Superresolved fluorescence microscopy of soft matter: from the development of novel photoswitches to the visualization of compartmentalized microgels*

The elucidation of the structure of nano-compartmentalized soft matter systems, such as microgels (nanogels) or block copolymer assemblies is challenging since they do not possess strong contrast for electron microscopy, and since classical fluorescence microscopy fails due to the fact that their structures are significantly smaller than the diffraction limit of optical light. However, modern superresolved fluorescence microscopy methods such as dSTORM (direct stochastic optical reconstruction microscopy) reach resolutions down to typically 10-30 nm and, therefore, are highly suited to fill this gap of structural in situ imaging. Many concepts and dye classes can be readily transferred from the experience of biological or medical superresolution imaging. The fact that synthetic soft matter systems are often non-aqueous and apolar, however, creates challenges that have to be overcome.
In this contribution, the success of superresolution fluorescence imaging methods to address research questions concerning microgels and their application in determining the spatial distribution of implemented functionalities will be demonstrated. In addition, the challenge to gain reliable 3D information in fluorescence and transmission electron microscopy (TEM) imaging will be discussed. Novel diarylethene photoswitches will be presented for superresolution fluorescence microscopy of the apolar nanostructures within block copolymer assemblies allowing their imaging with PALM (photoactivated localization microscopy), SOFI (super-resolution optical fluctuation imaging) and RESOLFT (reversible saturable optical linear fluorescence transitions) microscopy.