Multiple labeling and time-resolvable fluorophores.

A new time-resolved fluorescence immunoassay system involving use of the europium chelate of 4,7-bis(chlorosulfophenyl)-1,10-phenanthroline-2,9-dicarboxylic acid as label is reviewed. This stable chelate by itself is not very fluorescent but, used in multiple labeling strategies, improves the achievable detection limits. By using multiple labeling, streptavidin tailing, and Eu3+ activation, one can create a very stable, easy-to-use reagent that is suitable for devising highly sensitive immunoassays and other biotechnological assays. This reagent, a streptavidin-based macromolecular complex, is able to detect approximately 300,000 molecules (approximately 0.5 amol) of alpha-fetoprotein in a model noncompetitive immunoassay.