Controlling the properties of solid-liquid interfaces in silica nanopores via surface functionalization

This dissertation explores how functionalization of mesoporous silicas affects their solid-liquid interfacial properties. The research work is focused on carefully modifying pore surfaces of mesoporous silica with organic functional groups to create local environments that differ from the bulk medium. Chapter 1 is a general introduction to mesoporous silica nanoparticles (MSN) and a literature review of previous attempts to modify silica-water interface for different applications. Chapter 2 describes an effort to control local polarity at silica-water interface via surface functionalization of MSN. A local polarity scale was created using solvatochromic dye Prodan and interfacial polarity values were assigned to functionalized MSN pores. The effects of pore polarity on quenching of Nile Red fluorescence and on the vibronic band structure of pyrene were also studied. The results showed that the dielectric properties in the pores are different from the bulk water. We found that the catalytic activity of TEMPO for the aerobic oxidation of furfuryl alcohol in water improved when decreasing pore polarity. This work demonstrated that the activity of a nanoconfined catalyst can be modified by controlling the local polarity around it. Chapter 3 further explores the interfacial control of catalytic activity inside the nanometer pores of MSN. The activity of aminopropyl-functionalized mesoporous silica nanoparticles (AP-MSN) for the aldol condensation can be improved by using either a non-polar solvent or an aqueous media. In this work, a novel AP-MSN based catalytic system with combined action of water and low-local polarity environment is presented. Local polarity was tuned by introducing different surface densities of hexyl groups on AP-MSN. The dielectric constants of the hexyl modified silica-water interfaces were determined using the solvatochromic probe Prodan as discussed in Chapter 1. The activity of hexyl-modified AP-MSN in water increased with decreasing interfacial dielectric constants. In addition, aldol reactions with substituted substrates, and other C-C bond forming reactions such as Henry and Vinylogous aldol catalyzed by hexyl-modified AP-MSN in water were enhanced compared to those catalyzed by AP-MSN in water. An improved performance of AP-MSN for aldol condensation and similar reactions were achieved by combining the effects of hydrophobic environments and water at the catalyst-solvent interface. Chapter 4 demonstrates how the orientation and mobility of surface groups affects the strength of non-covalent interactions between a guest molecule and the mesoporous silica surface. In this study, we created different phenyl functionalized mesoporous silica samples with different orientations of phenyl groups relative to the pore surface, i.e. rigid perpendicular, variable orientation derived from a flexible ethylene linker, and rigid co-planar. The release of adsorbed Ibuprofen into simulated body fluid from these phenyl-functionalized silicas was analyzed using an adsorption-diffusion model. All phenyl-bearing materials showed lower Ibuprofen initial release rates than bare MSN. The materials with conformationally locked upright and co-planar phenyl groups had stronger interactions with Ibuprofen than those with mobile groups and bare MSN. The obtained results were consistent with DFT calculations. We demonstrated that we could control the kinetics and extent of Ibuprofen release by tuning the type and geometry of non-covalent interactions at the solid-liquid interface. Chapter 5 introduces an approach for controlling interfacial acid-based properties inside nanopores. We demonstrated that the silica-water interfacial pH of MSN can be tuned by functionalizing the pores with different acids and bases. To probe the interfacial pH, we grafted a modified pH sensitive dual emission fluorescent probe, SNARF-AP on silica surfaces. The fluorescence intensity ratio (I588/I635) of the probe at different bulk pH served as a calibration to assign pH values for functionalized mesoporous silica-water interfaces. We showed that interfacial pH varied as a function of the surface groups’ pKa and that it was different from the bulk pH. We attributed the differences to altering protonation/deprotonation equilibria on surface and to the interfacial potential that results from the surface charges. We demonstrated that effective screening of surface charges can be achieved by increasing the ionic strength of the solution. In addition, replacing MSN with a wider pore MSN-10 showed a similar effect. Both these factors affect the proton concentration in the vicinity of surface