Another concern is that, for a 9-coordinate lanthanide ion in
aqueous solution bound to a single tridentate DPA ligand, the
six coordinated water molecules will strongly quench the
luminescence ( 92 ). To eliminate the potential for solvent
quenching and improve the sensitivity of the assay, we explored
the use of a hexadentate ancillary ligand to encapsulate the Tb^3 þ
ion and exclude solvent from the coordination sphere without
impeding dipicolinate chelation.
We selected the macrocyclic ligand 1,4,7,10-
tetraazacyclododecane-1,7-bisacetate (DO2A) as our helper
ligand, as this chelator meets our initial criteria for designing
a terbium-containing dipicolinate receptor site. Macrocyclic lig-
ands often have semirigid backbone ring structures. These lig-
ands vary in terms of ring diameter and the extent of
substituent functionalization on the ring scaffold. Most
macrocycles have a hydrophilic cavity in which an ionic substrate
such as a metalion can nest and be shielded from the environ-
ment by its lipophilic envelope (3,93). Ligands such as the
octadentate DOTA (1,4,7,10-tetrakiscarboxymethyl-1,4,7,10-
tetraazacyclododecane) have found a convenient niche in the field
of bioimaging as magnetic resonance contrast agents due to their
tendency to bind gadolinium with high affinity (94,95). In
fact, most macrocyclic ligands seem to exhibit an unprecedented
selectivity for lanthanide ions, and the dissociation of these
lanthanide–macrocycle complexes appears to be independent of
OEnergy tranferOO–O–N Tb3+[Tb(D PA)+ complex544 nm
Emission278 nm
AbsorptionFIG. 5. The method of bacterial spore detection using terbium.
Dipicolinic acid (DPA) is released from endospores via lysis or germina-
tion and binds to Tb^3 þwith high affinity. The resulting [Tb(DPA)]þ
complex exhibits intense luminescence in the visible region (544 nm)
under UV excitation (278 nm) that is much greater than Tb^3 þalone.
LUMINESCENT LANTHANIDE SENSORS 13