Enhanced brightness and photostability of cyanine dyes by supramolecular containment

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.