29 Septembre – Thesis defense - Hanna Anop

14 h30 Amphi CRPP, Centre de Recherche Paul-Pascal (Pessac)

Directing through low ionic strength, free polymers and metallic nanoparticles the self-organization of viral rod-shaped colloids.

Filamentous bacteriophages, due to their unique physical properties, such as size monodispersity and high colloidal stability, are widely used in soft condensed matter as a system of rod-shaped colloids. In aqueous dispersions, self-organization of these viruses has been shown to be essentially driven by entropy, which means purely repulsive (hard core) interactions between viral particles. In this thesis, by varying the nature of the interactions between viral rods, we have studied their resulting self-organization into liquid crystalline phases. For this purpose, we have first investigated the system of purely repulsive rods at very low ionic strength, where thick electric double layers are present. The phase behavior of virus suspensions at very low ionic strength has been determined using small angle X-ray scattering (SAXS) and optical microscopy techniques. We have found that the Smectic-A phase is not stable in case of high electrostatic repulsion between viral particles and that the system undergoes a direct Cholesteric to Smectic-B phase transition by increasing rod concentration. Moreover, our results evidence that viruses with thick double layers do not form colloidal glasses at high concentrations, which contradicts recently reported findings for the same system.
In a second part, we have tuned viral particle interactions from purely repulsive to attractive ones by adding non-adsorbing polymers in their suspensions, which act as depletant agent. By using polymers with coil size comparable to the rod diameter, virus self-organization initiated from the Cholesteric liquid crystalline phase results in a growth of original chiral superstructures, called helical bundles. Viruses are mostly oriented along the main bundle axis and exhibit long-range positional order, as proved by SAXS and by single particle tracking using optical microscopy. Phase diagrams of virus/polymer two-component mixtures as well as the stability with time of the resultant helical superstructures have been determined and compared for two different polymer sizes.
In the last part, we have increased Van der Waals attractive interactions in our viral system by introducing gold nanoparticles into self-assembled hybrid virus-based colloids. Thus, different hybrid virus-based colloids consisting of one (scepter-like) or two (diblocks) viral filaments attached to the same gold nanoparticle have been produced. This approach using gold nanobead has been extended to link together two bacteriophages of different lengths to achieve asymmetric colloidal diblocks. Self-organization of scepter-like particles and symmetrical diblocks driven by soft effective attraction has been explored and the corresponding phase diagrams have been established. We have found that Van der Waals attractive interactions between gold nanobeads incorporated into hybrid colloids favor formation of Smectic-B like fibrils in which virus particles are organized in periodic layers separated by layers of gold nanobeads. Finally, we have studied the effect of introducing a weak asymmetry into hybrid virus-based colloids and investigated the possible segregation of their respective blocks through the formation of the lamellar Smectic-A phase.
Overall, we have demonstrated an efficient way to control self-organization of virus-based colloids by varying interactions between them, which results in formation of various original self-assembled morphologies.

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