Highly oriented mesoporous silica channels synthesized in microgrooves and visualized with single-molecule diffusion.

A novel synthesis method for large-pore, well-aligned 2D hexagonal mesoporous silica thin films is reported. The alignment was achieved by confinement in poly(dimethylsiloxane) (PDMS) microgrooves without the necessity of additional forces (such as electric fields). We describe the influence of various experimental conditions including the way the grooves are filled, surface modification at the solid/liquid interfaces, and the height-to-width ratio of the microgrooves on mesopore alignment. With this technique, highly oriented mesoporous silica channels can be obtained at a length scale of several millimeters. For a nondestructive, detailed, and wide-ranging structural and dynamic characterization of the as-synthesized mesochannel silica network, dye molecules were incorporated into the channels at concentrations suitable for single-molecule microscopy. A "maximum projection" of individual frames recorded with a fluorescence microscope immediately gives a global overview ("map") of the pore structure, thus providing direct feedback for tuning synthesis conditions. In addition, deeper insights into the real nanoscale structure of the mesoporous silica framework were obtained through high-accuracy single-molecule tracking experiments. The high spatial accuracy of the experiments allowed for the direct observation of jumps of single dye molecules between individual channels in the mesoporous silica host. Nevertheless, due to the low concentration of defects, the diffusion could be described as a 1D random walk where the molecules diffuse along the highly oriented, parallel channels and sometimes switch from channel to channel through small defects in the pore walls. Furthermore, it could be shown with single-molecule microscopy that template removal and calcination of the aligned films results in an increased defect concentration; however, the overall order of the structures remained intact.

The complexity of mesoporous silica nanomaterials unravelled by single molecule microscopy.

Mesoporous silica nanomaterials are a novel class of materials that offer a highly complex porous network with nanometre-sized channels into which a wide amount of differently sized guests can be incorporated. This makes them an ideal host for various applications for example in catalysis, chromatography and nanomedicine. For these applications, analyzing the host properties and understanding the complicated host-guest interactions is of pivotal importance. In this perspective we review some of our recent work that demonstrates that single molecule microscopy techniques can be utilized to characterize the porous silica host with unprecedented detail. Furthermore, the single molecule studies reveal sample heterogeneities and are a highly efficient tool to gain direct mechanistic insights into the host-guest interactions. Single molecule microscopy thus contributes to a thorough understanding of these nanomaterials enabling the development of novel tailor-made materials and hence optimizing their applicability significantly.

Nanostructured silica materials as drug-delivery systems for Doxorubicin: single molecule and cellular studies.

We apply mesoporous thin silica films with nanometer-sized pores as drug carriers and incorporate the widely used anticancer drug Doxorubicin. Through single-molecule based measurements, we gain mechanistic insights into the drug diffusion inside the mesoporous film, which governs the drug-delivery at the target-site. Drug dynamics inside the nanopores is controlled by pore size and surface modification. The release kinetics is determined and live-cell measurements prove the applicability of the system for drug-delivery. This study demonstrates that mesoporous silica nanomaterials can provide solutions for current challenges in nanomedicine.

Tuning single-molecule dynamics in functionalized mesoporous silica.

Mesoporous silica materials are promising host structures for diverse applications in nanoscience. Many applications can profit significantly from the ability to influence guest dynamics in the host matrix. To this end, we introduce covalently attached organic functionalization into the walls of mesoporous silica networks. Using single-molecule fluorescence microscopy, we study the diffusion behavior of single terrylene diimide dye molecules in functionalized mesoporous silica films. We show that, through variation of the chemical nature and density of the functional groups, the diffusion dynamics of the dye molecules, in the presence of the surfactant template, can be controlled precisely. The mean diffusion coefficient of the dye molecules increases or decreases depending on the functional group attached to the silica wall. This allows fine-tuning of the diffusion dynamics of the dye by approximately one order of magnitude. The observed changes in the mean diffusion coefficients can be explained by shielding of hydroxyl groups on the silica surface in combination with changes in the rigidity of the micellar packing in the film, as well as direct interactions between the functional groups and the dye molecules.