Investigating Molecular Interactions in Biosensors Based on Fluorescence Resonance Energy Transfer

We describe here molecular sensors based on fluorescence resonance energy transfer (FRET) between sulforhodamine 101 (donor) and polydiacetylene (PDA, acceptor) for selective detection of biomolecules (streptavidin) in solution. These novel FRET-based systems primarily utilize changes in J values (the spectral overlap between the emission of the donor and absorption of the acceptor) for the modulation of FRET efficiency between donors and acceptors. The biotin−streptavidin interactions were used as a sensing model system to test our sensor response. In this paper, four different biotin-tagged lipids were used as receptors to investigate the effect of interactions between ligand−receptors on the FRET efficiency. The biotin was covalently linked to the liposome surface when using biotin-tagged diacetylene; whereas the biotin-tagged lipids with hydrophobic chains but without diacetylene functionalities provided noncovalently inserted lipids in liposomes. The polymerized liposomes, consisting of sulforhodamine-tagged-diacetylene and receptors linked to lipids in different molar ratios, were investigated using UV−vis and steady-state emission spectroscopy. The liposome solution yielded a weak emission from the donor after photopolymerization of diacetylene monomers due to energy transfer from the donor to PDA backbone chains (acceptors). The addition of streptavidin resulted in increase of the donor emission, which was due to a decrease in the FRET efficiency from donor to PDA. These studies are intended to enhance our basic understanding of the interactions at the molecular level and will guide us for the fabrication of highly sensitive and selective biosensors.