New Class of Tetradentate β-Diketonate-Europium Complexes That Can Be Covalently Bound to Proteins for Time-Gated Fluorometric Application

Luminescent lanthanide complexes that can be covalently bound to proteins have shown great utility as biolabels for highly sensitive time-gated luminescence bioassays in clinical diagnostics and biotechnology discoveries. In this work, three new tetradentate beta-diketonate europium complexes that can be covalently bound to proteins to display strong and long-lived Eu3+ luminescence, 1,2-bis[4'-(1 '',1 '',1 '',2 '',2 '',3 '',3 ''-heptafluoro-4 '',6 ''-hexanedion-6 ''-yl)-benzyl]-4-chlorosulfobenzene-Eu3+ (BHHBCB-Eu3+), 1,2-bis[4'-(1 '',1 '',1 '',2 '',2 ''-pentafluoro-3 '',5 ''-pentanedion-5 ''-yl)-benzyl]-4-chlorosulfo-benzene-Eu3+ (BPPBCB-Eu3+), and 1,2-bis[4'-(1 '',1 '',1 ''-trifluoro-2 '',4 ''-butanedion-4 ''-yl)-benzyl]-4-chlorosulfobenzene-Eu3+ (BTBBCB-Eu3+), have been designed and synthesized as biolabels for time-gated luminescence bioassay applications. The luminescence spectroscopy characterizations of the aqueous solutions of three complex-bound bovine serum albumin reveal that BHHBCB-Eu3+ has the strongest luminescence with the largest quantum yield (40%) and longest luminescence lifetime (0.52 ms) among the complexes, which is superior to the other currently available europium biolabels. The BHHBCB-Eu3+-labeled streptavidin was prepared and used for both the time-gated luminescence immunoassay of human prostate specific antigen and the time-gated luminescence microscopy imaging of a pathogenic microorganism Cryptosporidium muris. The results demonstrated the practical utility of the new Eu3+ complex-based biolabel for time-gated luminescence bioassay applications.