Europium in the Limelight

Home , Europium, Georges Urbain

in your element Europium in the limelight Jean-claude Bünzli sheds light on why europium — an element that is neither abundant in the earth’s crust nor involved in biological processes — has nevertheless attracted a great deal of interest from chemists.

he story starts at the end of the Paris (at that time, neither francium nor sensitive biomedical analyses that use nineteenth century, when gifted lutetium were known). Europium metal time-resolved luminescence. Such analyses Tscientists were systematically filling is now known to be highly reactive; the are routinely carried out in most hospitals in gaps in Mendeleev’s table by deciphering element’s most stable oxidation state is +3, and medical laboratories. Luminescent atomic optical spectra. Today this task but the +2 state also occurs in solid-state bioprobes based on europium and other seems rather easy, and could be carried out compounds and water. lanthanides are now ubiquitous in the life by undergraduate students. At the time, Georges Urbain, a brilliant young sciences, including bio-imaging. Fortunately, however, poorly performing instruments chemist who inherited Demarçay’s one kilogram of europium is sufficient for and difficulties in purifying samples spectroscopic equipment, observed in almost one billion analyses — which means were major hindrances. As a result, the 1906 a very bright red emission that these applications are not history of lanthanide discovery is full of for yttrium oxide doped with threatened by the shortage of incorrect claims and heated disputes among europium3. This was the start rare-earth elements feared by would-be discoverers. of a long career for europium industrialized nations after the The first, somewhat furtive, signal as an active component recent restrictions on exports by from element 63 was recorded in 1885 by in phosphorescent the Chinese government. Sir William Crookes, who found an materials — not only A recently anomalous red line (609 nm) in the as a red, but also a proposed application emission spectrum of a samarium blue emitter, because that could have sample. In 1892–3, Paul-émile LeCoq de its reduced divalent far-reaching Boisbaudran — the discoverer of gallium, form (EuII) emits in this importance to the samarium and dysprosium — confirmed spectral range. Phosphors fast-growing world population the existence of this band and detected based on red EuIII, green is in agriculture. Plastics doped a further green band (535 nm). Enter TbIII and blue EuII emitters, with traces of divalent europium Eugène-Anatole Demarçay who, in 1896, or a combination thereof, and monovalent copper have after patient fractionation of samarium can convert UV radiation into been shown to efficiently convert oxide, determined the presence of a new visible light. These materials play the UV portion of solar energy into visible rare-earth element between samarium essential roles worldwide in applications light. This process is rather ‘green’ (the and gadolinium. He isolated it in 1901, such as X-ray-intensifying screens, complementary colour of red). Using these and ended the note in which he reported cathode-ray tube or plasma-display panels, plastics to cover greenhouses has enhanced this discovery with “I propose for the new and have also been used more recently in the amount of visible light received by element the name europium, with symbol energy-saving fluorescent lamps and light- plants, leading to crop yields roughly Eu, and atomic weight 151 (approx.)”1. emitting diodes. 10% higher. The reason why Demarçay decided on The luminescence of trivalent europium Such an increase could cover the growing this name remains a mystery. Interestingly, can also be sensitized by organic aromatic need for food over several decades without he was not affiliated to any university, molecules, making such complexes useful in expanding the cultivated surface — showing and had been running an independent a variety of highly sensitive applications such that europium is possibly heading towards laboratory after an unsuccessful application as security inks and bar codes. For example, another bright future. ❐ to the Academy of Sciences. He was a when the European Union launched its rather eclectic scientist who had dealt with single currency in 2002, the euro bills JeAn-CLAUde BünzLi is at the Swiss organic, organometallic and inorganic were printed with an anticounterfeiting Federal institute of Technology Lausanne chemistry before becoming a talented ink at least one component of which is (ePFL), CH-1015 Lausanne, Switzerland; spectroscopist. He had also toured several a europium phosphor, most probably a and at the Center for next Generation countries to study their geology and tris(β-diketonate), yielding an orange–red Photovoltaic Systems, Korea University, culture2. It is plausible that this opening to emission under UV light. The greenish-blue Chungnam-do 339-700, South Korea. all aspects of chemistry and to the world emission from the same banknotes could e-mail: [email protected] led him to chose Europe over France or well arise from divalent europium but this is References mere conjecture. 1. Demarçay, A. E. C. R. Acad. Sci. Paris 132, 1484–1486 (1901) Since the beginning of the 1980s, 2. http://www.chem.unt.edu/Rediscovery/Demarcay.pdf europium has played a major role in highly 3. Urbain, G. C. R. Acad. Sci. Paris 142, 205–207 (1906). Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta W Re Os Ir Pt Au Hg 696 nature chemistry | VOL 2 | AUGUST 2010 | www.nature.com/naturechemistry