Researchers from Seoul National University and the University of Manchester have found that a graphene coating on biological samples helps dissipate the charge build-up that tends to occur on the surface of these samples during non-destructive electron microscopy imaging. Such build-ups are often damaging and prevent high-resolution images from being obtained.
Currently used gold or platinum coatings mean that researchers cannot obtain high-resolution images of the samples or perform further quantitative and qualitative chemical analyses with techniques such as energy dispersive spectroscopy (EDS). Now, the research team discovered that a layer of graphene on biological samples can dissipate the charge accumulation on the non-conducting surfaces of biological samples thanks to the high electrical conductivity of graphene. The researchers explain that as soon as excessive charges appear on the sample surfaces, the graphene membrane provides conducting channels for these charges to disappear quickly and so allows to obtain high-resolution EM images. Furthermore, the high thermal conductivity of graphene allows it to dissipate excess heat produced by the high-energy electrons in the microscope, thus preventing thermal damage or deformation of biological specimens as well.Since graphene is also highly transparent to an electron beam, the electrons easily pass through it and penetrate deep into the sample. These electrons excite various atoms inside the sample and eventually produce characteristic X-rays, which can then escape very easily from the sample through the graphene membrane and to an EDS detector, for example. In contrast, electrons are blocked from entering metal-coated biological samples and any X-rays produced inside the sample cannot easily escape either because of the large electron-scattering cross-section of the metal coatings.
The team also found that graphene could be suitable for encapsulating macromolecules (DNAs or proteins, for example) in a liquid environment and that it was stable in a vacuum. “This allows us to obtain in situ and real-time EM images of biological processes occurring in these molecules, something that cannot easily be done in EM samples prepared using conventional methods, which only work in high vacuum.”
The researchers say that their technique provides them with a new way to explore biological phenomena. “For example, we are planning to observe how biological objects encapsulated with graphene self-assemble in liquids”. “Attempts to do this with other techniques, such as cryogenic transmission electron microscopy (TEM) and topographic imaging by atomic force microscopy (AFM) have proved unsuccessful for when it comes to observing these real-time processes, so we believe that our technique will help shed new light on these.”