Spatial distribution of organic functional groups supported on mesoporous silica nanoparticles (2): a study by 1H triple-quantum fast-MAS solid-state NMR

The distribution of organic functional groups attached to the surface of mesoporous silica nanoparticles (MSNs) via co-condensation was scrutinized using 1D and 2D 1H solid-state NMR, including the triple-quantum/single-quantum (TQ/SQ) homonuclear correlation technique. The excellent sensitivity of 1H NMR and high resolution provided by fast magic angle spinning (MAS) allowed us to study surfaces with very low concentrations of aminopropyl functional groups. The sequential process, in which the injection of tetraethyl orthosilicate (TEOS) into the aqueous mother liquor was followed by dropwise addition of the organosilane precursor, resulted in deployment of organic groups on the surface, which were highly clustered even in a sample with a very low loading of ∼0.1 mmol g−1. The underlying mechanism responsible for clustering could involve fast aggregation of the aminopropyltrimethoxysilane (APTMS) precursor within the liquid phase, and/or co-condensation of the silica-bound molecules. Understanding the deposition process and the resulting topology of surface functionalities with atomic-scale resolution, can help to develop novel approaches to the synthesis of complex inorganic–organic hybrid materials

Interfacial Control of Catalytic Activity in the Aldol Condensation: Combining the Effects of Hydrophobic Environments and Water

Aminopropyl-functionalized mesoporous silica nanoparticles (AP-MSN) catalyze aldol condensations. The activity of AP-MSN decreases with increasing solvent polarity due to the stabilization of ion pairs formed between acidic silanol groups and the amines, which ultimately decreases the number of catalytically active amine sites. However, the reaction in water is faster than expected based on polarity, because water limits the formation of Schiff bases that are also responsible for blocking active sites. In this work, we combined the action of water with a low-local-polarity environment around the catalytic sites of AP-MSN to maximize active site availability and catalyst performance. We specifically demonstrate how the local polarity of AP-MSN can be controlled by modifying its surface with varying concentrations of hexyl groups, and how the dielectric constant of the silica-water interface can be determined using the solvatochromic probe Prodan. The catalytic activities of hexyl-modified AP-MSN in water were inversely proportional to their interfacial dielectric constants, and were significantly higher (roughly by a factor of 4) than those of AP-MSN in anhydrous solvents of comparable polarities. Producing low-local-polarity environments in aqueous AP-MSN also enhanced the sensitivity of the aldol reaction to the electronic effects of substituents in the substrate. The enhancement of catalytic activity by low interfacial polarity was also observed in other amine-catalyzed C-C bond forming reactions such as the Henry and Vinylogous aldol reactions. Overall, our results demonstrate that the catalytic activity of AP-MSN can be controlled by the synergistic action of water and a low interfacial dielectric constant

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

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.</p

Control of interfacial pH in mesoporous silica nanoparticles via surface functionalization

The pH at silica-water interfaces (pHint) was measured by grafting a dual emission fluorescent probe (SNARF) onto the surface of mesoporous silica nanoparticles (MSN). The values of pHint of SNARF-MSN suspended in water were different from the pH of the bulk solution (pHbulk). The addition of acid or base to aqueous suspensions of SNARF-MSN induced much larger changes in pHbulk than pHint, indicating that the interface has buffering capacity. Grafting additional organic functional groups onto the surface of SNARF-MSN controls the pHint of its buffering region. The responses of pHint to variations in pHbulk are consistent with the acid/base properties of the surface groups as determined by their pKa and are affected by electrostatic interactions between charged interfacial species as evidenced by the dependence of ζ-potential on pHbulk. Finally, as a proof of principle, we demonstrate that the hydrolysis rate of an acid-sensitive acetal can be controlled by adjusting pHint via suitable functionalization of the MSN surface. Our findings can lead to the development of nanoreactors that protect sensitive species from adverse conditions and tune their chemical reactivity.</p

Spatial distribution of organic functional groups supported on mesoporous silica nanoparticles (2): a study by 1H triple-quantum fast-MAS solid-state NMR

The distribution of organic functional groups attached to the surface of mesoporous silica nanoparticles (MSNs) via co-condensation was scrutinized using 1D and 2D 1H solid-state NMR, including the triple-quantum/single-quantum (TQ/SQ) homonuclear correlation technique. The excellent sensitivity of 1H NMR and high resolution provided by fast magic angle spinning (MAS) allowed us to study surfaces with very low concentrations of aminopropyl functional groups. The sequential process, in which the injection of tetraethyl orthosilicate (TEOS) into the aqueous mother liquor was followed by dropwise addition of the organosilane precursor, resulted in deployment of organic groups on the surface, which were highly clustered even in a sample with a very low loading of ∼0.1 mmol g−1. The underlying mechanism responsible for clustering could involve fast aggregation of the aminopropyltrimethoxysilane (APTMS) precursor within the liquid phase, and/or co-condensation of the silica-bound molecules. Understanding the deposition process and the resulting topology of surface functionalities with atomic-scale resolution, can help to develop novel approaches to the synthesis of complex inorganic–organic hybrid materials.</p

Fine-tuning the release of molecular guests from mesoporous silicas by controlling the orientation and mobility of surface phenyl substituents

Phenyl-functionalized mesoporous silica materials were used to explore the effect of non-covalent interactions on the release of Ibuprofen into simulated body fluid. Variations in orientation and conformational mobility of the surface phenyl groups were introduced by selecting different structural precursors: 1) a rigid upright orientation was obtained using phenyl groups directly bound to surface Si atoms (Ph-MSN), 2) mobile groups were produced by using ethylene linkers to connect phenyl groups to the surface (PhEt-MSN), and 3) groups co-planar to the surface were obtained by synthesizing a phenylene-bridged periodic mesoporous organosilica (Ph-PMO). The Ibuprofen release profiles from these materials and non-functionalized mesoporous silica nanoparticles (MSN) were analyzed using an adsorption-diffusion model. The model provided kinetic and thermodynamic parameters that evidenced fundamental differences in drug-surface interactions between the materials. All phenyl-bearing materials show lower Ibuprofen initial release rates than bare MSN. The conformationally locked Ph-MSN and Ph-PMO have stronger interactions with the drug (negative ΔG of adsorption) than the flexible PhEt-MSN and bare MSN (positive ΔG of adsorption). These differences in strength of adsorption are consistent with differences between interaction geometries obtained from DFT calculations. B3LYP-D3-optimized models show that π-π interactions contribute more to drug adsorption than H-bonding with silanol groups. The results suggest that the type and geometry of interactions control the kinetics and extent of drug release, and should therefore serve as a guide to design new drug delivery systems with precise release behaviors customized to any desired target.This is a manuscript of an article published as Manzano, J. Sebastián, Dilini Singappuli-Arachchige, Bosky L. Parikh, and Igor I. Slowing. "Fine-tuning the release of molecular guests from mesoporous silicas by controlling the orientation and mobility of surface phenyl substituents." Chemical Engineering Journal 340 (2018): 73-80. DOI: 10.1016/j.cej.2017.12.015. Copyright 2017 Elsevier B.V. Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0). Posted with permission