For the first time, researchers from the University of Illinois at Chicago have developed an innovative microscopy technique that enables the visualization of liquids at the nanoscale—nearly 10 times more resolution than achieved using conventional transmission electron microscopy.
Trapping very little amounts of liquid between two two-dimensional boron nitride layers enables the liquid sample to be imaged at a remarkably high resolution with the help of a conventional transmission electron microscope and spectroscopy techniques. This strategy could offer information related to the vibrational state of individual molecules.
The innovative technique can be employed to follow nano-sized tracers used in biological research, and also to visualize processes that take place at liquid-solid interfaces at extraordinary resolution. The researchers used their specialized sample holder, or boron nitride liquid cell, to detail the distinctive characteristics of water and heavy water at the nanoscale. The outcomes of the study have been published in the Advanced Materials journal.
While it may seem odd to focus on something as seemingly well-understood as water, there are still things we do not understand when it is confined at the nanoscale. So many applications in energy, catalysis, chemistry and biology depend on nanoscale interactions in water, which we haven’t been able to visualize using currently available measurement techniques.
Robert Klie, UIC Professor of Physics & Senior Author
“Using our specialized cell, we can look at the vibrational behavior of water and begin to explore how it acts in extremely tiny quantities confined within the boron nitride layers,” stated Jacob Jokisaari, corresponding author of the paper and a post-doctoral researcher in the department of physics at UIC.
Initially, the researchers had to overcome the challenge of finding a way to isolate very little amounts of liquid in preparation for scanning transmission electron microscopy, which involves using a focused beam of electrons for imaging samples. In general, samples should be frozen or encased in epoxy and subsequently sliced super-thin before being placed under the electron beam, where the user has only a few seconds at hand to captures images of the sample before it gets vaporized.
“We wanted to look at tiny amounts of liquid, and we turned to nanomaterials to encapsulate and support the liquid without affecting the measurements,” stated Klie. “Because the two-dimensional materials consist of only one layer of atoms, they barely influence the electron beam used to image the liquid, yet they are strong enough to hold the liquid bubble inside the microscope vacuum.”
After investigating a number of two-dimensional materials, the team finally decided to use boron nitride nanolayers. This material has the ability to contain the water molecules and is transparent to the infrared radiation produced by the vibrating water molecules. However, the progress was very gradual.
“These are extremely tiny and fragile pieces of material—just learning how to hold and manipulate them took months,” stated Klie.
It took nearly four years for the researchers to successfully sandwich water, and its cousin, heavy water, in between the boron nitride layers and place it in the exact position in the university transmission electron microscope.
“We could get down to about 350 milli-electron volts energy resolution with our microscope, but we knew we needed better resolutions to measure the vibrational properties of water. We needed access to a better microscope,” stated Klie. An electron volt is a measurement unit that can be used to describe the energy of vibrating particles.
The researchers took their boron nitride cell to the Department of Energy’s Oak Ridge National Laboratory in Tennessee, where scientists at the Center for Nanophase Materials Sciences, a DOE Office of Science User Facility, have access to a scanning transmission electron microscope with one of the best energy resolutions in the world. With the help of that microscope, Klie and coworkers could observe that water behaves differently when isolated in tiny amounts.
“We saw there was a shift in its vibrational frequency when confined in small amounts in our cell,” stated Jordan Hachtel, a post-doctoral researcher at Oak Ridge National Laboratory and one of the authors of the paper.
In general, water vibrates at 420 milli-electron volts when in large quantities. However, Klie noted that water confined in his cell vibrated at 406 milli-electron volts.
The team also visualized heavy water using the high-energy resolution electron microscope. In the case of heavy water, the two hydrogen atoms bound to an oxygen atom are replaced with deuterium, heavier than hydrogen. Heavy water is normally used for tagging molecules of interest in experiments. Although researchers have been able to identify the location of heavy water in cells, it has never been visualized with the level of resolution provided by Klie’s innovative technique.
Earlier research has investigated the electrochemistry of water at a macro- or micrometer level, where properties are averaged out over a large volume. However, electrochemical reactions look very different when investigated at a sufficiently small scale.
“Measuring how water bonds and interacts with other substances, such as at an interface where water touches something else, or interactions that take place in water such as corrosion of metals, has been impossible at the nanoscale level until now,” stated Jokisaari. “This work paves the way for the examination of electrochemistry and the atomic level, where theory based on computer modeling has forged way ahead of experimental techniques.”
“This new electron microscopy technique lets us see physical and chemical processes happening in a liquid environment at the nanoscale level—far smaller volumes than what can be measured by other currently available methods,” stated Klie. “At such small scales, the behavior of something we think of as basic, like water, changes as individual atomic bonds, local electric fields and the proximity of surfaces begin to affect its normal behavior.”
Xuan Hu, Arijita Mukherjee, and Canhui Wang of the University of Illinois at Chicago; Juan-Carlos Idrobo of Oak Ridge National Laboratory; Andrea Konecna and Javier Aizpurua of the Center for Material Physics in Spain; and Tracy Lovejoy, Niklas Dellby, and Ondrej Krivanek from the Nion Company in Kirkland, Washington, are co-authors on the paper.
This study was partially funded by National Science Foundation grants (DMR-0959470 and DMR-1626065) and by a grant (FIS2016-80174-P) from the Spanish Ministry.