Ancillary ligands can dramatically improve detection
strategies based on sensitized Ln^3 þluminescence by stabilizing
the lanthanide to pH variations. Such enhancements in stability
and reproducibility allow the use of lanthanide sensorsin situ
and also potentiallyin vivo, where free lanthanide ions might
precipitate and/or have toxic effects.
D. SELECTIVITY
Ancillary ligands also impart selectivity to lanthanide-based
detection strategies, improving resistance to common environ-
mental interferents. For lanthanides, which rely on ionic
interactions for analyte binding, the greatest threat is from
charged species. Anions, particularly those containing oxygen
donors or otherwise have the ability to complex metal ions, could
compete with the target analyte for the lanthanide and produce a
false negative result. If these anionic interferents are aromatic
and capable of transferring energy to the lanthanide, we might
also encounter false positives. By encapsulating the lanthanide
and limiting the surface area exposed to solvent, the receptor
ligand can reduce these undesired interactions and improve ana-
lyte detection limits.
A good example of a lanthanide-based detection system made
more robust by the inclusion of an ancillary ligand is that of
the aspirin metabolite salicylurate (SU). Acetylsalicylic acid
(ASA), commonly known as aspirin, is used widely as an anti-
inflammatory agent, an analgesic to relieve minor aches and
pains, and an antipyretic to reduce fever( 142 ). Aspirin is also
the primary medication used to treat chronic rheumatic fever,
rheumatoid arthritis, and osteoarthritis ( 143 ). Further, recent
studies have shown the antithrombotic benefits of an aspirin reg-
imen in stroke prevention (144,145). In the body, ASA is
hydrolyzed to salicylic acid (SA) by carboxylesterases in the gut
walls and liver, with an elimination half-life of 15–20 min ( 146 ).
SA is then converted primarily to salicyluric acid (SU) and other
metabolites, which are excreted in urine (147,148). The elimina-
tion rate constant for SU is much greater than SA ( 149 ), and
endogenous SU formation only occurs in a limited manner
(150,151). We can therefore use SU detected in urine as an indi-
cator of SAin vivo, and thus we have a noninvasive means of
monitoring aspirin dosage and residence in the body. In addition,
unusually high or low concentrations of SU in urine have been
correlated to a variety of diseases and conditions, such as appen-
dicitis, anemia, abdominal trauma, liver diseases, uremia, and
LUMINESCENT LANTHANIDE SENSORS 27