Polymer nanotubes that can be loaded with water-soluble substances may find applications in drug delivery and as highly specific templates for inorganic synthesis, suggest chemists at the University of Basel, in Switzerland, who prepared the materials.
“We developed a new, efficient, and easy method for the preparation of chemically and mechanically stable water-filled tubes with diameters of about 50 nm and lengths up to several tens of micrometers,” chemistry professor Wolfgang P. Meier says. He carried out the work with Ph.D. student Julie Grumelard and postdoc Andreas Taubert [Chem. Commun., 2004, 1462].
“These structures are based on self-assembled amphiphilic block copolymers that can be permanently fixed by cross-linking polymerization,” Meier explains. “The tubes have a large inner volume and can be filled with any desired water-soluble species.”
The macromonomers that self-assemble to form the nanotubes are amphiphilic ABA-triblock copolymers. The two A blocks are hydrophilic poly(2-methyloxazoline) (PMOXA), and the B block consists of hydrophobic poly(dimethylsiloxane) (PDMS).
The Basel team synthesized the copolymers by cationic ring-opening polymerization of 2-methyloxazoline onto activated PDMS. The PMOXA blocks have hydroxyl end groups that allow functionalization with methacrylic acid. In aqueous solution, the triblock macromonomers form supramolecular assemblies that can be chemically cross-linked by polymerization of the methacrylic acid groups.
The team showed that the inner surfaces of the nanotubes are hydrophilic by encapsulating carboxyfluorescein, a water-soluble fluorescent dye, in the nanotubes and examining them by fluorescence microscopy.
“I particularly admire the simple synthesis of a triblock copolymer with polymerizable end-caps that can be cross-linked to produce robust assemblies,” comments William J. Brittain, professor of polymer science at the University of Akron, in Ohio.
University of Toronto chemistry professor Ian Manners points out that although spherical structures are still the most common supramolecular structures generated by self-assembly of block copolymers, a remarkable variety of other morphologies have now been demonstrated, such as the new nanotubes. “Hollow tubes are rare, and this paper is significant in that it reports the formation of tubes in aqueous solution by a simple self-assembly process,” he says.
Meier notes that polymer chemistry allows a wide variety of possible modifications of the individual parts of synthetic polymers. In the future, “the use of amphiphilic ABC triblock copolymers, for example, should lead to nanotubes with chemically different inner and outer surfaces or the introduction of additional functional groups that allow for selective interactions with their environment,” he says. “Such nanotubes might be suitable as synthetic mimics of protein channels.”
The Basel chemists are now working on chemical modification of their ABA triblock copolymer and the nanotube surfaces. “We are also exploring the potential use of the nanotubes as vehicles for the controlled release of materials and as a matrix for the controlled formation of inorganic nanowires,” Meier says.