pH-sensitive fluorescent sensors based on europium(iii) complexes

Abstract

New europium(III) complexes Eu(TTA)₂-DSQ and Eu(TTA)₃-DR1 were designed and synthesized as new fluorescent pH probes (where HDSQ = 5-(dimethylamino)-N-(4-(2-((8-hydroxyquinolin-2-yl)methylene)hydrazinecarbonyl)phenyl)naphthalene-1-sulfonamide, DR1 = N¹-(4-(dimethylamino)benzylidene)-N²-(rhodamine-6G) lactamethylene-diamine and TTA = thiophentrifluoroacetone). Eu(TTA)₂-DSQ exhibited high sensitivity in monitoring pH changes in neutral aqueous solution with negligible background fluorescence. Eu(TTA)₃-DR1 comprised a green light emitting Rhodamine 6G fluorophore and a Eu^III moiety as the origin of red light. These pH-sensitive emitter components have pK_a values of 5.0 and 7.2 respectively, and exhibit isolated protonated steps within one molecule. Luminescence titrations demonstrate that Eu(TTA)₃-DR1 was able to detect pH values at both near neutral pH and acidic pH ranges, and was also able to detect pH in both cultured cells and in vivo.