ligand-induced enhancement is lanthanide dependent and
is governed by the gadolinium break. We believe this property is
due to an induced anisotropy in the electron density of the lantha-
nide (Fig. 8). Our evidence is that the improvement in
bindingaffinityisdirectlyrelatedtotheionizationenergyofthelan-
thanide in question. The greatest effect is seen with terbium, as this
lanthanide has the lowest ionization energy because it has exactly
one more electron than a half-filled f-shell. Previous explanations
for this trend include theories based on a change in basicity of the
complex with the helper ligand bound, which our experiments do
not support. We conclude that ligand-induced anisotropy of lantha-
nide ion electron density is a plausible explanation for the observed
binding affinity variations.
C. SENSITIVITY
By enhancing the binding affinity between analyte and recep-
tor complex, the ancillary ligand confers additional stability to
FIG.8 Enhanced receptor effect on analyte binding affinity. Upon
chelation, the electronegative ancillary ligand causes a polarization of
the Ln^3 þion, inducing an increased positive charge at the binding site.
This anisotropy results in an increased binding affinity for the anionic
analyte. Limit of detection (LOD) values shown are for the Tb/DO2A/
dipicolinate system.
LUMINESCENT LANTHANIDE SENSORS 21