dipicolinate selectively, even in the presence of similar oxygen-
donor ligands such as acetate, carbonate, and citrate. Phosphate,
in particular, has been shown to severely inhibit DPA binding
and/or decrease luminescence intensity; we have found that not
only does DO2A improve resistance to phosphate by several
orders of magnitude, but also it is far more effective than alumi-
num chloride, the current recommended technique for phosphate
mitigation (158,159).
Other examples of ancillary ligands used to enhance analyte
selectivity include amide-modified DO3A (1,4,7,10-
tetraazacyclododecane-1,4,7-trisacetate) complexed to Tb^3 þ,
which selectively binds the bidentate analytes
p-dimethylaminobenzoic acid (DMABA) and SA( 160 – 162 ). The
binary complex of Tb^3 þwith EDTA can effectively detect SA,
4-aminosalicylic acid and 5-fluorosalicylic acid ( 163 ). [Tb(EDTA)]
also has been used to detect catalysis of hydroxybenzoic acid
(HBA) by hemin via formation of a ternary complex with the
HBA oxidation product ( 164 ). Diaza-crown ethers have been
utilized with Tb^3 þ and Eu^3 þ to detect phthalate, benzoate,
dibenzoylmethide, and picolinate ( 165 ). The future of these types
of sensors lies in the development of lanthanide receptor
complexes with greater analyte affinities, perhaps via stronger
host–guest interactions such as p-stacking or modification of
receptor site topology to generate a“lock and key”hydrophobic
pocket.
Ancillary ligands improve the selectivity of lanthanide-based
detection techniques by shielding the luminescent reporter from
solvent and interferents, thereby reducing the potential for false
positives or negatives. By improving selectivity in detection
technologies, these receptor ligands make such technologies
more robust and broaden the applications for which such
schemes may be used.
We have examined the attributes that helper ligands impart to
lanthanide-based detection by encapsulating the lanthanide ion
and shifting Ln^3 þ ion polarization. Protecting the lanthanide
from the quenching effects of solvent molecules as well as
detrimental chelation of environmental interferents improves
both sensitivity and selectivity. Generation of a more electropos-
itive region on the lanthanide surface by an electronegative
ancillary ligand can enhance analyte binding affinity by an
order of magnitude or more. We next turn to recent findings
concerning steric considerations and oxophilicity that can further
optimize detection strategies based on sensitized lanthanide
luminescence.
LUMINESCENT LANTHANIDE SENSORS 29