Excitonic Processes in a Conjugated Polyelectrolyte Complex

In aqueous solution, a di-sulfonated phenylenevinylene polymer (DPS-PPV) forms a complex with non-ionic poly(vinyl alcohol) (PVA) leading to absorption spectroscopic shifts and a dramatic (6-fold) increase in DPS-PPV fluorescence intensity. Spectroscopic investigations demonstrate that the complexation with PVA and other neutral polymers results in conformational changes in the DPS-PPV chains that lead to the removal of non-fluorescent energy traps and results in the observed increase in fluorescence in the bulk solution. Single molecule fluorescence measurements of DPS-PPV chains dispersed on glass and in PVA films confirm that efficient exciton energy transfer occurs within each photo-excited DPS-PPV chain and that the observed increase in fluorescence intensity in the PVA film environment is also associated with fewer quenching sites. The results highlight the importance of conjugated polyelectrolyte conformation on exciton relaxation pathways.

Protein cages and synthetic polymers: a fruitful symbiosis for drug delivery applications, bionanotechnology and materials science

Protein cages have become essential tools in bionanotechnology due to their well-defined, monodisperse, capsule-like structure. Combining them with synthetic polymers greatly expands their application, giving rise to novel nanomaterials fore.g.drug-delivery, sensing, electronic devices and for uses as nanoreactors.

Highly fluorescent conjugated polyelectrolyte for protein sensing and cell-compatible chemosensing applications

Using a highly fluorescent, water-soluble polymer derived from a triazine-bridged copolymer (DTMSPV), we explored the tunable fluorescence properties of the water-soluble DTMSPV by solvent polarity to function as a fluorescence sensory probe for protein sensing. The green-blue fluorescence from DTMSPV was significantly enhanced in the presence of bovine serum albumin through hydrophobic interactions. Meanwhile, complete quenching of the fluorescence from DTMSPV occurred in the presence of hemoglobin through iron complexation with the polyelectrolyte. In addition, the DTMSPVs were highly fluorescent and permeated into living mesenchymal stem cells (MSCs), enabling effective imaging of the MSCs. This permeation into stem cells is crucial to the detection of Al(3+) in living MSCs. The interaction between the triazine units in DTMSPV with the Al(3+) ions allows for the detection of Al(3+) in living cells. Thus, a strong fluorescence from living MSCs pretreated with DTMSPV was quenched as a function of the Al(3+) concentration, confirming that DTMSPV is a cell-permeable fluorescent polymer that can function as a versatile probe to detect Al(3+) in living cells.

Self-assembling semiconducting polymers--rods and gels from electronic materials

In an effort to favor the formation of straight polymer chains without crystalline grain boundaries, we have synthesized an amphiphilic conjugated polyelectrolyte, poly(fluorene-alt-thiophene) (PFT), which self-assembles in aqueous solutions to form cylindrical micelles. In contrast to many diblock copolymer assemblies, the semiconducting backbone runs parallel, not perpendicular, to the long axis of the cylindrical micelle. Solution-phase micelle formation is observed by X-ray and visible light scattering. The micelles can be cast as thin films, and the cylindrical morphology is preserved in the solid state. The effects of self-assembly are also observed through spectral shifts in optical absorption and photoluminescence. Solutions of higher-molecular-weight PFT micelles form gel networks at sufficiently high aqueous concentrations. Rheological characterization of the PFT gels reveals solid-like behavior and strain hardening below the yield point, properties similar to those found in entangled gels formed from surfactant-based micelles. Finally, electrical measurements on diode test structures indicate that, despite a complete lack of crystallinity in these self-assembled polymers, they effectively conduct electricity.

Relative size selection of a conjugated polyelectrolyte in virus-like protein structures

A conjugated polyelectrolyte poly[(2-methoxy-5-propyloxy sulfonate)-phenyl-ene vinylene] (MPS-PPV) drives the assembly of virus capsid proteins to form single virus-like particles (VLPs) and aggregates with more than two VLPs, with a relative selection of high molecular weight polymer in the latter.

Using polymer conformation to control architecture in semiconducting polymer/viral capsid assemblies

Cowpea chlorotic mottle virus is a single-stranded RNA plant virus with a diameter of 28 nm. The proteins comprising the capsid of this virus can be purified and reassembled either by themselves to form hollow structures or with polyanions such as double-stranded DNA or single-stranded RNA. Depending on pH and ionic strength, a diverse range of structures and shapes can form. The work presented here focuses on using these proteins to encapsulate a fluorescent polyanionic semiconducting polymer, MPS-PPV (poly-2-methoxy-5-propyloxy sulfonate phenylene vinlyene), in order to obtain optically active virus-like particles. After encapsulation, fluorescence from MPS-PPV shows two distinct peaks, which suggests the polymer may be in two conformations. A combination of TEM, fluorescence anisotropy, and sucrose gradient separation indicate that the blue peak arises from polymer encapsulated into spherical particles, while the redder peak corresponds to polymers contained in rod-like cages. Ionic strength during assembly can be used to tune the propensity to form rods or spheres. The results illustrate the synergy of hybrid synthetic/biological systems: polymer conformation drives the structure of this composite material, which in turn modifies the polymer optical properties. This synergy could be useful for the future development of synthetic/biological hybrid materials with designated functionality.

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.

Encapsulation of semiconducting polymers in vault protein cages

We demonstrate that a semiconducting polymer [poly(2-methoxy-5-propyloxy sulfonate phenylene vinylene), MPS-PPV] can be encapsulated inside recombinant, self-assembling protein nanocapsules called "vaults". Polymer incorporation into these nanosized protein cages, found naturally at approximately 10,000 copies per human cell, was confirmed by fluorescence spectroscopy and small-angle X-ray scattering. Although vault cellular functions and gating mechanisms remain unknown, their large internal volume and natural prevalence within the human body suggests they could be used as carriers for therapeutics and medical imaging reagents. This study provides the groundwork for the use of vaults in encapsulation and delivery applications.