The constrained diffusion of molecules, macromolecules and nanoparticles in nanoporous media plays an important role in wide range of applications ranging from water purification and drug delivery to tissue engineering. With increasing complexity of such systems, it’s become evident that the diffusion behavior depends strongly on the diffusant size, its interactions with the pore walls and the length scale probed. Using fluorescence correlation spectroscopy (FCS) such studies can be performed on a single molecule level which provides an important inside on the corresponding processes and interactions. For example we have employed FCS to study the diffusion of various dye molecules and spherical nanoparticles in liquid filled silica inverse opals. The inverse opals consisted of a nanoporous silica scaffold spanning a hexagonal crystal of spherical voids with variable sizes and openings in the nanometer domain. The experimental results were compared with Brownian dynamics simulations performed by C. Licos (Vienna) that helped to gain better understanding of the relevant physical processes.

Currently we are interested in the effect of the tracer shape and rigidity, e.g. elongated solid particles, polymers, DNA on the diffusion behavior. Furthermore we are working on coating the inverse opals internal surface with stimuli responsive polymer brushes (e.g. PNIPAAm) and exploring the possibility to control diffusion by external stimuli.

• T. Cherdhirankorn, M. Retsch, U. Jonas, H.-J. Butt, K. Koynov: Tracer Diffusion in Silica Inverse Opals. Langmuir, 26, 10141-10146 (2010)
• R. Raccis, A. Nikoubashman, M. Retsch, U. Jonas, K. Koynov, H.-J. Butt, C. Likos, G. Fytas: Confined Diffusion in Periodic Porous Nanostructures. ACS Nano, 5, 4607–4616 (2011)