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Encyclopedia of Physical Science and Technology EN014A-653 July 28, 2001 20:55

18 Rare Earth Elements and Materials

(REPs): color purity and device processing flexibility. 2. Luminescent Immunological
First, the emission occurs at a well-defined wavelength Assays and DNA Labeling
and the spectral bands are very narrow. Second, since the
The detection and quantification of important biological
ligands have little influence on the core they can be mod-
molecules rely on immunoassays in solution. A typical
ified without altering the emission wavelength. The lig-
analytical system consists of two components: a targeting
ands, being on the surface of the REP, largely determine
group to recognize selectively and bind strongly to the
its physical properties, such as solubility and volatility,
target molecule of interest, and a reporter group which
which are so important for volume manufacture. Hence,
incorporates a radioisotope to allow detection at extremely
these key determinants of the ease of processing can be
low level of the target species. This approach provides
“tuned” independently of the emission characteristics.
high sensitivity for detection but also presents problems,
An additional and potentially great advantage of REPs
includingthepotentialofradiationexposure,highdisposal
is that devices containing REPs could operate with in-
costs, and limited shelf-lives of radioisotopes.
creased efficiency over those containing purely organic
It has long been recognized that luminescence can po-
materials. The electrons in an electroluminescent device
tentially provide an equally sensitive detection method
can be excited in two different states, singlets and triplets.
where the obvious drawbacks and problems associated
In OLEDs containing purely organic systems only those
with the radioactive substances do not arise. Using lumi-
electrons which are in singlet states can be responsible
nescent rare earth compounds not only eliminates these
for the emission of light, imposing a limit of 25% on the
problems but also offers several advantages over the fluo-
efficiency of the device. The mechanism of light emission
rescent organic dyes currently used in substitution of ra-
in REP materials is such that both singlet and triplet states
dioisotopes.
can emit. The theoretical ceiling to device efficiency is
The emission bandwidths of the lanthanide ions are
therefore 100%.
narrow, even at room temperature in fluid solution. As
a comparison, the fluorescent dyes have emission band-
widths that overlap, sometimes making it difficult to dis-
B. Biological and Biomedical Applications
tinguish between target species. In addition, because the
of Rare Earth Luminescence
lifetimes of the excited states of rare earth ions are rela-
In luminescence research, attention has recently shifted tively long (due to the forbidden nature of the f -electronic
from the development of solid-state phosphors (in tele- transitions), emission detection can be time-gated. In other
vision screens) and lasers (e.g., yttrium aluminum garnet words, a time delay is introduced between excitation and
(YAG) lasers) to solution-state methods exemplified by detection so that sources of interfering light such as scat-
the development of probes for bioinorganic chemistry and tered excitation light, Raman scattering, and impurity flu-
the development of time-resolved luminescent labelings orescence have died down, allowing us to identify target
and assays. molecules with greater accuracy.
The luminescent probe concept is by no means limited
to immunological assays. It can be conveniently utilized in
1. Rare Earth Ion as Probe for Metal the labeling and mapping of DNA. Sequence information
Binding Sites in Proteins can offer insight into the location and function of genes,
provide an understanding of the relationship between the
The importance of identifying Ca 2+ in living systems is
variations in sequence and different genetic diseases, and
that it is an important intracellular secondary messenger
serve as a basis for the prediction, treatment, and cure of
of signal transduction. Changes in Ca 2+ concentration
those disease conditions.
trigger changes in cellular metabolism and are respon-
sible for cell growth and regulation. Unfortunately, the
study of Ca 2+ is very difficult because it lacks any conve- C. Magnetic Applications
nient spectroscopic property by which it can be studied. 1. Permanent Magnets
Because of the similar size and chemistry, it is possible
to replace Ca 2+ with a rare earth ion, such as Eu 3+ or Permanentmagnetsarematerialsthatretaintheirmagnetic
Tb , which does have convenient spectroscopically use- properties after having been exposed to a magnetic field.
3+
ful traits. Study of the rare earth ions then gives the in- They are found in a great variety of materials used in a sig-
formation otherwise unavailable directly for Ca . Such nificant and increasing number of industrial and commer-
2+
replacements are a popular trick in bioinorganic chem- cial applications. These include micromotors and capac-
istry and have yielded very useful structural information itors used in computers, audiovisuals (speakers, VCR’s,
of metal-containing biomolecules. etc.), automobiles (directional aids, power windows,