Coffee - a ubiquitous substitute for uranyl acetate in staining of biological ultrathin sections for electron...

The use of uranyl acetate, a staining agent successfully used for decades in electron microscopy of biological specimens, is now strictly regulated by law due to its toxicity and radioactivity. It is even banned in some laboratories. In the meantime, there are a number of substitutes on the market, none of which comes close to the very good staining results of uranyl acetate, or only partially, and some of which are also toxic. In this paper, two alternatives to uranyl acetate are presented, namely coffee, which is used in countless households, and pure chlorogenic acid, which is a component of coffee. We used the well-known zebrafish as biological test material, focusing on the mitochondrial membranes. The staining ability of coffee and chlorogenic acid compared with commercially available staining agents as well as uranyl acetate is assessed by the interference contrast between membranes and their environment. This work also describes how a subjective impression of good or bad contrast can be cast into an objective and comparable numerical value.