WAFER-THIN WALLS: A liquid capsule made of graphene allows researchers to peer inside with an electron microscope. Image: Image courtesy of Alivisatos, Lee and Zettl research groups, Lawrence Berkeley National Laboratory and KAIST
Graphene, the wonder material that earned two researchers the 2010 Nobel Prize in Physics and that has been the subject of countless scientific studies and news articles, has certainly spent some time in the limelight. Now it may take center stage under the microscope.
Now researchers have harnessed graphene’s novel attributes to alleviate one of the challenges of investigating the workings of the nanoscale world—that of applying the tools of high-resolution microscopy to objects in liquid. Electron microscopy can image solid structures down to the scale of single atoms by irradiating small objects with a beam of electrons. But achieving the same resolution for fluid specimens is a tricky enterprise, because electron microscopes require a vacuum to keep air molecules from interfering with the irradiating beam. And vacuums cause liquids to vaporize.
In the April 6 issue of Science, a team from Lawrence Berkeley National Laboratory, the University of California, Berkeley, and the Korea Advanced Institute of Science and Technology in Daejeon, South Korea, reports that liquids fare just fine inside the vacuum of an electron microscope when encapsulated in graphene. The researchers sandwiched nanoscale pockets of liquid between two sheets of graphene and then used a transmission electron microscope to peer inside.
They found that the graphene capsules shielded the fluid from vacuum while also allowing for atomic-resolution imaging, which had been a challenge for other liquid capsules fashioned from materials such as silicon nitride. “The problem with that is the silicon nitride is already 25 nanometers thick. It’s a lot thicker than graphene,” says Jungwon Park, a U.C. Berkeley graduate student and a co-author of the new study. “It scatters a lot of the electron beam out, and it reduces the resolution and contrast a lot.”
The walls of the graphene liquid capsule, on the other hand, are so slim—less than a nanometer thick—that the researchers could resolve individual platinum atoms inside.
Park and his colleagues are using the graphene capsules to track in real time the growth of platinum nanoparticles in solution. “We know how to make nanoparticles in the wet lab, but we still don’t understand why they form such a nice shape and structure,” he says. “We wanted to run exactly the same reaction that we do in the lab, and watch what happens inside in real time.”
Graphene could also help facilitate ultrahigh-resolution microscopy of biological specimens, although the potential for radiation damage from an electron microscope’s particle beam imposes some limits. “The graphene liquid capsule can be used for taking biological samples in water, potentially,” Park says. In fact, another group recently demonstrated graphene’s applicability for biological imaging, albeit in a more restrictive capacity. Last year researchers at Kansas State University exposed water-rich bacteria to electron microscopy after wrapping them in graphene treated with a protein that binds to the bacterium’s cell wall.
“One could think to encapsulate proteins or protein complexes” in graphene bubbles, says Niels de Jonge, a microscopist at the Leibniz Institute for New Materials in Saarbrücken, Germany. “I’m actually curious to see those kind of studies.” One unresolved question, he says, is whether liquid microscopy would reduce or increase the electron beam’s radiation effects compared with an alternative approach in which researchers prepare biological samples for imaging by freezing them.