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  • Mesporous Silica
    Dec 07, 2017 | ACS MATERIAL LLC

    Mesoporous silica materials have gained much attention due to its ordered pore structures and ability to be controlled at micrometer and nanometer scales. The material made excellent candidates to many applications such as drug delivery, adsorption and waste water treatment. The high surface area and narrow pore size distribution make this material suitable as catalysts and templates for other materials. Here, we will discuss what makes this material so unique, the different methods that was used to synthesize them and some of their applications.

    Introduction

    Mesoporous silicas (MS) are solid materials with a porous structure and many empty channels (mesopores) that absorb relatively large amounts of bioactive molecules. MS have originated from different sources of silica (sodium silicate, alkoxides-like tetraethylorthosilicate, etc.) and can decompose into relatively harmless silicic acid by-products which is less challenging for long-term use.1 The silica surface contains a high density of silanol groups that can be modified with multiple organic functional groups.2 This material contains a high specific surface area of 500-1000 m2/g, large pore volume greater than 0.9 cm3/g, chemical/thermal stability and tunable pore size with a narrow distribution of 2-10 nm.3 They are known for their ordered pore structures and various morphologies that appear as either rods, spheres, powders and so on. The mesopores also have their own shapes (i.e. hexagonal, cubic) with diameters that are between 2-50 nm. Differences in the final outcome of the material are based on the changes that were made to the properties of the original molecule.

    Many variations of MS (i.e. MCM, SBA, KIT) exist with different wall thicknesses, pore sizes and shapes. The corresponding numbers for each type of MS represent its own specific pore structure and surfactant that sets them apart from each other. For example, MCM-41, MCM-48 and MCM-50 (Mobil Crystalline Material) have pore geometries that are hexagonal, cubic and lamellar, respectively, with different wall thicknesses. The characteristics of each MS species depend on the synthesis since the specifications are tunable.

    Synthesis

    MS are synthesized with surfactants used as templates for the polycondensation of silica species that have originated from different silica sources such as sodium silicate, tetraethyl- (TEOS) or tetramethylorthosilicate (TMOS). Synthesis conditions depend on the source of silica, type of surfactant, ionic strength, and composition of the reaction mixture which will then determine the characteristics of the porous structure and macroscopic morphology.1 Surfactants play an important role in synthesizing MS as it will ultimately determine the pore size and structure. Many types of ionic and non-ionic surfactants have been used to obtain different pore structures and morphologies.4 In fact, the SBA-type (Santa Barbara Amorphous) silica materials were first synthesized by Zhao et al. using non-ionic triblock copolymers as a template since it is nontoxic, biodegradable and inexpensive.5

    The sol-gel method has also been applied to synthesize MS which involves a colloidal solution (sol) where micelles are formed by surfactants and dispersed in an aqueous solution. After a silica precursor has been added, the sol gradually transitions into a gel-like network containing both the solid and liquid phase. After the sol-gel reaction, the hydrothermal method has been applied as a synthesis and post-synthesis treatment of mesoporous silica as it can potentially increase hydrothermal stability, improve mesoscopic regularity and extend pore size.6 Adding the hydrothermal treatment promotes the gelation of the silica sol and extension of the 3D network to fabricate a rigid silica framework.7 ACS Material has utilized the hydrothermal method to synthesize some mesoporous silica products as shown in Figure 1 for MCM-41.

    TEM MCM-41

    Figure 1. ACS Material TEM image of MCM-41 that was synthesized using the hydrothermal method.

    Applications

    Such range of mesoporous silica materials have led to its use in promising applications such as drug delivery, adsorption and waste water treatment. In particular, mesoporous silica nanoparticles (MSNs) with a large pore volume has been highly coveted for their application in biomedicine. With its pore size ranging from 2-50 nm, MSNs make excellent candidates for drug adsorption and loading within the pore channels that will enable better control of drug loading and release kinetics. SBA-16 was just one of the porous materials tested to have a 3D cage-like cubic mesoporous structure that had a fast drug dissolution rate based on the interconnected pore structure which reduced the diffusion hindrance and facilitated drug diffusion into a dissolution medium.8  

    With a high specific surface area, MS carry an excellent adsorption capacity in which a large amount of material touching its surface may remain attached until a high-temperature desorption process is applied.9 Since MS can be enhanced with appropriate functional groups, amine-functionalized MCM-41, SBA-12 and SBA-15 are just some of the materials that exhibited carbon dioxide capture but the sorption capacity depended on their different pore sizes and surface density of amine groups.10   

    Other functionalized MS have also been used to remove heavy metal ion and phosphate from waste water based on their large pore volumes and controlled pore diameters. With this mesoporous material, specific metal ion pollutants can be selectively removed from aqueous or organic systems with large uptakes. Not only are MS environmentally friendly, but they have robust mechanical stability and the ability to adsorb these ions.11

    Conclusion

    Silica-coated mesoporous materials have proven to be very useful in a wide range of applications such as drug delivery and reducing gas emissions with carbon dioxide capture. The unique properties of these materials show that they are becoming more established in different fields due to their large surface areas, large pore volumes and wide range of morphologies that can be synthesized into various shapes. The ordered pore geometries are what sets them apart from traditional porous silicas which makes them highly desirable for multiple purposes. 

    ACS Material Products:

    SBA-type Series

     MCM-type Series

     KIT-type Series

     Mesoporous Silica Nanosphere

     References

    1. Giraldo, L. F., et al. “Mesoporous Silica Applications.” Macromolecular Symposia, vol. 258, no. 1, 20 Nov. 2007, pp. 129–141., doi:10.1002/masy.200751215.

    2. Huang, Xinyue, et al. “Characterization and Comparison of Mesoporous Silica Particles for Optimized Drug Delivery.” Nanomaterials and Nanotechnology, vol. 4, 2014, p. 2., doi:10.5772/58290.

    3. Slowing, I, et al. “Mesoporous silica nanoparticles as controlled release drug delivery and gene transfection carriers.” Advanced Drug Delivery Reviews, vol. 60, no. 11, 2008, pp. 1278–1288., doi:10.1016/j.addr.2008.03.012.

    4. Wei, Liangming, et al. “Synthesis of Polymer—Mesoporous Silica Nanocomposites.” Materials, vol. 3, no. 7, 2010, pp. 4066–4079., doi:10.3390/ma3074066.

    5. Rivera-Muñoz, Eric M., and Rafael Huirache-Acuña. “Sol Gel-Derived SBA-16 Mesoporous Material.” International Journal of Molecular Sciences, vol. 11, no. 9, 2010, pp. 3069–3086., doi:10.3390/ijms11093069.

    6. Yu, Qiyu, et al. “Hydrothermal synthesis of mesoporous silica spheres: effect of the cooling process.” Nanoscale, vol. 4, no. 22, 2012, p. 7114., doi:10.1039/c2nr31834b.

    7. Kato, Noritaka, and Nao Kato. “High-Yield hydrothermal synthesis of mesoporous silica hollow capsules.” Microporous and Mesoporous Materials, vol. 219, 2016, pp. 230–239., doi:10.1016/j.micromeso.2015.08.015.

    8. Wang, Ying, et al. “Mesoporous silica nanoparticles in drug delivery and biomedical applications.” Journal of Membrane Science, vol. 218, no. 1-2, 1 July 2003, pp. 55–67., doi:10.1016/S0376.7388(03)00136-4.

    9. Zamani, Cyrus, et al. “Mesoporous Silica: A Suitable Adsorbent for Amines.” Nanoscale Research Letters, vol. 4, no. 11, 2009, pp. 1303–1308., doi:10.1007/s11671-009-9396-5.

    10. Mello, Marília R., et al. “Amine-Modified MCM-41 mesoporous silica for carbon dioxide capture.” Microporous and Mesoporous Materials, vol. 143, no. 1, 2011, pp. 174–179., doi:10.1016/j.micromeso.2011.02.022.

    11. Zhai, S. “Bi-Functionalized xerogel adsorbent for selective removal of Pb2 from aqueous solution.” JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, vol. 13, June 2001, pp. 733–736.