The solution flow through the nanochannel of MCM-41: a spin-probe study

A spin-probe electron spin resonance (ESR) study was made on the alcoholic solution flowing through a quartz column packed with MCM-41 to clarify the dynamics of the liquid molecules in the nanochannel. The ESR spectra of a few hydrophobic spin probes showed that they undergo rotational diffusion preferentially along the longest molecular axis, indicating that the nanochannel is effectively narrowed further for these radicals by the influence of the solvent. Since almost identical ESR spectra were observed for the static samples, which were prepared in vacuo by introducing the solutions into the quartz tube with the MCM-41 powder and sealing off, the solution in the above-mentioned experiment should really flow through the nanochannel of MCM-41. Although a laminar flow is expected from the classical theory, the calculated flow rate is almost zero. In addition, the duration for the spin-probe molecules to flow through the column was basically not dependent on their affinity to the silica surface. To explain all these phenomena, we propose a model that the liquid molecules flow collectively by slipping on the surface of the nanochannel.     

Comparison of silver nanoparticles confined in nanoporous silica prepared by chemical synthesis and by ultra-short pulsed laser ablation in liquid

Hexagonally ordered mesoporous silica materials, MCM-41 and SBA-15, have been synthesized and loaded with Ag nanoparticles, utilizing both chemical synthesis and ultra-short pulsed laser ablation in liquid. In laser ablation, a silver target, immersed in aqueous suspension of ordered mesoporous silica SBA-15, was irradiated by ultra-short laser pulses to generate silver nanoparticles. For comparison, samples of similar silver contents were prepared either by incorporating silver into the SBA-15 during a hydrothermal synthesis or by introducing silver in MCM-41 by template ion-exchange. Samples were characterized by XRD, N₂ physisorption, TEM and UV–vis spectroscopy. All preparations contained significant amount of 5–50 nm size silver agglomerates on the outer surface of the silica particles. The laser ablation process did not cause significant destruction of the SBA-15 structure and metallic silver (Ag⁰) nanoparticles were mainly generated. It is demonstrated that by laser ablation in aqueous silica suspension smaller and more uniform metallic silver particles can be produced and loaded on the surface of the silica support than by synthesis procedures. Catalytic properties of the samples have been tested in the total oxidation of toluene. Because of its favorable Ag dispersity, the Ag/SBA-15 catalyst, generated by the laser ablation method, had better catalytic stability and, relative to its Ag load, higher activity than the conventional Ag/SBA-15 preparations.     

CP MAS Kinetics Study of Ionic Liquids Confined in Mesoporous Silica: Convergence of Non-Classical and Classical Spin Coupling Models

The high data point density measurements of ¹H→¹¹B cross-polarization (CP) kinetics upon magic-angle spinning (MAS) of [bmim][BF₄] confined in mesoporous SBA-15 and MCM-41 were carried out. The complex shaped ¹¹B CP MAS signals were observed in both silica and decomposed into two Lorentz components. This points towards the possibility of bimodal distribution of [bmim][BF₄] in the studied confinements. The convergence of classical and non-classical spin coupling models was deduced processing CP kinetic curves. A good fit of the theoretical curves to the experimental data was achieved using both models without any non-random deviations between theory and experiment to appear. The convergence of spin coupling models was discussed in terms of relatively high mobility of BF₄ ^? anion respect to the cation and the dynamics of anions in pores. These factors delete the borders between spin clusters. The spin diffusion along the pore surfaces in MCM-41 is more than twice faster than in SBA-15.     

Optical oxygen sensing materials based on a novel dirhenium(I) complex assembled in mesoporous silica

A new dirhenium(I) complex fac-[{Re(CO)₃(4,7-dinonadecyl-1,10-phenanthro -line)}₂ (4,4′-bipyridyl)] (trifluoromethanesulfonate)₂ (denoted as D-Re(I) ) is assembled in MCM-41 and SBA-15 type mesoporous silica support. The emission peaks of D-Re(I) in D-Re(I)/MCM-41 and D-Re(I)/SBA-15 are observed at 522 and 517 nm, respectively. Their long excited lifetimes, which are of the order of microseconds, indicate the presence of phosphorescence emission arising from the metal to ligand charge-transfer (MLCT) transition. The luminescence intensities of D-Re(I)/MCM-41 and D-Re(I)/SBA-15 decrease remarkably with increase in the oxygen concentration, meaning that they can be used as optical oxygen sensing materials based on luminescence quenching. The ratios I₀/I₁₀₀ of D-Re(I)/MCM-41 and D-Re(I)/SBA-15 are estimated to be 5.6 and 20.1, respectively. The obtained Stern-Volmer oxygen quenching plots of the mesoporous sensing materials could be fitted well to the two-site Demas model and Lehrer model.     

Applicability of classical methods of pore size analysis for MCM-41 and SBA-15 silicas

Two most popular ordered mesoporous silicas, MCM-41 and SBA-15, exhibiting uniform mesopores of approximately cylindrical shapes, have been used as model adsorbents for verification, improvement and/or development of adsorption-based methods for characterization nanoporous materials. While the applicability of the classical methods for pore size analysis was widely examined by employing MCM-41 materials, the large-pore materials such as SBA-15 did not find adequate usage for this type of studies. The current work addresses the issue of applicability of classical methods such as Barrett-Joyner-Hallenda (BJH) and Broekhoff-de Boer (BdB) methods for pore size analysis of mesoporous silicas by using MCM-41 and SBA-15 materials as model adsorbents. In addition, the Kruk-Jaroniec-Sayari (KJS) method, which is based on the BJH algorithm and experimental relations for the pore width and statistical film thickness, is discussed too. While the MCM-41 materials cover the range of small mesopores (about 2-7 nm), the inclusion of SBA-15 materials allowed us to examine the range of the pore diameters up to about 12 nm. The high quality MCM-41 and SBA-15 samples are used to discuss the applicability and limitations of the aforementioned characterization methods and to propose some recommendations for pore size analysis of these materials.     

Research progress of nanomaterial-mediated photodynamic therapy in tumor treatment

Journal of Nanoparticle Research - In modern methods of synthesis of nanostructured materials based on mesoporous silica matrices (MCM-41, SBA-15), it is not possible to simultaneously fine adjust...     

ESR and TPD Investigations of the Adsorption of Di-tert-butyl Nitroxide on Au(111) and NiO(111). Evidence for Long-Range Interactions

Electron spin resonance and temperature-programmed desorption spectra of thin layers of DTBN (di-tert-butyl nitroxide) adsorbed on Au(111) and NiO(111)/Au(111) surfaces have been measured. The temperature-programmed desorption data show a weak chemisorption of the DTBN molecules in the monolayer on both surfaces. On Au(111) as well as on NiO(111)/Au(111), the ESR signal from monolayer coverages is totally suppressed. This suppression continues into the multilayer regime on both substrates. Disturbances of the substrate/adsorbate interface have a strong influence on the range of the signal suppression. Possible reasons for this behavior are discussed.     

Naproxen drug delivery using periodic mesoporous silica SBA-15

In this paper, we present the release of naproxen from hexagonal periodic mesoporous silica SBA-15, which serves as a drug delivery system. Naproxen, the well-known nonsteroidal anti-inflammatory drug (NSAID), was loaded into the pores of SBA-15 silica modified with aminopropyl groups. The physicochemical properties of the modified sample (A-SBA-15/napro) were compared with the unmodified SBA-15 mesoporous silica loaded with the drug (SBA-15/napro). The kinetic of the naproxen release into the physiological solution was studied. The released amount of naproxen represented 90.7% from the unmodified SBA-15 in 72 h, while from the sample A-SBA-15/napro the released amount represented about 80.9%. The prepared materials were characterized by nitrogen adsorption/desorption, Small angle X-ray scattering (SAXS), Fourier-transform infrared spectroscopy (FT-IR) and the thermoanalytical methods (TG/DTA). Thin layer chromatography (TLC) was used for quantitative determination of the released naproxen.     

Modification of functionalized mesoporous silica on the formation and the catalytic performance of platinum nanocatalysts

Several functionalized mesoporous silicas (MCM-41 and SBA-15 terminated with amino groups by multistep graft) are prepared and used as carriers for platinum nanoparticles. Modification of the grafted molecules is investigated on the component of the silicas and the formation of nanoparticles. Catalytic performance of the loaded platinum nanocatalysts is tested on selective hydrogenation of ortho-chloronitrobenzene (o-CNB). Results indicate that SBA-15 can be grafted with more amino groups than MCM-41. In most cases, small quantity of platinum precursor benefits the formation of small platinum nanoparticles on the functionalized silicas. The platinum nanocatalysts loaded on the one-step grafted silicas hold better catalytic activity and slightly lower selectivity to ortho-chloroaniline (o-CAN) than those loaded on the two-step grafted silicas. Besides the advantage of reuse, the loaded platinum catalysts on the functionalized silicas have similar catalytic property as the colloidal platinum nanocatalysts.     

Structure of MnO nanoparticles embedded into channel-type matrices

X-ray diffraction experiments were performed on MnO confined in mesoporous silica SBA-15 and MCM-41 matrices with different channel diameters. The measured patterns were analyzed by profile analysis and compared to numerical simulations of the diffraction from confined nanoparticles. From the lineshape and the specific shift of the diffraction reflections it was shown that the embedded objects form ribbon-like structures in the SBA-15 matrices with channels diameters of 47–87 Å, and nanowire-like structures in the MCM-41 matrices with channels diameters of 24–35 Å. In the latter case the confined nanoparticles appear to be narrower than the channel diameters. The physical reasons for the two different shapes of the confined nanoparticles are discussed.     

Oxidation of ferrocene molecules adsorbed in MCM-41

A mesoporous silica material MCM-41 was synthesized under two different conditions, and ferrocene molecules were adsorbed into one-dimensional pores formed in MCM-41. The pore sizes were determined to be 0.9 and 2.8 nm for the MCM-41 synthesized under an acidic and a basic condition, respectively. The formation of paramagnetic ferricinium ions was observed after the adsorption into pores by ESR, magnetic susceptibility and ⁵⁷Fe Mössbauer measurements. Spin densities attributable to ferricinium ions in the pores are estimated to be 0.74 and 0.065 in MCM-41 synthesized under an acidic and a basic condition, respectively. It was shown that the oxidation of ferrocene molecules to ferricinium ions in MCM-41 prepared in an acid solution is faster than that in base at ambient temperature and pressure.     

Wide range humidity sensing of LiCl incorporated in mesoporous silica circular discs

Lithium chloride (LiCl) incorporated MCM-41 has been synthesised by sol-gel method using tetraethyl orthosilicate as a precursor in basic medium. 5, 10, 15, 20, 25, 30 and 35 wt% of LiCl were incorporated in mesoporous silica to investigate the humidity sensing. With increasing wt% of LiCl broadening of O–H peak is observed in the Fourier Transform Infrared spectra, indicating greater adsorption of hydroxyl groups on porous silica. The surface area of the MCM-41 circular discs was determined by Brunauer?Emmett?Teller (BET). Scanning electron microscopy images suggest that incorporation of LiCl leads to coalescence of grains in mesoporous silica. 25 wt% LiCl incorporated MCM-41 showed a wide range linear response of impedance change for 11%–90% RH exhibiting 3.5-order drop in impedance at a 1 kHz frequency. The Nyquist plots for all compositions showed increased ionic conduction with increasing relative humidity.     

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.     

Partition coefficient of small spin probes: An improved ESR determination method

We report on the improved determination of the partition coefficient,P _e, of the small hydrophobic spin probe di-tert-butyl nitroxide (DTBN) between the aqueous and the fluid hydrophobic phase of dipalmitoylphosphatidyl choline (DPPC) multilayers, made in terms of the second integral evaluation of the isotropic ESR resonance lines. The calculation of the second integral was carried out taking into account both the width and amplitude of the resonance lines and a shape factor related to the degree of inhomogeneous line broadening. The expression ofP _c obtained in this way has been tested by computer simulation using Voightian lineshapes for the ESR absorption resonance lines. The method has been applied to the determination of the degree of cooperativity of the gel → liquid-crystal phase transition of DPPC multilamellae, and the results were compared with those obtained using the traditional expression ofP _e. Finally, the calculation of the cooperativity parameter of the main phase transition of DPPC multilayers results improved of about 70% by using the new expression ofP _c.     

Fluorescence properties of riboflavin-functionalized mesoporous silica SBA-15 and riboflavin solutions in presence of different metal and organic cations

Riboflavin was covalently linked to mesoporous SBA-15 silica surface via grafting technique. Then fluorescence properties of the system obtained were analyzed in the presence of several metal and organic cations. Both quenching and strengthening of fluorescence as well as significant changes in the maximum fluorescence wavelength were observed. The results were compared with absorption and fluorescence data obtained for riboflavin water solutions.     

Capacitance properties of ordered porous carbon materials prepared by a templating procedure

The electrochemical performance of carbon materials with a highly ordered nanoporous structure is investigated in two-electrode supercapacitors. The materials were prepared by a templating procedure using a silica matrix (type MCM-48 or SBA-15) with an organized porosity in which carbon was inserted, either by chemical vapor decomposition of propylene or by impregnation with a sucrose solution followed by carbonisation. After the removal of silica, a micro-mesoporous carbon residue is recovered which displays an uniform pore size distribution. Such a well-defined nanostructure is interesting for a fundamental study of the double layer capacitance behavior. The performance of supercapacitors built with electrodes prepared from the templated carbon was tested in acidic, alkaline and organic electrolyte solutions. High values of capacitance in aqueous and organic media were obtained with a rectangular shape of the voltammograms over a wide range of scan rates indicating a quick charge propagation. Especially, the templated carbons prepared by the impregnation of sucrose in MCM-48 display high capacitance values due to the formation of an adequate micro-mesoporous network during their formation. A marked shift of capacitance drop at higher values of frequency is clearly observed for the materials rich in mesopores; the mesopores make easier the diffusion of the ions to the active surface.     

The aqueous reference for ESR oximetry

The interaction of molecular oxygen with derivatives of nitroxide EPR spin labels has been investigated using nuclear spin-relaxation spectroscopy in aqueous and nonaqueous solvents. The proton spin-lattice relaxation rate induced by oxygen provides a measure of the local concentration of oxygen, which we find is dependent on solvent. In water, the hydrophobic effect increases the local concentration of oxygen in the nonpolar portions of solute molecules. For nitroxides reduced to the hydroxylamine in aqueous solutions, we find that the local concentration of oxygen is approximately twice that associated with a free diffusion hard sphere limit, while in octane, this effect is absent. These results show that nitroxide based ESR oximetry may suffer a reference concentration shift of order a factor of two if the aqueous nitroxide spectrum or relaxation is used as the reference.     

Cu(II) recognition materials: Fluorophores grafted on mesoporous silica supports

There were designed and synthesized naphthalene and pyrene derivatives consisting of fluorophore group and of receptor fragment with donor N and O atoms. These fluorosensors were covalently attached by grafting carboxyl group to surfaces of silica xerogel or mesoporous silicas (MCM-41 and MCM-48) functionalized either with 3-aminopropyl or 3-glycidoxypropyl groups. The pyrene derivatives 2 and 3 covalently grafted on MCM-48 silica functionalized with 3-aminopropyl groups are potential recognition elements of a fluorescence chemical sensor.Fluorescence emission of the prepared recognition materials is quenched specifically owing to photoinduced electron transfer (PET) effect after coordination reactions with Cu(II) ions. Moreover, both the materials exhibit selectivity for Cu(II) ions in aqueous solutions in presence of such metal ions as: alkali, alkaline earth and transition. During UV irradiation the studied recognition elements undergo slowly photochemical degradation.     

pH自旋探针分子的ESR波谱性质和量子化学研究─pH自旋探针应用研究之二

系统研究了咪唑类氮氧自由基作为pH自旋探针的ESR波谱特性．用量子化学方法AM1研究了pH自旋探针分子的几何结构、电子结构及电子自旋密度分布．aN理论值与实验结果基本一致．得到了pka值与电子密度关系曲线．指出了它们在生命科学中的应用前景．     

Thermal conductivity of a kind of mesoporous silica SBA-15

Mesoporous silica SBA-15 consists of uniform hexagonal, unconnected cylindrical channels with diameters that can be tuned within a range of 1.5 nm-30 nm, and is thought to have a special thermal conductivity. The theoretical investigation of the shell thermal conductivity of the mesoporous silica is performed in the relaxation time approximation in this paper and an available one-dimensional heat transfer model is used to predict the effective thermal conductivity （ETC） of the mesoporous silica. The experimental result of the ETC is also presented for comparison. The shell thermal conductivity of the mesoporous silica decreases with mesochannel radius increasing or wall thickness decreasing, but does not strictly decrease with porosity increasing. The thermal radiation possibly plays a primary role in heat transfer at the large porosity scale. The predicted ETC of SBA-15 with only conduction considered is quite low at the large porosity, even lower than the thermal conductivity of the silica aerogels. To realize it, doping carbon or other matters which can strongly absorb infrared light into SBA-15 is a possible way.