RESUMEN
Vesicles are self-assembled nanocontainers (size â¼100 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.
RESUMEN
Many applications of hydrogels rely on their ability to deliver encapsulated solutes, such as drugs; however, small hydrophilic solutes rapidly leak out of gels by diffusion. A need exists for a way to regulate solute release out of gelsâto ensure zero release until a desired time (the OFF state) and thereafter for the release to be switched ON at a high rate. This should ideally be a repeatable switch; i.e., the gel should be cyclable repeatedly between the ON and OFF states. Such perfect, cyclical ON-OFF release of solutes from gels is demonstrated for the first time through a "smart skin" that is synthesized rapidly (in â¼10 min) around an entire gel. The thin (â¼100 µm) and transparent polymer skin is endowed with redox-responsive properties through the use of urethane and acrylate monomers, one of which contains a thioether group. Initially, the skin is hydrophobic (water contact angle 102°), and it completely prevents hydrophilic solutes from leaking out of the gel. When contacted with oxidants such as hydrogen peroxide (H2O2), the thioethers are converted to sulfoxides, making the skin hydrophilic (water contact angle 42°) and thereby turning ON the release of solutes. Conversely, solute release can be turned OFF subsequently by adding a reducing agent such as vitamin C that reverts the sulfoxides to thioethers and thus returns the skin to its hydrophobic state. The release rate in the ON state can be tuned via the skin thickness as well as the oxidant concentration. The ability to regulate solute delivery from gels using smart skins is likely to prove significant in areas ranging from separations to agriculture and drug delivery.