Eu^III and Tb^III upconversion intermediated by interparticle energy transfer in functionalized NaLnF₄ nanoparticles

Lanthanide (Ln^III)-doped sodium gadolinium tetrafluoride (NaGdF₄) nanoparticles have been excelled as attractive upconversion systems for anti-counterfeiting or energy conversion for instance, with a special interest in the visible upconversion of Eu^III and Tb^III. The core@shell architecture has enabled the bright upconversion of Eu^III and Tb^III in this matrix by interfacial energy transfer sensibilized by the Tm^III/Yb^III pair. Another approach to enable Eu^III and Tb^III upconversion could be the interparticle energy transfer (IPET) between Ln^III-doped sensitizer and acceptor nanoparticles. Yet, the low molar absorptivity of the Ln^III through 4f ↔ 4f electronic transitions and the large distance between the nanoparticles are shortcomings that should decrease the energy transfer efficiency. On the other hand, it is feasible to predict that the association of organic ligands displaying large molar absorptivity on the acceptor nanoparticle surface could help to overcome the absorption limitation. Inspired by this exciting possibility, herein, we present the Eu^III/Tb^III upconversion intermediated by IPET between the donor Tm^III, Yb^III-doped NaGdF₄ nanoparticle and the acceptor Ln^III-doped NaGdF₄ (Ln = Eu and/or Tb) nanoparticles functionalized with a series organic ligands on the surface (tta^- = thenoyltrifluoroacetonate, acac^- = acetylacetonate, or 3,5-bbza^- = 3,5-dibromebenzoate). Either in solid state or in suspension, upon excitation at 980 nm, visible Eu^III/Tb^III upconversion could be observed. This emission comes from the absorption of the Tm^III, Yb^III pair in the donor nanoparticle, followed by IPET from the Tm^III excited levels to the ligand singlet/triplet states on the acceptor nanoparticle surface, ligand-to-Eu^III/Tb^III energy transfer, and upconversion emission. Spectroscopic evidences from the analysis of the donor level lifetimes indicate the contribution of non-radiative energy transfer for the IPET mechanism; the radiative mechanism also contributes for the IPET. Moreover, the design herein introduced enables the development of luminescence temperature probes with relative thermal sensitivity as high as 1.67% K^-1 at 373 K. Therefore, this new upconversion pathway opens an avenue of possibilities in an uncharted territory to tune the visible upconversion of Ln^III ions.