Monte Carlo simulations bring new focus to electron microscopy
New findings enable first direct, real-time images of radiation-sensitive
soft nanomaterials in organic solvents

Date:
February 17, 2022
Source:
Northwestern University
Summary:
A new method is using Monte Carlo simulations to extend the
capabilities of transmission electron microscopy and answer
fundamental questions in polymer science.



FULL STORY ==========================================================================
With highly specialized instruments, we can see materials on the nanoscale
- - but we can't see what many of them do. That limits researchers'
ability to develop new therapeutics and new technologies that take
advantage of their unusual properties.


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Now, a new method developed by researchers at Northwestern University is
using Monte Carlo simulations to extend the capabilities of transmission electron microscopy and answer fundamental questions in polymer science.

"This has been an unmet need in chemistry and materials science,"
said Northwestern's Nathan C. Gianneschi, who led the research. "We
can now look at nanomaterials in organic solvents and watch these
dynamic systems self- assemble, transform and respond to stimuli. Our
findings will provide a valuable guide for researchers in microscopy."
The research was published online today (Feb. 17) in the journal Cell
Reports Physical Science.

Gianneschi is the Jacob and Rosaline Cohn Professor of Chemistry in Northwestern's Weinberg College of Arts and Sciences and associate
director of the International Institute for Nanotechnology. Joanna
Korpanty, a graduate student in Gianneschi's laboratory, is the paper's
first author.

Limitations to imaging Transmission electron microscopy (TEM) allows researchers to see materials at the nanoscale, which is smaller than the wavelength of visible light. The microscope fires a beam of electrons
at a specimen, which is held in a vacuum; by studying how the electrons
scatter off the specimen, an image can be developed.



==========================================================================
This foundational imaging technique has limitations, though. Drying out
a specimen for use in the vacuum of TEM will distort its appearance,
and can't be used for specimens that exist in a liquid solution or
organic solvent.

Cryogenic-TEM allows researchers to examine specimens that have been
frozen in a solution, but it doesn't allow researchers to watch the
specimens respond to heat, chemicals and other stimuli.

That's a major problem for the study of radiation-sensitive soft
nanomaterials, which are enormously promising for applications such as
"smart" drug delivery systems, catalysis, and ultra-thin films. In order
to harness their potential, scientists need to see how these nanomaterials behave under different conditions -- but conventional TEM and cryo-TEM
can only show the dried-out or frozen aftereffects.

Liquid-cell TEM (LCTEM) is an attempt to solve that. Northwestern has
been the site of several advances in this rapidly developing field of microscopy, which inserts solvated nanoscale materials into a closed
liquid cell that protects them from the vacuum of the microscope. The
liquid cell is enclosed in a silicon chip with small but powerful
electrodes that can serve as heating elements to induce thermal reactions,
and the chip has a tiny window -- 200 x 50 nanometers in size -- that
allows an electron beam to pass through the liquid cell and create
the image.

However, being hit by a beam of electrons will leave a mark. In this
case, using more electrons would lead to a clearer picture -- since
there would be more of them to scatter -- but it would also lead to a
damaged specimen, especially in the case of radiation-sensitive soft nanomaterials. Suspending the specimen in an organic solvent could
protect it from damage, but little is known about how electron beams
interact with different solvents.

That's where Monte Carlo comes in.



========================================================================== "There's no other imaging that gives us this level of understanding"
Monte Carlo simulations are used to predict outcomes of highly uncertain events. Named for the Mediterranean casino and Formula One racing
destination, the technique was actually invented in the 1940s at Los
Alamos National Laboratory, where scientists working on nuclear weapons
had limited supplies of uranium and an extremely low threshold for trial
and error.

Since then, Monte Carlo simulations have become a staple of financial
risk assessment, supply chain management, and even search-and-rescue operations.

Typically, Monte Carlo simulations use thousands or even tens of
thousands of random samples to account for unknown variables and model
the likelihood of a range of results.

Gianneschi's team used software to model a liquid-cell transmission
electron microscope, and then adapted the Monte Carlo simulation to
focus on the electrons' trajectories through three solvents -- methanol,
water, and dimethylformamide (DMF) -- and assess interactions between
electrons and solvents. The simulations suggested that water would be
the most radiolytically sensitive of the three solvents -- meaning that
it will react to the electrons and change or even damage the specimen --
while methanol would be the most stable, likely to scatter the fewest
electrons and generate a clearer image.

These modeled findings were then verified using actual LCTEM, where the researchers could observe the soft nanomaterials as they transformed
into worms, micelles and other shapes dictated by solvent conditions --
and take detailed notes on their behavior and properties.

But more important than learning about these three solvents is the
creation of a method for testing the suitability of any solvent.

"We can use this adapted Monte Carlo method to model the radiolysis of any organic solvent," Korpanty said. "Then you could understand the solvent
effect for any experiment you wanted to do. It's a huge increase in the
scope of what you can study with this form of microscopy." "Our findings
show that LCTEM is a fantastic way to study soft, solvated nanomaterials," Gianneschi said. "There's no other imaging method that gives us this
level of understanding of what is happening, how these nanomaterials
behave differently from their bulk counterparts, and what we can do to
perturb them to access new, as yet undiscovered materials properties." Gianneschi also is a professor of biomedical engineering and materials
science and engineering in the McCormick School of Engineering and a
member of the Chemistry of Life Processes Institute, Simpson Querrey
Institute, and Robert H.

Lurie Comprehensive Cancer Center of Northwestern University.

========================================================================== Story Source: Materials provided by Northwestern_University. Original
written by Mark Heiden.

Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Joanna Korpanty, Karthikeyan Gnanasekaran, Cadapakam Venkatramani,
Nanzhi
Zang, Nathan C. Gianneschi. Organic solution-phase transmission
electron microscopy of copolymer nanoassembly morphology and
dynamics. Cell Reports Physical Science, 2022; 100772 DOI:
10.1016/j.xcrp.2022.100772 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/02/220217122329.htm

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