Morphology control of mesoporous silica particles using bile acids as co

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Morphology control of mesoporous silica particles using bile acids as co-surfactants Leana Travaglini, and Luisa De Cola Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b01873 • Publication Date (Web): 30 May 2018 Downloaded from http://pubs.acs.org on May 31, 2018

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Chemistry of Materials

Morphology control of mesoporous silica particles using bile acids as cosurfactants Leana Travaglini,a* Luisa De Cola a,b* a

Université de Strasbourg, CNRS, ISIS UMR 7006, 8 allée Gaspard Monge, 67000 Strasbourg, France.

b

Institut für Nanotechnologie (INT) - Building 640, Karlsruhe Institute of Technology (KIT) - Campus Nord,

Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany.

Email: [email protected]; [email protected]

Abstract. Morphology control and tuning of nanomaterials are crucial to determine their properties and applications. Solutions based on different synthetic methodologies have been proposed and in general they required variation of several parameters. Here, a new facile and cost-effective bottom up strategy to control the morphology of mesoporous silica particles is presented. Specifically, catanionic templating systems composed of bile acids and CTAB enable the production of submicrometer MCM-41 particles of various shapes, high porosity and remarkable features. The variation of a single component, the bile acid, leads to the preparation of particles with different morphologies. For instance, small (1 have shown improved cellular uptake in drug delivery.3, 10, 21, 23

In addition, our platelets are very well-separated contrary to the hexagonal platelets reported to

date, which are SBA-15 materials that are

32, 54-58

larger in size and show in most cases a high

degree of intergrowth and/or twinned aggregation.11b, 22 Such aggregation prevents their grafting onto surfaces and limits their use in several applications. In order to demonstrate that our hMPs can overcome such limitations a monolayer assembly of hMPs was obtained through their covalent linkage on glass plates (Figure 3), following a well-established procedure to prepare assembled monolayer of zeolite crystals.59-62

Figure 3. SEM images of the monolayer formed by hMPs-NH2 onto a glass slide. The image taken at higher magnification (b) clearly shows that the platelets are disposed in such a way that the pores are perpendicular to the glass surface.

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Briefly, the monolayer was prepared by reacting glass plates, previously tethered with isocyanate groups, with amino-functionalized hMPs (hMPs-NH2, see SI for details on synthesis and characterization). The monolayer is characterized by good coverage of the surface and coating homogeneity, and the particles lie parallel to the glass surface with no significant tilting or stacking , thus exposing the accessible pores and providing a large porous surface. XPS survey analysis confirmed the presence of hMPs-NH2 onto the glass plate surface (Figure S6). To prove the main role played by LCA in the preparation of the hMPs, a control experiment was performed, in identical experimental conditions but employing only CTAB as a template. In this case particles of spherical geometry (Figure 4a) with a mean diameter of 246 ± 29 nm (Figure S7) were observed. The particles show facets, reflecting the internal ordering well-visible in TEM micrographs (Figures 5a and S8) and corresponding to a MCM-41 hexagonal mesostructure, as shown by SAXS analysis (Figure S9). The estimated pore diameter is 3.0 nm, confirmed by porosimetry measurements (Figure S10). The data clearly reveals that, while mesostructure and porosity are retained, the introduction of LCA has a fundamental effect in determining the shape of the particles. In our synthesis the material phase separation occurs at an early stage, ca. 5 min after the addition of TEOS, thus according to the CPSM model the ∆G is expected to be dominant over F, and the final overall morphology is expected to develop together with the mesostructure, reflecting its ordering.27 This would explain the fact that both the particles with spherical geometry, showing facets, and more evidently the hMPs, reflect the mesostructure order. Nevertheless, to better understand the underlying cause of such a dramatic morphology variation, it is necessary to consider the role of lithocholate anion (LC-) and its possible interactions in the system. We propose that at the utilised concentration LC- ions intercalate the hydrophobic steroid backbone into the micelle, exposing the carboxylate groups towards the exterior where they interact through electrostatic attractive interactions with the positively charged CTA+ head groups (Figure 6), analogous to what has been described for mixed micelles of CTAB/DCA and CTAB/CA.46


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Figure 4. SEM images of as-prepared particulate materials obtained using as templates (a) CTAB, (b) CTAB and salicylic acid, (c) CTAB and 2-naphtoic acid, (d) CTAB and DCA, (e) CTAB and CA and performing the reaction at 50 °C using as templates (f) CTAB, (g) CTAB and LCA, (h) CTAB and DCA, (i) CTAB and CA. In panels (h) and (i) the arrows indicate the hexagonal cross-section. Scale bar = 500 nm.



Figure 5. TEM images of calcined particulate material obtained using as templates (a) CTAB, (b) CTAB and DCA, (c) CTAB and CA and performing the reaction at 50 °C using as templates (d) CTAB and LCA, (e) CTAB and DCA, (f) CTAB and CA. In panel (e) and (f) the arrows indicate the fringes. Scale bars = 100 nm.


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Figure 6. Schematic representation of the probable interactions between LC- and CTA+ micelles.

This would lead to a decrease of charge density of the micelles that would inevitably influence the interactions with the growing silicates. It is in fact well-known that in alkaline conditions, anions can act as a block towards the adsorption of silicates onto the micellar surface, hence delaying their polymerization, to an extent that depends on their nature.63, 64 In the case of LCthe effect would be particularly pronounced, given its high tendency to insert into the micelles due to the bulky hydrophobic steroidal moiety. This means that when only CTAB is used, an isotropic growth of the particles is favoured, since the mesophase formation and silica condensation are driven by electrostatic interactions between the cationic head groups and the growing polysilicates without interferences.24-26,

36, 65

Conversely, LC-, delaying the silica

polymerization, favours a non-isotropic growth of the particles, and a side-on growth of the composite micelles leads to the formation of hMPs with pore channels running along the short dimension. Given the complexity of the system, investigating particle formation is not trivial and further work is ongoing to fully unravel the mechanism. To prove the presence of the BA within the micellar templates, we analysed the non-extracted particles. The as-prepared hMPs were extracted using MeOH at 65 °C (details in the SI), recovered by centrifugation, and the collected supernatant solution analysed by liquid chromatography-mass spectrometry (LC-MS). The analysis was conducted operating the electrospray ionization (ESI) interface in positive and negative ion mode and the mass spectra (Figure S11) showed peaks at 284.38 and 375.29 m/z revealing the presence of both hexadecyltrimethylammonium and lithocholate ions, respectively, within the non-extracted particles. To verify that the effect of LCA is not merely electrostatic, but is also strongly related to the unique molecular structure of the BA and the specific interactions with the cationic surfactant,



LCA was replaced by equimolar amounts of salicylic and 2-naphtoic acid. The synthesis was then performed in the presence of organic carboxylates bearing a hydrophobic moiety, smaller than the steroid and known to interact with CTAB micelles in a similar fashion.66, 67 The particles formed in the presence of salicylate are mostly spherical (Figure 4b), some oblate and only a few ones show a hexagonal geometry. In the presence of 2-naphthoate anions the particles are mostly oblate, but a few hexagonal plate-like particles are visible (Figure 4c). This suggests that the electrostatic effect is related to the tendency of the anion to insert into the micelles and vary the charge density on the micelle surface. At equal concentrations, it is clear that the electrostatic effect is not enough to modulate the shape of the particles, but the hydrophobicity and bulkiness of the organic anion play a fundamental role in determining the particle morphology. To further elucidate the correlation between the effect on the morphology and the molecular structure of the co-surfactant we replaced LCA by DCA and CA possessing an increasing hydrophilic character, bearing two and three OH groups, respectively. This allows not only for a good comparison with LCA in terms of size and shape, but also tests the significant effect of the amphiphilicity to modulate the shape of the particles. Deoxycholate (DC-) had an effect similar to that of LC-, leading to hexagonal plate-like particles of comparable width (Figures 4d and S12), but slightly larger height (ca. 200 nm). In the presence of cholate (C-) rugby-ball-like particles were formed being the hexagonal profile less pronounced (Figure 4e). The width was comparable (423 ± 43 nm, Figure S14), while the height larger, ranging from 180 to 350 nm. In both cases TEM (Figures 5b,c and S13,15) revealed pore channels parallel to the particle short dimension and the SAXS pattern showed a main peak at q = 1.84 and 1.90 nm-1, respectively. A smaller broad peak at higher q values is also visible, suggesting the presence of slightly smaller pores in the structure (Figure S16). This is supported by the broader pore width distributions calculated from N2 sorption (Figure S17) and the slightly smaller average pore diameters calculated as 2.8 and 2.7 nm for particles templated by DCA- and CA-containing mixtures, respectively. Both materials show high porosity since BET surface areas correspond to 1228 and 1509 m2/g, and total pore volumes to 0.64 and 0.79 cm3/g. These results suggest that DC- and C- interact less with CTA+ micelles. The higher solubility in water would determine the intercalation of the molecules to a lesser extent, leading to fewer interactions and therefore to a less pronounced inhibition towards the adsorption of the growing silicates onto the micelles, which would be the weakest for C-. Such an effect would hence 10 ACS Paragon Plus Environment

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explain the gradual increase in thickness of the particles upon replacement of LCA with DCA and CA. The loss in long range order suggested by the absence of (110) and (200) plane peaks in SAXS patterns may be instead explained by the fact that DCA and CA molecules could intercalate into the micelles as dimers to minimize the energy.49 The OH groups borne on the hydrophilic face of one molecule could therefore interact through H-bonding with the hydroxyl groups of the next molecule, preventing the presence of polar groups within the hydrophobic micelle core. Most likely this intercalation would not occur always to the same extent, having the possible consequence of reducing the framework ordering. Similarly to what has been done for the hMPs, the presence of the DCA and CA within the templating micelles was proven by analysing by LC-MS the solutions collected after extraction of the non-calcined particles. The spectra reported in Figures S18 and S19 showed the presence of both deoxycholate and cholate anions together with the hexadecyltrimethylammonium ion. The effect of temperature was also evaluated both in the presence and absence of the BAs by performing the reactions at 50 °C, where hydrolysis and polymerization of the silica source occur at an increased rate. Morphology, mesostructure and porosity of the material obtained at 25 °C were retained (Figures 4f, S20-S22) when only CTAB was used. Conversely, the presence of BAs led to twisted rod-like particles (Figures 4g-1). Those formed in the presence of LCA are characterized by length up to 1 µm and ca. 80 nm cross-section diameter (Figures 4g and S23). TEM showed 2D chiral pores running along the rods and a twist pitch of ca. 40 nm can be estimated from the fringes (Figures 5d and S24). As inferred from the SAXS pattern, 3.4 nm pores are arranged in p6mm symmetry (Figure S25). The pore width was confirmed by N2 adsorption (Figure S26), which also provided a BET surface area of 1319 m²/g and total pore volume of 0.92 cm3/g. Twisted rods prepared in the presence of DCA were considerably shorter and characterized by a larger cross-section diameter (Figures 4h and S24), and a larger twist pitch of ca. 100 nm (Figures 5e and S27). They showed a hexagonal cross-section, and 3.4 nm pores hexagonally ordered (Figures S28, S30) running along the particles. BET surface area and pore volume were estimated as 1215 m²/g and 1.09 cm3/g. The use of CA led to further shorter and wider rod-like particles (Figures 4i and S31) for which the twisting could be clearly appreciated by TEM (Figures 5f and S32), but did not affect either mesostructure or pore properties (Figure S33, BET 1130 m²/g, total pore volume 0.99 cm3/g). Again, proof of the presence of the BAs within the non-extracted particles was provided by LC-MS analysis (Figures 11 ACS Paragon Plus Environment


S34-S36) on the solutions collected after extraction with MeOH, since the spectra clearly showed the presence of the lithocholate, deoxycholate and cholate anions. The elongated twisted shape suggests that at 50 °C the presence of BAs favours the growth of the particles along the channel direction to produce rod-like particles with hexagonal crosssection. This is most probably the result of the interplay of several factors. As already mentioned, it is known that the insertion of BA molecules into a CTAB micelle lowers its curvature favouring the growth into elongated micelles.46 This effect depends on the extent to which the intercalation occurs, dictated by the solubility in water of the BA monomers. It means that this effect would be stronger in the order for LCA > DCA > CA. As stated above, at 25 °C elongated particles are not observed because the screening of positive charges due to electrostatic interactions likely delays the silicates adsorption and subsequent polymerization. The different behaviour at 50 °C is most probably due to the concomitance of more favourable hydrophobic interactions and higher speed rate of silica condensation. In fact, hydrophobic interactions between amphiphilic molecules in water are generally stronger at higher temperature, causing the aggregates to compact and grow in size and length. The faster silica polymerization would instead counterbalance the delay in adsorption and condensation due to charge screening, further favouring the growth of elongated objects. The insertion of the chiral rigid curved steroidal moiety may contribute to induce strain in the system and impart the twist, which could also be induced by the tendency of the system to reduce the surface energy. Further, it is possible to notice that the more hydrophilic the BA, the shorter and wider are the resulting twisted rods, in accordance with the less tendency towards the BA insertion into the micelles. It follows that a higher number of inserted BA molecules would lead to longer rods and to a higher degree of twisting in order to minimize the lateral surface. Besides the elucidation of the role of BAs in the process, it is also worth noting that our templating system allows easy access to chiral mesoporous silica and special attention has been recently paid to the synthesis of chiral mesoporous silica particles, due to their potential as templates for metal nanowires with tuneable properties or as supports in catalysis or chiral recognition. The first example of chiral mesoporous silica was reported by Che et al. who prepared chiral rods employing chiral anionic surfactants as to impart chirality to the particles.19 Notably, this work represents also the first example in which the morphology control is obtained through the introduction of co-templates inducing variation of the silica-template interactions. 12 ACS Paragon Plus Environment

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Since then, various examples of chiral mesoporous silica prepared by using chiral or achiral structure directing agents,68-71 or simply upon addition of swelling agents to the system,1 in conditions that favoured the growth of fibres that eventually twist into chiral structures in order to minimize the lateral surface.72, 73 However, our system certainly provides an alternative costeffective way to synthesise these types of materials.

Conclusions In summary, we showed that using CTAB/BAs binary mixtures as templating agents it is possible to attain good control of MSPs overall shape, in alkaline aqueous conditions. For the first time the use of BAs in the synthesis of mesoporous silica is reported and provides a simple method to control the morphology of the particles varying a single parameter. The specific interaction between the two surfactants is responsible for the described variation of shape, size and aspect ratio. Specifically, the bulkiness and the amphiphilicity of the bile acids play a major role. The contribution of temperature was also evaluated, but yet the tuning of the structures depends on the bile acid employed. This method enabled us to obtain MCM-41 submicron MSPs of various shapes, with wellordered pores and high porosity, and remarkable features. As we showed, small hexagonal platelets and twisted chiral rods with tuneable aspect ratio were obtained with our method and both are particularly promising in the design of functional materials. The first are ideal to be integrated onto surfaces, while the latter ones are attractive templates or supports for separations. This work certainly paves the way to a new cost-effective and efficient strategy for the morphology modulation of mesoporous silica particles.

Associated content The Supporting Information contains experimental details, description of materials and instruments and additional figures.

Notes The authors declare no competing financial interest.



Acknowledgments This work was financially supported by the European Commission through the SACS Project (grant N° 310651). Prof. L. De Cola acknowledges AXA Research Fund for financial support. The authors would like to thank Dr. S. Silvestrini for XPS measurement on the monolayer and Dr. P. Chen for XPS measurements on pristine and amino-functionalized platelets and nitrogen sorption measurements on hexagonal platelets and spherical particles.

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Morphology control of mesoporous silica particles using bile acids as co

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