Enhancing the photostability of poly(phenylene ethynylene) for single particle studies

Enhanced photostability of conjugated polyelectrolytes achieved by using anti-fading agents opens the way for advanced single molecule fluorescence imaging studies.

Exploiting Conjugated Polyelectrolyte Photophysics toward Monitoring Real-Time Lipid Membrane-Surface Interaction Dynamics at the Single-Particle Level

Herein we report the real-time observation of the interaction dynamics between cationic liposomes flowing in solution and a surface-immobilized charged scaffolding formed by the deposition of conjugated polyanion poly[5-methoxy-2-(3-sulfopropoxy)-1,4-phenylenevinylene (MPS-PPV) onto 100-nm-diameter SiO2 nanoparticles (NPs). Contact of the freely floating liposomes with the polymer-coated surfaces led to the formation of supported lipid bilayers (SLBs). The interaction of the incoming liposomes with MPS-PPV adsorbed on individual SiO2 nanoparticles promoted the deaggregation of the polymer conformation and led to large emission intensity enhancements. Single-particle total internal reflection fluorescence microscopy studies exploited this phenomenon as a way to monitor the deformation dynamics of liposomes on surface-immobilized NPs. The MPS-PPV emission enhancement (up to 25-fold) reflected on the extent of membrane contact with the surface of the NP and was correlated with the size of the incoming liposome. The time required for the MPS-PPV emission to reach a maximum (ranging from 400 to 1000 ms) revealed the dynamics of membrane deformation and was also correlated with the liposome size. Cryo-TEM experiments complemented these results by yielding a structural view of the process. Immediately following the mixing of liposomes and NPs the majority of NPs had one or more adsorbed liposomes, yet the presence of a fully formed SLB was rare. Prolonged incubation of liposomes and NPs showed completely formed SLBs on all of the NPs, confirming that the liposomes eventually ruptured to form SLBs. We foresee that the single-particle studies we report herein may be readily extended to study membrane dynamics of other lipids including cellular membranes in live cell studies and to monitor the formation of polymer-cushioned SLBs.

Cy3 photoprotection mediated by Ni2+ for extended single-molecule imaging: old tricks for new techniques

The photostability of reporter fluorophores in single-molecule fluorescence imaging is of paramount importance, as it dictates the amount of relevant information that may be acquired before photobleaching occurs. Quenchers of triplet excited states are thus required to minimize blinking and sensitization of singlet oxygen. Through a combination of single-molecule studies and ensemble mechanistic studies including laser flash photolysis and time-resolved fluorescence, we demonstrate herein that Ni(2+) provides a much desired physical route (chemically inert) to quench the triplet excited state of Cy3, the most ubiquitous green emissive dye utilized in single-molecule studies.

Interaction of anionic phenylene ethynylene polymers with lipids: from membrane embedding to liposome fusion

Here we report spectroscopic studies on the interaction of negatively charged, amphiphilic polyphenylene ethynylene (PPE) polymers with liposomes prepared either from negative, positive or zwitterionic lipids. Emission spectra of PPEs of 7 and 49 average repeat units bearing carboxylate terminated side chains showed that the polymer embeds within positively charged lipids where it exists as free chains. No interaction was observed between PPEs and negatively charged lipids. Here the polymer remained aggregated giving rise to broad emission spectra characteristic of the aggregate species. In zwitterionic lipids, we observed that the majority of the polymer remained aggregated yet a small fraction readily embedded within the membrane. Titration experiments revealed that saturation of zwitterionic lipids with polymer typically occurred at a polymer repeat unit to lipid mole ratio close to 0.05. No further membrane embedding was observed above that point. For liposomes prepared from positively charged lipids, saturation was observed at a PPE repeat unit to lipid mole ratio of ∼0.1 and liposome precipitation was observed above this point. FRET studies showed that precipitation was preceded by lipid mixing and liposome fusion induced by the PPEs. This behavior was prominent for the longer polymer and negligible for the shorter polymer at a repeat unit to lipid mole ratio of 0.05. We postulate that fusion is the consequence of membrane destabilization whereby the longer polymer gives rise to more extensive membrane deformation than the shorter polymer.

Enhanced luminescence of photosensitizer-based mesoporous silica nanocomposites via energy transfer from conjugated polymer

A unique assembly approach was developed to fabricate conjugated polymer and photosensitizer-doped mesoporous silica nanoparticles with effective Förster resonance energy transfer (FRET) from poly[(9,9-di(3,30-N,N0-trimethylammonium)-propylfluorenyl-2,7-diyl)-alt-co-(1,4-phenylene)] (PFP) to porphyrin-based photosensitizers (PSs). PFP and silica nanoparticles form a complex through electrostatic interactions, and efficient energy transfer from PFP to porphyrin-based PSs occurs upon irradiation. This approach is stable, effective, and diversified. PS-doped mesoporous silica nanoparticles showed three- to four-fold enhanced emission with the excitation of the maximum absorption wavelength of PS in the presence of PFP in comparison to the case without PFP. Doping fluorescence dyes into the nonporous core and adjusting the content of PS conjugated with the shell can endow the silica nanoparticles with a combinational optical signal of dyes and PS. These silica nanoparticles exhibit further improved performance on the basis of enhanced energy transfer offered by light-harvesting conjugated polymers.