Cholesterol-mediated lipid interactions are thought to have a functional role in many membrane-associated processes such as signalling events. Although several experiments indicate their existence, lipid nanodomains ('rafts') remain controversial owing to the lack of suitable detection techniques in living cells. The controversy is reflected in their putative size of 5-200 nm, spanning the range between the extent of a protein complex and the resolution limit of optical microscopy. Here we demonstrate the ability of stimulated emission depletion (STED) far-field fluorescence nanoscopy to detect single diffusing (lipid) molecules in nanosized areas in the plasma membrane of living cells. Tuning of the probed area to spot sizes approximately 70-fold below the diffraction barrier reveals that unlike phosphoglycerolipids, sphingolipids and glycosylphosphatidylinositol-anchored proteins are transiently ( approximately 10-20 ms) trapped in cholesterol-mediated molecular complexes dwelling within <20-nm diameter areas. The non-invasive optical recording of molecular time traces and fluctuation data in tunable nanoscale domains is a powerful new approach to study the dynamics of biomolecules in living cells.

We undertake a comprehensive study of the inverse square root dependence of spatial resolution on the saturation factor in stimulated emission depletion (STED) microscopy and generalize it to account for various focal depletion patterns. We used an experimental platform featuring a high quality depletion pattern which results in operation close to the optimal optical performance. Its superior image brightness and uniform effective resolution <25 nm are evidenced by imaging both isolated and self-organized convectively assembled fluorescent beads. For relevant saturation values, the generalized square-root law is shown to predict the practical resolution with high accuracy.

Fluorescent proteins that can be reversibly photoswitched between a fluorescent and a nonfluorescent state are important for innovative microscopy schemes, such as protein tracking, fluorescence resonance energy transfer imaging, sub-diffraction resolution microscopy and others. However, all available monomeric reversibly switchable fluorescent proteins (RSFPs) have similar properties and switching characteristics, thereby limiting their use. Here, we introduce two bright green fluorescent RSFPs, bsDronpa and Padron, generated by extensive mutagenesis of the RSFP Dronpa, with unique absorption and switching characteristics. Whereas bsDronpa features a broad absorption spectrum extending into the UV, Padron displays a switching behavior that is reversed to that of all green fluorescent RSFPs known to date. These two RSFPs enable live-cell fluorescence microscopy with multiple labels using a single detection color, because they can be distinguished by photoswitching. Furthermore, we demonstrate dual-color fluorescence microscopy with sub-diffraction resolution using bsDronpa and Dronpa whose emission maxima are separated by <20 nm.

By combining the photoswitching and localization of individual fluorophores with spectroscopy on the single molecule level, we demonstrate simultaneous multicolor imaging with low crosstalk and down to 15 nm spatial resolution using only two detection color channels. The applicability of the method to biological specimens is demonstrated on mammalian cells. The combination of far-field fluorescence nanoscopy with the recording of a single switchable molecular species at a time opens up a new class of functional imaging techniques.

We demonstrate nanoscale resolution in far-field fluorescence microscopy using reversible photoswitching and localization of individual fluorophores at comparatively fast recording speeds and from the interior of intact cells. These advancements have become possible by asynchronously recording the photon bursts of individual molecular switching cycles. We present images from the microtubular network of an intact mammalian cell with a resolution of 40 nm.