  Electron tomography has become a commonly used tool to investigate the
three-dimensional (3D) structure of nanomaterials, including colloidal
nanoparticle assemblies. However, electron microscopy is typically carried out
under high vacuum conditions. Therefore, pre-treatment sample preparation is
needed for assemblies obtained by (wet) colloid chemistry methods, including
solvent evaporation and deposition on a solid TEM support. As a result of this
procedure, changes are consistently imposed on the actual nanoparticle
organization. Therefore, we propose herein the application of electron
tomography of nanoparticle assemblies while in their original colloidal liquid
environment. To address the challenges related to electron tomography in
liquid, we devised a method that combines fast data acquisition in a commercial
liquid-TEM cell, with a dedicated alignment and reconstruction workflow. We
present the application of this method to two different systems, which
exemplify the difference between conventional and liquid tomography, depending
on the nature of the protecting ligands. 3D reconstructions of assemblies
comprising polystyrene-capped Au nanoparticles encapsulated in polymeric shells
revealed less compact and more distorted configurations for experiments
performed in a liquid medium compared to their dried counterparts. On the other
hand, quantitative analysis of the surface-to-surface distance of
self-assembled Au nanorods in water agrees with previously reported dimensions
of the ligand layers surrounding the nanorods, which are in much closer contact
when in similar but dried assemblies. This study, therefore, emphasizes the
importance of developing high-resolution characterization tools that preserve
the native environment of colloidal nanostructures.
