Nanoscale tubular and sheetlike superstructures from hierarchical self-assembly of polymeric janus particles.

Inspired by hierarchical protein self-assembly in biological systems, the self-assembly of nanoparticles into superstructures has attracted much attention because of potential applications of these superstructures in the fields such as electronic or optical materials and novel nanodevices. Template-free self-assembly of nanoparticles with anisotropic interactions is of particular interest because it can lead to tailor-made complex superstructures. In fact, Janus nanoparticles are nano-objects with anisotropic interactions. The facile preparation, applications, and self-assembly of Janus nanoparticles have attracted extensive interest, and Janus particles with various sizes, structures, and compositions have been recently reported. Several kinds of Janus particles are capable of self-assembling into regular superstructures. For example, M ller and co-workers reported that amphiphilic Janus micelles prepared from ABC triblock copolymers could self-assemble into spherical supermicelles. 20] Granick and co-workers reported that amphiphilic and zwitterionic Janus colloidal spheres could assemble in water to form ordered clusters. In a recent study, we prepared Janus nanoparticles by using hybrid organic/inorganic nanotubes as a desymmetrization tool and the as-prepared amphiphilic Janus nanoparticles self-assembled into narrowly size-distributed flowerlike supermicelles in water. Despite these results, assembly of Janus nanoparticles into superstructures other than spherical supermicelles or clusters, such as nanowires, tubular and sheetlike superstructures, still remains challenging. Herein, we report a novel mechanism for the formation of polymeric Janus particles from mixed-shell micelles (MSMs) and the template-free self-assembly of the Janus particles into tubular superstructures and nanosheets. Micelles with mixed P2 VN/PEO shells were prepared by noncovalent crosslinking of poly(acrylic acid) (PAA) blocks by addition of 1,2-propanediamine (PDA) to a solution of PEO3500-bPAA3800/P2VN38000-b-PAA24000 (1:1 (w/w), PEO = poly(ethylene oxide), P2VN = poly(2-vinyl naphthalene), and the subscripts denote the molecular weights of the respective blocks) in DMF (Figure 1 a). The molar ratio of AA/PDA was 1:5, and the total polymer concentration was 1.0 mgmL . MSMs with PEO/P2VN as the mixed shell and a PDAcross-linked PAA network as the core were thus formed by noncovalent cross-linking. After switching the solvent from DMF to water (pH 7) by using dialysis, P2 VN in the mixed shell collapsed into separated microdomains (Figure 1b). In water, P2VN microdomains were surrounded and protected by solvated PEO chains so that the MSMs were individually dispersed. By decreasing the pH value of the aqueous solution to 3.1, intramicellar complexation occurred between PEO and PAA (Figure 1c), which resulted in an asymmetric intramicellar phase separation between the PEO/PAA complex and P2VN. As a result, amphiphilic Janus nanoparticles with a hydrophobic P2VN domain (formed by aggregation of all the P2VN small domains in a MSM) on one side and a hydrophilic PEO/PAA complex domain on the opposite side were formed (Figure 1d). These Janus nanoparticles were able to self-assemble in water to produce tubular and sheetlike superstructures. The average hydrodynamic radius, hRhi, of MSMs in DMF was 190 nm (Figure 2a, curve 1). In water, MSMs were individually dispersed with an hRhi value of 140 nm (Figure 2a, curve 2) because of the protection of solvated PEO chains in the mixed shell. The decrease in the hRhi value of the MSMs should result from the collapse of P2 VN chains in water. In the transmission electron microscopy (TEM) image of MSMs cast from neutral water stained with RuO4 (Figure 2b), contrast within each MSM was observed. The darker domains were assigned to P2VN microdomains (2–