Improved nanoscale electron microscopy technique

US scientists have developed a new electron microscopy technique to see the process by which ‘smart’ molecules are formed at the nanoscale.

Professor Nathan Gianneschi from Northwestern University said that with their method it is now possible to see the reaction taking place and how nanostructures are formed, as well as learning how to harness their amazing capabilities.

The team of researchers say their method could revolutionize dispersion polymerization, a process used to make drugs, cosmetics, latex and other items on an industrial scale. At the nanoscale, polymerization can be used to create nanoparticles with unique and valuable properties.

Nanoparticles hold great promise for the environment, as they can be used to clean up pollutants, such as absorbing oil or other spills, without harming marine life. They could also be used in medicine, for example in ‘smart’ drug delivery systems (SDDS) nanomaterials can be designed to penetrate human cells and release therapeutic molecules under appropriate conditions.

Creating nanomaterials is a complex and very time-consuming process. Although the polymerization-induced self-assembly (PISA) technique used to make nanoparticles links the steps together and saves time, the behavior of the molecules during the process has proven difficult to predict for one simple reason: scientists have not been able to observe what actually happens during the polymerization process. Reactions at the nanoscale cannot be seen with the naked eye. Traditional imaging methods only capture the end result of polymerization, not the actual process by which it occurs.

Scientists have tried to get around this problem by studying and analyzing the different stages of the process, but using snapshots alone has failed to depict the full story of the transformations occurring during the chemical and physical processes.

“It’s like comparing a few snapshots of a football match with the information contained in the video of the entire game,” – Prof Gianneschi said. “If you understand the formation process of a chemical, you can learn how to speed it up or figure out how to disrupt it to get a different effect.”

Transmission electron microscopy (TEM) does not work well for chemical reactions, even though it allows imaging even with sub-nanometer resolution. In TEM, a beam of electrons is shot through a vacuum towards the sample; by examining the electrons coming out the other side, an image can be obtained. However, the quality of the image depends on the number of electrons fired by the beam – and firing too many electrons will affect the outcome of the chemical reaction. In other words, this is a case of the observer effect – that is, observing the self-organization of nanostructures can alter or even distort that self-organization process. The end result will be different from what we would get without observing the process.

To solve this problem, the researchers placed the nanoscale polymer materials in a closed liquid cell, protecting the materials from the vacuum inside the electron microscope. The materials were designed to respond to a change in temperature. Thus, the initiation of self-organization inside the liquid cell occurred only after a certain temperature was reached. The liquid cell was encapsulated in a silicon chip with small electrodes, but with high power and performing heating functions. A small window – measuring 200 x 50 nanometers – was embedded in the chip, allowing the low-energy beam to pass through the liquid cell. When the chip was inserted into the electron microscope holder, the temperature inside the liquid cell rose to 60 degrees Celsius, beginning the self-organization of polymerization. Through a small window, the behavior of the block copolymers and their formation process could be recorded.

Once the process was complete, Gianneschi’s team tested the resulting nanomaterials and found that they were the same as comparable nanomaterials produced outside the liquid cell. This confirms that the technique – which they call variable temperature liquid cell transmission electron microscopy (VC-LCTEM) – can be used to understand nanoscale polymerization occurring under ordinary conditions.

The shapes that are formed during polymerization are also of particular interest. At different stages, nanoparticles can resemble spheres, insects or even jellyfish – each of which gives the nanomaterial different properties. By understanding what happens during self-organization, scientists can begin to develop methods to induce specific shapes and tune their effects.

“Nanoparticles evolve over time, forming and then changing as they grow,” Sumerlin said. “The amazing thing is that we are able to see how and when these transitions occur in real time.”

The team of researchers has high hopes for the future of VC-LCTEM, saying it is a technique with the potential to transform medicine and the environment.

These findings are described in the article entitled “Probing Thermoresponsive Polymerization-Induced Self-Assembly with Variable-Temperature Liquid-Cell Transmission Electron Microscopy” published in the journal Matter, 22.12.2020(Authors: Georg M. Scheutz, et. al.)
Translation was done with the assistance of DeepL translator.