Superresolution microscopy of the volume phase transition of pNIPAM microgels

• Superresolution microscopy applied to study the structure of microgel colloids. • Microgel radial density profiles revealed in real space. • Work set's the stage for the use of superresolution microscopy in materials research. • dSTORM may allow for nanoscale characterization of distinct polymeric subunits in 3D. Hierarchical polymer structures such as pNIPAM microgels have been extensively studied for their ability to undergo structural and physical transformations that can be controlled by external stimuli such as temperature, pH or solvent composition. However, a direct three-dimensional visualization of individual particles in-situ has so far been hindered by insufficient resolution, with optical microscopy, or contrast, with electron microscopy. In recent years superresolution microscopy techniques have emerged that can provide nanoscopic optical resolution. Here we report on the in-situ superresolution microscopy of dye-labelled submicron sized pNIPAM microgels revealing the internal density profile during swelling and collapse of individual particles. Using direct STochastic Optical Reconstruction Microscopy (dSTORM) we demonstrate a lateral optical resolution of 30 nm and an axial resolution of 60 nm.