first step, together with the short total reaction time, is the main reason for the retention of the PL efficiency. The size of the glass beads can be easily controlled over the wide range by adjusting the injection speed and the ratio of chemicals used for the reverse micelle preparation. Since the original efficiency was maintained in the beads and is the highest ever reported for QD-containing silica beads, the method presented here is of significant importance for applications of silica beads to biological probes.     

Preparation and self-assembly of carboxylic acid-functionalized silica

A simple method for the fabrication of silica nanoparticle film based on the covalent-bonding interaction between carboxylic acid-functionalized silica nanoparticles (SiO(2)-COOH) and amino-terminated silicon wafer was developed. Prior to assembly, silica nanoparticles with an average diameter 80 nm were prepared using the St?ber method, amino-functionalized silica nanoparticles (SiO(2)-NH(2)) were prepared by a silanization with 3-aminopropyltriethoxysilane (APTES), while carboxylic acid-functionalized silica nanoparticles (SiO(2)-COOH) were prepared by a ring opening linker elongation reaction of the amine functions with succinic anhydride, at the same time, amino-terminated silicon wafer (Si-NH(2)) was obtained by self-assembling 3-aminopropyltriethoxysilane, then one layer relative close-packed carboxylic acid-functionalized silica nanoparticles (SiO(2)-COOH) was arranged on silicon wafer through amidation reaction under DCC coupling agent.     

Preparation and characterization of silica aquasols

Aquasols containing silica nanoparticles with diameters of 75 to 95 nm were obtained directly by hydrolysis of 2 wt.% tetraethoxysilane (TEOS) in water in the presence of a non-ionic surfactant. The reaction was catalyzed by hydrochloric acid, ammonia, or sodium hydroxide. The particle size, which mainly depends on the concentration of TEOS in water, was determined by dynamic light scattering (DLS). Whereas the catalysts have almost no influence on the particle size, they very strongly affect the morphology of the silica particles formed. The dried SiO(2) particles obtained via the HCl-catalyzed reaction have film-forming properties and show no measurable BET surface area. SiO(2) particles prepared with ammonia as catalyst form nanoporous films on glass, and the BET surface area of the freeze-dried particles is 540 m(2)/g. Using sodium hydroxide as catalyst results in some agglomeration of uniform spherical particles with a BET surface area of 237 m(2)/g. (29)Si MAS NMR investigations of the freeze-dried particles provide information about the degree of condensation and the ratio of "free" hydroxyl groups. The silica aquasols described have a surprisingly high hydrophilizing effect on hydrophobic fibers (PP, PET). Silica nanoparticles of comparable diameters, prepared by the "St?ber method", dispersed in alcohol do not show any hydrophilizing properties worth to mention.     

Synthesis and self-assembly of monodisperse silver-nanocrystal-doped silica particles

A simple method based on the St?ber reaction was developed to prepare silver-nanocrystal-doped silica composite particles. The silane coupling agent N-[3-(trimethoxysilyl)dropyl]ethylene diamine (TSD) incorporated Ag+ into a siloxane framework and a further chemical reducer reduced Ag+ to silver nanoparticles. TEM images showed that, in the presence of TSD, silver nanocrystals (fcc) of 2-8 nm were homogeneously doped in the silica particles, which showed a typical surface plasmon resonance (SPR) peak. The as-prepared Ag/SiO2 composite particles can be self-assembled into long-range ordered lattices (or photonic crystals) over large areas.     

Direct coating of gold nanoparticles with silica by a seeded polymerization technique

Gold nanoparticles prepared through a conventional citrate-reduction method were directly coated with silica by means of a seeded polymerization technique based on the St?ber method. The method required no surface modification. The addition of tetraethylorthosilicate and water prior to ammonia was found to be critical to obtain a proper coating. The silica shell thickness was varied from 30 to 90 nm for TEOS concentrations of 0.0005-0.02 M at 10.9 M of water and 0.4 M of ammonia. The optical spectra of the core-shell gold-silica composite particles agreed with predictions of Mie theory.     

Synthesis and crystallization of hybrid spherical colloids composed of polystyrene cores and silica shells

The St?ber method has been adopted to prepare hybrid core-shell particles by coating the surfaces of monodisperse polystyrene beads with uniform silica shells. Polystyrene beads with diameters in the range of 0.1-1.0 microm have been successfully demonstrated for use with this process, and the thickness of the silica coating could be controlled in the range of 50-150 nm by adjusting the concentration of tetraethoxysilane, the deposition time, or both. The morphology and surface smoothness of the deposited silica were found to strongly depend on a number of parameters such as the surface functional groups on the polymer beads, the pH value of the medium, and the deposition time. Hollow spheres made of silica could be obtained by selectively removing the polymer cores via calcination in air at an elevated temperature or by wet etching with toluene. These core-shell colloids were also explored as building blocks to fabricate long-range ordered lattices (or colloidal crystals) that exhibited stop bands different from those assembled from spherical colloids purely made of either polystyrene or silica.     

Well-defined surface imido amido tantalum(v) species from ammonia and silica-supported tantalum hydrides

The MCM-41 supported hydrides [([triple bond]SiO)(2)TaH(3)], 1a, and [([triple bond]SiO)(2)TaH(3)], 1b, cleave N-H bonds of ammonia at room temperature to yield the well-defined imido amido surface complexes [([triple bond]SiO)(2)Ta(NH)(NH(2))], 2, and 2xNH(3). Additionally, the surface silanes [[triple bond]Si-H] that exist in close proximity to 1a and 1b also react with ammonia at room temperature to give the surface silylamido [Si-NH(2)]. Such reaction is tantalum assisted: surface silanes were synthesized independently and in absence of tantalum by reaction of highly strained silica, SiO(2-1000), with SiH(4) and no reaction with ammonia was observed. Surface-supported complexes 2, 2xNH(3), and [[triple bond]Si-NH(2)] have been characterized by, inter alia, solid-state NMR, IR, and EXAFS and independent synthesis of [[triple bond]Si-NH(2)]. The NMR studies on the fully 15N-labeled samples have led to unambiguous discrimination between imido, amido, and amino resonances of 2*, 2*x(15)NH(3), and [[triple bond]Si-15NH(2)] through the combination of solid-state magic angle spinning (MAS), heteronuclear correlation (HETCOR), 2D proton double-quantum (DQ) single-quantum (SQ) correlation, and 2D proton triple-quantum (TQ) single-quantum (SQ) correlation spectra. The in situ IR monitoring of the reaction of 1a and 1b with regular NH(3) and 15NH(3), and after H/D exchange has yielded the determination of all the NH(x) vibration and deformation modes, with their respective H/D and 14N/15N isotopic shifts. EXAFS study yielded the bond distances in 2 of 1.79(2) Angstrom for Ta=N, 1.89(1) Angstrom for Ta-O, and 1.98(2) Angstrom for Ta-N.     

Synthesis of a Au/silica/polymer trilayer composite and the corresponding hollow polymer microsphere with a movable Au core

Gold/silica/poly(N,N'-methylenebisacrylamide) (Au/SiO2/polyMBAAm) trilayer composite materials were prepared by distillation precipitation polymerization of N,N'-methylenebisacrylamide (MBAAm) in the presence of Au/SiO2 particles as seeds, in which the seeds were prepared by a combination of gold-complexing and silane coupling agent with a further modified St?ber method. The polymerization of MBAAm was performed in neat acetonitrile with 2,2'-azobisisobutyronitrile as an initiator to encapsulate the Au/SiO2 seeds driven by the hydrogen-bonding interaction between the hydroxyl group on the surface of the seeds and the amide unit of polyMBAAm without modification of the Au/SiO2 surface in the absence of any stabilizer or surfactant. Hollow polyMBAAm microspheres with movable Au cores were further developed by the selective removal of the middle silica layer with hydrofluoric acid. The resultant trilayer Au/SiO2/polyMBAAm composite and hollow polyMBAAm microspheres with movable Au cores were characterized by transmission electron microscopy. The diffusion of chemicals across the polyMBAAm shell was investigated by a catalytic reduction of 4-nitrophenol to 4-aminophenol in the presence of sodium borohydride as a reductant.     

Rapid evaporation-induced synthesis of monodisperse budded silica spheres

Budded silica spheres have been synthesized by a novel rapid evaporation-induced self-assembly combined with the well-known St?ber method. The morphology of budded silica spheres were examined by transmission electron microscopy, and their mean size and size distribution were also estimated. Both the temperature of the sol-gel reaction and following post-treatment were found to play crucial roles in determining the surface morphology of obtained silica spheres and the yield of budded silica spheres. The possible formation mechanism was also proposed on the basis of experimental observations. The budded silica spheres would have higher surface areas than smooth silica spheres, and significant potentials for catalyst supports, building blocks of photonic crystals, and for constructing superhydrophobic and superhydrophilic surfaces.     

Facile preparation of highly monodisperse small silica spheres (15 to >200 nm) suitable for colloidal templating and formation of ordered arrays

Highly monodisperse spherical silica nanoparticles with diameters ranging from ca. 15 to 200 nm were prepared using an environmentally friendly water-based synthesis. The size of the spheres can be precisely controlled by using a facile regrowth procedure in the same reaction media. Furthermore, these monodisperse silica spheres can be successfully used as seeds in the well-established St?ber silica preparation. The regrowth approach allows for easy incorporation of functional additives. High monodispersity and charge stabilization renders these nanoparticles highly suitable for close-packed array formation and colloidal templating.     

Coating of Nanosize Silver Particles with Silica

The formation of silica shells on core silver particles by a modified St?ber process was investigated and the coated particles were characterized using UV-visible spectroscopy, transmission electron microscopy, and electrophoresis. The deposition conditions, such as reagent concentrations and reaction time, were optimized in order to obtain uniform surface layers of silica and to avoid excess precipitation of the latter. Copyright 2000 Academic Press.     

Architecture of micron-scale hollow spheres composed of silica nanoparticles and approach to luminescent spheres

Micron-scale hollow spheres were successfully constructed with silica nanoparticles by templating of polymer spheres. Subsequently, the use of 3-aminopropyltriethoxysilane (APTES) introduces carbon and oxygen defects in the silica nanoparticles resulting from calcination of the aminopropyl group. In this approach, the template of micron-scale polymer spheres was prepared from dispersion polymerization. Subsequent St?ber process results in the formation of a silica layer attached to the polymer sphere surfaces. After calcination, the obtained micron-scale hollow silica spheres were then studied on the relationship between the particle diameter and the surface morphology. The luminescence of hollow spheres was prepared through using APTES in St?ber process, and which of related the appearance of luminescence to the APTES concentration and calcination temperature. The results of this study can provide useful information for the structure of micron-scale hollow spheres and their application to luminescent materials.     

Synthesis of poly(ethylene glycol) (PEG)-grafted colloidal silica particles with improved stability in aqueous solvents

The known grafting procedures of colloidal silica particles with poly(ethylene glycol) (PEG) lead to grafting layers that detach from the silica surface and dissolve in water within a few days. We present a new grafting procedure of PEG onto silica with a significant improvement of the stability of the grafting layers in aqueous solvents. Moreover, the procedure avoids any dry states or other circumstances leading to strong aggregation of the particles. To achieve the improved water stability, St?ber silica particles are first pre-coated with a silane coupling agent (3-aminopropyl)triethoxysilane (APS) to incorporate active amine groups. The water solubility of the pre-coating layer was minimized using a combination of APS with bis-(trimethoxysilylpropyl)amine (BTMOSPA) or bis-(triethoxysilyl)ethane (BTEOSE). These pre-coated particles were then reacted with N-succinimidyl ester of mono-methoxy poly(ethylene glycol) carboxylic acid to form PEG-grafted silica particles. The particles form stable dispersions in aqueous solutions as well as several organic solvents.     

Encapsulation of Inorganic Particles by Dispersion Polymerization in Polar Media: 2. Effect of Silica Size and Concentration on the Morphology of Silica–Polystyrene Composite Particles

Following a previous work (Bourgeat-Lami, E., and Lang, J.,J. Colloid Interface Sci.197, 293 (1998)), encapsulation of silica beads has been achieved by dispersion polymerization of styrene in an aqueous ethanol medium using poly(N-vinyl pyrrolidone) as stabilizer. Silica beads, prepared according to the Stöber method, were coated prior to polymerization by grafting 3-(trimethoxysilyl)propyl methacrylate onto the surface. A great number of silica beads per composite particle were previously found using beads that had diameters between 49 and 120 nm. In the present work, larger silica beads with diameters between 191 and 629 nm are investigated. We demonstrate by transmission electron microscopy that, consequently, only a small number of silica beads are contained in the composite particles. By counting the composite particles containing precisely zero, one, two, three, four, and more than four silica beads, it clearly appears that the encapsulation of only one silica bead can be obtained simply by increasing the size of the beads. Under our experimental conditions, the optimal bead diameter for achieving composite particles containing only one silica bead turns out to be around 450 nm. We show that increasing the silica bead size above this value results in an increased number of composite particles without silica beads. In contrast, the number of composite particles with two, three, four, or more than four silica beads increases with decreasing silica bead size. In addition to the above variations in composition of the composite particles, changes in particle shapes were also observed as a function of the size of the silica beads and the styrene concentration in the polymerization medium. Hypotheses concerning these variations are presented.     

Silane-based poly(ethylene glycol) as a primer for surface modification of nonhydrolytically synthesized nanoparticles using the Stöber method

This paper describes the use of methoxy-poly(ethylene glycol) silane (MPEG-sil) as a linker molecule for the synthesis of silica-coated nanoparticles by the St?ber method. While short alkane chain-based siloxanes including (acryloxypropyl)trimethoxysilane and 3-methacryloxypropyl-trimethoxysilane are popular molecules used in surface modification, they are not efficient for the silica coating of nanoparticles synthesized from organic solvents containing long carbon chain carboxylic acids or amines as capping agents. Here, we report the utilization of MPEG-sil to bridge this gap. Our approach is based on a two-phase system, in which ligand exchange takes place in a hydrophobic environment and the surface modification with silica is conducted in an ethanol-water mixture. Our results show that this two-phased approach was effective to coat monodisperse Fe2O3 nanoparticles capped with oleic acid and Ag nanoparticles capped with oleylamine with uniform SiO2 shells. The process was also demonstrated for double-shell nanostructures to produce SiO2-coated Pt@Fe2O3 core-shell nanoparticles. The results described in this work represent a new approach for the surface modification with silica coating of monodisperse nanoparticles synthesized from nonhydrolytic solutions and can potentially have a broad ramification in the development of water-dispersible nanoparticles for biological applications.     

Synthesis and characterization of large colloidal cobalt particles

Large colloidal environmentally stable silica-coated cobalt particles were synthesized by combining the sodium borohydride reduction in aqueous solution and the St?ber method. Low size polydisperse cobalt spheres with an average size of 95 nm were synthesized by using a borohydride reduction method and were subsequently coated with a thin layer of silica by means of hydrolysis and condensation of tetraethylorothosilicate (TEOS) in an aqueous/ethanolic solution. The large uniform cobalt spheres consist of smaller metallic Co clusters, explaining the superparamagnetic behavior of the spheres. The particles were investigated by transmission electron microscopy (TEM) and vibrating sample magnetometry (VSM).     

Mechanism of a chemical classic: quantum chemical investigation of the autocatalyzed reaction of the serendipitous wöhler synthesis of urea

The detailed reaction pathways for the ammonium cyanate transformation into urea (W?hler's reaction) in the gas phase, in solution, and in the solid state have exhaustively been explored by means of first-principles quantum chemical calculations at the B3LYP level of theory using the 6-31G(d,p) basis set. This serendipitous synthesis of urea is predicted to proceed in two steps; the first step involves the decomposition of the ammonium cyanate to ammonia and isocyanic or cyanic acid, and the second one, which is the main reaction step (and probably the rate-determining step), involves the interaction of NH(3) with either isocyanic or cyanic acid. Several alternative pathways were envisaged for the main reaction step of W?hler's reaction in a vacuum involving the formation of "four-center" transition states. Modeling W?hler's reaction in aqueous solution and in the solid state, it was found that the addition of NH(3) to both acids is assisted (autocatalyzed) by the active participation of extra H(2)O and/or NH(3) molecules, through a preassociative, cooperative, and hydrogen-transfer relay mechanism involving the formation of "six-center" or even "eight-center" transition states. The most energetically economic path of the rate-determining step of W?hler's reaction is that of the addition of NH(3) to the C=N double bond of isocyanic acid, directly affording urea. An alternative pathway corresponding to the anti-addition of ammonia to the Ctbd1;N triple bond of cyanic acid, yielding urea's tautomer HN=C(OH)NH(2), seems to be another possibility. In the last case, urea is formed through a prototropic tautomerization of its enolic form. The energies of the reactants, products, and all intermediates along with the barrier heights for each reaction path have been calculated at the B3LYP/6-31G(d,p) level of theory. The geometry optimization and characterization of all of the stationary points found on the potential energy hypersurfaces was performed at the same level of theory.     

负载型氧化锆催化剂上甲醇脱氢制甲醛

甲醇在无氧条件下脱氢,可以制得含水量极低的甲醛。在上述反应中,主要采用以硅胶为载体的负载型氧化物催化剂,其中以周期表ⅠB和ⅡB族金属,如铜、银或锌为主要组分。这些金属的氧化物在高温下易还原、烧结和表面积炭而使催化剂失活。添加P,S,Se或Te等组分作为助催化剂,在一定程度上可以改善催化性能。最近的发展倾向是采用非负载的碱金属盐作为催化剂,如Na_2CO_3,Na_2MoO_4,或Na_xLi_(1-x)AlO_2(0≤x≤1)。这类催化剂要求过高的反应温度,如高于650℃,甚至900℃条件下使用。     

单分散高纯硅胶色谱柱填料的制备

在氨水和氨气催化下使单晶硅粉水解, 合成了单分散、高纯的纳米二氧化硅水溶胶; 再利用聚合诱导胶体凝聚法(PICA)制备单分散脲醛二氧化硅复合微球, 经过高温煅烧后得到球形硅胶色谱柱填料. 通过电感耦合等离子-质谱(ICP-MS)、电子显微镜和BET比表面积测试等手段对球形硅胶的纯度、粒径分布及比表面积进行了表征, 并通过色谱分离对硅胶填料的性能进行了评价. 实验结果表明, 该方法合成的色谱柱填料具有纯度高、粒径分布均匀、机械强度高及分离能力强等优点.     

Direct formation of thermally stabilized amorphous mesoporous Fe2O3/SiO2 nanocomposites by hydrolysis of aqueous iron III nitrate in sols of spherical silica particles

Nanocomposite materials containing 10% and 20% iron oxide/silica, Fe2O3/SiO2 (w/w), were prepared by direct hydrolysis of aqueous iron III nitrate solution in sols of freshly prepared spherical silica particles (St?ber particles) present in their mother liquors. This was followed by aging, drying, calcination up to 600 degrees C through two different ramp rates, and then isothermal calcinations at 600 degrees C for 3 h. The calcined and the uncalcined (dried at 120 degrees C) composites were characterized by thermogravimetric analysis, differential scanning calorimetry, Fourier transform infrared spectroscopy, X-ray diffraction (XRD), N2 adsorption/desorption techniques, and scanning electron microscopy as required. XRD patterns of the calcined composites showed no line broadening at any d-spacing positions of iron oxide phases, thereby reflecting the amorphous nature of Fe2O3 in the composite. The calcined composites showed nitrogen adsorption isotherms characterizing type IV isotherms with high surface area. Moreover, surface area increased with the increasing of the iron oxide ratio and lowering of the calcination ramp rate. Results indicated that iron oxide particles were dispersed on the exterior of silica particles as isolated and/or aggregated nanoparticles. The formation of the title composite was discussed in terms of the hydrolysis and condensation mechanisms of the inorganic FeIII precursor in the silica sols. Thereby, fast nucleation and limited growth of hydrous iron oxide led to the formation of nanoparticles that spread interactively on the hydroxylated surface of spherical silica particles. Therefore, a nanostructured composite of amorphous nanoparticles of iron oxide (as a shell) spreading on the surface of silica particles (as a core) was formed. This morphology limited the aggregation of Fe2O3 nanoparticles, prevented silica particle coalescence at high temperatures, and enhanced thermal stability.