Sensitized luminescence from water-soluble LaF3:Eu nanocrystals via partially-capped 1,10-phenanthroline: time-gated emission and multiple lifetimes

Abstract

Water dispersible citrate-capped LaF₃:Eu(5%) nanocrystals (NCs) have been partially surface-functionalized by 1,10-phenanthroline (phen) via a ligand exchange method to produce novel water dispersed citrate/phen-capped LaF₃:Eu(5%) NCs in which citrate ligands preserve the water dispersibility of the NCs and phen ligands act as sensitizers of surface Eu³⁺-dopant sites. The partial ligand exchange and the formation of water dispersed NCs have been monitored by ¹H NMR spectroscopy, as well as luminescence measurements at different time intervals during the reaction. These NCs display a distinct phen-sensitized Eu³⁺-emission profile with enhanced intensity in water as compared to the emission profile and intensity obtained upon direct excitation. Time-resolved (or time-gated) emission spectroscopy (TRES) has been used to probe PL dynamics of Eu³⁺-sites of LaF₃:Eu(5%) NCs by taking advantage of selectively sensitizing surface Eu³⁺-dopant sites by phen ligands as well as by exciting all the Eu³⁺-sites in the NCs upon direct excitation. TRES upon direct excitation of the citrate-capped LaF₃:Eu(5%) NCs reveals that Eu³⁺-dopants occupy at least three different sites, each with a different emission profile and lifetime, and emission from purely interior Eu³⁺-sites has been resolved due to their long lifetime as compared to the lifetime of purely surface and near surface Eu³⁺-sites. In contrast, the phen-sensitized emission from citrate/phen-capped LaF₃:Eu(5%) NCs displays similar emission profiles and lifetimes in TRES measurements, which reveal that phen truly sensitizes purely surface dopant sites of the NCs in water, all of which have nearly the same local environment. The phen-sensitized Eu³⁺-emission of the NCs in water remains stable even upon addition of various buffer solutions at physiological pH, as well as upon addition of water-miscible organic solvents. Furthermore, the two-photon excitation (λ_ex. = 720 nm) of these water-soluble phen-capped NCs produces bright red Eu³⁺ emission, which reveals that these NCs are promising for potential applications in biological imaging.