Fluorescence correlation spectroscopy (FCS) is an experimental technique allowing studies of the diffusion properties of fluorescent tracers in various environments with very high sensitivity, down to a single molecule level. The method is based on detecting the fluctuations of the fluorescent light intensity caused by the diffusion of the tracers through a small observation volume (<1 μm³), formed by the focus of a confocal microscope. A correlation analysis of these fluctuations provide information on the diffusion rate of the tracers and therefore on their size and the properties of the environment. Tracers can be single fluorescent molecules or macromolecules, quantum dots, fluorescent nanoparticles, etc.

In particular we use FCS to study diffusion in inhomogeneous media such as polymer melts, cross-linked polymer networks and nano-porous materials, where the diffusion coefficient can depend on the tracer size and the length scale probed.
Furthermore, we employ FCS to investigate the size, conformational changes and aggregation behavior of fluorescently labeled macromolecules such as: synthetic polymers and copolymers, dendrimers, proteins or DNAs. We pay special attention on the formation of fluorescently labeled vesicles and micelles. The technique is also extremely suitable tool to probe dynamics on liquid-liquid interfaces. We apply FCS to stydy the diffusion of quantum dots, dendrimers and small dye molecules on water-alcane interfaces at single molecule level.
Recently, we have developed a special setup for total internal reflection fluorescence cross-correlation spectroscopy, in order to measure the flow of a liquid close (< 200 nm) to a solid surface. The aim is to better understand the hydrodynamic boundary condition of a liquid at a solid surface.

• T. Cherdhirankorn, G. Floudas, H.-J. Butt, K. Koynov:
  Effects of Chain Topology on the Tracer Diffusion in Star Polyisoprenes.
  Macromolecules, 42, 9183-9189 (2009)
• T. Cherdhirankorn, M. Retsch, U. Jonas, H.-J. Butt, K. Koynov
  Tracer Diffusion in Silica Inverse Opals.
  Langmuir, 26, 10141-10146 (2010)
• S. Yordanov, A. Best, H.-J. Butt, K. Koynov:
  Direct studies of liquid flows near solid surfaces by total internal reflection fluorescence cross-correlation spectroscopy.
  Optics Express, 17, 21149 (2009)
• D. Wang, S. Yordanov, H. Paroor, A. Mukhopadhyay, C. Li, H.-J. Butt, K. Koynov:
  Probing Diffusion of single nanoparticles at water-oil interfaces with fluorescence correlation spectroscopy.
  Small, 7, 3502-3507 (2011)