Tech Briefs Magazine - May 2021 - PIT-27

Initially, production was hampered by the time-consuming
Variable-Temperature Liquid-Cell
process required to create and
TEM
then activate them. A technique called polymerizationinduced self-assembly (PISA)
combines steps and saves time,
HEAT
HEAT
HEAT
COOL
but the molecules' behavior
during this process has proven
difficult to predict for one simple reason: Scientists were
unable to observe what was
RAFT Polymerization
In Situ Assembly
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
500 nm
500 nm
taking samples at various points
in the process and analyzing
them, but using only snapshots
failed to tell the full story of (Credit: Northwestern University)
chemical and physical changes
occurring throughout the process.
would allow a low-energy beam to pass
Transmission electron microscopy
through the liquid cell.
(TEM) is capable of taking images at a
With the chip inserted into the holder
sub-nanometer resolution, but it is genof the electron microscope, the tempererally used for frozen samples, and
ature inside the liquid cell is raised to
doesn't handle chemical reactions as
60°C, initiating the self-assembly.
well. With TEM, an electron beam is
Through the tiny window, the behavior
fired through a vacuum, toward the
of the block copolymers and the process
subject; by studying the electrons that
of formation could be recorded.
come out the other side, an image can
When the process was complete the
be developed. However, the quality of
team tested the resulting nanomaterials
the image depends on how many elecand found they were the same as comtrons are fired by the beam-and firing
parable nanomaterials produced outtoo many electrons will affect the outside a liquid cell. This confirmed that
come of the chemical reaction. In other
the technique-which they call variablewords, it's a case of the observer
temperature liquid-cell transmission
effect-watching the self-assembly
electron microscopy (VC-LCTEM)-
could alter or even damage the selfcan be used to understand the
assembly. What you end up with is difnanoscale polymerization process as it
ferent from what you would have had if
occurs under ordinary conditions.
you weren't watching.
Of particular interest are the shapes
To solve the problem, the researchers
that are generated during polymerizainserted the nanoscale polymer materition. At different stages the nanopartials into a closed liquid cell that would
cles may resemble spheres, worms or
protect the materials from the vacuum
jellyfish-each of which confers differinside the electron microscope. These
ent properties upon the nanomaterimaterials were designed to be responal. By understanding what is happensive to changes in temperature, so the
ing during self-assembly, researchers
self-assembly would begin when the
can begin to develop methods to
inside of the liquid cell reached a set
induce specific shapes and tune their
temperature.
effects.
The liquid cell was enclosed in a siliThese intricate and well-defined
con chip with small, but powerful, elecnanoparticles evolve over time, forming
trodes that serve as heating elements.
and then morphing as they grow.
Embedded in the chip is a tiny winAccording to the researchers, " What's
dow-200 × 50 nanometers in size-that
incredible is that we're able to see both
O

Photonics & Imaging Technology, May 2021

how and when these transitions occur in real time. "
Professor Nathan 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. "
For more information, contact Lila Reynolds
at lila.reynolds@northwestern.edu.

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Tech Briefs Magazine - May 2021

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