Simple and aberration-free 4color-STED--multiplexing by transient binding.

Most fluorescence microscopy experiments today require a multicolor-capable setup, e.g. to study the interaction between different proteins. Multicolor capabilities are also well desirable for superresolution images. However, especially for STED (Stimulated Emission Depletion) microscopy, which requires two laser lines for a single color, multicolor imaging is technically challenging. Here we present a straightforward, easy-to-implement method to extend a single-color fluorescence (STED) microscope to a multichannel microscope without the need of modifying the optical setup. Therefore, we use a labeling technique based on complementary DNA sequences: a single-stranded short DNA sequence is attached to each structure to be imaged, different colors for labeling different features are represented by different sequences. Within the imaging process, the corresponding complementary sequence labeled with an organic fluorophore is added and transiently binds to the corresponding structure. After imaging, the labeled sequence is washed away and replaced by a second fluorescently labeled DNA strand complementary to the sequence bound to another feature. This way, multiplexing is achieved using only one arbitrary fluorophore, therefore aberrations are avoided.