The liposome is a self-assembled structure where a lipid bilayer surrounds an aqueous compartment. With a typical volume on the order of one thousandth of a cubic micron, this interior compartment has been used to carry drugs, peptides, proteins and DNA for applications in molecular biology, pharmaceuticals, and cosmetics. 1 Beyond the simple containment of molecules, the confined interior of a liposome is also an interesting space to explore supramolecular chemistry. In the literature, supramolecular structures that have been formed inside liposomes include actin fibers2 and fibril-shaped precipitates of the cancer drug doxorubicin. 3 These examples, however, typically use ion injection or pH gradients that depend on the invasive diffusion of ions through the liposomal membrane in order to stimulate aggregation of molecules. In this report, we have investigated the use of light to non-invasively induce the selfassembly of encapsulated molecules into nanofiber networks inside liposomes. Bulk nanostructure formation by light has been observed in the form of photosensitive gels and liquid crystals. 4 Additionally, UV irradiation has been used to change the chemical structure of molecules in order to trigger their self-assembly into nanofibers. 5 However, to the best of our knowledge, no cases exist where irradiation has been used to directly create nanofibers inside liposomes by self-assembly.

In our work, peptide amphiphiles (PAs) have served as instructive models to study self-assembly inside liposomes. PAs have afforded many supramolecular structures including micelles, 6 membranes7 and nanofibers. 8 The PA molecules used here comprise three segments: a linear aliphatic chain, a peptide segment that promotes b-sheet formation, and a peptide terminal sequence which can be modified according to the application of interest and in some cases may be an epitope for biological signaling. 8 In aqueous media, selfassembly of these PA monomers into cylindrical nanofibers can be triggered by a change in pH or salt concentration. 8, 9 Networks of these nanofibers can further form three-dimensional gel scaffolds resembling extracellular matrices. 8 For our studies, various model PAs designed to self-assemble under acidic conditions were prepared using standard Fmoc solid-phase peptide synthesis (Scheme 1 and Supporting Information†). By sequestering PA monomers inside