Single-minded approach to imaging nanoparticles

Individual nanoparticles in solution can now be resolved in three dimensions thanks to a new approach that combines developments in electron microscopy, biology, and computation devised by researchers at Lawrence Berkeley National Laboratory (LBL) and other institutions.

The arrangement of atoms on the surface and in the core of colloidal nanoparticles is central to determining their chemical and physical properties, which can be very different from the same elements in bulk form. Existing microscopy techniques are limited in their applicability to individual nanoparticles, so the team from LBL, the University of California Berkeley, Harvard University, Princeton University, Monash University in Australia, Ulsan National Institute of Science and Technology, and Amore-Pacific Co. R&D Center in South Korea turned to biology instead.

The researchers were inspired by a number of recent improvements in electron microscopy, particularly in resolving complex biological molecules like proteins. Both single-particle cryo-electron microscopy (cryo-TEM), which is widely used to determine the structure of biological molecules, and electron tomography rely on capturing multiple images from different angles to reconstruct a single three-dimensional representation. In combination with improvements to conventional transmission electron microscopy through aberration correction, which compensates for beam-induced motion, and novel approaches to isolating very small objects from the microscope’s high vacuum, the team led by A. Paul Alivisatos at LBL and Hans Elmlund from the ARC Centre of Excellence in Advanced Molecular Imaging at Monash University have come up with a new approach [Park et al., Science 349 (2015) 290].

The new approach is called SINGLE or 3D structure identification of nanoparticles by graphene liquid cell electron microscopy. The set up uses a graphene liquid cell (GLC) to seal nanoparticles suspended in a liquid within the high vacuum chamber of a TEM. Images are captured by an aberration corrected direct electron detector—which enables multiple images to be taken per millisecond at high resolution—and reconstructed using ab initio single-particle three-dimensional calculations that can cope with noisy individual images from unknown angles.

The hybrid technique can resolve the three-dimensional structure of sub-2 nm diameter Pt nanocrystals in solution at 300 kV (as shown). The reconstructed images show near atomic-scale resolution gathered from relatively small collections of noisy experimental TEM images. The researchers believe SINGLE will be able to reveal the structural principles underpinning the assembly and morphology of small, stable nanoparticles in solution. The approach could provide an insight into the three-dimensional nature of many types of particle in solution. 

‘‘Understanding structural details of colloidal nanoparticles is required to bridge our knowledge about their synthesis, growth mechanisms, and physical properties to facilitate their application to renewable energy, catalysis and a great many other fields,’’ says Alivisatos.

The team now plans to use more advanced imaging equipment to capture 400 frames-per-second with even better image quality.

‘‘We plan to image defects in nanoparticles made from different materials, core shell particles, and also alloys made of two different atomic species,’’ says LBL co-author of the study, Peter Ercius.

Although there have been many previous reports of nanoparticle structures, points out Angus I. Kirkland of the University of Oxford, this combination of interdisciplinary methods represents the first time that an individual particle has been characterized in real space away from a high vacuum environment.

‘‘It opens up the possibility of studying nanomaterials under operating conditions for a wide range of applications including catalysis, biomarkers, and sensors,’’ he says.

This story was originally published in Nano Today (2015), doi:10.1016/j.nantod.2015.08.002