Toward an Axial Nanoscale Ruler for Fluorescence Microscopy

Graphene- and metal-induced energy transfer for single-molecule imaging and live-cell nanoscopy with (sub)-nanometer axial resolution

Super-resolution fluorescence imaging that surpasses the classical optical resolution limit is widely utilized for resolving the spatial organization of biological structures at molecular length scales. In one example, single-molecule localization microscopy, the lateral positions of single molecules can be determined more precisely than the diffraction limit if the camera collects individual photons separately. Using several schemes that introduce engineered optical aberrations in the imaging optics, super-resolution along the optical axis (perpendicular to the sample plane) has been achieved, and single-molecule localization microscopy has been successfully applied for the study of 3D biological structures. Nonetheless, the achievable axial localization accuracy is typically three to five times worse than the lateral localization accuracy. Only a few exceptional methods based on interferometry exist that reach nanometer 3D super-resolution, but they involve enormous technical complexity and restricted sample preparations that inhibit their widespread application. We developed metal-induced energy transfer imaging for localizing fluorophores along the axial direction with nanometer accuracy, using only a conventional fluorescence lifetime imaging microscope. In metal-induced energy transfer, experimentally measured fluorescence lifetime values increase linearly with axial distance in the range of 0-100 nm, making it possible to calculate their axial position using a theoretical model. If graphene is used instead of the metal (graphene-induced energy transfer), the same range of lifetime values occurs over a shorter axial distance (~25 nm), meaning that it is possible to get very accurate axial information at the scale of a membrane bilayer or a molecular complex in a membrane. Here, we provide a step-by-step protocol for metal- and graphene-induced energy transfer imaging in single molecules, supported lipid bilayer and live-cell membranes. Depending on the sample preparation time, the complete duration of the protocol is 1-3 d.

Simultaneous Two-Angle Axial Ratiometry for Fast Live and Long-Term Three-Dimensional Super-Resolution Fluorescence Imaging

The application of optical microscopy in four-dimensional (spatial and temporal) super-resolution imaging poses challenges because of the requirement of a long acquisition time or high illumination intensity. In this paper, we introduce simultaneous two-angle axial ratiometry (STARII) for <20 nm axial super-resolution imaging and for fast and long-term imaging of live cells up to hundreds of frames per second. This method involves recording two raw images in two incident angle channels in the context of evanescent wave illumination and obtaining the corresponding intensity ratio. Furthermore, we demonstrate the combination of STARII with the lateral super-resolution method to resolve three-dimensional nanoscale structures of microtubules and to visualize the long-term dynamical plasma membrane curvature and fast remodeling of endoplasmic reticulum tubule meshwork and three-way junctions. These demonstrations indicate an important potential application of STARII in investigating nanoscale cellular complex processes in the native state.