One-step synthesis of dye-incorporated porous silica particles

Fluorescent nanoparticles have a variety of biomedical applications as diagnostics and traceable drug delivery agents. Highly fluorescent porous silica nanoparticles were synthesized in a water/oil phase by a microemulsion method. What is unique about the resulting porous silica nanoparticles is the combination of a single-step, efficient synthesis and the high stability of its fluorescence emission in the resulting materials. The key of the success of this approach is the choice of a lipid dye that functions as a surrogate surfactant in the preparation. The surfactant dye was incorporated at the interface of the inorganic silica matrix and organic environment (pore template), and thus insures the stability of the dye?Csilica hybrid structure. The resulting fluorescent silica materials have a number of properties that make them attractive for biomedical applications: the availability of various color of the resulting nanoparticle from among a broad spectrum of commercially dyes, the controllablity of pore size (diameters of ~5?nm) and particle size (diameters of ~40?nm) by adjusting template monomer concentration and the water/oil ratio, and the stability and durability of particle fluorescence because of the deep insertion of surfactant??s tail into the silica matrix.     

The effect of aging temperature on structure characteristics of ordered mesoporous silicas

A series of mesoporous silica materials were synthesized by applying Pluronic type polymers as pore creating agents. In order to differentiate the characteristics of porous structure of the obtained sorbents the temperature of aging process was changed in the synthesis. The parameters characterizing the pore structure were estimated from nitrogen adsorption/desorption isotherms. The changes of pore arrangement of the obtained materials being a result of different synthesis conditions were investigated by using a small angle X-ray scattering method. Correlations between the values of structure parameters and aging temperature were found.     

Iron oxide nanoparticles hosted in silica aerogels

The investigated hybrid materials consist of non-agglomerated iron oxide particles hosted in silica aerogels. The composite material can be produced as a monolith, in any shape, and with different dilutions of the iron oxide phase. Two sol–gel chemistry routes have been followed: a solution of Fe(NO₃)₃·9H₂O has been added either to the silica gel or to the initial sol; in the latter, the iron salt provided the water required for the gel polymerisation. To obtain monolithic aerogels, the gels were dried by hypercritical solvent evacuation. On the other hand, some gels were dried by slow and controlled evaporation of the solvent, resulting in xerogels. Several heat treatments have been performed and the iron oxide particle phase, growth mechanism and crystallinity have been analysed. The composite materials were characterised by means of X-ray diffraction, M?ssbauer spectrometry and superconducting quantum interference device magnetometry. It was observed that the particle sizes (in the nanometre range) and the thermal stability of the iron oxide phases strongly depend on the preparation method that determines the microstructure of the host material. Consequently, the magnetic properties of the nanoparticles can be controlled via synthesis conditions, matrix properties and thermal treatments. Received: 5 March 2001 / Accepted: 16 June 2001 / Published online: 30 August 2001     

Synthesis and Spectroscopy of Silica Nanoparticles as Scatter Centers in Random Gain Porous Media

This work was carried out to synthesis a silica matrix by sol-gel technique, which used as host to Kiton Red laser dye doped with silica nanoparticles, which also prepared by sol-gel technique, to obtain KR-SiO₂ nanoparticles confined in silica xerogel matrix. The rods at different pH values were successfully synthesized. The different values of pH cause different size of obtained nanoparticles, these nanoparticles act as scatter centers in the matrix. Amplified spontaneous emission (ASE), threshold pumping energy (E_th), and mean free path (l_t) for photons in the rods have been reported. the results show that the values of bandwidth at full width half-maximum (FWHM) and the threshold energy are about 8.7 nm and 12 mJ respectively.     

Investigation of self-assembled fractal porous-silica over a wide range of length scales using a combined small-angle scattering method

The unique structure of a set of self-assembled porous silica materials was characterized through a combined small-angle scattering (CSAS) method using small- and ultra-small angle neutron scattering as well as small-angle X-ray scattering. The porous silica specimens investigated were prepared by a sol-gel method under the presence of alkylketene dimer (AKD) template particles and through calcination, which leads to the development of porous silica having a mass-fractal structure over length scales from ~ 100 nm to ~ 10 μm. Furthermore, the specimens posses a hierarchical structure, which consist of a fractal porous structure, and also contain primary silica particles less than 10 nm in size, which form a continuous silica matrix. To characterize these complex structures, observation over a broad range of length scales is indispensable. We propose a CSAS technique that serves this purpose well.     

Density profile of water confined in cylindrical pores in MCM-41 silica

Recently, water absorbed in the porous silica material MCM-41-S15 has been used to demonstrate an apparent fragile to strong dynamical crossover on cooling below ～220 K, and also to claim that the density of confined water reaches a minimum at a temperature around 200 K. Both of these behaviours are purported to arise from the crossing of a Widom line above a conjectured liquid-liquid critical point in bulk water. Here it is shown that traditional estimates of the pore diameter in this porous silica material (of order 15 ?) are too small to allow the amount of water that is observed to be absorbed by these materials (around 0.5 g H(2)O/g substrate) to be absorbed only inside the pore. Either the additional water is absorbed on the surface of the silica particles and outside the pores, or else the pores are larger than the traditional estimates. In addition the low Q Bragg intensities from a sample of MCM-41-S15 porous silica under different dry and wet conditions and with different hydrogen isotopes are simulated using a simple model of the water and silica density profile across the pore. It is found the best agreement of these intensities with experimental data is shown by assuming the much larger pore diameter of 25 ? (radius 12.5 ?). Qualitative agreement is found between these simulated density profiles and those found in recent empirical potential structure refinement simulations of the same data, even though the latter data did not specifically include the Bragg peaks in the structure refinement. It is shown that the change in the (100) peak intensity on cooling from 300 to 210 K, which previously has been ascribed to a change in density of the confined water on cooling, can equally be ascribed to a change in density profile at constant average density. It is further pointed out that, independent of whether the pore diameter really is as large as 25 ? or whether a significant amount of water is absorbed outside the pore, the earlier reports of a dynamic crossover in supercooled confined water could in fact be a crystallization transition in the larger pore or surface water.     

A general and low-cost synthetic route to high-surface area metal oxides through a silica xerogel template

Porous metal oxides with a large surface area are synthesised by means of a procedure based on the templating approach. An inexpensive porous silica xerogel synthesised at moderate temperatures (∼100 °C) in order to preserve the silanol superficial groups was used as template. In a first step, the silica porosity was filled with a concentrated solution containing a metallic salt. Then, the impregnated sample was calcined in air at a temperature of 600 °C. Under these conditions, the metal oxides were synthesised within the confined space provided by the silica pores. Finally, the product was recovered after dissolution of the silica framework in 2 M NaOH solution. The materials obtained by this procedure are made up of aggregates of nanoparticles and/or 3D solid structures containing confined pores. In this work, the synthetic route proposed is illustrated by the preparation of various binary metal oxides (i.e. Fe₂O₃, Cr₂O₃, NiO, CeO₂, Mn₂O₃, Co₂O₃ and Al₂O₃). The BET surface areas measured for these materials are in the range of 100-270 m² g⁻¹. The proposed method is not restricted to the binary metal oxides. It can also be used in the preparation of other inorganic materials such as metal sulphides or mixed metal oxides.     

Ionic liquid confined in silica nanopores: molecular dynamics in the isobaric–isothermal ensemble

Molecular dynamics simulations in the isobaric–isothermal ensemble are used to investigate the structure and dynamics of an ionic liquid confined at ambient temperature and pressure in hydroxylated amorphous silica nanopores. The use of the isobaric–isothermal ensemble allows estimating the effect of confinement and surface chemistry on the density of the confined ionic liquid. The structure of the confined ionic liquid is investigated using density profiles and structural order parameters while its dynamics is assessed by determining the mobility and ionic conductivity of the confined phase. Despite the important screening of the electrostatic interactions (owing to the small Debye length in ionic liquids), the local structure of the confined ionic liquid is found to be mostly driven by electrostatic interactions. We show that both the structure and dynamics of the confined ionic liquid can be described as the sum of a surface contribution arising from the ions in contact with the surface and a bulk-like contribution arising from the ions located in the pore centre; as a result, most properties of the confined ionic liquid are a simple function of the surface-to-volume ratio of the host porous material. In contrast, the ionic conductivity of the confined ionic liquid, which is a collective dynamical property, is found to be similar to the bulk. This study sheds light on the complex behaviour of hybrid materials made up of ionic liquid confined in inorganic porous materials.     

Synthesis and study of fluorescence properties of Cu nanoparticles

ZnO nanoparticles, of average size of 10–15 nm, homogeneously dispersed in a silica matrix were prepared by a two stage citric acid/sol–gel process and thermal treatments up to 700 °C. The precursors formed at the early stages of the synthesis and their thermal evolution were investigated by FTIR, ²⁹Si NMR MAS and CPMAS, ¹³C CPMAS and T_1ρ(¹H). A unidentate complex was revealed in the gel, together with other complexes in which citrate carboxylate groups are bound to one Zn²⁺ ion or act as a bridge between two Zn²⁺ ions. A comparison of the results from nanocomposite and silica samples prepared by the same method showed that chemical interactions between amorphous silica and zinc ions are not present either in the precursors or in the final materials. As a consequence, ZnO particles do not react with silica matrix when they are heated up to 700 °C. This result is ascribed to a nanophase segregation of zinc citrate complexes from the host matrix.     

Hierarchically Structured Porous Silica Spheres by Microemulsion/Vesicle Templating for Hydrodesulfurization of Fluid Catalytic Cracking Diesel

A series of hierarchically structured porous silica sphere (HSPSS) materials are successfully fabricated by a facile, one‐step microemulsion/vesicle bimodal method in a multicomponent microemulsion system of P123/n‐butanol/1,3,5‐trimethylbenzene/KCl/H₂O (surfactant/cosurfactant/oil/salt/water). The pore structures of the obtained HSPSS products consist of mesocellular foam and mesostructured vesicles. In contrast to the traditional porous silica materials the new structures combine two separate, distinct mesophases with different‐sized mesovoids in a single porous sphere. Moreover, the proportion of every mesophase in obtained HSPSS can be easily adjusted by tuning the initially added amount of n‐butanol or KCl in this multicomponent microemulsion system. When the molar ratio of KCl/tetraethoxysilane is 2.15, the obtained HSPSS material is turned into uniform mesostructured vesicle silica spheres, which consist of many small diameter vesicle particles. The hydrodesulfurization (HDS) activity of fluid catalytic cracking diesel over the HSPSS was tested. HSPSS‐0.75‐1.43 catalyst support with multiple mesoporous structures shows the highest HDS efficiency (98.5%) among all the studied catalysts.     

Synthesis and characterization of double –SO3H functionalized Brönsted acidic hydrogensulfate ionic liquid confined with silica through sol-gel method

Imidazolium-based metal and halogen-free Brönsted acidic ionic liquid (BAIL) (3,3′-(hexane-1,6-diyl)bis(2-methyl-1-(3-sulfopropyl)-1H-benzimidazolium) hydrogensulfate [HbMBIM-PS][HSO₄] was synthesized. The physicochemical properties of this BAIL were investigated using a variety of different analytical and spectroscopic techniques such as ¹H and¹³C-NMR, FT-IR, mass, UV–vis and TGA spectra. A porous silica matrix has been synthesized using BAIL and tetraethoxysilane (TEOS) as silica source by nonhydrolytic sol–gel method. The properties of IL confined silica gel matrix have been studied using FTIR, TGA, SEM, N₂-sorption measurement (BET characterization for determining pore parameters), and NH₃-TPD techniques. From the N₂-sorption measurement, it has been found that BET surface area decreased while pore volume, average pore size and porosity decreased. The thermal stability of the IL has been found to increase upon confinement in silica gel matrix. The results suggested that IL had been successfully confined on silica gel. This IL confined silica gel catalyst is environment friendly and useful for alkane isomerization and esterification reactions.     

Synthesis and spectroscopic characterization of Kiton Red-620 dye doped silica gel rods

The work was carried out to achieve two different but interdependent objectives; one to synthesis a transparent silica matrix with enough strength and thermal stability, by sol–gel technique, to host an organic dye molecule without quenching its fluorescence and two to find the probability if the said material is suitable for the construction of solid state dye lasers. Crack-free transparent silica gel rods with good mechanical strength, thermal stability and dimensions were successfully synthesized by sol–gel techniques. The rods were doped with Kiton Red-620 dye in different concentration separately. Effect of various synthesis parameters like time, temperature and aging condition was extensively studied to obtain crack-free silica rods doped with dye. Optical properties of prepared rods were studied by FTIR, UV/VIS–NIR and fluorescence spectroscopy. It is observed that Kiton Red-620 dye doped silica gel rods show good fluorescence with sharp peaks in the visible range. Their UV–VIS spectrum indicates the absorption in visible range. Thermal stability of rods were studied by DSC/TGA methods. Eventually it is found that these dye doped silica gel materials explore the possibility for new solid-state dye laser materials.     

Continuous laser direct-writing of PbS nanoparticles inside transparent silica monoliths

Direct space-selective growth of PbS nanoparticles, embedded inside a transparent porous sol–gel derived silica matrix, has been achieved using continuous laser irradiation. Before the irradiation, the porous silica host has been soaked in a PbS precursor solution. The effect of the concentration of PbS precursors and of the incident laser power on the mean particle size was studied. Absorption spectroscopy, X-ray diffraction analysis, and TEM measurements were used to identify the PbS crystallites inside the xerogel and to estimate the average particle size. It has been shown that PbS crystallite sizes range between 3 and 13 nm depending on the PbS precursors concentration.     

Pore-size determination of mesoporous materials by1H NMR spectroscopy

The pore-size distributions of a series of mesoporous silica materials were determined by measuring the¹H nuclear magnetic resonance (NMR) signal from the nonfrozen fraction of organic probe molecules as a function of temperature. The melting point distribution curves of confined benzene reveal 2–3 transition points. The high-temperature transition point, corresponding to the temperature at the first maximum of the melting point distribution curve, is interpreted as the average depressed melting point of the confined substance. However, the intensity data reveal that a measurable portion of the confined benzene apparently remains nonfrozen even 120 K below the bulk melting point in the 4–10 nm pore systems. The component at lowest temperature is largely attributed to the liquidlike molecules at the pore wall, while the component at the intermediate temperature might result from pockets in the solid matrix or even a bimodal pore-size distribution. The average pore-size distributions obtained by NMR agree fairly well with those obtained by N₂ sorption. However, NMR gives a more detailed picture of the distribution, revealing two or three well-defined peaks. The peak at the smallest pore size, however, reflects the surface layer rather than a pore-size distribution.     

Water confined in mesoporous silica glasses: Influence of temperature on adsorption/desorption hysteresis loop and fluid structure

In natural as well as industrial processes, water is frequently confined in silica porous materials with pore sizes in the nanometer scale. Understanding the confinement effects on the fluid properties is a fundamental issue, helpful to optimize the industrial processes. The molecular simulation is a powerful tool to study complex polar fluid like water at the atomic scale. The water adsorption/desorption properties in a mesoporous silica glass are investigated by means of Grand Canonical Monte Carlo simulations (GCMC). The SPC and PN TrAZ potential are used to describe water-water and water-silica interactions. The numerical sample of mesoporous silica glass (pore size: 3.6nm) was obtained by off-lattice reconstruction, known to reproduce in a realistic way the geometrical complexity of high specific surface Vycor (pore size distribution, pore interconnections, etc). The intermolecular potential is shown to reproduce the experimental data at 300K (adsorption isotherm and isosteric heat of adsorption). The water structure is analyzed and confinement effects are emphasized. The temperature influence is studied: the hysteresis loop is shown to shrink with an increase in temperature.     

Preparation and characterization of carbamoylphosphonate(CMPO) silane grafted on various mesoporous silicas

The carbamoylphosphosphonate silane (CMPO analogue; 2-(diphenylphosphoryl)-N-(3-(triethoxysilyl)propyl) acetamide) modified mesoporous silica was prepared via a post-synthesis grafting method for the effective purification of rare earth elements. The guest CMPO analogue was synthesized by direct coupling reaction of 2-(diphenylphosphoryl) acetic acid and 3-(triethoxysilyl)propan-1-amine. Various mesoporous silicates such as MCM-41, SBA-15, or amorphous silica nanoparticles were adopted as host materials. The resulting surface-modified mesoporous materials were characterized with respect to their structural integrity, surface area, and pore size and the concentration of the CMPO silane species. These CMPO functionalized periodic mesostructured silicates offer the potential of applications as catalysts, sensors, or environmental sorbents.     

Does confined water exhibit a fragile-to-strong transition?

The dynamics of supercooled confined water has recently been shown to have a pronounced, apparent fragile-to-strong transition (FST). Here we use broadband dielectric spectroscopy (10^-2–10⁹ Hz) to study the dynamics of water confined in silica matrices MCM-41 C10 and C18, with pore diameter of 21.4 and 36.1 ?, respectively. The local dynamics of water molecules and the dynamics of the hydroxyl groups on the inner wall of the pores are followed up to over 240 K. We argue that the reported FST for confined water is due to the vanishing of the cooperative α relaxation, which implies that it should not be interpreted as a true FST.     

Solid state polymorphism of liquid crystals in confined geometries

Solid polymorphism of 4-alkyl-4'-cyanobiphenyl (nCB) was studied so far as a function of thermal history. In this paper we show that metastable solid phases of 4-octyl-4'-cyanobiphenyl (8CB) are also formed when the mesogens are confined in porous silica matrices and we study their structure by neutron diffraction and by Raman spectroscopy. Three metastable solid states are identified: one crystalline phase K', two frozen-in smectic-like phases K(s) and K'(s). We discuss the relation between the structure of the metastable solid phases and that of the mesomorph phases.     

Synthesis of hollow silica-sulfur composite nanospheres towards stable lithium-sulfur battery

Herein, porous hollow silica nanospheres were prepared via a facile sol-gel process in an inverse microemulsion, using self-assemblies of chiral amphiphile as a soft template and fine water droplets as a hard template. The shells of the hollow silica nanospheres are composed of flake-like nanoparticles with dense big holes on the surface. After covering a layer of sulfur on the silica nanospheres, followed by hydrothermal treatment in a D-glucose aqueous solution, silica-sulfur and silica-sulfur-carbon nanospheres were successfully fabricated. The silica-sulfur composites exhibit a stable capacity of 454 mAh g^?1 at current density of 335 mA g^?1 after 100 cycles with capacity retention of 85%, demonstrating a promising cathode material for rechargeable lithium-sulfur batteries. We believe that the approach for synthesis of porous hollow silica nanospheres and its carbon spheroidal shell can also be applicable for designing other electrode materials for energy storage.     

Universal nuclear spin relaxation and long-range order in nematics strongly confined in mass fractal silica gels

We show how the low-frequency dependence of the proton spin-lattice relaxation time T1(nu) of octylcyanobiphenyl liquid crystals confined in high-density silica gels evidences a long-range order nematic phase in spite of the strong confinement and random disorder of the gels. The universal value and frequency dependence observed, T1(nu) proportional, variant nu(2/3), is interpreted within a relaxation model due to director fluctuations in nematic liquid crystals confined to mass fractal porous media. The model provides a relation T1(nu) proportional, variant nu(2-d/2), giving a reliable value of the structural fractal dimension d(f)=2.67 for all the host silica gels.