1. Use Nano-W stain (2 % Methylamine Tungstate) or 2% uranyl acetate (UA)

2. Loaded 4 μL sample solution onto carbon film for 1 min, and removed the excess solution was by filter paper

3. Washing one time with ddH_2O

4. Sample was stained with 4 μL 2 % Nano-W or 2 % uranyl acetate (UA) for 1 min (incubate time depends on sample concentration).

   Because UA is a light-sensitive stain, prevent the UA solution from light (especially UV light)

5. The residual stain solution was removed by filter paper

6. Stored in humidity control box and dry overnight