Department of Physics, Freie Universität Berlin, Berlin, Germany
Apart from standard visualization techniques more recent developments such as Fluorescence Lifetime Imaging Microscopy (FLIM) and Single Molecule Microscopy become important tools to meet the needs of investigating newly developed nanocarriers and drug delivery [1, 2]. FLIM gains its information from the excited state fluorescence decay curves that depend on the fluorescing molecule. In addition, time-resolved fluorescence is sensitive to the environment and biomolecular interactions [3, 4]. Thereby FLIM has the potential to provide image contrast enhancement and high sensitivity compared to conventional fluorescence microscopy, and at the same time, it allows for the discrimination of target molecule fluorescence (e.g. of nanocarrier or drug) from the autofluorescent background [5-7]. Furthermore, our novel FLIM analysis method provides excellent statistical significance and high spatial resolution, resulting in unprecedented insights into the mechanism of action of the target molecules. Recent results from investigations of newly developed nanocarriers in topical skin therapy will be presented. Furthermore, studying fluorescent molecules in turbid tissue, such as skin, at the single molecule level is challenging. Two examples of detecting drugs/drug mimetics at the single molecule level will be given: 1) the diffusion of a model drug in the stratum corneum of the skin , and 2) visualization of drug distribution in lipid nanoparticles  for skin applications.
 Vogt, A et al.: J Control Release 2016, 242: 2-15.
 Alexiev, U et al.: Eur J Pharm Biopharm 2017, http://dx.doi.org/10.1016/j.ejpb.2017.01.005.
 Boreham, A et al.: Langmuir 2014, 30: 1686-1695.
 Boreham, A et al.: Molecules 2015, 21:22.
 Boreham, A et al.: ASC Med.Chem. Lett. 2011, 2: 724–728.
 Alnasif, N et al.: J Control Release 2014, 185: 45-50.
 Ostrowski, A et al.: Nanomedicine 2015, 10: 1571-1581.
 Volz, P et al.: Int J Mol Sci 2014,16: 6960-6977.
 Boreham, A et al.: Eur J Pharm Biopharm 2017, 110:31-38.