Dynamics and Inclusion Chemistry in Mesoporous Channel Systems

Mesoporous materials made through the cooperative self-assembly and condensation of precursor building blocks and liquid-crystalline surfactants offer very large surface areas and pore diameters, control of the internal surface chemistry through molecular functionalization, and great flexibility regarding their morphology. For example, we study ways to impart orientation into the channel system of hexagonal mesoporous silica by means of host-guest chemistry in the larger channels of anodized alumina channels. Such structures are interesting hosts for inclusion chemistry such as the formation of templated periodic metal nanowire arrays in an insulating host. Mesoporous thin films are being studied as model host for the diffusion of guest molecules. In collaboration with the group of Prof. Christoph Bräuchle, we investigate the diffusion and stabilization of dye molecules and bioactive molecules in the silica nanochannels. Unprecedented insights into the diffusional behavior correlated with the real space structure of the channel system have been obtained. These studies also provide insights into the behavior of biomelecules such as DNA in silica hosts, which is of great interest for the design of nanoscale drug carriers for targeted drug delivery.

Key publications:

  • Periodic Mesoporous Organosilica in Confined Environments: A. Keilbach, M. Döblinger, R. Köhn, H. Amenitsch, T. Bein, Chemistry-a European Journal 2009, 15, 6645-6650.
    Periodic mesoporous organosilica (PMO) mesophases based on bis(triethoxysilyl)ethane were synthesized within the confined tubular environment of anodic alumina membrane (AAM) channels. The resulting mesophases were investigated by transmission small angle X-ray scattering (SAXS), nuclear magnetic resonance (NMR), nitrogen sorption, and transmission electron microscopy (TEM).

  • Tuning Single-Molecule Dynamics in Functionalized Mesoporous Silica: T. Lebold, L. A. Mühlstein, J. Blechinger, M. Riederer, H. Amenitsch, R. Köhn, K. Peneva, K. Müllen, J. Michaelis, C. Bräuchle, T. Bein, Chemistry-a European Journal 2009, 15, 1661-1672.
    Diffusion of single molecules of a substituted terrylene diimide dye in functionalized mesoporous silica films was monitored by single-molecule fluorescence microscopy. By varying the chemical nature and density of the functional groups, the diffusion dynamics of the dye molecules can be controlled precisely. The picture shows a sketch of a dye molecule in a pore, diffusion data for different phenyl functionalization densities, and the trajectory of one molecule in a cyanopropyl-functionalized film.

  • Vertical Columnar Block-Copolymer-Templated Mesoporous Silica via Confined Phase Transformation: B. Platschek, N. Petkov, D. Himsl, S. Zimdars, Z. Li, R. Köhn, T. Bein, J. Am. Chem. Soc. 2008, 130, 17362-17371.
    Upon the basis of a systematic investigation of the effects of interfacial interactions and different synthesis parameters on the resulting hierarchical mesophase, a salt-induced phase transformation was developed for efficient structural control. Samples with a columnar hexagonal 2D structure along the vertical channels of the AAM can be produced with ionic CTAB as template. However, when nonionic surfactants (Pluronic P123 and Brij 56) are used, samples with a circular hexagonal 2D structure perpendicular to the channels or phase mixtures of circular and columnar orientations are obtained. The behavior of ionic CTAB can be mimicked by adding inorganic salt to the nonionic template precursor solution, thus leading to a phase transformation toward columnar orientation.

  • Visualizing single-molecule diffusion in mesoporous materials: A. Zürner, J. Kirstein, M. Döblinger, C. Bräuchle, T. Bein, Nature 2007, 450, 705.
    Materials with mesoscale pores (that is, with diameters of several hundred nanometres) have many potential applications — including storage, separation and catalytic conversion — most of which depend on the diffusion of ‘guest’ molecules through the pores. Yet no method existed to correlate the guests’ motion with the structure of the material. This looks set to change with the development of a technique combining electron microscopy with optical single-molecule tracking to visualize molecular diffusion in a mesoporous material. For the first time, guest molecules (dyes in this case) can be ‘seen’ changing speed or direction in response to structural features of the host on a nanometre scale. This opens the way to a deeper understanding of the properties of mesoporous solids.