Luminescence lifetime measurements were performed, and
the decay curves fit to a monoexponential model to obtain the
lifetime of each terbium complex in H 2 O and D 2 O. The
calculated hydration numbers verify that [Tb(DO2A)(DPA)]–
excludes all water from the lanthanide coordination sphere,
eliminating solvent-quenching effects (Table I)( 99 ). Further,
encapsulation of the lanthanidealso causes a marked improve-
ment in emission lifetime, from 600ms to nearly 2 ms. This
allows for application of time-gated detection techniques,
reducing short-lived background fluorescence considerably
in environmental samples and demonstrating a significant
advantage in achievable limits of detection.
Another example of improved sensitivity due to modulation of
lanthanide photophysics by ancillary ligands can be found in the
europium and terbium chelates used in time-resolved
fluorescence resonance energy transfer (TR-FRET) immunoassays
(100,101). Due to their line-type emissions and long decay times,
the lanthanide chelate is used as a donor, with some visible-absorb-
ing dye such as AlexaÒ647 or a rhodamine derivative as the accep-
tor. Without the helper ligand, the lanthanides would be
unprotected from solvent and have much shorter decay times,
making them unsuitable for such an assay.
Ancillary ligands improve the photophysical properties of lumi-
nescent lanthanide-based assays by encapsulating the cation and
protecting it from solvent-quenching effects. In addition to
improving signal intensity, the helper ligand also improves the
lifetime of the emitting lanthanide. But enhancement of lantha-
nide photophysics is only one of several improvements observed
with use of such receptor ligands.
B. STABILITY
Ancillary ligands have been used in lanthanide-based sensors
to improve signal intensity by making the lanthanide more
TABLE I
LUMINESCENCELIFETIMEMEASUREMENTS ONVARIOUSTERBIUMCOMPLEXES( 100 ).Complex tH 2 O(ms) tD 2 O(ms) q
[Tb(H 2 O) 9 ]^3 þ 0.4 3.4 8.81.1
[Tb(DPA)(H 2 O) 6 ]þ 0.6 3.5 5.60.7
[Tb(DO2A)(H 2 O) 3 ]þ 1.1 2.6 2.40.3
[Tb(DO2A)(DPA)]– 1.9 2.2 0.30.0
LUMINESCENT LANTHANIDE SENSORS 15