New findings enable first direct, real-time images of soft, radiation-sensitive nanomaterials in organic solvents

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With highly specialized instruments, we can see materials at the nanoscale, but we can’t see what many of them are doing. This limits the ability of researchers to develop new therapies and technologies that take advantage of their unusual properties.

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

“It’s been an unmet need in chemistry and materials science,” said Nathan C. Gianneschi of Northwestern, who led the research. “We can now observe nanomaterials in organic solvents and watch these dynamic systems self-assemble, transform, and respond to stimuli. Our findings will provide valuable guidance to microscopy researchers.”

The research was published online today (February 17) in the journal Physical Sciences Cell Reports.

Gianneschi is the Jacob and Rosaline Cohn Professor of Chemistry at 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 lab, is the first author of the paper.

Imaging limitations

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 electrons scatter across the specimen, an image can be developed.

However, this fundamental imaging technique has limitations. Drying a specimen for use in a TEM vacuum will distort its appearance and cannot be used for specimens that exist in a liquid solution or organic solvent. Cryogenics-TEM allows researchers to examine specimens that have been frozen in solution, but it does not allow researchers to watch specimens react to heat, chemicals, and other stimuli.

This is a major problem for the study of soft radiation-sensitive nanomaterials, which hold tremendous promise for applications such as “smart” drug delivery systems, catalysis and ultra-thin films. In order to exploit their potential, scientists need to see how these nanomaterials behave under different conditions – but conventional TEM and cryo-TEM can only show the dried or frozen after-effects.

Liquid cell TEM (LCTEM) is an attempt to solve this problem. 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 shields them from the vacuum of the microscope. The liquid cell is encased in a silicon chip with small but powerful electrodes that can act as heating elements to induce thermal reactions, and the chip has a tiny window – 200 x 50 nanometers – that allows a beam of electrons to pass through the liquid cell and create the image.

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

This is where Monte Carlo comes in.

“No other imagery gives us this level of understanding”

Monte Carlo simulations are used to predict the outcomes of highly uncertain events. Named after the Mediterranean casino and Formula 1 racing destination, the technique was actually invented in the 1940s at Los Alamos National Laboratory, where nuclear weapons scientists had a limited supply 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 probability of a range of outcomes.

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

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

But more important than learning these three solvents is creating a method to test the suitability of any solvent.

“We can use this adapted Monte Carlo method to model the radiolysis of any organic solvent,” Korpanty said. “So you could figure out the solvent effect for any experiment you wanted to do. That’s a huge increase in the scope of what you can study with this form of microscopy.”

“Our results show that LCTEM is a fantastic way to study soft and solvated nanomaterials,” said Gianneschi. “No other imaging method gives us this level of understanding of what’s going on, how these nanomaterials behave differently from their bulk counterparts, and what we can do to disrupt them to gain access to new new properties of materials still unknown.”

Gianneschi is also a professor of biomedical engineering and materials science and engineering at the McCormick School of Engineering and a member of the Chemistry of Life Processes Institute, the Simpson Querrey Institute, and the Robert H. Lurie Comprehensive Cancer Center at Northwestern University.

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