Mesoporous silica templated by polyion complex micelles: A versatile approach for controlling the mesostructure
Graphical abstract
Introduction
Since their discovery in the early 1990s [1], [2], [3], Ordered Mesoporous Materials (OMMs) have gained an increasing attention in research and development, mainly due to their unique physicochemical properties, i.e. high specific surface area, ordered pore network at the mesoscale with narrow pore size distribution, large pore volume and rich variety of morphologies, proving a wide range of potential applications in fields such as adsorption [4], separation [5], catalysis [6], [7], [8], [9], drug delivery [10], sensors [11] and energy conversion and storage [12], [13]. Of particular interest are OMMs obtained through the soft templating approach using non-ionic amphiphilic block copolymers as the structure directing agent (SDA) of silica, which besides obtaining larger pores size and thicker walls compared to surfactants, allowed tuning of the ordered mesostructures from cubic to lamellar [14], [15], [16]. This can be achieved by controlling factors influencing the self-assembly properties of the amphiphilic system through modification of its chemical composition (hydrophobic/hydrophilic volume ratio, hydrophobic length …), by adjusting the processing variables (pH, temperature, concentration, ionic strength, silica source …) and by swelling one of the blocks by additives such as an aromatic hydrocarbon, a long-chain alkane or a cosolvent [17], [18], [19], [20], [21], [22].
However, despite the new possibilities they were opening up, OMMs have only been scarcely exploited in the industry. The major barriers to their industrial development originate from a high cost of the raw materials, a possible inhomogeneity within the material associated with scaling-up production, a still insufficient hydrothermal stability and a lack of environmentally friendly processes. During the last decade, many efforts have been made on the development of ecodesign of mesoporous silica, e.g. by the use of renewable raw materials or by the implementation of eco-strategies both for material synthesis and removal of the template, which usually proceeds through calcination or chemical extraction by organic solvents [23]. Within that context, we developed a new strategy to prepare OMMs using a pH-stimulable SDA, whose formation can successively be induced for mesostructuring silica and hindered for generating porosity in water and at room temperature, thus avoiding high energy consumption during calcination step and the use of unfriendly organic solvents [24], [25]. This successful sustainable strategy could be achieved by using polyion complex (PIC) micelles as silica-structure directing agents, which are constituted of a double hydrophilic block copolymer (DHBC), possessing a neutral block and an ionizable one (poly(ethylene oxide)-b-poly(acrylic acid), PEO-b-PAA), and a homopolyelectrolyte (oligochitosan, OC) of charge opposite to that of the DHBC.
PIC (or complex coacervate) micelles are micelles presenting a core–corona nanostructure in aqueous solution [26], [27]. The core consists of an electrostatic complex formed by the charged parts of the polymers whereas the corona is constituted by the neutral block (often a polyethylene oxide block) of the DHBC, which ensures the sterical hindrance needed to the stabilization of the assembly. During materials synthesis, the neutral block allows for the interaction with silica species (through the N°/I° pathway) necessary to create the hybrid organic-inorganic interface primary to mesostructuration [25]. When the DHBC/homopolyelectrolyte system is constituted of a weak polyacid/polybase couple, it exhibits smart pH-responsive assembly/disassembly properties, which have been widely used for drug delivery purpose from PIC colloids [28], in our ecodesigned mesoporous material synthesis [24], [25] and were more recently exploited for the release of aminoglycoside antibiotics after promoting the mesostructuring of silica [29].
A major question arising from the ordered structuring process by PIC micelles is whether the mesostructure of silica can be varied as easily as with non ionic amphiphilic block copolymers, which will make this type of SDA at least as exciting and promising as those. The few studies that reported up to now a mesostructure variation of PIC-silica materials rely either on the modification of the pH of the synthesis medium [25] or on a change of the polymer asymmetry degree aiming at varying the Vcore/Vcorona ratio controlling the system curvature [30]. However, adopting this last strategy to modulate the material mesostructure requires the synthesis of several double hydrophilic block copolymers with well-controlled architecture and block lengths, which is time consuming and far from cost-effective. In this study, the DHBC/homopolyelectrolyte system we focused on is constituted of PEO-b-PAA and poly-L-lysine (PLL), which is expected to be a good candidate for such PIC micelle formation. Indeed the rather similar poly(ethylene oxide)-b-poly(methacrylic acid) (PEO-b-PMAA) DHBC, although somewhat more hydrophobic, was shown to efficiently form PIC micelles in the presence of PLL [31], [32]. PLL-based PIC micelles are particularly interesting as drug carriers for drug delivery applications owing to their stability under physiological conditions and their likely dissociation in slightly acidic endosomal compartment. The present study based on a PEO-b-PAA/PLL system aims at demonstrating the general behavior of PIC micelles to act as efficient structure-directing agent (SDA) of silica. Moreover, we propose to explore in details the influence of relevant synthesis parameters on the structuring process of silica using a unique PEO-b-PAA polymer of fixed block lengths. We show that by varying the molar ratio R of opposite charges born by the homopolyelectrolyte and the DHBC (R = LL/AA), the molar ratio between units involved in the formation of the hybrid organic-inorganic interface (Si/EO) and the concentration of the system (DHBC wt%), it is possible to control the mesostructure of PIC-templated mesoporous silica. This study also highlights the role of the competitive interactions in the determination of the nature of the hybrid mesophase and the necessity to consider the affinity between poly-L-lysine and silica in the formation process. Finally the present study will help to better estimate the potential of PIC micelles as versatile systems to control the mesostructure of silica by a simple adjustment of the synthesis conditions and thus to provide materials with handily adjustable properties.
Section snippets
Materials
Poly(ethylene oxide)-b-poly(acrylic acid) copolymer (PEO-b-PAA, MPEO = 5000 g mol−1, MPAA = 2700 g mol−1) was purchased from Polymer Source Inc. (USA). Poly-L-lysine hydrobromide (PLL) of 24000 g mol−1 molecular weight, tetraethoxysilane (TEOS), hydrochloric acid (HCl) and sodium hydroxide (NaOH) were from Sigma Aldrich. All products were used without any further purification.
Preparation of PIC micelles
Micelle formation as a function of pH was studied by dynamic light scattering (DLS) and circular dichroism (CD,
Polyion complex micelle formation
The poly(ethylene oxide)-b-poly(acrylic acid) copolymer (PEO-b-PAA, MPEO = 5000 g mol−1, MPAA = 2700 g mol−1) and poly-L-lysine (PLL, 24000 g mol−1) system we used in this study is expected to exhibit the typical associative phase separation behavior leading to PIC micelles, whose formation can be controlled by any parameter affecting the electrostatic interactions, such as the ionic strength, pH and mixing fraction. The formation of PIC micelles as a function of pH (4 < pH < 13) was studied by
Conclusions
In summary, the variation of the synthesis parameters investigated (R = LL/AA, Si/EO, mass concentration of the reacting medium) allowed modulating the mesostructure of silica-based materials prepared with PEO-b-PAA/PLL PIC micelles as SDA. The mesostructure changes were ascribed to modifications of the chemical composition of the as-synthesized material, resulting from the interplay between four competing chemical equilibria and strongly dependent on the reaction media composition. The
Acknowledgments
The authors would like to acknowledge Philippe Dieudonné (Institut Charles Coulomb Montpellier, UMR 5221) for SAXS analysis of the materials, Martin Cohen-Gonsaud (Centre de Biochimie Structurale de Montpellier, UMR 5048) for help during CD measurements and “Laboratoire de Mesures Physiques de l'Université de Montpellier” for elemental analyses. Authors are also thankful to MENRT (Ministry of National Education, Research and Technology) for financial support.
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