Researchers
can now fill in missing information about nanoscale polymerization and “smartâ€
materials for medicine and the environment.
Northwestern
researchers have developed a new microscopy method that allows scientists to
see the building blocks of “smart†materials being formed at the nanoscale.
The
chemical process is set to transform the future of clean water and medicines
and for the first time people will be able to watch the process in action.
“Our
method allows us to visualize this class of polymerization in real time, at the
nanoscale, which has never been done before,†said Northwestern’s Nathan
Gianneschi. “We now have the ability to see the reaction taking place, see
these nanostructures being formed, and learn how to take advantage of the
incredible things they can do.â€
The research was published today (Dec. 22) in the journal Matter.
Nathan Gianneschi
The paper
is the result of a collaboration between Gianneschi, the associate director of
the International Institute for Nanotechnology and the Jacob and Rosalind Cohn
Professor of Chemistry in the Weinberg College of Arts and Sciences, and Brent
Sumerlin, the George and Josephine Butler Professor of Polymer Chemistry in the
College of Liberal Arts & Sciences at the University of Florida.
Dispersion
polymerization is a common scientific process used to make medicines,
cosmetics, latex and other items, often on an industrial scale. And at the
nanoscale, polymerization can be used to create nanoparticles with unique and
valuable properties.
These
nanomaterials hold great promise for the environment, where they can be used to
soak up oil spills or other pollutants without harming marine life. In
medicine, as the foundation of “smart†drug delivery systems, it can be
designed to enter human cells and release therapeutic molecules under specified
conditions.
There have
been difficulties in scaling up production of these materials. Initially,
production was hampered by the time-consuming process required to create and
then activate them. A technique called polymerization-induced self-assembly
(PISA) combines steps and saves time, but the molecules’ behavior during this
process has proven difficult to predict for one simple reason: Scientists were
unable to observe what was actually happening.
Reactions
at the nanoscale are far too small to be seen with the naked eye. Traditional
imaging methods can only capture the end result of polymerization, not the
process by which it occurs. Scientists have tried to work around this by taking
samples at various points in the process and analyzing them, but using only
snapshots failed to tell the full story of chemical and physical changes
occurring throughout the process.
“It’s like
comparing a few photos of a football game to the information contained in a
video of the whole game,†said Gianneschi. “If you understand the pathway by
which a chemical forms, if you can see how it occurred, then you can learn how
to speed it up, and you can figure out how to perturb the process so you get a
different effect.â€
Transmission
electron microscopy (TEM) is capable of taking images at a sub-nanometer
resolution, but it is generally used for frozen samples, and doesn’t handle
chemical reactions as well. With TEM, an electron beam is fired through a
vacuum, toward the subject; by studying the electrons that come out the other
side, an image can be developed. However, the quality of the image depends on
how many electrons are fired by the beam – and firing too many electrons will affect
the outcome of the chemical reaction. In other words, it’s a case of the
observer effect – watching the self-assembly could alter or even damage the
self-assembly. What you end up with is different from what you would have had
if you weren’t watching.
To solve
the problem, the researchers inserted the nanoscale polymer materials into a
closed liquid cell that would protect the materials from the vacuum inside the
electron microscope. These materials were designed to be responsive to changes
in temperature, so the self-assembly would begin when the inside of the liquid
cell reached a set temperature.
The liquid
cell was enclosed in a silicon chip with small, but powerful, electrodes that
serve as heating elements. Embedded in the chip is a tiny window – 200 x 50
nanometers in size – that would allow a low-energy beam to pass through the
liquid cell.
With the
chip inserted into the holder of the electron microscope, the temperature
inside the liquid cell is raised to 60ËšC, initiating the self-assembly. Through
the tiny window, the behavior of the block copolymers and the process of
formation could be recorded.
When the
process was complete, Gianneschi’s team tested the resulting nanomaterials and
found they were the same as comparable nanomaterials produced outside a liquid
cell. This confirmed that the technique – which they call variable-temperature
liquid-cell transmission electron microscopy (VC-LCTEM) – can be used to
understand the nanoscale polymerization process as it occurs under ordinary
conditions.
Of
particular interest are the shapes that are generated during polymerization. At
different stages the nanoparticles may resemble spheres, worms or jellyfish –
each of which confers different properties upon the nanomaterial. By
understanding what is happening during self-assembly researchers can begin to
develop methods to induce specific shapes and tune their effects.
“These
intricate and well-defined nanoparticles evolve over time, forming and then
morphing as they grow,†Sumerlin said. “What’s incredible is that we’re able to
see both how and when these transitions occur in real time.â€
Gianneschi
believes that insights gained from this technique will lead to unprecedented
possibilities for the development and characterization of self-organizing soft
matter materials – and scientific disciplines beyond chemistry.
“We think
this can become a tool that’s useful in structural biology and materials
science too,†said Gianneschi. “By integrating this with machine learning
algorithms to analyze the images, and continuing to refine and improve the
resolution, we’re going to have a technique that can advance our understanding
of polymerization at the nanoscale and guide the design of nanomaterials that
can potentially transform medicine and the environment.â€