Colloidal Porous Nanoparticles - Synthesis and Functionalization of Nanostructured Aluminosilicates and Silicas

Colloidal porous hosts in the form of microporous aluminosilicate (zeolite) or mesoporous silica nanoparticles are attractive materials for a wide range of potential applications, i.e. controlled release drug delivery systems. However, many fundamental challenges still remain in this relatively young research field. The following work focuses on overcoming some of the present limitations by developing new concepts for the synthesis and functionalization of porous nanoparticles. The number of zeolite structures available for the synthesis of stable colloidal suspensions is very limited when compared to the large number of known frameworks in bulk materials. A novel class of zeolite templates in the form of metal ammines was developed by taking advantage of the unique synthesis conditions typical in colloidal zeolite systems, i.e. low temperatures and low alkalinity. Square planar copper(II) tetraammine complexes were employed as co-templates in the synthesis of nanosized EDI-type molecular sieves. It was shown that the complexes are the key elements responsible for formation and growth of the zeolite nanoparticles, and their role in the crystallization process was thoroughly investigated. Substitution of the copper complexes by isostructural palladium and platinum species was demonstrated. By employing templates with similar shapes but different effects on the nucleation rate it was possible to drastically decrease the particle size by several factors in comparison to previously known colloidal zeolite systems and to generate stable suspensions of non-agglomerated EDI-type nanocrystals with diameters below 20 nm. The size and morphology of mesoporous silica nanoparticles was controlled by cocondensation with additives, i.e. phenyltriethoxysilane, and subsequent simultaneous removal of the functional groups and template molecules by oxidation with hydrogen peroxide in a simple one-pot reaction. Conversion of colloidal mesoporous silica systems with metalorganic reagents was demonstrated. The key step for avoiding particle agglomeration and coalescence processes involves the removal of water from the mesopores at temperatures below 90 °C either by hydrolysis of triethyl orthoformate or by vapour adsorption from the gas phase. In a joint project with Alexander Darga from our group, thin films of different phenylsubstituted mesoporous silica nanoparticles were deposited on quartz crystal microbalance chips in order to probe the intrapore surfaces by toluene sorption. It was shown that samples prepared by grafting and co-condensation approaches bearing similar surface densities of functional groups display considerably different toluene heats of adsorption. Furthermore, a novel concept for the selective functionalization of mesoporous silica nanoparticles was developed. By using a time-delayed co-condensation approach, functional groups can be completely dispersed inside the channels, concentrated in parts of the mesopores, or exclusively placed on the external surface depending on the time of addition. Aminopropyl was used as a representative functionality in order to determine the density of functional groups on the outer surface via zeta potential measurements. Staining with iridium cations and subsequent scanning transmission electron microscopy studies allowed the visualization of metal clusters with different radial distributions depending on the addition time of the organosilane component. In contrast to grafting approaches, it was possible to easily adjust the concentration of functional groups on the outer surface by variation of the organosilane to silane ratio.