Tailor-Made Nanostructures

CONTROLLING THE FORMATION of complex polymeric nanostructures is no easy task. As the polymer molecules self-assemble, they tend to form particles in an unruly mess of different shapes and sizes. Two new advances offer scientists the ability to tailor these properties, thereby offering nanostructure design strategies that could be useful for a number of applications, including drug delivery and nanolithography (Science 2007, 317, 644 and 647).

Both strategies use block copolymers, which contain segments, or blocks, of different compositions and solubilities. In solution, these amphiphilic molecules organize into micelles, where the insoluble segments come together to form a core surrounded by a corona of the more soluble segments. A group led by Ian Manners of the University of Bristol, in England, and Mitchell A. Winnik of the University of Toronto demonstrated that they can control the length of cylindrical micelles using a novel polymer growth mechanism.

The researchers first prepare short, cylindrical polyferrocenyldimethylsilane-containing block copolymers in a specific solvent. Then they add the same polymer to a different solvent in which both types of polymer segments are soluble. With this method, the researchers can dial in the length of the micelles: The more polymer they add, the longer the micelles become.

In the other work, a team led by Karen L. Wooley of Washington University, St. Louis, and Darrin J. Pochan of the University of Delaware used electrostatic interactions to manipulate how charged, amphiphilic block copolymers aggregate into supramicellar assemblies. The researchers employ a triblock copolymer that contains a carboxylic acid unit. By varying the solvent or the counterion to the acid groups, they can drive the organization of the block copolymers into specific nanostructures.

"By controlling the kinetics of assembly, we can get many nanostructures from the same molecule, as opposed to having to design completely new molecules to get different nanostructures," Pochan explains.

Both reports "demonstrate how the self-assembly of block copolymer amphiphiles can be controlled with unprecedented precision and how the marriage of organic, inorganic, and polymer chemistries can yield fascinating new and exotic nanometer-scale assemblies," notes the University of Minnesota's Marc A. Hillmyer in an accompanying Science commentary.