  Single molecule applications, saturated pattern excitation microscopy, or
stimulated emission depletion (STED) microscopy demand for bright and highly
stable fluorescent dyes1,2. Despite of intensive research the choice of
fluorphores is still very limited. Typically a stable fluorescent dyes is
covalently attached to the target. This methodology brings forward a number of
limitations, in particular, in case of protein labeling. First of all the
fluorescent probes need to be attached selectively and site-specifically to
prevent unspecific background. This often requires single cysteine mutations
for covalent protein modification. Employing quantum dots allows overcoming
problems of photo-bleaching3-6. However, the downsides are their large size,
rendering the probe inaccessible to spatially confined architectures, issues in
biocompatibility due to proper particle coating, and cellular toxicity6-8. Here
we propose a new method to overcome the above outlined problems.
