January
20, 2004
Nano
Chemists Make Curved Structures
EVANSTON, Ill. --- The natural world is full of curves and three
dimensions, but the ability to deliberately and rationally construct
such complex structures using nanoscale building blocks has eluded
nanotechnologists who are eager to add curved structures to their
toolbox.
Now a team of Northwestern University chemists report they have
discovered ways to construct nanoscale building blocks that assemble
into flat or curved structures with a high level of predictability,
depending on the architecture and composition of the building blocks.
The results are published in the Jan. 16 issue of the journal Science.
Using hybrid nanorods consisting of segments of gold and conducting
polymers as their building blocks, the researchers created a number
of unusual structures, including bundles, sheets and tubes of varying
diameters. The extraordinary control that they were able to demonstrate
over the process holds promise for building new and powerful drug
delivery systems, electronic circuits, catalysts and light-harvesting
materials.
“We are trying to mimic life itself,” said Chad A.
Mirkin, director of Northwestern’s Institute for Nanotechnology,
who led the research team. “Much like proteins which must
fold into complex structures in order to function properly, we
have designed new materials that also form complex structures through
the process of self-assembly.”
Mirkin and his team made the different structures by varying
the diameter of the gold-polymer rods or adjusting the ratio of
polymer segment to gold segment in the rods. Both methods should
enable researchers to design structures with interesting electronic
and optical properties.
“We also discovered that the alumina template we used to
build the rods initially is essential in guiding the assembly process,” said
Mirkin, also George B. Rathmann Professor of Chemistry. “Without
the orientation the template provides, the rods do not form bundles,
sheets or tubes.”
The nanorods were made by the sequential deposition of gold and
conducting polymer into the pores of an aluminum template. After
the gold-polymer rods were synthesized, the template was dissolved,
leaving the rods parallel to one another, gold end to gold end
and polymer end to polymer end. The strong interactions between
the polymer ends built stress, causing curves to form.
In a subsequent experiment, the researchers observed that self-assembly
did not take place when the rods were randomly dispersed in solution.
“The research clearly shows that some unnatural building
blocks, such as the gold-polymer rods, need assistance in order
to form higher-ordered structures,” said Mirkin. “This
means that when we work with building blocks that are larger than
molecules but smaller than macroscopic objects, we should consider
building materials in a completely new way -- by using templates
to help guide the assembly process and reduce the large number
of assembly pathways potentially available to the building blocks.”
In addition to Mirkin, other authors on the Science paper are
Sungho Park (lead author), Jung-Hyurk Lim and Sung-Wook Chung,
all from Northwestern University. The research was supported by
the National Science Foundation and the U.S. Air Force Office of
Scientific Research. |