the lanthanide ion from solvent-quenching effects and interfering
ions, improving assay sensitivity and selectivity. The ligand-
induced enhancement in binding affinity appears to be the result
of an increase in positive charge at the analyte binding site due
to the electronegative ancillary ligand bound on the opposite
hemisphere of the lanthanide. We have elucidated the effects of
ancillary ligands for various lanthanide/analyte systems and
shown how such effects can greatly improve sensor performance
for medical, planetary science, and biodefense applications.
Keywords:Lanthanide; Sensor; Sensitized luminescence;
Dipicolinate; Macrocycle; Ternary complex; Bacterial spore;
Ancillary ligand; Gadolinium break; Catecholamine; Salicylic
acid; Salicylurate.
I. IntroductionA. LANTHANIDES
The lanthanides, or lanthanoids, are elements with lives of
their own—enigmatic, difficult to separate and purify— and
most often pictured as a row orphaned from the rest of
the periodic table along with their more radioactive siblings,
the actinides. Also known as“rare earth elements”due to the
etymology of the term “lanthanide” (derived from the Greek
lanthanein, meaning “to lie hidden”), they include the 15
elements of the top row in the “f-block” and have electronic
configurations [Xe] 4fn5s^2 5p^6 , wherenvaries from 0 to 14. Inter-
estingly, lanthanides are neither rare nor“earths,”an old term
used to describe certain metal oxides such as lime and magnesia
( 1 ). Even the rarest lanthanides—thullium and lutetium—are
two orders of magnitude more abundant than gold ( 2 ).
Lanthanides have found uses as catalysts, ceramics, and perma-
nent magnets, as well as in optics and electronics (3,4). Solid
phosphors containing europium, cerium, and terbium are found in
many common fluorescent lighting and color displays. Various
lanthanide ions are used in lasers, with neodymium as the
most famous in yttrium aluminum garnet (Nd-YAG). The green,
blue, and red luminescent bands in Euro banknotes are from
europium complexes ( 5 ). Certain lanthanides are used as tracers
in wine chemistry to discriminate wines according to geographical
region ( 6 ). The ratio of europium, which is almost entirely formed
in stars, to other rare earth elements in meteorites has helped
2 MORGAN L. CABLEet al.