Methodology for the immobilization of enzymes onto mesoporous materials

Cytochrome c and xylanase were adsorbed onto two mesoporous materials, SBA-15 (a pure silicate) and MSE (an organosilicate), with very similar physical properties but differing chemical compositions. A methodical order was developed whereby the influences of surface area, pore size, extent of order, particle size, surface potentials, isoelectric points, pH, and ionic strength on immobilization were explored. In silico studies of cytochrome c and xylanase were conducted before any immobilization experiments were carried out in order to select compatible materials and probe the interactions between the adsorbents and the mesoporous silicates. The stabilities of the mesoporous materials at different pH values and their isoelectric points and zeta potentials were determined. Electrostatic attraction dominated protein interactions with SBA-15, while weaker hydrophobic interactions are more prominent with MSE for both cytochrome c and xylanase. The ability of the immobilized protein/enzyme to withstand leaching was measured, and activity tests and thermostability experiments were conducted. Cytochrome c immobilized onto SBA-15 showed resistance to leaching and an enhanced activity compared to free protein. The immobilized cytochrome c was shown to have higher intrinsic activity but lower thermostability than free cytochrome c. From an extensive characterization of the surface properties of the silicates and proteins, we describe a systematic methodology for the adsorption of proteins onto mesoporous silicates. This approach can be utilized in the design of a solid support for any protein.     

Effects of Morphology and Structural Characteristics of Ordered SBA-15 Mesoporous Silica on Release of Ibuprofen

Three kinds of highly ordered SBA-15 mesoporous materials with different pore sizes and morphologies denoted as LPS-SBA-15 (stick-like with pore size 7.28 nm), CPS-SBA-15 (stick-like with pore size 5.96 nm) and T-SBA-15 (tablet-like with pore size 4.64 nm) have been prepared, characterized and employed as carrier materials. The release behaviors of the ibuprofen in a simulated body fluid from these mesoporous silica materials were studied. The influences of pore size and exterior morphologies of mesoporous silica on the release behaviors of ibuprofen have been investigated. It has been found that the release becomes fast with increasing of pore size and slow with extending of transport pathway, and that the release rate of ibuprofen from the three kinds of SBA-15 is LPS-SBA-15 > T-SBA-15 > CPS-SBA-15. The results show that the inner structure as well as the exterior morphologies of SBA-15 mesoporous silica can seriously affect the release behaviors of ibuprofen.     

Rapid synthesis of SBA-15 rods with variable lengths, widths, and tunable large pores

Dispersed SBA-15 rods have been synthesized with varying lengths, widths, and pore sizes in a low-temperature synthesis in the presence of heptane and NH(4)F. The pore size of the material can systematically be varied between 11 and 17 nm using different hydrothermal treatment times and/or temperatures. The particle length (400-600 nm) and width (100-400 nm) were tuned by varying the HCl concentration. All the synthesized materials possess a large surface area of 400-600 m(2)/g and a pore volume of 1.05-1.30 cm(3). A mechanism for the effect of the HCl concentration on the particle morphology is suggested. Furthermore, it is shown that the reaction time can be decreased to 1 h, with well-retained pore size and morphology. This work has resulted in SBA-15 rods with the largest pore size reported for this morphology.     

Effects of surface curvature and surface chemistry on the structure and activity of proteins adsorbed in nanopores

The interactions of proteins with the surface of cylindrical nanopores are systematically investigated to elucidate how surface curvature and surface chemistry affect the conformation and activity of confined proteins in an aqueous, buffered environment. Two globular proteins, lysozyme and myoglobin, with different catalytic functions, were used as model proteins to analyze structural changes in proteins after adsorption on ordered mesoporous silica SBA-15 and propyl-functionalized SBA-15 (C(3)SBA-15) with carefully controlled pore size. Liquid phase ATR-FTIR spectroscopy was used to study the amide I and II bands of the adsorbed proteins. The amide I bands showed that the secondary structures of free and adsorbed protein molecules differ, and that the secondary structure of the adsorbed protein is influenced by the local geometry as well as by the surface chemistry of the nanopores. The conformation of the adsorbed proteins inside the nanopores of SBA-15 and C(3)SBA-15 is strongly correlated with the local geometry and the surface properties of the nanoporous materials, which results in different catalytic activities. Adsorption by electrostatic interaction of proteins in nanopores of an optimal size provides a favorably confining and protecting environment, which may lead to considerably enhanced structural stability and catalytic activity.     

Bifunctionalized SBA-15 as a novel micropipette tip sorbent for selective removal and enrichment of biomolecules

Selective enrichment or removal of specific proteins prior to analysis can minimize nonspecific interferences as well as information loss, which has been an important issue in current proteomics fields. In this work, two kinds of mesoporous silica SBA-15 (mean pore diameter of 5 nm and 7 nm), bifunctionalized with quaternary ammonium and C18 groups via silylanization reaction, was used to investigate the adsorption behavior for different proteins (bovine serum albumin (BSA), ovalbumin (OVA), hemoglobin (Hb), lysozyme (Lys) and cytochrome c (cyt c)). As possessing anion exchange (quaternary ammonium) groups, the bifunctionalized SBA-15 showed selective adsorption of the negative charged proteins, BSA and OVA. Based on these properties, sequential fractionation based on different SBA-15 materials as micropipette tip sorbents was developed for sample preparation prior to protein analysis. As expected, after the sequential treatment of the sample, the detection signal of the high abundant BSA was significantly decreased and that of the low abundant cyt c was obviously enlarged, which solved the problem that low abundant protein was suppressed by adjacent high abundant protein. The sample preparation technique significantly improved protein identification and this sequential fractionation approach could be potentially applied to extend information on the protein identification for biological samples with a wide dynamic range.     

片/板状短孔道SBA-15的控制合成及应用

有序介孔材料家族中,SBA-15因具有较大的孔径尺寸、规则可调的孔道结构以及卓越的机械和水热稳定性而备受关注。最近,片/板状短孔道SBA-15由于更有利于分子快速扩散和质量传递成为介孔材料研究领域的热点。目前合成片/板状短孔道SBA-15的主要方法有有机物辅助、金属离子诱导及多重模板剂协同效应。根据不同类别添加剂的作用机制,本文综述了片/板状短孔道SBA-15的控制合成方法及其在催化、吸附和生物分子固定等不同领域的最新进展,并对其应用前景进行了展望。     

Pore size distribution analysis of selected hexagonal mesoporous silicas by grand canonical Monte Carlo simulations

We combine here a regularization procedure with individual adsorption isotherms obtained from grand canonical Monte Carlo simulations in order to obtain reliable pore size distributions. The methodology is applied to two hexagonal high-ordered silica materials: SBA-15 and PHTS, synthesized in our laboratory. Feasible pore size distributions are calculated through an adaptable procedure of deconvolution over the adsorption integral equation, with two necessary inputs: the experimental adsorption data and individual adsorption isotherms, assuming the validity of the independent pore model. The application of the deconvolution procedure implies an adequate grid size evaluation (i.e., numbers of pores and relative pressures to be considered for the inversion, or kernel size), the fulfillment of the discret Picard condition, and the appropriate choice of the regularization parameter (L-curve criteria). Assuming cylindrical geometry for both porous materials, the same set of individual adsorption isotherms generated from molecular simulations can be used to construct the kernel to obtain the PSD of SBA-15 and PHTS. The PSD robustness is measured imposing random errors over the experimental data. Excellent agreement is found between the calculated and the experimental global adsorption isotherms for both materials. Molecular simulations provide new insights into the studied systems, pointing out the need of high-resolution isotherms to describe the presence of complementary microporosity in these materials.     

有机胺修饰具有较大孔径介孔材料的二氧化碳吸附性能

以非离子表面活性剂P123为模板剂,正硅酸甲酯为硅源,通过加入不同的扩孔剂制得具有较大孔径的SBA-15类介孔材料,并采用粉末X射线衍射(XRD)、低温氮气吸附-脱附、扫描电镜(SEM)、傅里叶变换红外(FTIR)光谱等手段对所得样品进行了表征.加入扩孔剂可以明显增大介孔材料的孔容和孔径,而异辛烷为扩孔剂的扩孔效果明显优于四氯化碳.经四乙烯五胺(TEPA)镀饰后,这些样品均表现出良好的CO2吸附性能.其中对于除去模板剂后再镀胺的样品,其CO2吸附能力与介孔材料孔道结构关系不大,而对于未除模板剂的原粉镀胺样品,CO2吸附能力则随孔道的变大而增强.此外,通过吸附等温线和CO2-程序升温脱附(TPD)手段比较了温度和压力对CO2吸附的影响,发现在较高温度下吸附时CO2的吸附能力随压力的变化存在显著差别,因而在这类TEPA修饰的介孔材料上可通过变压吸附的途径来实现对环境气流中CO2的吸附和分离.     

富含羧基的球形介孔分子筛SBA-15的合成及药物释放性能

本文采用原位合成法合成了富含羧基的SBA-15球形介孔分子筛. 研究了修饰剂（三烷氧基氰乙基硅烷 CTES）的用量对介孔分子筛SBA-15形貌、孔径及BET比表面积的影响. 用粉末X射线衍射(XRD)、扫描电镜(SEM)、红外（IR）和氮气吸附/脱附对样品进行了详细的表征. 该材料展示了尺寸在0.5-1 μm规则的球形形貌、有序的二维六方相介孔结构、较大的比表面积和孔容、并且随着修饰剂用量的增加, SBA-15的孔径变小, 比表面积下降. 药物组装及缓释性能测试表明, 该材料具有较好的药物组装及缓释释放性能. 该材料在催化、药物载体和色谱分析填料等领域将具有潜在的应用.     

Modification of MCM-41 and SBA-15 mesoporous silicas by imidazolium ionic liquids

MCM-41 and SBA-15 mesoporous molecular sieves are modified with imidazolium ionic liquid (IL) via both physical adsorption and covalent grafting. The considerable effect of IL grafting on the porous structure and the particle morphology of mesoporous supports is shown. The pore size of these supports is found to be a key parameter determining the possibility of loading IL into such support materials.     

Adsorption of C7 hydrocarbons on biporous SBA-15 mesoporous silica

In our recent studies (Vinh-Thang, H.; Huang, Q.; Eic, M.; Trong-On, D.; Kaliaguine, S. Langmuir 2005, 21, 2051-2057; Vinh-Thang, H.; Huang, Q.; Eic, M.; Trong-On, D.; Kaliaguine, S. Stud. Surf. Sci. Catal. 2005, in press), a series of synthesized SBA-15 materials were characterized using nitrogen adsorption/desorption isotherms at 77 K and SEM images. In the present paper, four of them (MMS-1-RT, MMS-1-60, MMS-1-80, and MMS-5-80) were further investigated with regard to their equilibrium characteristics using n-heptane and toluene as sorbates by the standard gravimetric technique. SBA-15 materials proved to have a broad pore size distribution within the micropore/small-mesopore range in the walls of their main mesoporous channels. The adsorption capacities for toluene were found to be higher than for n-heptane. The isosteric heats of adsorption, estimated by the Clausius-Clapeyron equation, are also higher for toluene compared to n-heptane. They were found to depend on framework microporosity of the relevant SBA-15 samples. The isosteric heats of adsorption for all sorbates decrease with increased loading and approach the heats of evaporation of the respective sorbate. The adsorption capacities of SBA-15 samples are significantly higher than those of silicalite, i.e., the MFI zeolite silica analogue. In contrast to that, the isosteric heats of adsorption in the mesopore channels of SBA-15 were found to be much smaller. This result also suggests that SBA-15 can potentially be a good candidate for separation of C(7) hydrocarbons.     

Epoxy-functionalized large-pore SBA-15 and KIT-6 as affinity chromatography supports

Mesoporous silica supports are proposed as an alternative to polymeric stationary phases for fast affinity chromatography due to their better mechanical strength compared to polymers. Ideal supports should combine high surface area and large pore size to allow a high loading capacity of large molecules, such as proteins, and favor their accessibility. Increasing the pore size of large-surface area micelle-templated silicas (SBA-15, KIT-6) has been achieved by swelling the micelles by the addition of organic molecules and increasing synthesis time and temperature. The pore size of hexagonal silica mesostructured SBA-15 has been increased up to 35 nm. These materials could find therefore application as affinity chromatography for immunoextraction.     

Molecular simulation of RMM: ordered mesoporous SBA-15 type material having microporous ZSM-5 walls

SBA-15 is a novel porous material with uniform size mesopores arranged in a regular pattern. The adjacent mesopores are connected to each other by microporous walls. The major disadvantages of these materials are that the walls are amorphous and have low thermal, hydrothermal, and mechanical stability. Recently, there have been a few attempts to either coat the walls of SBA-15 by microporous crystalline zeolites or to fabricate SBA-15 using CMK-3 in such a way that the walls are made up of ZSM-5. The present work provides a first-ever study of RMM (replicated mesoporous materials, which are SBA-15 like ordered mesoporous materials with walls made up of ZSM-5) using molecular modeling. A random orientation of the unit cells and the distribution of sizes of the supercells located at nucleation sites would be ideal to model the RMM. However, such a study would introduce more uncertainties with regard to voids between the individual supercells, noncrystalline silica, and the location of active sites where the nucleation occurs. In a simpler model studied in the present work, the walls of SBA-15 were made up of regularly arranged ZSM-5 having the same orientation. The structure was characterized by estimating the nitrogen accessible area/volume by Connolly surfaces, small-angle and wide-angle X-ray diffraction patterns, methane adsorption, and ice as a probe to study the pore structure. It was found that RMMs have significantly higher methane adsorption capacity compared to SBA-15 and the majority of methane is adsorbed in the microporous walls of RMM. Further research in the field of RMM is needed to obtain the details of zeolitic wall structure.     

Highly efficient enrichment and subsequent digestion of proteins in the mesoporous molecular sieve silicate SBA-15 for matrix-assisted laser desorption/ionization mass spectrometry with time-of-flight/time-of-flight analyzer peptide mapping

Based on a previous study of protein digestion inside the nanoreactor channels of the mesoporous molecular sieve silicate SBA-15 (Chem. Eur. J. 2005, 11: 5391), we have developed a highly efficient enrichment and subsequent tryptic digestion of proteins in SBA-15 for matrix-assisted laser desorption/ionization mass spectrometry with time-of-flight/time-of-flight analyzer (MALDI-TOF/TOF) peptide mapping. The performance of the method is exemplified with myoglobin and cytochrome c. First, protein adsorption isotherms for two standard proteins with a range of initial concentration of proteins were investigated at room temperature. The results revealed that the kinetic adsorption rate of a protein within SBA-15 was independent of initial protein concentration, and a 15-min protein enrichment within SBA-15 could be enough for protein identification in biological samples. It was noticed that no washing steps were needed to avoid protein loss due to desorption from the mesochannels into solution. Second, protein digestion inside the channels of SBA-15 was also optimized. After adsorption of proteins into SBA-15 in 15 min, the trypsin solution (pH 8) was directly added to the SBA-15 beads with immobilized proteins by centrifugation, and then the digestion was performed for 15 min at 37 degrees C. It was observed that a higher peptide sequence covering of 98% for myoglobin was obtained by MALDI-TOF/TOF analysis, compared to in-solution digestion. So the protein digestion inside SBA-15 was proved to be significantly faster and yielded a better sequence coverage. The new procedure allows for rapid protein enrichment and digestion inside SBA-15, and has great potential for protein analysis.     

Engineered complex emulsion system: toward modulating the pore length and morphological architecture of mesoporous silicas

In the complex alkane/P123/TEOS/H2O emulsion system, an emulsion engineering method to modulate pore length and morphological architecture of mesoporous materials has been built. With fine tuning of the synthetic parameters (e.g., the composition of the synthetic mixtures, temperature, stirring, etc.), a series of chemically significant mesostructures (i.e., short-pore SBA-15 materials) with tunable pore length and morphological architecture have been successfully constructed. The effects of alkane solubilizates on pore length and particle morphology are discussed. The resulting short-pore materials would have potential applications in the fields of adsorption/separation of biomolecules and inclusion chemistry of guest species, etc.     

Direct synthesis of well-ordered and unusually reactive MnSBA-15 mesoporous molecular sieves with high manganese content

The mesoporous MnSBA-15 materials with different n(Si)/n(Mn) ratios of 4, 8, 20, and 50 have been synthesized, for the first time, using manganese nitrate tetrahydrate and Pluronic 123 triblock polymer [(EO)20(PO)70(EO)20] by simply adjusting the molar ratio of water to hydrochloric acid (n(H2O)/n(HCl)) under direct hydrothermal conditions. For the effect of structural and textural properties with incorporation of manganese, the MnSBA-15 has been synthesized with different synthesis temperatures at the fixed molar ratios of n(Si)/n(Mn) = 4 and n(H2O)/n(HCl) = 295 in the synthesis gel. The hydrothermal and thermal stabilities of MnSBA-15 have also been investigated. The calcined MnSBA-15 materials prepared have been characterized by ICP-AES, XRD, N2 adsorption, ESR, FE-SEM, and TEM. The ICP-AES studies show a higher amount of manganese incorporation on the silica pore walls, as MnSBA-15 with a n(Si)/n(Mn) ratio up to 2.2 can be successfully prepared at a fixed n(H2O)/n(HCl) molar ratio of 295 by adjusting the ratios of n(Si)/n(Mn) in the synthesis gel. The structural and textural properties of calcined MnSBA-15 prepared can be found by the results of XRD and N2 adsorption. The investigation of ESR results clearly describe the effect of structure and Mn species coordination on the SBA-15 silica pore walls while the uniform pore diameter and rope-like hexagonal mesoporous structure of MnSBA-15 can be identified by TEM and FE-SEM images. With increasing synthesis temperature, an increase the unit cell parameter, pore size, and pore volume and a decrease the specific surface area and pore wall thickness of MnSBA-15 can be obviously noted by the results of XRD and N2 adsorption. The hexagonal MnSBA-15 materials prepared could be tested as catalysts in epoxidation of trans-stilbene to produce trans-stilbene oxide under various optimal conditions while their catalytic properties could also be compared to the results of MnMCM-41 and ZrMnMCM-41.     

Gas adsorption in mesoporous micelle-templated silicas: MCM-41, MCM-48, and SBA-15

This paper reports a molecular simulation and experimental study on the adsorption and condensation of simple fluids in mesoporous micelle-templated silicas MCM-41, MCM-48, and SBA-15. MCM-41 is described as a regular cylindrical silica nanopore, while SBA-15 is assumed to be made up of cylindrical nanopores that are connected through lateral channels. The 3D-connected topology of MCM-48 is described using a gyroid periodic minimal surface. Argon adsorption at 77 K is calculated for the three materials using Grand Canonical Monte Carlo simulations. Qualitative comparison with experiments for nitrogen adsorption in mesoporous micelle-templated silicas is made. The adsorption isotherm for SBA-15 resembles that for MCM-41. In particular, capillary condensation and evaporation are not affected by the presence of the connecting lateral channels. In contrast, the argon adsorption isotherm for MCM-48 departs from that for MCM-41 having the same pore size. While condensation in MCM-41 is a one-step process, filling of MCM-48 involves two successive jumps in the adsorbed amounts which correspond to condensation in different domains of the porosity. The condensation pressure for MCM-48 is larger than that for MCM-41. We attribute this result to the morphology of the MCM-48 surface (made up of both concave and convex regions) that differs from that for MCM-41 (concave only). Our results suggest that the pore connectivity affects pore filling when the size of the connections is comparable to that of the nanopores.     

纯硅介孔分子筛SBA-15表面的手性移植

聚环氧乙烯醚-聚环氧丙烯醚-聚环氧乙烯醚(EO20PO70EO20)三嵌段高分子为模板剂制备了SBA-15分子筛. 用3-氨丙基-三乙氧基硅烷对SBA-15进行改性, 改性后SBA-15表面上的氨基再与(+)-O,O'-二苯甲酰基-L-酒石酸酐(DBTA)进行酰化反应, 以酰胺键将该手性羧酸连接在SBA-15表面上. XRD和氮气吸附结果表明, 材料经过处理后仍然保持良好的孔性质; 13C和29Si魔角旋转核磁共振(MAS NMR)谱图表明, SBA-15与氨丙基化合物的作用是共价成键, 表面修饰度达25%; 从傅里叶变换红外(FTIR) 光谱可见, 有部分修饰氨基与DBTA成功地进行了酰化反应, 以一个羧基裸露的形式将该二元羧酸化合物连接在表面上; 从孔径分布图可知, 胺丙基修饰之后孔减小了1.5 nm, 与DBTA修饰后孔尺寸又减少0.5 nm, 说明胺丙基化合物是头对头垂直连接在Si表面上, 而酒石酸分子是采取平行方式侧卧在氨基表面.     

Unusual mesoporous SBA-15 with parallel channels running along the short axis

Submicrometer-sized mesoporous silica SBA-15 with cuboidlike morphology has been prepared by using excess amounts of decane as cosolvent in the presence of NH4F. The channels of the resulting material run parallel to the short axis of the cuboid-like SBA-15. It has also been found that the materials have a well-ordered channel structure with pore size ca. 12 nm.     

Preparation and catalytic performances of ultralarge-pore TiSBA-15 mesoporous molecular sieves with very high Ti content

Highly ordered TiSBA-15 mesoporous molecular sieves with different nSi/nTi ratios and tunable pore diameters have been prepared through direct synthesis under various hydrochloric acid concentrations and synthetic temperatures. The structure and the textural parameters of the materials were investigated by powder X-ray diffraction and nitrogen adsorption/desorption measurements. Decrease of the acid concentration and nSi/nTi ratio in the synthetic gel enhanced the amount of Ti incorporation in SBA-15 materials without affecting their structural order and textural parameters. Highly ordered mesoporous TiSBA-15 with a very high Ti content up to a nSi/nTi ratio of 1.9 was prepared for the first time under the optimized synthesis conditions. Control of synthetic temperature resulted in tuning of pore geometries without structural deterioration and Ti content. Ultralarge-pore TiSBA-15 with a pore size of 12.6 nm and a pore volume of 1.3 cm3 g-1 was also synthesized. The nature and the coordination of the Ti atoms in SBA-15 prepared under various synthesis conditions were investigated by UV-vis spectroscopy. It has been found that the Ti atoms are well-dispersed and mostly occupy the tetrahedral coordination under the optimized synthesis conditions. Catalytic performance of the obtained TiSBA-15 materials was also investigated through oxidation of styrene by hydrogen peroxide and tert-butylhydroperoxide as oxidants.