Tech Briefs Magazine - September 2021 - PIT-22

New Electron Microscopy Technique Offers First Look at Previously
Hidden Processes
Northwestern University (Evanston, IL)
N
orthwestern 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.
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.
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
22
Cov
ToC
(Credit: Northwestern University)
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 selfassembly.
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 selfassembly
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 ¥ 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
60°C, 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 the
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 polymerizaPhotonics
& Imaging Technology, September 2021
http://www.abpi.net/ntbpdfclicks/l.php?202109TBNAV
Tech Briefs Magazine - September 2021

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