Spontaneous Formation of Artificial Vesicles in Organic Media through Hydrogen-Bonding Interactions

The unusual spontaneous formation of submicrometer-sized vesicles from asmall, nonamphiphilic bis-biuret difluorene derivative upon dissolution of the solid in ananhydrous organic solvent was investigated using multiple scattering techniques. Timeresolvedlight scattering (TLS) measurements confirm that the self-assembly process isdriven by hydrogen-bonding interactions, leading to the formation of vesicles at a criticalconcentration ∼1 × 10−4 M in tetrahydrofuran as determined by absorbance and surfacetension measurements. Results from cryogenic-scanning electron microscopy (cryo-SEM),dynamic light scattering (DLS), and small-angle X-ray scattering (SAXS) experiments areconsistent with the existence of vesicle-like aggregates in solution. DLS studies indicate abroad distribution of aggregates with a mean hydrodynamic radius ⟨RH⟩ = 303 nm(polydispersity =0.49). SAXS profiles show two decay regimes (low-Q decay, very largeaggregates; large-Q decay, smaller species). The analysis models the large aggregates asvesicles (hollow spheres) with a mean external radius Ro = 750 nm and an internal radiusRi = 720 nm while the smaller aggregates have a mean radius R = 2.2 nm. The results obtained by cryo-SEM show sphericalaggregates of vesicles size in the range ca. 100 nm to 1 μm. Transmission electron microscopy (TEM) micrographs evidence thepresence of aggregates whose morphology is compatible with budding and pearling processes as possible mechanisms for theformation of vesicles.