Re: What is special about the make-up of US currency, and the Chemistry in it?

Ashley, your question is interesting and I've thought about it myself on 
occasion.  Obviously, there is something added to the paper that reacts 
with the iodine ink pens that keeps the ink from changing color.  
Unfortunately, the government doesn't make the ingredients of currency 
paper readily available.  I did a literature search and I was able to find 
a couple of patents dealing with the iodine and counterfeit currency.  

Keep in mind that these patents don't necessarily mean that the government 
uses this particular technology; just that someone patented the idea.  

The first patent I found (U.S. pat 5,393,556 issued July 13, 1993) states 
that the iodine tests for starch content in the paper using an aqueous 
alcohol solution of iodine and phenolphthalein (phenolphthalein is a common 
indicator used in many chemistry labs).  

The second patent (U.S. pat 5,662,735 issued September 2, 1997) discusses 
the iodine test where, as you mentioned, the ink does not mark the genuine 
currency and leaves a mark on the counterfeit money.  This method uses 
metallic iodine in any number of solvents such as alcohols, ketones, esters 
of alcohols, or ethers of alcohols.  Some chemicals it mentions are methyl 
acetate, isopropyl alcohol (rubbing alcohol), ethylene glycol (used in 
antifreeze for the car), and acetone.  Water is then used to dilute the 
solution.  

I'm sorry I couldn't give you a more solid answer, but the information 
above should point you in the right direction if you want to do some more 
research.