Imaging of Nanoscale Gold in "Intact" Biological Cells by Environmental Electron Microscopy

The ability to monitor the cellular uptake and distribution of engineered nanomaterials is a basic requirement in nanomedicine and nanotoxicology. This is commonly achieved by using optical and electron microscopies. Optical microscopy allows for easy sample preparation and live cell imaging but is inherently limited in resolution to typically hundreds of nanometers, making it unsuitable for detailed investigation of nanoscale objects. On the other hand, electron microscopy allows for imaging with subnanometer resolution but requires a dedicated sample preparation that is usually destructive and can be cumbersome and costly. Here we demonstrate direct imaging of engineered nanomaterials in biological cells under preservation of cellular ultrastructure by means of environmental scanning electron microscopy (ESEM) and wet scanning transmission electron microscopy (wet-STEM). Specifically, we outline protocols for imaging fully hydrated cells on glass slides or TEM grids, requiring no prior processing steps. This enables high-throughput analysis of structurally uncompromised biological samples with nanometer resolution by using a minimal electron dose (<1 e(-)/angstrom(2)) at low electron energy (<= 30 keV). Here we refer to these cells as "intact", which should not be interpreted as "alive", although the cells are alive before being exposed to the electron beam. Our approach can be a viable alternative to the established electron microscopy methods for cellular imaging.