The dissociative enhancement principle

The excited triplet level of the ligand in the enhancement system must lie above the emissive level of the lanthanide ion for efficient transfer of energy. The emission levels of Eu3+ (613 nm) and Sm3+ (643 nm) are at lower energy than Tb3+ (545 nm) and Dy3+ (573 nm) and hence it is difficult to have one optimum universal ligand for energy transfer in the case of all four lanthanides. Emission levels of lanthanides along with the triplet levels of aliphatic and aromatic /3-diketones are shown in Fig. 12.34. 

A great number of enhancement solution constituents have been studied [185-188] including fluorinated aliphatic, and aromatic /3-diketones and dipicolinic acid derivatives. Fluorination of /3-diketone helps chelate formation at the low pH used for ion dissociation. 

The emission of luminescent chelates can be increased by having a large excess of structurally identical non-emissive chelates. An example of non-emissive chelates are those of Gd3+, Y3+. For example, the luminescence of Eu3+-/3-diketone can be increased by 1000-fold by the addition of Gd3+ chelates. 

MODERN ASPECTS OF RARE EARTHS AND THEIR COMPLEXES 

The decay times and detection limits of some lanthanides measured with time resolved fluorometry using different enhancement systems are given in Table 12.33. 

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With the DELFIA chelates, the sensitivity is, furthermore, increased because of the dissociation-enhancement principle the lanthanide ion in the chelate is dissociated and a new highly fluorescent chelate is formed inside a protective micelle (Figure 2). 

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