Large-pore Mesoporous Silicas Research Articles

PEG-Coated Large Mesoporous Silicas as Smart Platform for Protein Delivery and Their Use in a Collagen-Based Formulation for 3D Printing.

Silica-based mesoporous systems have gained great interest in drug delivery applications due to their excellent biocompatibility and high loading capability. However, these materials face challenges in terms of pore-size limitations since they are characterized by nanopores ranging between 6–8 nm and thus unsuitable to host large molecular weight molecules such as proteins, enzymes and growth factors (GFs). In this work, for an application in the field of bone regeneration, large-pore mesoporous silicas (LPMSs) were developed to vehicle large biomolecules and release them under a pH stimulus. Considering bone remodeling, the proposed pH-triggered mechanism aims to mimic the release of GFs encased in the bone matrix due to bone resorption by osteoclasts (OCs) and the associated pH drop. To this aim, LPMSs were prepared by using 1,3,5-trimethyl benzene (TMB) as a swelling agent and the synthesis solution was hydrothermally treated and the influence of different process temperatures and durations on the resulting mesostructure was investigated. The synthesized particles exhibited a cage-like mesoporous structure with accessible pores of diameter up to 23 nm. LPMSs produced at 140 °C for 24 h showed the best compromise in terms of specific surface area, pores size and shape and hence, were selected for further experiments. Horseradish peroxidase (HRP) was used as model protein to evaluate the ability of the LPMSs to adsorb and release large biomolecules. After HRP-loading, LPMSs were coated with a pH-responsive polymer, poly(ethylene glycol) (PEG), allowing the release of the incorporated biomolecules in response to a pH decrease, in an attempt to mimic GFs release in bone under the acidic pH generated by the resorption activity of OCs. The reported results proved that PEG-coated carriers released HRP more quickly in an acidic environment, due to the protonation of PEG at low pH that catalyzes polymer hydrolysis reaction. Our findings indicate that LPMSs could be used as carriers to deliver large biomolecules and prove the effectiveness of PEG as pH-responsive coating. Finally, as proof of concept, a collagen-based suspension was obtained by incorporating PEG-coated LPMS carriers into a type I collagen matrix with the aim of designing a hybrid formulation for 3D-printing of bone scaffolds.

Exploring the confinement of polymer nanolayers into ordered mesoporous silica using advanced gas physisorption

Over the last two decades, in parallel to the rise of ordered mesoporous silica, porous nanostructured polymer-silica composites have attracted the interest of material scientists due to their promising perspectives of application as sorbents, ion-exchangers, supports, and catalysts. While knowledge is available regarding their synthesis and applications, understanding and controlling their pore properties in order to rationalize their performances remain challenging tasks. Greater knowledge is therefore needed regarding their precise characterization, especially using gas adsorption. To this aim, mesoporous polymer-silica nanocomposites were synthesized from two ordered mesoporous silica materials using a pore-surface restricted polymerization technique. Hydrophobic polystyrene, PS, and hydrophilic poly(2-hydroxyethyl methacrylate), PHEMA, were specifically confined and polymerized in the pores of high-quality SBA-15 and KIT-6 silicas of different pore sizes. The physico-chemical characteristics of the resulting hybrid materials were probed in detail using gas physisorption at cryogenic temperatures (Ar at 87 K and N2 at 77 K). The polymer loadings and the interactions between the silica host and the polymer were investigated using thermogravimetric analysis coupled with differential thermal analysis (TGA-DTA) and attenuated total reflection infrared spectroscopy (ATR-FTIR). The effects of the pore structure, mode pore size and presence or absence of intra-wall pores in the silica hosts on the final composite characteristics were assessed as a function of the polymer type and loading. Two different polymer filling mechanisms were identified as a function of the polymer-silica interactions, resulting in important changes on the pore topology of the composites. The results of this study allow a better understanding of the nature of the confined interactions between hydrophilic and hydrophobic polymers and large pore mesoporous silicas and shed some light on fundamental aspects regarding the design of silica-based composites.

Phase Domain of the Cubic Im3̄m Mesoporous Silica in the EO106PO70EO106−Butanol−H2O System

n-butanol has been chosen as an organic additive in the SiO(2)-EO(106)PO(70)EO(106)-H(2)O system at low HCl concentrations, for the generation of large-pore mesoporous silicas with easily tailored textural and structural properties. As opposed to previous reports on syntheses of cubic mesoporous silica that are usually performed in a very narrow composition range, we report now the possibility of preparing large-pore cagelike mesoporous silicas in a wide range of synthesis mixture compositions. Particularly, the cubic Imm silica with large interconnected cagelike pores (SBA-16) can easily be synthesized with controlled pore sizes and wall thicknesses, depending upon the synthesis mixture composition. The primary mesopore volume of the SBA-16 cages can be tuned from 0.27 to 0.56 cm(3) g(-)(1), and the mesopore size is shown to range from 4.7 to 7.2 nm, by performing a simple adjustment of the starting mixture composition. With the synthesis parameters varied, we describe the first complete diagram of the product phase domains obtained for silica mesophases in a SiO(2)-EO(106)PO(70)EO(106)-butanol-H(2)O system. Other ordered mesophases also observed in this system are the face-centered cubic Fmm silica mesophase and a 2D hexagonal-like mesostructure. Importantly here, the use of a low acid catalyst concentration regime allowed the preparation of silica mesophases in almost thermodynamically controlled conditions because of slow condensation kinetics of the inorganics. Such conditions enabled the introduction of n-butanol as the phase-controlling agent in the system, providing efficient tuning of the mesophase topology. The description of the phase domains provides a future basis for the design of large-pore mesoporous silicas with tailored textural and structural properties. Mesoporous samples obtained within the composition ranges of the phase domains are characterized by powder X-ray diffraction (PXRD) and nitrogen physisorption measurements.

Design of Mesoporous Silica at Low Acid Concentrations in Triblock Copolymer-Butanol-Water Systems

Assembly of hybrid mesophases through the combination of amphiphilic block copolymers, acting as structuredirecting agents, and silicon sources using low acid catalyst concentration regimes is a versatile strategy to produce large quantities of high-quality ordered large-pore mesoporous silicas in a very reproducible manner. Controlling structural and textural properties is proven to be straightforward at low HCl concentrations with the adjustment of synthesis gel composition and the option of adding co-structure-directing molecules. In this account, we illustrate how various types of large-pore mesoporous silica can easily be prepared in high phase purity with tailored pore dimensions and tailored level of framework interconnectivity. Silica mesophases with two-dimensional hexagonal (p6mm) and three-dimensional cubi (Fm<TEX>$\overline{3}$</TEX>m, Im<TEX>$\overline{3}$</TEX>m and Ia<TEX>$\overline{3}$</TEX>d) symmetries are generated in aqueous solution by employing HCl concentrations in the range of 0.1−0.5 M and polyalkylene oxide-based triblock copolymers such as Pluronic P123 <TEX>$(EO_{20}-PO_{70}-EO_{20})$</TEX> and Pluronic F127 <TEX>$(EO_{106}-PO_{70}-EO_{106})$</TEX>. Characterizations by powder X-ray diffraction, nitrogen physisorption, and transmission electron microscopy show that the mesoporous materials all possess high specific surface areas, high pore volumes and readily tunable pore diameters in narrow distribution of sizes ranging from 4 to 12 nm. Furthermore, we discuss our recent advances achieved in order to extend widely the phase domains in which single mesostructures are formed. Emphasis is put on the first synthetic product phase diagrams obtained in <TEX>$SiO_2$</TEX>-triblock copolymer-BuOH-<TEX>$H_2O$</TEX> systems, with tuning amounts of butanol and silica source correspondingly. It is expected that the extended phase domains will allow designed synthesis of mesoporous silicas with targeted characteristics, offering vast prospects for future applications.

Assembly of Large-Pore Silica Mesophases with Wormhole Framework Structures from α,ω-Diamine Porogens

Large-pore mesoporous silicas with wormhole framework structures have been assembled through hydrogen-bonding pathways from sodium silicate or tetraethyl orthosilicate (TEOS) as the silica source and amine-terminated Jeffamine surfactants of the type H2NCH(CH3)CH2[OCH2CH(CH3)]xNH2 as the structure-directing porogen. Depending on the molecular weight of the α,ω-diamine surfactant (x ∼ 33 and 68 for Jeffamine D2000 and D4000, respectively) and the synthesis temperature (25−65 °C), the mean pore size distributions of the mesostructured silicas (denoted MSU-J) were centered between 4.9 and 14.3 nm, making these materials comparable to hexagonal SBA-15 mesostructures in average framework pore size. In addition to the BET surface areas of MSU-J silicas (408−1127 m2/g) being comparable to those of SBA-15 (630−1040 m2/g), MSU-J silicas exhibit larger pore volumes (1.37−2.29 cm3/g) than SBA-15 silicas (0.56−1.23 cm3/g) prepared in the absence of mesitylene. MSU-J wormhole mesostructures represent the largest pore sizes observed to date for a fully three-dimensional mesoporous framework assembled from a single micellar porogen. Only mesostructured micellar foam structures exhibit larger pore sizes, but the preparation of foam structures requires the use of more complex microemulsions or latex polymers as porogens.

Functionalization of large-pore mesoporous silicas with organosilanes by direct synthesis

Functionalization of porous solids plays an important role in many technical aspects such as enzyme immobilization. In this study, functionalized large-pore mesoporous materials with a P6 mm symmetry were synthesized by co-condensation of tetraethyl orthosilicate (TEOS) and organosilanes 3-aminopropyltriethoxysilane (APTES), 3-mercaptopropylmethoxysilane (MPTMS), phenyltrimethoxysilane (PTMS), vinyltriethoxysilane (VTES), and 4-(triethoxysilyl)butyronitrile (TSBN) in the presence of nonionic triblock co-polymer Pluronic P123 under acidic conditions. The effect of the organosilanes present in the initial synthesis mixtures on the mesostructural properties of the resultant materials was systematically studied by varying the molar ratios of organosilane to TEOS. Solid-state nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and elemental analysis data demonstrated the presence of the functional groups both on the pore surface and inside the solid matrixes. Nitrogen adsorption, small-angle X-ray scattering (SAXS), and transmission electron microscopy (TEM) results showed different levels of disorder of the mesostructures upon incorporation of the functional groups, which are dependent upon the type and amount of the organosilanes present in the initial synthesis mixtures. Among the organosilanes studied, their disruptive effects on the formation of the mesostructure follow VTES < TSBN < PTMS ≅ MPTMS < APTES. Such different effects are interpreted in terms of their different behaviors under acidic synthetic conditions and steric molecular sizes and shapes, which have direct impact on interactions of P123 with silicate species and on micellation of P123 template.

Functionalization of large-pore mesoporous silicas with organosilanes by direct synthesis

Functionalization of porous solids plays an important role in many technical aspects such as enzyme immobilization. In this study, functionalized large-pore mesoporous materials with a P6mm symmetry were synthesized by co-condensation of tetraethyl orthosilicate (TEOS) and organosilanes 3-aminopropyltriethoxysilane (APTES), 3-mercaptopropylmethoxysilane (MPTMS), phenyltrimethoxysilane (PTMS), vinyltriethoxysilane (VTES), and 4-(triethoxysilyl)butyronitrile (TSBN) in the presence of nonionic triblock co-polymer Pluronic P123 under acidic conditions. The effect of the organosilanes present in the initial synthesis mixtures on the mesostructural properties of the resultant materials was systematically studied by varying the molar ratios of organosilane to TEOS. Solid-state nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and elemental analysis data demonstrated the presence of the functional groups both on the pore surface and inside the solid matrixes. Nitrogen adsorption, small-angle X-ray scattering (SAXS), and transmission electron microscopy (TEM) results showed different levels of disorder of the mesostructures upon incorporation of the functional groups, which are dependent upon the type and amount of the organosilanes present in the initial synthesis mixtures. Among the organosilanes studied, their disruptive effects on the formation of the mesostructure follow VTES < TSBN < PTMS ≅ MPTMS < APTES. Such different effects are interpreted in terms of their different behaviors under acidic synthetic conditions and steric molecular sizes and shapes, which have direct impact on interactions of P123 with silicate species and on micellation of P123 template.