This review compares the chemical and physical properties of lanthanide ion complexes and of other narrow-emitting species that can be used as labels for cytometry. A series of luminescent lanthanide ion macrocyclic complexes, Quantum Dyes Ò , which do not release or exchange their central lanthanide ion, do accept energy transfer from ligands, and are capable of covalent binding to macromolecules, including proteins and nucleic acids, is described and their properties are discussed.Two methods are described for increasing the luminescence intensity of lanthanide ion complexes, which intrinsically is not as high as that of standard fluorophores or quantum dots. One method consists of adding a complex of a second lanthanide ion in a micellar solution (columinescence); the other method produces dry preparations by evaporation of a homogeneous solution containing an added complex of a second lanthanide ion or an excess of an unbound antenna ligand. Both methods involve the Resonance Energy Transfer Enhanced Luminescence, RETEL, effect as the mechanism for the luminescence enhancement. q 2006 International Society for Analytical Cytology Key terms: Quantum Dye; RETEL; lanthanide; macrocycle; luminescence; columinescence; time-delayed; europium; terbium Only one article published in Cytometry, by Seveus et al.(1), has described in detail the use of a lanthanide ion complex as a luminescent label for an antibody, and this article, published in 1992, dealt primarily with the instrumentation for time-gated microscopy. Thus, it is appropriate for a special issue of Cytometry to include a focused review of this class of luminescent labels. Extensive reviews of the clinical and other uses of lanthanide complexes have previously been published by Hemmil€ a and coworkers (2-4).This review presents a comparison of the spectral properties, relative sizes, and essential chemical features of lanthanide complexes and other narrow emitting labels. It also provides a critical description of two related approaches that can be used to overcome the comparatively low molar extinction coefficients (molar absorptivities) of lanthanide ion complexes: 1) the columinescence effect, where the luminescence of a lanthanide complex is increased in a micellar solution by energy transfer from a complex of a non-emitting lanthanide to a complex of an emitting lanthanide, and 2) the Resonance Energy Transfer Enhanced Luminescence, RETEL, effect (5) where the energy transfer occurs in the solid state.A companion Technical Note (6) describes the experimental aspects of the RETEL effect (5), which resulted in a major increase of the luminescence intensity of a specific type of lanthanide macrocycles, the Quantum Dyes Ò . This increase in luminescence facilitates the use of the Quantum Dyes as labels, either with fluorescent microscopes conventionally illuminated by a Mercury-Xenon (Hg-Xe) arc or-when it is helpful to eliminate contamination from the emissions of conventional fluorophoreswith new cost-effective time-gating instrumentation.The emissions...

Background: Luminescent lanthanide complexes produce emissions with the narrowest-known width at half maximum; however, their significant use in cytometry required an increase in luminescence intensity. The companion review, Leif et al., Cytometry 2006;69A:767-778, described a new technique for the enhancement of lanthanide luminescence, the Resonance Energy Transfer Enhanced Luminescence (RETEL) effect, which increases luminescence and is compatible with standard slide microscopy. Methods: The luminescence of the europium ion macrocyclic complex, EuMac, was increased by employing the RETEL effect. After adding the nonluminescent gadolinium ion complex of the thenoyltrifluoroacetonate (TTFA) ligand or the sodium salt of TTFA in ethanol solution, the EuMac-labeled sample was allowed to dry. Both a conventional arc lamp and a time-gated UV LED served as light sources for microscopic imaging. The emission intensity was measured with a CCD camera. Multiple time-gated images were summed with special software to permit analysis and effective presentation of the final image.Results: With the RETEL effect, the luminescence of the EuMac-streptavidin conjugate increased at least six-fold upon drying. Nuclei of apoptotic cells were stained with DAPI and tailed with 5BrdUrd to which a EuMac-anti5BrdU conjugate was subsequently attached. Time-gated images showed the long-lived EuMac luminescence but did not show the short-lived DAPI fluorescence. Imaging of DNA-synthesizing cells with an arc lamp showed that both S phase and apoptotic cells were labeled, and that their labeling patterns were different. The images of the luminescent EuMac and fluorescent DAPI were combined to produce a color image on a white background. This combination of simple chemistry, instrumentation, and presentation should make possible the inexpensive use of the lanthanide macrocycles, Quantum Dyes Ò , as molecular diagnostics for cytological and histopathological microscopic imaging. q 2006 International Society for Analytical Cytology Key terms: quantum dye; columinescence; RETEL; lanthanide; macrocycle; luminescence; time-delayed; europium; terbium Lanthanide ion complexes have found a variety of significant applications in laboratory diagnostics (1-3); however, the weak luminescence of these complexes, which is due to their comparatively low molar extinction coefficients (molar absorptivities), was a major impediment to their use in cytometry (4). The problem of increasing the emission intensity of lanthanide ion complexes has been solved by the use of the resonance energy transfer enhanced luminescence (RETEL) effect, formerly called FRE-TEL (5), combined with measurement in the dry state. In the RETEL effect, as described by Leif, Vallarino, and coworkers in the companion Review article published in this issue of Cytometry (6), light energy originally absorbed by a nonluminescent photon acceptor (a complex or an unbound ligand) is transferred to the central lanthanide ion of a macrocyclic complex, Quantum Dye Ò , thus enhancing its emission int...

Methods for increasing the luminescence intensity of lanthanide macrocycles, Quantum Dyes®, by the Fluorescence Resonance Energy Transfer Enhanced Luminescence (FRETEL) effect in the solid state have been developed. A homogeneous solution containing the europium or terbium Quantum Dye and an excess of selected energy transfer species is evaporated to dryness, resulting in a thin film that surrounds and embeds the Quantum Dye or its conjugates. Under these conditions, in the presence of the gadolinium-thenoyltrifluoroacetonate complex as the energy transfer species, the luminescence of the europium Quantum Dye increased approximately 6-fold upon drying. However, the presence of a nonemitting lanthanide such as gadolinium is not always required for this effect. In studies employing the 2,6-pyridinedicarboxylate ion as the energy transfer species, where both the terbium and the europium Quantum Dyes could be simultaneously excited at 280 nm, the presence of gadolinium actually decreased the luminescence compared to that obtained with the 2,6-pyridinedicarboxylate alone. The simplest explanation for the FRETEL effect is that fluorescence resonance energy transfer occurs between the photo-trapping energy transfer species, either unbound or complexed with the nonluminescent gadolinium ion. The energy being finally transferred to the luminescent lanthanide ion complexes with consequent increase in emission intensity. This new method for the enhancement of luminescence intensity in the solid state has the significant advantage of eliminating the need for the previously required aqueous emulsion, which was difficult to make and transport.

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