Cryo‐electron microscopy and cryo‐electron tomography of nanoparticles

Cryo‐transmission electron microscopy (cryo‐ TEM or cryo‐ EM ) and cryo‐electron tomography (cryo‐ ET ) offer robust and powerful ways to visualize nanoparticles. These techniques involve imaging of the sample in a frozen‐hydrated state, allowing visualization of nanoparticles essentially as they exist in solution. Cryo‐ TEM grid preparation can be performed with the sample in aqueous solvents or in various organic and ionic solvents. Two‐dimensional ( 2D ) cryo‐ TEM provides a direct way to visualize the polydispersity within a nanoparticle preparation. Fourier transforms of cryo‐ TEM images can confirm the structural periodicity within a sample. While measurement of specimen parameters can be performed with 2D TEM images, determination of a three‐dimensional ( 3D ) structure often facilitates more spatially accurate quantization. 3D structures can be determined in one of two ways. If the nanoparticle has a homogeneous structure, then 2D projection images of different particles can be averaged using a computational process referred to as single particle reconstruction. Alternatively, if the nanoparticle has a heterogeneous structure, then a structure can be generated by cryo‐ ET . This involves collecting a tilt‐series of 2D projection images for a defined region of the grid, which can be used to generate a 3D tomogram. Occasionally it is advantageous to calculate both a single particle reconstruction, to reveal the regular portions of a nanoparticle structure, and a cryo‐electron tomogram, to reveal the irregular features. A sampling of 2D cryo‐ TEM images and 3D structures are presented for protein based, DNA based, lipid based, and polymer based nanoparticles. WIREs Nanomed Nanobiotechnol 2017, 9:e1417. doi: 10.1002/wnan.1417 This article is categorized under: Therapeutic Approaches and Drug Discovery > Emerging Technologies Nanotechnology Approaches to Biology > Nanoscale Systems in Biology