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.