Porous Silica Matrix Research Articles

Sonochemical deposition of platinum nanoparticles on polymer beads and their transfer on the pore surface of a silica matrix

This study reported the sonochemical deposition of platinum on the surface of polystyrene beads (PSBs) and the transfer of obtained Pt nanoparticles into a porous silica matrix using the PSB as a sacrificial template. Platinum nanoparticle deposition was ensured by the sonochemical reduction of Pt(IV) at room temperature in latex solutions containing polystyrene beads in the presence of formic acid under Ar or under Ar/CO atmosphere without any additives. After ultrasonic treatments for few hours, well dispersed Pt nanoparticles within the range of 3–5nm deposited on PSB were obtained in both studied conditions. Samples were then mixed with TEOS, dried, and heated at 450°C to ensure the PSB removal from the silica matrix. TEM and SEM results clearly show that final silica pore size is within the same order of magnitude than initial PSB. Finally, platinum decorated silica matrix with chosen pore sizes was successfully prepared.

HIGH DIELECTRIC CONSTANT COMPOSITE BASED ON CHLOROPHYLL A ENTRAPPED NANOPOROUS SILICA GEL

Chlorophyll a (naturally occurring Mg porphyrene) has been entrapped in nano/porous silica gel using sol–gel method at room temperature, producing a stable composite. HRTEM observation reveals regular nanoscale [around 15–20 nm diameter] distribution of aggregated polycrystalline chlorophyll a within porous silica matrix. UV-vis study also corroborates the presence of various aggregated chlorophyll a species within the system. Low field measurement shows almost 400 times enhancement of dielectric constant (1700) with incorporation of only 0.125 mg/ml of chlorophyll and the loss is 0.5 at room temperature at 100 Hz. The dielectric constant of the composite reaches 2500 as chlorophyll concentration becomes 1 mg/ml. Observed strong space charge response to the external field and strong frequency dispersion of the dielectric properties of the composite can be attributed to the long-range electron delocalization [nomadic polarization] in chlorophyll a aggregates. The electric modulus (M*) formalism used in this study enabled us to distinguish and separate various relaxation processes. It is found that with increasing chlorophyll concentration D.C. relaxation time decreases exponentially at room temperature. It is shown that observed relaxations do not perfectly follow the Debye response in high frequency region due to heterogeneous distribution of chlorophyll aggregates. The low values of room temperature activation energy calculated from Arrhenius plot reveal that polaronic hopping phenomena is absent at grain-interfacial region due to low thermal energy.

A label-free amperometric immunosensor for alpha-fetoprotein determination based on highly ordered porous multi-walled carbon nanotubes/silica nanoparticles array platform

In this contribution, we develop a novel type of multi-walled carbon nanotubes/silica nanoparticles array (MWCNTs/SiO2) composite by the galvanostatic deposition technique. In this process, MWCNTs were mixed with ammonium fluorosilicate to form a doped precursory sol solution, the electrochemically generated hydroxyl ions at negative potentials promote the hydrolysis of ammonium fluorosilicate, and the simultaneously generated hydrogen bubbles assist the formation of three-dimensional porous silica matrix, further facilitating the construction of SiO2 NPs array with uniform distribution, as confirmed by scanning electron microscopy. The formation of SiO2 NPs array provides a higher surface area and biocompatible microenvironment for retaining the native activity of the immobilized biomolecules. Further incorporation of MWCNTs into SiO2 NPs array improves the electronic conductivity and enhances the electroactive surface area of the film, making the fabricated three-dimensional porous MWCNTs/SiO2 electrode an ideal platform for further immobilization of alpha-fetoprotein (AFP), as a model protein. This method achieves ultrasensitive detection of AFP antigen with ferricyanide as a probe. The obtained results provided a linear response range from 0.1 to 30.0ngmL−1 AFP antigen with a lower detection limit of 0.018ngmL−1. This work implies that the biocompatible and controllable three-dimensional porous SiO2 NPs array possessed potential applications for biosensing.

Temperature dependent thermal conductivity of pure silica MEL and MFI zeolite thin films

This paper reports the temperature dependent cross-plane thermal conductivity of pure silica zeolite (PSZ) MFI and MEL thin films measured using the 3ω method between 30 and 315 K. PSZ MFI thin films were b-oriented, fully crystalline, and had a 33% microporosity. PSZ MEL thin films consisted of MEL nanoparticles embedded in a nonuniform and porous silica matrix. They featured porosity, relative crystallinity, and particle size ranging from 40% to 59%, 23% to 47%, and 55 to 80 nm, respectively. Despite their crystallinity, MFI films had smaller thermal conductivity than that of amorphous silica due to strong phonon scattering by micropores. In addition, the effects of increased relative crystallinity and particle size on thermal conductivity of MEL thin films were compensated by the simultaneous increase in porosity. Finally, thermal conductivity of MFI zeolite was predicted and discussed using the Callaway model based on the Debye approximation.

One-step immobilization of Ru(bpy)32+ in a silica matrix for the construction of a solid-state electrochemiluminescent sensor with high performance

An electrochemically induced sol-gel process has been used to efficiently immobilize Ru(bpy)(3)(2+) in a 3D porous silica film matrix deposited on a glassy carbon electrode (GCE), forming a solid state electro-chemiluminescence (ECL) sensor. In this approach, electrolysis of the GCE at cathodic voltages from a solution of ammonium fluorosilicate containing Ru(bpy)(3)(2+) results in the reduction of water to hydroxyl ions and hydrogen bubbles. The former product catalyzes the hydrolysis of ammonium fluorosilicate to form a silica film; while the hydrogen bubbles act as a dynamic template in forming a porous silica matrix. Therefore, a large quantity of Ru(bpy)(3)(2+) ions can be efficiently encapsulated in the porous silica matrix, and the formed porous structure offers a good mass transport path. The fabricated [Ru(bpy)(3)](2+) solid-state ECL sensor shows high sensitivity and stability towards the determination of tripropylamine (TPA). The electrochemically generated luminescence signal shows a good linear relationship to TPA concentration ranging from 3.46 × 10(-10) to 3.70 × 10(-6) M and 3.70 × 10(-6) to 3.60 × 10(-4) M, with an extremely low detection limit of 17 pM (S/N = 3). The present approach is effective for encapsulation of various molecules and could find wide application in the construction of various sensors.

Effect of Ultrasonic Irradiation on Preparation and Properties of Ionogels

Silica‐gel matrices containing ionic liquid (IL) 1‐butyl‐3‐methylimidazolium hexafluorophosphate viz. ionogels have been synthesized using one‐pot nonhydrolytic sol‐gel method and taking tetraethyl orthosilicate (TEOS) as starting precursor. Effect of ultrasonic irradiation on pore parameters of ionogels and vibrational properties of the IL upon confinement in the porous matrix has been investigated. The synthesized gels have been characterized by BET, DSC, TGA, and FTIR. BET analysis shows some changes in the pore parameters due to ultrasonic irradiation. DSC results indicate shift in glass transition temperature upon confinement of the IL. The FTIR spectra show changes in vibrational bands on confinement, particularly, the bands related to the imidazolium ring; aliphatic chain and anion PF6 − of the IL are found to shift upon confinement in porous silica matrix obtained due to ultrasonic irradiation. Ultrasonic irradiation has been found to affect the gelation dynamics and kinetics and pore parameters.

Density minimum in supercooled confined water

Recent papers in PNAS, here labeled I (1) and II (2), have proposed that when water confined in the porous silica matrix MCM-41-S with a 15-A pore diameter is supercooled, it reaches a density minimum at around 200 K. The evidence for this assertion is based on the observed intensity of the (100) Bragg diffraction peak, which arises from the hexagonal arrangement of the cylindrical pores, as a function of temperature. In I, it is further found that the density profile as function of temperature shows significant hysteresis between heating and cooling at different pressures. Both results are used to speculate on a possible liquid-liquid transition in supercooled confined water.

Nanocrystalline Pentaerythritoltetranitrate using Sol-Gel Process

The secondary explosives developed with reduced particle size tend to be more insensitive for mechanical stimuli and may release energy with faster rate and gaining more importance nowadays. Therefore, aiming to reduce the particle size of one of the popular explosives, viz., pentaerythritoltetranitrate (PETN) to the nanometer range, a method for preparation of nanocrystalline PETN in the silica (SiO2) gel matrix using sol-gel process has been demonstrated. The PETN-SiO2 xerogels were prepared containing PETN content ranging from 50 per cent to 90 per cent (w/w) and the xerogels were characterised using different techniques. An exothermic peak at around 185 oC preceded by an endotherm in thermal analysis accompanied with weight loss in the temperature range from 150 oC to 200 oC for the xerogel confirmed the presence of PETN in xerogel. Infrared spectra of xerogels showed peaks at around 1285 cm-1 and 1700 cm-1 assigned to O-NO2 and C-O bond representing PETN. Small angle x-ray scattering measurements on xerogels indicated that PETN entered in the pores of silica matrix. Transmission electron microscopy revealed that cystalline PETN with particle size of around 15 nm dispersed in silica xerogel. The specific surface area for the PETN-SiO2 (90:10) xerogels was found to be 75 m2/g. Defence Science Journal, 2011, 61(6), pp.534-539 , DOI:http://dx.doi.org/10.14429/dsj.61.594

Fabrication and Photoluminescent Properties of ZnO/Mesoporous Silica Composites Templated by a Chelating Surfactant

A novel anionic surfactant-templated synthesis of ZnO/mesoporous silica nanocomposites has been carried out by using N-hexadecylethylenediamine triacetate (HED3A), a triprotic surfactant, as the structure-directing agent. The chelating template can capture zinc ions in solution and then direct the mesophase formation, enabling an amount of zinc oxide to be embedded in the porous silica matrix during calcination. With variation of the molar ratio of Zn(2+) to HED3A in the template, a series of composites with different doping amounts were obtained after the removal of organic components. The variation of the zinc ion concentration in the initial template solution induces an evolution of the silica mesophase, presumably due to the change in electronegativity of the HED3A headgroup caused by the chelating effect. Spectroscopic studies show a strong host-guest interaction between the silica pore walls and ultrafine ZnO nanoparticles. The photoluminescence properties of the resulting composites exhibit a size-dependent light emission and quantum-confinement effect of ZnO, accompanied by an infrequent violet emission originating from the ZnO-SiO(2) interface.

Direct-writing of PbS nanoparticles inside transparent porous silica monoliths using pulsed femtosecond laser irradiation

Pulsed femtosecond laser irradiation at low repetition rate, without any annealing, has been used to localize the growth of PbS nanoparticles, for the first time, inside a transparent porous silica matrix prepared by a sol-gel route. Before the irradiation, the porous silica host has been soaked within a solution containing PbS precursors. The effect of the incident laser power on the particle size was studied. X-ray diffraction was used to identify the PbS crystallites inside the irradiated areas and to estimate the average particle size. The localized laser irradiation led to PbS crystallite size ranging between 4 and 8 nm, depending on the incident femtosecond laser power. The optical properties of the obtained PbS-silica nanocomposites have been investigated using absorption and photoluminescence spectroscopies. Finally, the stability of PbS nanoparticles embedded inside the host matrices has been followed as a function of time, and it has been shown that this stability depends on the nanoparticle mean size.

Colorimetric determination of copper(II) ions by filtration on sol–gel membrane doped with diphenylcarbazide

A simple and sensitive colorimetric method for the determination of trace copper(II) ions in aqueous solution has been developed using diphenylcarbazide (DPC) immobilized sol–gel matrices. To enhance the odds of chelating interactions between copper(II) ions and DPC, a peristaltic pump was utilized to drive copper(II) ions solution pass through the cellulose acetate/nitrate membrane, which was coated with DPC immobilized sol–gel matrices. The membrane was sandwiched in a home-made flow cell. The porous silica matrix serves as a preconcentrator, and enriches the trace copper(II) ions. Meanwhile, the filtration increases the chelating interaction probability between copper(II) ions and DPC, thereby improves the sensitivity significantly. After the formation of purple complex compound, the color changes of the DPC immobilized sol–gel membrane were recorded using a flatbed scanner. The images were digitized, and the red ( R), green ( G), and blue ( B) values were investigated. The colorimetric method provides a rapid and reliable determination of trace copper(II) ions with a detection limit as low as 0.16 μM and a kinetic range up to 1.6 μM in neutral medium. Moreover, the method shows good selectivity, and the color changes of the DPC immobilized sol–gel membranes are remarkable for the copper(II) ions detection even in the presence of other metal ions.

Ionic and electronic conductivities in carbon nanotubes – ionogel solid device

Easy immobilisation of carbon nanofillers in a solid state device featuring mesopores filled with ionic liquid (IL) is presented. Such a route allows a good dispersion of carbon nanotubes (CNTs) and carbon fibers in N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide, subsequently followed by the confinement of the whole within a porous silica matrix. We thus obtain herein a new type of hybrid ionogel within a solid device containing a percolating IL network as its real liquid state, as well as presenting a percolating carbon network. The process is carried out within a single step, without any chemistry for covalent grafting. We show that an IL with a non π-conjugated cation allows good dispersion of the carbon nanotubes, with a subsequent percolation for a loading level lower than 3.6 wt%. Together with the dynamic IL properties of ionogels, we thus present herein an inorganic-solid host containing carbon nanofillers, which gives the solid device ionic and electronic conductivities.

Thermal conductivity of pure silica MEL and MFI zeolite thin films

This paper reports the room temperature cross-plane thermal conductivity of pure silica zeolite (PSZ) MEL and MFI thin films. PSZ MEL thin films were prepared by spin coating a suspension of MEL nanoparticles in 1-butanol solution onto silicon substrates followed by calcination and vapor-phase silylation with trimethylchlorosilane. The mass fraction of nanoparticles within the suspension varied from 16% to 55%. This was achieved by varying the crystallization time of the suspension. The thin films consisted of crystalline MEL nanoparticles embedded in a nonuniform and highly porous silica matrix. They featured porosity, relative crystallinity, and MEL nanoparticles size ranging from 40% to 59%, 23% to 47% and 55 nm to 80 nm, respectively. PSZ MFI thin films were made by in situ crystallization, were b-oriented, fully crystalline, and had a 33% porosity. Thermal conductivity of these PSZ thin films was measured at room temperature using the 3ω method. The cross-plane thermal conductivity of the MEL thin films remained nearly unchanged around 1.02±0.10 W m−1 K−1 despite increases in (i) relative crystallinity, (ii) MEL nanoparticle size, and (iii) yield caused by longer nanoparticle crystallization time. Indeed, the effects of these parameters on the thermal conductivity were compensated by the simultaneous increase in porosity. PSZ MFI thin films were found to have similar thermal conductivity as MEL thin films even though they had smaller porosity. Finally, the average thermal conductivity of the PSZ films was three to five times larger than that reported for amorphous sol-gel mesoporous silica thin films with similar porosity and dielectric constant.

Radiolysis of Water Confined in Porous Silica: A Simulation Study of the Physicochemical Yields

We investigate the radiolysis of liquid water confined in a porous silica matrix by means of an event-by-event Monte Carlo simulation of electron penetration in this composite system. We focus on the physical and physicochemical effects that take place in the picosecond range, before the radicals start to diffuse and react. We determine the radiolytic yields of the primary species for a system made of cylindrical pores filled with water over a wide range of pore radii $R_{\rm C}$. We show that the relative position of the conduction band edge $V_0$ in both materials plays a major role in the radiolysis of composite systems. Due to its lower $V_0$ as compared to that of silica, water acts as a collector of low-energy electrons, which leads to a huge enhancement of the solvated electron yields for $R_{\rm C} \leq$ 100 nm. The confinement has also a marked effect on the spatial distribution of the radicals, which become isolated in a very large number of pores as $R_{\rm C}$ decreases.

Properties of Ionic Liquid Confined in Porous Silica Matrix

Porous silica matrices of different pore sizes with confined ionic liquid (1-butyl-3-methylimidazolium hexafluorophosphate) [BMIM] [PF(6)] were prepared by sol-gel technique using a tetraethyl orthosilicate (TEOS) precursor with an aim to study the changes in physico-chemical properties of ionic liquid on confinement. It is found that on confinement 1) melting point decreases, 2) fluorescence spectra shows a red shift and 3) the vibrational bands are affected particularly those of imadazolium ring, which interacts more with the walls of the silica matrix. Preliminary theoretical calculations suggest that SiO(2) matrix interact more with the heterocyclic group of [BMIM] cation than the tail alkyl chain end group resulting in significant changes in the aromatic vibrations.

Functional Composite Membranes Based on Mesoporous Silica Spheres in a Hierarchically Porous Matrix

We demonstrate a synthetic strategy toward functional, composite porous materials by incorporating functionalized porous silica spheres into a hierarchically porous silica matrix. Monodisperse mesoporous silica spheres decorated with organic probe molecules for visual Cd2+ ion detection were coated with a thin protecting poly(methyl methacrylate) (PMMA) layer to form core−shell particles. When these particles were redispersed in a mixture of a silica precursor, surfactant and hydrochloric acid, hierarchically structured composite films could be prepared by spin-coating and aging. The PMMA capsule protected the mesoporous silica spheres from being exposed to the silica precursor in the assembly step, which could have blocked the mesopores and/or reacted with the surface functional species. Once the polymer and surfactant components were removed by extraction while preserving the organic probe molecules within the silica spheres, composite porous structures were derived in which the mesoporous silica sphere...

Design of photochemical materials for carbohydrate production via the immobilisation of whole plant cells into a porous silica matrix

Photochemical materials that act as bioreactors by exploiting the photosynthesis mechanism have been fabricated by entrapping whole plant cells within a porous silica matrix. The immobilisation step has been achieved via the in situ co-polymerisation of an aqueous silica precursor and a biocompatible trifunctional silane directly around cells. The cells remain undivided whilst the photochemical activity of the cells is well preserved over time. The design of a photochemical material that can act like a leaf, converting water into O2 and produce valuable organic compounds from CO2 under light irradiation is described. In particular, the increased excretion of polysaccharides by this photochemical material has been highlighted. The organic compounds formed have been extracted and analysed. The success of this work could open the door to new exciting photochemical materials with long-term photosynthetic activity and stability and to new green chemical processes for the conversion of solar energy into chemical energy with a concomitant reduction in CO2.

Photocatalytic activity of mesoporous films based on N-doped TiO2 nanoparticles

TiO2 based photocatalytic coatings are currently restricted to outdoor applications due to the requirement of UV light. Nitrogen doping of TiO2 was used here to activate them in indoor conditions for self-cleaning applications. We developed an original process, allowing nitridation of anatase nanoparticles dispersed within a porous silica matrix, at high temperatures (up to 800 °C), with neither structural change, nor aggregation and growth of the nanoparticles. After chemical dissolution of the silica, N-doped TiO2 particles were dispersed in water allowing the preparation of both concentrated colloidal aqueous dispersions and transparent sol–gel films in which N-doped particles are dispersed in a sol–gel silica binder with a controlled porosity. The N-doped TiO2 nanoparticles present visible light photocatalytic activity due to the presence of bulk nitrogen species. We showed that the photocatalytic activity of the films can be improved by an additional oxidative thermal treatment suppressing the Ti3+ ions which act as recombination centers in TiO2. We thus succeeded in preparing coatings twenty times more active than the reference under irradiation at 390 nm by monitoring the degradation of the Rhodamine 6G dye.

Influence of Surface Effects on Magnetic Behavior of Hematite Nanoparticles Embedded in Porous Silica Matrix

Unique magnetic properties of superparamagnetic iron oxide (hematite) nanoparticles prepared by nanocasting in a periodic nanoporous silica (PNS) matrix are described in this work. The nanoparticles were prepared via a novel approach, when the external surface of PNS was modified to become more hydrophobic. The prepared composite sample was characterized by the synchrotron related techniques small-angle X-ray scattering (SAXS), wide-angle X-ray scattering (WAXS), and X-ray absorption near edge spectra (XANES) measurements, by nitrogen adsorption/desorption, and by high resolution transmission electron microscopy (HRTEM). The investigation of the magnetic properties by superconducting quantum interference device (SQUID) magnetometry at the temperatures 2−300 K shows the superparamagnetic relaxation of the particles with a blocking temperature of TB ∼32 K. Below TB, the ferromagnetic interactions are present as suggested by coercivity HC ∼1900 Oe. The value of magnetic moment mP = 296 μB of the hematite particle was estimated by distribution of Langevin functions. This magnetic moment originates in uncompensated surface spins of Fe3+ ions due to the small size of hematite particles (5 nm) and due to the loading of nanoparticles into silica matrix. The magnetic behavior of the Fe2O3@PNS nanocomposite is mainly related to the surface effects (spin canting and different surface to volume spin ratio).

Dielectric Relaxations in Confined Hydrated Myoglobin

In this work we report the results of a broadband dielectric spectroscopy study on the dynamics of a globular protein, myoglobin, in confined geometry, i.e. encapsulated in a porous silica matrix, at low hydration levels, where about only one or two water layers surround the proteins. In order to highlight the specific effect of confinement in the silica host, we compared this system with hydrated myoglobin powders at the same hydration levels. The comparison between the data relative to the two different systems indicates that geometrical confinement within the silica matrix plays a crucial role in protein-water dielectric relaxations, the effect of sol-gel encapsulation being essentially a suppression of cooperative relaxations that involve the coherence/cooperativity of solvent motions and solvent-coupled protein dynamics. We also provide direct evidence that protein relaxations inside the gel depend on the hydration level and are "slaved" to the solvent beta-relaxation.