Bourg, N. et al. Nat. Photonics 9, 587–593 (2015).

Although super-resolution microscopy methods such as photoactivated localization microscopy and direct stochastic optical reconstruction microscopy (dSTORM) can improve lateral resolution by tenfold compared to traditional light microscopy, their axial resolution is still diffraction-limited. To bypass this limitation, Bourg et al. developed direct optical nanoscopy with axially localized detection (DONALD). DONALD combines conventional dSTORM with the detection of a fluorescent probe's evanescent light—specifically, its supercritical-angle fluorescence emission. The latter measurement allows for accurate distance measurement between the probe and up to ∼150 nm from the coverslip, allowing for an overall isotropic resolution of 20 nm. Using this method, the authors were able to demonstrate isotropic super-resolution imaging of actin and microtubules in mammalian cells.