Self-Assembly and Photo-Responsive Capacity of Star-Type Block Azopeptoids

Star polymers, as an appealing category of branched polymers, have garnered significant attention due to their distinctive physicochemical properties and topological configurations, demonstrating a variety of applications in biomedicine and nanotechnology. Nevertheless, the development of stimulus-responsive sequence-controlled star polymers remains a particularly formidable challenge. This paper reported the design and synthesis of a novel photo-responsive sequence-defined amphiphilic star-type block azopeptoids of Star-(N_AZO4-b-N_ME4-b-N_COOH)₃ using the solid-phase submonomer synthesis technique, where N_AZO represented hydrophobic monomer of N-(2-azophenyl)ethyl glycine and N_ME represented hydrophilic monomer of N-(2-carboxyethyl) glycine, respectively. The subsequent molecular self-assembly of Star-(N_AZO4-b-N_ME4-b-N_COOH)₃ was carried out in the mixture of water and acetonitrile (volume ratio 1∶1), maintaining undisturbed at 4 ℃ to slowly evaporate the organic solvent. Consequently, uniform spherical micelles with an average diameter of approximately 20 nm were successfully formed. The photo-response behavior of these spherical micelles was thoroughly characterized using transmission electron microscopy (TEM) and dynamic light scattering (DLS). An alternating irradiation with the ultraviolet (UV) and visible light resulted in a reversible diameter change toward the spherical micelles from approximately 20 nm to 10 nm, due to the photoisomerization feature of the azobenzene moiety. The carboxyl-modified amphiphilic star azopeptoids were utilized as building blocks to generate the functional spherical micelles, which could serve as the substrates to load the platinum nanoparticles for fabricating the organic-inorganic hybrid catalysts. As a proof-of-concept, the catalytic activity of the as-prepared hybrid nanocomposites was evaluated toward the borohydride-mediated reduction from 4-nitrophenol to 4-aminophenol, demonstrating a photo-controllable reversible and recyclable catalytic activity for several cycles. This work paves a significant route to fabricate stimulus-responsive sequence-controlled star polymers for the potential on nanocatalysis.