RESUMEN
In this paper, we report on the luminescence of europium by directly exciting europium ions with visible light in aqueous medium. Upon replacing all the water molecules that coordinate around a central europium ion with a ditopic ligand 1,11-bis(2,6-dicarboxypyridin-4-yloxy)-3,6,9-trioxaundecane (L2EO4), the quenching from water molecules is efficiently eliminated, offering considerable europium emission. By stoichiometrically mixing with a positively charged block polyelectrolyte, the negatively charged L2EO4-Eu coordinating complex can be transformed into a coordination 'polymer', which simultaneously forms electrostatic micelles with further enhanced europium fluorescence emission, owing to the increased fraction of L2EO4-coordinated Eu(III) as revealed by the fluorescence lifetime measurements. This approach avoids the use of the antenna effect that often utilizes UV light as the irradiation source. We further use those micelles for bio-imaging, and for the first time demonstrate the use of directly excited Eu-containing nano-probes for in vivo fluorescence imaging in small animals under visible excitation. Although literature results have shown that the direct excitation of europium ions in water may lead to emissions in the presence of coordinating ligands, those emissions were too weak to be applied due to the remaining water molecules in the coordination sphere. Our work points out that the direct excitation of europium can generate considerable europium emission given that all the water molecules in the coordination sphere are excluded, which does not only greatly reduce tedious lab work in synthesizing antenna molecules, but also facilitates the application of europium in aqueous medium under visible light.
RESUMEN
In this paper, we report on the impact of the structure of ligands on the luminescence enhancement of Eu(III) by directly exciting Eu(III) with visible light in aqueous media. Upon replacing the water molecules that coordinated around a Eu(3+) ion with a ditopic ligand 1,11-bis(2,6-dicarboxypyridin-4-yloxy)-3,6,9-trioxaundecane (L2EO4) or ethylenediaminetetraacetic acid disodium salt (EDTA), significant luminescence can be obtained. L2EO4 may occupy all 9 coordinating sites of a Eu(3+) ion at proper L2EO4/Eu ratios, whereas EDTA only occupies 6 of them with 3 sites left for water at various EDTA/Eu ratios. These coordinated water molecules quench the fluorescence of EDTA-Eu complexes drastically so that the luminescence is about 30 times lower than that of the L2EO4-Eu system. Furthermore, the negatively charged L2EO4/Eu = 3/2 coordinated complex can be further transformed into coordination 'polymers' by mixing with a positively charged block polyelectrolyte, which forms electrostatic micelles with further enhanced luminescence. The emission of the EDTA-Eu complex is not influenced by the addition of polymers due to the formation of stable small 1 : 1 EDTA-Eu complex which doesn't change with increasing concentration. Our work points out that the L2EO4-Eu system is superior to the EDTA-Eu system in creating visible light sensitized Eu(III) luminescence, and the emission of Eu(III) can be indeed significantly enhanced to an applicable level by excluding all the water molecules in the coordination sphere of Eu(III).