When amorphous carbon support films are originally made, they are very hydrophilic. However, as the films age, they become more and more hydrophobic. A simple test to show whether a particular grid is hydrophilic or hydrophobic is simply to place a small (1-2 μl) droplet of water onto the carbon surface: If the carbon surface is hydrophilic, the water droplet will collapse and spread across the surface. If the carbon surface is hydrophobic, the droplet will remain hemi-spherical and resist spreading.
It even sometimes happens that part of the carbon surface of a transmission electron microscope (TEM) grid will be hydrophilic and part will be hydrophobic. The degree of hydrophilicity or hydrophobicity can also vary with position on a single grid and from grid to grid, even when the grids were made at the same time. These changing surface properties of the carbon film are one of the factors that can cause negative staining of protein samples to be extremely variable. Such variation can also affect the reproducibility of sample freezing for cryoTEM and the dispersion of materials science samples such as metallic nanoparticles on a TEM grid.
The surface properties of the carbon film will control how well a specimen will stick to it. Most biological specimens prefer...
There are a number of different ways to affect the surface properties of TEM grids. Here are some of them:
- Ultraviolet (UV) light
- Depositing fresh carbon
- Plasma cleaning
- Denton DV502 Carbon Evaporator
- Clean room
- Wish list