Subdiffraction-limit two-photon fluorescence microscopy for GFP-tagged cell imaging.

We report applications of two-photon excitation fluorescence (2PEF) microscopy with subdiffraction-limit resolution for green-fluorescent-protein-tagged cell imaging. The microscope integrates 2PEF microscopy and stimulated emission depletion microscopy in one microscope that has the benefits of both techniques: intrinsic three-dimensional resolution, confined photobleaching, and subdiffraction-limit resolution. The subdiffraction-limit resolution was demonstrated by resolving green-fluorescent-protein-tagged caveolar vesicles located within a distance shorter than the diffraction limit of a regular 2PEF microscope, which is approximately 250 nm even with the best optics. The full width at half-maximum of the effective point-spread function for the 2PEF microscope was estimated to be approximately 54 nm.