Vesicle-micelle transitions driven by ROS, light and heat.

Abstract

Vesicles are self-assembled nanocontainers (size ∼100 nm) in which solutes such as drugs can be encapsulated. There is great interest in triggering vesicle-micelle transitions (VMTs) because such transitions will result in the release of encapsulated solute. Here, we focus on reactive oxygen species (ROS) as a trigger for VMTs. ROS arise in our body within cells, and ROS levels are known to be high near a tumor. Thus, ROS-responsive vesicles are of interest. We make such vesicles by combining the cationic amphiphile (4-phenylthiophenyl)diphenyl-sulfonium triflate (PDST), and the anionic surfactant sodium dodecylbenzene sulfonate (SDBS). By simply mixing these two commercially available molecules in water, we prepare 'catanionic' vesicles in an easy, low-cost, and scalable way. When exposed to ROS such as hydrogen peroxide (H2O2), the thioether in the PDST tail gets oxidized to a hydrophilic sulfoxide. As a result, the vesicles are transformed into spherical or short, cylindrical micelles. Evidence for the VMT comes from turbidity, light scattering, and cryo-TEM measurements. The same vesicles are also sensitive to other stimuli, specifically light and temperature: i.e., a VMT can also be induced by irradiation with UV light or heating above a critical temperature. We explain the origin of the VMT in each case based on changes in the driving forces for amphiphile assembly.