Lanthanide probes

Lanthanide probes are a non-invasive analytical tool commonly used for biological and chemical applications. Lanthanides are metal ions which have their 4f energy level filled and generally refer to elements cerium to lutetium in the periodic table. The fluorescence of lanthanide salts is weak because the energy absorption of the metallic ion is low; hence chelated complexes of lanthanides are most commonly used. The term chelate derives from the Greek word for “claw,” and is applied to name ligands, which attach to a metal ion with two or more donor atoms through dative bonds. The fluorescence is most intense when the metal ion has the oxidation state of 3+. Not all lanthanide metals can be used and the most common are: Sm(III), Eu(III), Tb(III), and Dy(III). It has been known since the early 1930s that the salts of certain lanthanides are fluorescent. The reaction of lanthanide salts with nucleic acids was discussed in a number of publications during the 1930s and the 1940s where lanthanum-containing reagents were employed for the fixation of nucleic acid structures. In 1942 complexes of europium, terbium, and samarium were discovered to exhibit unusual luminescence properties when excited by UV light. However, the first staining of biological cells with lanthanides occurred twenty years later when bacterial smears of E. coli were treated with aqueous solutions of a europium complex, which under mercury lamp illumination appeared as bright red spots. Attention to lanthanide probes increased greatly in the mid-1970s when Finnish researchers proposed Eu(III), Sm(III), Tb(III), and Dy(III) polyaminocarboxylates as luminescent sensors in time-resolved luminescent (TRL) immunoassays. Optimization of analytical methods from the 1970s onward for lanthanide chelates and time-resolved luminescence microscopy (TRLM) resulted in the use of lanthanide probes in many scientific, medical and commercial fields. There are two main assaying techniques: heterogeneous and homogeneous.