Assembly of zwitterionic phospholipid/conjugated polyelectrolyte complexes: structure and photophysical properties

We report on the formation of complexes between zwitterionic phospholipid vesicles and an anionic fluorescent conjugated polyelectrolyte and the effect of mono- and divalent cations on the photophysical properties of these complexes. Our goal is to gain an understanding of the interplay of morphology and exciton transport in these complexes, information that is critical to designing efficient lipid/conjugated polymer-based sensors. Our studies further underscore the potential application of lipid/conjugated polymer complexes in light-harvesting devices. Our work focuses on the negatively charged conjugated polyelectrolyte poly[5-methoxy-2-(3-sulfopropoxy)-1,4-phenylenevinylene] (MPS-PPV) and its interaction with the zwitterionic lipid dioleoylphosphatidylcholine (DOPC). We utilize monovalent and divalent cations as a tool to control and explore the interaction of MPS-PPV with lipids. We show that Ca(2+) ions promote the complexation of zwitterionic lipids and MPS-PPV in comparison to Na(+) ions. The addition of increasing amounts of zwitterionic phospholipids in the form of vesicles gradually disrupts MPS-PPV aggregates albeit vesicle structure is preserved in Na(+) buffer. Lipid complexation and the resulting MPS-PPV aggregate disruption produces an intensity enhancement and blue shifting of the MPS-PPV emission peak. In the absence of Ca(2+), the intensity enhancement and blue shift reach a plateau at larger than a 10:1 lipid/MPS-PPV monomer mole ratio. In the presence of Ca(2+), a plateau is reached at equimolar concentrations of MPS-PPV and lipid. Vesicle particle coalescence and agglomerate formation are observed herein. Lipid complexation and concomitant MPS-PPV shielding is shown to diminish the quenching of MPS-PPV emission by water-soluble quencher methyl viologen. FRET experiments conducted with membrane-intercalating acceptor dye DiD further underscore the large lipid/polymer interaction mediated by Ca(2+). We observe efficient light harvesting and MPS-PPV-amplified emission quenching in Ca(2+) buffer and to a lesser extent in Na(+) buffer. Our results highlight how the interplay of a zwitterionic lipid, cations, and buffer, in combination with the conjugated polyelectrolyte MPS-PPV, provides rich diversity in architecture and photophysical properties.