Science 349, 290–295 (2015)

Electron microscopes are frequently used to characterize nanoparticles. The samples probed are, however, typically in the solid state, as the electron beam of the microscope requires a high-vacuum environment, while nanoparticles are often synthesized and utilized (as catalysts, for example) in solution. Paul Alivisatos and colleagues have now shown that an electron microscope can generate 3D images of platinum nanoparticles that are floating freely in solution.

The researchers — who are based at institutes in the US, Australia and South Korea — used a transmission electron microscope to examine nanoparticles encapsulated in a liquid cell made from two graphene sheets; the graphene acts as an ultrathin cover for the samples, allowing liquid conditions to be maintained in the vacuum of the electron microscope. With the set-up, numerous 2D images of individual nanoparticles were taken as the particles rotated in the solution. These images were then combined into 3D reconstructions of the nanoparticles that had near-atomic resolution, with the help of an algorithm previously developed to analyse cryo-electron microscopy images of biomolecules.