  Amphiphilic molecules such as surfactants, lipids and block copolymers can be
assembled into bilayers and form vesicles. Fluorescent membrane labeling
methods require the use of dye molecules that can be inserted in the bilayers
at different stages of the synthesis. To our knowledge there is no generalized
method for labeling preformed vesicles. Herein we develop a versatile protocol
that is suitable to both surfactant and lipid preformed vesicles and requires
no separation or purification steps. Based on the lipophilic carbocyanine green
dye PKH67, the methodology is assessed on zwitterionic phosphatidylcholine
vesicles. To demonstrate its versatility, it is applied to dispersions of
anionic or cationic vesicles, such as a drug administrated to premature infants
with respiratory distress syndrome, or a vesicle formulation used as a fabric
softener for homecare applications. By means of fluorescence microscopy we then
visualize the interaction mechanisms of nanoparticles crossing live cell
membranes and of surfactant adsorbing on cotton fabric. These results highlight
the advantages of a membrane labeling technique that is simple and applicable
to a large number of soft matter systems.
