advantages include better spectral resolution due to large Stokes
shifts and narrow emission lines and increased temporal resolu-
tion due to long lifetimes. The latter property allows for the use
of time-gated techniques and bandpass filters to reduce interfer-
ence from native or autofluorescence in the sample, which occurs
on the nanosecond timescale(21,43). Lanthanides also are more
resistant to photobleaching than organic dyes, as they are
effective quenchers of triplet states ( 45 ). These qualities
make lanthanides particularly attractive for optical detection
applications, although ways must be found to improve sensitivity,
stability, and selectivity. In our work, we have constructed stable
and selective luminescence lanthanide sensors containing recep-
tor ancillary ligands designed to detect target analytes of interest.
C. LANTHANIDERECEPTORS
A number of factors must be considered when designing lantha-
nide-based receptors. The choice of lanthanide is paramount. Ionic
radius varies across the lanthanide series, and the size of this
cation can influence the relative binding affinity of the target ana-
lyte. The lanthanide excited state must also align correctly with
the triplet excited state of the target analyte; if the energy differ-
ence is too great, the EnT efficiency will be low, but if it is too
small, quenching effects such as back transfer will dominate.
The emission properties of the lanthanide, such as luminescence
lifetime and quantum yield, must also be considered to produce
an adequate signal. Of all the lanthanides, Eu^3 þ, Tb^3 þ, and
Gd^3 þare the best ions in terms of efficient excited state popula-
tion, with energy gaps of 12,300 cm–^1 (^5 D 0!^7 F 6 ), 14,800 cm–^1
(^5 D 4!^7 F 0 ), and 32,200 cm–^1 (^6 P7/2!^8 S7/2), respectively ( 12 ).
While europium and terbium both emit in the visible region,
gadolinium emits in the ultraviolet, disfavoring its use in lumines-
cence-based sensing applications due to significant absorption and
emission interference at these high-energy wavelengths. (The
exactly half-filled f-shell of Gd^3 þ makes this lanthanide better
suited to magnetic resonance-based sensing technologies, a rich
and broad field beyond the scope of this review.) Dy^3 þand Sm^3 þ
also emit in the visible region, though their emission intensities
are muted due to multiple nonradiative decay pathways from
several energy-accepting excited states (39,46). We therefore focus
on terbium and europium as the primary lanthanides used in
sensitized luminescence assays, with dysprosium and samarium
employed where necessary to elucidate trends based on variations
in ionic radii.
LUMINESCENT LANTHANIDE SENSORS 9