Counterfeiting is a serious problem. According to the United States Secret Service (who fall under the Department of the Treasury, and are tasked with chasing down counterfeiters) fake money “represent(s) a … danger to the nation’s economy and its citizens”. The increased sophistication of copier and scanning technology has enabled crooks to produce near-perfect replicas of United States money, and so the United States Treasury is forced to stay one step ahead by the invention of metallic inks, magnetic strips, microprinting, and similar techniques.
A recent development has been released in a scientific journal, and it may finally put counterfeiters out of business. Chemists from the University of Sheffield have developed a way of layering colorless polymers (plastics) so that the end result is a vividly-colored material. These materials can be printed or layered into the structure of a dollar bill. The materials used by the scientists are called block copolymers. The technology can be understood with the use of a thought experiment. Imagine a length of rope that has random segments painted brown, or made of metal instead of fibers. That’s a reasonable analogy for one molecular strand of a block copolymer. Now, imagine getting a bunch of similiar ropes, and piling them into a heap. If they were actually just ropes, the heap would just stay like that until someone came along and cleaned it up.
However, the analogy of a rope only goes so far before it breaks down. Actual block copolymers behave differently than ropes that are large enough to see and touch. Due to the peculiarities of the electrostatic and steric interactions between polymer segments of adjacent chains, the polymers (ropes) “self-organize” into an ordered arrangement. It may be that all the painted segments only want to associate with other painted segments, and so they’ll all shift (dragging the rest of the rope along with it) and clump together. It may be the case that the “metal” segments don’t want to be next to any other metal segments, so they’ll push away any adjacent polymer strands that they don’t like and associate only with the ones that they agree with. It’s this “self-organization” behavior that gives these new materials their pigmented appearance.
As a beam of light shines through the (colorless) block copolymers, it hits these different self-organized regions, and the light beam is shifted a fraction in a random direction. The light will then continue on and hit the next polymer layer, which has a different refractive index because it has a different chemical composition (as it’s a different bunch of polymer segments), and so the light beam gets shifted in a different direction. By the time the light beam exits the material, it’s been transformed into a rainbow of colors because it’s passed through thousands and thousands of different block copolymer segments, all of which are different from each other.
These types of materials are very, very difficult to “fake”. Basically, anyone trying to exactly duplicate the precise rainbow of colors (with all of the associated wavelengths and intensities) would have to precisely duplicate the exact chemical composition of the block copolymers used in the layers embedded in the money. Because of the nature of organic chemistry, which is an extremely vast and flexible field, that particular task can be made impractical to the point of impossibility.
Fake money will always exist; crooks can always pass a fake $1 bill off at the local gas station without too much trouble. However, technology is finally reaching the point where the true experts will always be able to determine if a piece of money is authentic. It’s going to make life very difficult for counterfeiters.
The source of this article can be found at:
Andrew J. Parnell, Andrew Pryke, Oleksandr O. Mykhaylyk, Jonathan R. Howse, Ali. M. Adawi, Nicholas J. Terrill, J. Patrick A. Fairclough. “Continuously tuneable optical filters from self-assembled block copolymer blends”. Soft Matter, 2011; 7 (8): 3721.