I have heard of two such reagent, one is sulfuric acid, another one is iodine, both of them are claimed to have been able to stain all carbohydrates, amino acids, and fat. But I'm interested in the mechanism, or chemical reactions involved.

 

For the sulfuric acid as a reagent, possibly, though I'm not quite sure, is due to dehydration of targets resulting in carbon formation making the spots visible.

 

For the iodine, I know iodine could stain starch by complexing, but I can't think of a staining mechanism for other carbohydrates. For fat, maybe the iodine vapor dissolves into fat and such dissolved iodine appears purple. The amino acids again I have no idea how it works.

 

By the way I'm also not sure if there is no particular chemical reaction involved, may it just because of higher affinity of iodine to those organic substance? But even if it is, why iodine would have a greater affinity for its targets?

I've never heard of using sulfuric acid as a TLC dip before to be honest.

 

I agree with your reasoning that the iodine complexes to carbohydrates so it becomes visible. Normally, KMnO4 is a universal TLC dip and that seems to show almost all compounds....onyl ones without oxidisable don't show up