  Ultrasensitive detection and real-time monitoring of biological processes can
benefit significantly from the improved brightness and photostability of the
popular organic dyes such as cyanines. Here, using a model cyanine dye, Cy3, we
demonstrate that brightness and photostability of the dye is significantly
altered when trapped in a molecular container, e.g. cucurbit[n]urils (CB[n])
and cyclodextrins (CD).Through computational modeling, we predicted that Cy3
forms a stable inclusion complex with three different hosts, CB[7], beta-CD,
and methyl-beta-CD, which was further confirmed by single-molecule diffusion
measurements using fluorescence correlation spectroscopy. The effect of
supramolecular encapsulation on Cy3 photophysical properties was found to be
highly host-specific. Up to three-fold increase in brightness of Cy3 was
observed when the dye was trapped in methyl-beta-CD, due to an increase in both
dye absorption and quantum yield. Steady-state and time-resolved spectroscopy
of the various complexes revealed that host polarizability and restricted
mobility of the dye in the host both contribute to the observed increase in
molecular brightness. Furthermore, entrapment of the dye molecule in CDs
resulted in a marked increase in dye photostability, whereas the dye degraded
more rapidly in CB[7]. These results suggest that the changes in photophysical
properties of the dye afforded by supramolecular encapsulation are highly
dependent on the host molecule. The reported improvement in brightness and
photostability together with the excellent biocompatibility of cyclodextrins
makes supramolecular encapsulation a viable strategy for routine dye
enhancement.
