Lanthanide luminescence, sensitizer

Sensitizers of near-IR lanthanide luminescence that are effective in the visible region of... 

Lanthanide chelates also can be used in FRET applications with other fluorescent probes and labels (Figure 9.51). In this application, the time-resolved (TR) nature of lanthanide luminescent measurements can be combined with the ability to tune the emission characteristics through energy transfer to an organic fluor (Comley, 2006). TR-FRET, as it is called, is a powerful method to develop rapid assays with low background fluorescence and high sensitivity, which can equal the detection capability of enzyme assays (Selvin, 2000). 

Due to the presence of hard anionic oxygen atoms, phenolate and carboxylate groups are often employed as donors in lanthanide coordination chemistry. Ligand [L18]4- is reported as an excellent triplet sensitizer for lanthanide luminescence (61). Indeed aqueous lifetimes of 0.57 and 1.61 ms are reported for europium and terbium, respectively quantum yields of 0.20 and 0.95 respectively refer to the efficiency of the energy transfer process alone. 

The use of europium chelates, with their unusually long fluorescence decay times, as labels for proteins and antibodies has provided techniques that are referred to as time-resolved fluoroimmunoassays (TRFIA). Fluorophores as labels for biomolecules will be the topic of Sect. 3. Nevertheless, TRFIAs always have to compete with ELISA (enzyme-linked immunosorbent assays) techniques, which are characterized by their great versatility and sensitivity through an enzyme-driven signal amplification. Numerous studies have been published over the past two decades which compare both analytical methods, e.g., with respect to the detection of influenza viruses or HIV-1 specific IgA antibodies [117,118]. Lanthanide luminescence detection is another new development, and Tb(III) complexes have been applied, for instance, as indicators for peroxidase-catalyzed dimerization products in ELISAs [119]. 

The spectroscopic characteristics of actinide and lanthanide luminescent probes are sensitive to numerous parameters, such as modifications of solvent composition, addition of supporting electrolytes, temperature changes etc. Therefore, TRES appears as an interesting tool for the chemist, because it provides sensitive experimental data. However, the interactions between the probe and the surrounding medium (in a wide sense) appear to be intricate and difficult to handle. In this sense, attempts to describe lifetime variations as a function of a unique parameter, the hydration sphere number, have shown their limitations. On the other hand, the open questions related to Forster s mechanism are a vivid and still not fully explored field. 

One of the advantages of lanthanide-doped semiconductor nanocrystals is that lanthanide luminescence can be efficiently sensitized by exciton recombination in the nanocrystals. The photoluminescence efficiency of Er3+ in Si nanocrystals in Si02 increases by more than five... 

Fig. 5. Schematic representation of the sensitization process of lanthanide luminescence via the surroundings of the...

Fig. 7. Simplified diagram showing the main energy flow paths during sensitization of lanthanide luminescence. From (Biinzli and Piguet, 2005), reproduced by permission of the Royal Society of Chemistry.

Keywords Lanthanide Sensor Sensitized luminescence Dipicolinate Macrocycle Ternary complex Bacterial spore Ancillary ligand Gadolinium break Catecholamine Salicylic acid Salicylurate. 

The first demonstration of sensitized lanthanide luminescence was due to the efforts of Bhaumik and El-Sayed, who foimd that if the lowest triplet level of europium tris-hexafluoroacetylacetonate. 

We have summarized the common attributes of ancillary ligands in improving detection techniques based on sensitized lanthanide luminescence and also introduced a new model for enhanced binding affinity. We suggest that binding affinities can be increased by an order of magnitude or more by ligand-induced anisotropy in... 

Functionalized organic dyes with polyaminocarboxylate have been widely utilized as feasible sensitizers to afford visible region excitation for sensitization of NIR lanthanide luminescence [36 5]. Verhoeven and coworkers [36, 37] first prepared a series of neodymium(III), erbium(III), and ytterbium(III) complexes with polyaminocarboxylate-functionalized fluorescein (21) and eosin (22) as sensitizing chromophores. These complexes show sensitized NIR... 

The preparation, characterization, aqueous stability, and photophysical properties of NIR emitting lanthanide complexes with tetradentate chelating ligands 36 and 37 were described by Raymond and coworkers [61, 62]. In aqueous solution, the chelating ligand 36 or 37 forms stable complexes with Ln(III) ions, and sensitized NIR lanthanide luminescence was detected for the complexes with Pr(III), Nd(III), Ho(III), or Yb(III) ions. For [Ln(36)2] complexes, the luminescence decay curves were biexponential due to partial hydrolysis of the complexes or alternately the presence of a slowly exchanging equilibrium mixture with a hydrated form of the complexes. For [Ho(37)2] , the NIR band due to Fs -> I transition of the Ho(III)... 

Porphyrin is one of the most widely studied macrocyclic systems suitable for complexation with lanthanide(III) ions. Porphyrins can absorb strongly in the UV-vis region so as to serve as efficient photo-sensitizers, making lanthanide(III)porphyrinate complexes ideal candidates for luminescence imaging agents. Indirect excitation of porphyrin antenna chromopheres in close proximity to lanthanide ions can make the energy in the triplet state of the porphyrin ligand transfer efficiently to the excited state of the lanthanide ion so as to sensitize the lanthanide luminescence, particularly NIR emission. 

Mono- or dinuclear platinum(II) complex capped with one or two [Pt(Bu3tpy)]+ units can be incorporated with Ln(hfac)3 units to produced a series of PtLn (Ln = Nd 88, Yb 89) and Pt2Ln (Ln = Nd 90, Yb 91) heteronuclear complexes [131]. With excitation at 350 nm < L < 550 nm, sensitized NIR lanthanide luminescence was detected in these Pt-Ln heteronuclear complexes with microsecond ranges of lifetimes, whereas Pt-based luminescence from the MLCT and LLCT states was mostly quenched. This reveals that fairly effective Pt Ln energy transfer is operating from the platinum(II) terpyridyl alkynyl chromophores to the lanthanide(III) centers. 

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