An emissive dual-sensitized bimetallic Eu-2(III)-bioprobe: design strategy, biological interactions, and nucleolus staining studies

A new strongly luminescent homodinuclear Eu-2(III)-based bioprobe, [Eu-2(tta)(6)(mu-pz(tpy)(2))] (1), was developed, where piperazine-4-methyl-phenyl terpyridine (L) (mu-pz(tpy)(2)) acts as a bridging ligand containing piperazine (pz) as a linker between two N-3-donor phenyl-terpyridine ends and thenoyltrifluroacetone (tta) acts as a capping antenna ligand. The Eu-2(III) bioprobe 1 was designed to show pH responsiveness via its acid-sensitive beta-diketonate (tta) ligand and to be a fluorescent cell imaging agent, utilizing the characteristic dual-antenna-sensitized red luminescence of Eu-III ions via intraconfigurational f -> f transitions. The structure of the mu-pz(tpy)(2) ligand utilizes the ideal disposition and potential binding capabilities of the terminal N-3-donor tpy moieties toward Eu-III ions. The dinuclear complex 1 is characterized using ESI-MS, FT-IR, and other spectroscopic methods; therein, each Eu-III atom is coordinated to six oxygen atoms from three anionic tta(-) ligands and one N-3-donor terminal terpyridine to achieve the proposed nine-coordinated {EuN3O6}-geometry. The coordinative saturation, thermodynamic stability, absence of exchangeable labile H2O molecules (q < 1), and the presence of C-F oscillators in the tta ligands minimize possible emission quenching via non-radiative vibrational energy transfer (VET). These design features of 1 result in bright luminescence, photostability, and a long luminescence lifetime in aqueous media, which are desirable for cellular imaging applications. It shows efficient binding interactions with CT-DNA (K-b: similar to 10(6) M-1) and serum albumin protein (BSA) (K-BSA: similar to 10(6) M-1) under physiological conditions. The subcellular localization of the Eu-2(III) bioprobe 1 was evaluated using the MCF-7 and MDA-MB-231 cancer cell lines via confocal laser scanning microscopy (CLSM). It showed significant cellular uptake and preferential nucleolus localization, utilizing intrinsic red emission derived from the f-f transitions of the Eu-III ions of the Eu-2(III) bioprobe 1.