Applications of Functionalized Silica Nanoparticles
Colloidal and monodispersed silica nanoparticles are one of the most applied materials due to their intrinsic properties like functionality, surface structure, optical properties, and biocompatibility (Noll 1956). Surface-functionalized silica nanoparticles have a wide range of applications in biomedical imaging, food and agriculture, electronics, paints, pigments, sensors, catalysis (Lim et al. 2010; Wang et al. 2010)(Figure 11.2). Bio-compatible nanoparticles are useful in drug delivery systems and bio-imag- ing (Muhammad et al. 2011; Bonacchi and Zaccheron 2010). However, different applications of silica nanoparticles depend on surface properties and size (Noll 1956).
Mesoporous silica has a high surface area and great shape selectivity, which is required in catalytic reactions. High surface area, better adsorption capacity, and fast transportation of molecules with a good thermal stability make porous material more useful for catalytic applications (Valtchev and Tosheva 2013; Walcarius and Mercier 2010). The silanol groups are easily functionalized to tune surface properties like hydrophilicity, hydrophobicity, biocompatibility, etc. The environmental applications of nanosilica are useful in air/water purification, pollution remediation, emission control, and biomass conversion to produce energy (Roy et al. 2009; Perego and Bosetti 2011; Taarning et al. 2011; Walcarius and Mercier 2010). The conversion of biomass has gained interest and development is reported across the globe (Huber et al. 2006; VandeVyver et al. 2011; Zhou et al. 2011; Dapsens et al. 2012). Lignin-enriched cellulose biomass is utilized for biomass conversion. Cellulose are made up of glucose and it is important to break p-glycosidic bonds for hydrolysis and biomass conversion. Catalysts are used for hydrolyzing the cellulose. Liquid catalyst is difficult to recycle, therefore a solid
FIGURE 11.2 Interdisciplinary application of mesoporous silica nanoparticles.
catalyst is used to convert bio-mass due to their easy removal and recycling (VandeVyver et al. 2011; Zhou et al. 2011). The use of a solid catalyst also overcomes the limited solubility of cellulose. Silica-based zeolites are mostly employed to convert biomass (Perego and Bosetti 2011; Taarning et al. 2011). Recently, zeolites are reported to transform biomass pyrolysis oil into hydrocarbons (Gayubo et al. 2004; Gayubo et al. 2005; Onda et al. 2008), olefin from ethanol (Nikolla et al. 2011; Bermejo-Deval et al. 2012), and glucose from cellulose (Onda et al. 2008). The study demonstrated that a higher ratio of Si/Al provides better selectivity for glucose and higher conversion. Research is on the way to establish better technology using silica-based zeolites for biomass conversion and energy production. Air and water purification and pollution remediation are also remarkable applications of silica nanoparticles. Surface functionalization of porous nanosilica endows their adsorption ability by enhancing electrostatic interactions and providing a binding site for heavy metal chelation. Functionalized silica nanoparticles are widely used in heavy metal adsorption (Chen et al. 2009; Li et al. 2011). Fryxell et al. demonstrated the recovery of radioactive elements from contaminated water by using silica-based materials (Fryxell et al. 2005). Magnetic mesoporous silica nanoparticles are also employed to adsorb metal elements. The use of magnetic silica enhances the separation of metal elements. There are innumer- ous reports that showed water purification by removing heavy and transition metal elements (Chen et al. 2009; Arruebo et al. 2006). Development in magnetic silica has reported the use of iron oxide nanoparticles as a core for mesoporous silica nanoparticle. Thus, a permanent magnet is utilized to separate magnetic solid adsorbents (Chen et al. 2009; Li et al. 2011). Thiol functionalized hybrid silica nanoparticles were reported to adsorb and remove Hg2+ because of a higher binding affinity of the Hg-S. They also showed that functionalized silica has better selectivity for Hg2+ ions over other metal ions (e.g., Pb2+, Ni2+, Zn2+, Fe3+, Co3+, and Cu2+) (Delacote et al. 2009: Antochshuk et al. 2003). On the other hand, N-donor functionalized silica materials are capable of binding with acids (Pb2+, Ni2+ Zn2+, Co2+, Cu2+ Cr3+, etc.) (Benhamou et al. 2009). Fabrication of carbamoylphosphonic acid was also studied to quench heavy and transition metal ions (Co2+, Cd2+, Cu2+, Pb2+, Cr3+, Ni2+, Mn2+, and Zn2+) (Yantasee et al. 2003). Amino- and carboxyl- functionalized nanoparticles are reported to adsorb organic compounds like methylene blue, phenosafranine, and rhodamine В (Deka et al. 2014). Photoactive compound-functionalized silica materials are widely studied as catalysis for environmental remediation (Zaccariello et al. 2014; Corma and Garcia 2004). Photocatalyst-functionalized hydrophobic silica nanoparticles are reported to be efficient adsorbent and also remediate impurities of water(Kuwahara et al. 2009).