Euros and europium
The article below describes the use of luminescent europium complexes in one of the counterfeit measures applied in Euro notes.
At the beginning of 2000, a number of European countries abandoned their old currencies and switched to the euro. You probably are aware of that. You also likely know that bank notes are often counterfeited and that the application of luminescence is one of the measures against this kind of falsification. But do you have an idea of what is behind that luminescence?
Freek Suyver and Andries Meijering, researchers at the Department of Chemistry at Utrecht University, were curious about this too. So they looked into what causes the luminescence of euro notes.
Guess what they found?
Wrapped europium metal in various stages of oxidation:
Eu is one of the rare earth elements (or lanthanides - and let's not discuss the definitions and differences here).
Most of the trivalent rare earth elements (REEs) are luminescent. This means that they can be excited by shining a light of a particular wavelength at them. When the ions relax again, they emit light - of a different wavelength. That is luminescence.
The luminescence of the REEs is unique: relatively long-lasting and producing sharp bands. It is therefore a widely used tracer and analytical tool, directly and indirectly (quenching by other ions). If you use a very intense excitation source, you can detect very low concentrations of fluorescent compounds.
You can enhance this luminescence by placing the REEs in a ligand cage that contains an antenna chromophore. One of the things the cage can do is provide a shielding effect against OH vibrations (in aqueous solutions). When you want the REE to luminesce, you hit the antenna and the antenna transfers to the REE. Much more efficient. (An example of the fun stuff that makes science so cool.)
(See also The Luminescent Lanthanides Homepage, which used to be at the University of Amsterdam and has moved numerous times; thankfully, Dr Werts is keeping it alive.)
For his work on Eu luminescence, Stephen Klink of the University of Twente recently received the Dutch Van Arkel award. The complex he uses absorbs UV (340 nm) and produces red light. The knowledge gained during this Eu project was later used to build a similar Nd complex (excited by green light, luminesces in the near-IR).
Euro notes luminesce in the red, green and blue (excited by 254 nm). The red light is clearly linked to europium and most likely to a Eu3+-β-diketone complex - according to the Utrecht researchers. They found it less easy to identify the source of the blue and green luminescence.
Still, they say that a likely candidate for the source of the green color is SrGa2S4:Eu2+.
There are many candidates for the blue color. Suyver and Meijering suspect, however, that the designers of the euro notes were really inspired. So the blue color may be caused by (BaO)x.6Al2O3:Eu3+.
The Dutch Central Bank's response to this? No comment.
Please note that any further investigation into what causes the luminescence of euro notes would constitute a violation of the law.
Klink, S. (2002) Licht converterende lanthanide-complexen. Chemisch2Weekblad, 98, 3, 17.
Suyver F. and Meijerink A. (2002) Europium beveiligt de Euro. Chemisch2Weekblad, 98, 4, 12-13.
Related publication (University of Twente):
Michels J.J., Huskens J. and Reinhoudt D.N. (2002) Noncovalent Binding of Sensitizers for Lanthanide(III) Luminescence in an EDTA-bis(-cyclodextrin) Ligand. J. Amer. Chem. Soc., 124, 2056-2064.