Electron tomography has become a commonly used tool to investigate the three-dimensional (3D) structure of nanomaterials, including colloidal nanoparticle assemblies. However, the nature of the electron microscopy technique typically requires such characterization to be carried out under high vacuum. Therefore, pre-treatment sample preparation is needed for assemblies prepared by (wet) colloid chemistry methods, including solvent evaporation and deposition on a solid substrate (TEM grid). As a result, changes are consistently imposed on the actual nanoparticle organization, which is largely responsible for the nanomaterial properties. Therefore, we propose herein the application of (fast) electron tomography of nanoparticle assemblies while in their original colloidal environment. To address the challenges related to electron tomography in liquid, we devised a method that combines fast data acquisition in a commercial liquid in situ TEM cell with a dedicated reconstruction workflow. We present the application of this method to two different systems, which exemplify the effects of drying and vacuum, 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 interparticle distance of self-assembled Au nanorods in water agrees with the previously reported dimensions of the ligand layers surrounding the nanorods, whereas the nanorods are in much closer contact in similar dried assemblies. This study, therefore, emphasizes the importance of developing high-resolution characterization tools that preserve the native environment of colloidal nanostructures.

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