Channeled membranes are one of nature's most clever and important inventions. Membranes perforated with subnanometer channels line the exterior and interior of a biological cell, controlling by virtue of size the transport of essential molecules and ions into, through, and out of the cell. This same approach holds enormous potential for a wide range of human technologies, but the challenge has been finding a cost-effective means of orienting vertically-aligned subnanometer channels over macroscopic distances on flexible substrates.
"Obtaining molecular level control over the pore size, shape, and surface chemistry of channels in polymer membranes has been investigated across many disciplines but has remained a critical bottleneck," Xu says. "Composite films have been fabricated using pre-formed carbon nanotubes and the field is making rapid progess, however, it still presents a challenge to orient pre-formed nanotubes normal to the film surface over macroscopic distances."
For their subnanometer channels, Xu and her research group used the organic nanotubes naturally formed by cyclic peptides - polypeptide protein chains that connect at either end to make a circle. Unlike pre-formed carbon nanotubes, these organic nanotubes are "reversible," which means their size and orientation can be easily modified during the fabrication process. For the membrane, Xu and her collaborators used block copolymers - long sequences or "blocks" of one type of monomer molecule bound to blocks of another type of monomer molecule. Just as cyclic peptides self-assemble into nanotubes, block copolymers self-assemble into well-defined arrays of nanostructures over macroscopic distances. A polymer covalently linked to the cyclic peptide was used as a "mediator" to bind together these two self-assembling systems
"The polymer conjugate is the key," Xu says. "It controls the interface between the cyclic peptides and the block copolymers and synchroniz
|Contact: Lynn Yarris|
DOE/Lawrence Berkeley National Laboratory