Excitation energy migration in a rare earth ions doped upconversion nanoparticle.

The application of upconversion nanoparticles (UCNPs), especially in vivo, has so far been hampered by their relatively low upconversion efficiency. In this work, a strategy of Li⁺ doping was revisited with the aim of enhancing UV to blue UC luminescence of NaYF₄ :Yb³⁺, Tm³⁺ nanocrystals. We have demonstrated that the short wavelength UC emission bands were indeed significantly enhanced. Compared to lithium-free NaYF₄ :Yb³⁺, Tm³⁺, the UC emission intensities of 452 nm and 479 nm of the NPs co-doped with 7 mol% Li⁺ ions were increased by 8 and 5 times, respectively. The mechanism of the enhancement was discussed and the improvement of the nanoparticles' crystallinity and the distortion of the local symmetry around the Tm³⁺ ions, when the Li⁺ ions were introduced, were confirmed to be the origin of the improvement.

The in vivo biological applications of upconversion nanoparticles (UCNPs) prefer excitation at 700–850 nm, instead of 980 nm, due to the absorption of water. Recent approaches in constructing robust Nd³⁺ doped UCNPs with 808 nm excitation properties rely on a thick Nd³⁺ sensitized shell. However, for the very important and popular Förster resonance energy transfer (FRET)-based applications, such as photodynamic therapy (PDT) or switchable biosensors, this type of structure has restrictions resulting in a poor energy transfer. In this work, we have designed a NaYF₄:Yb/Ho@NaYF₄:Nd@NaYF₄ core–shell–shell nanostructure. We have proven that this optimal structure balances the robustness of the upconversion emission and the FRET efficiency for FRET-based bioapplications. A proof of the concept was demonstrated for photodynamic therapy and simultaneous fluorescence imaging of HeLa cells triggered by 808 nm light, where low heating and a high PDT efficacy were achieved.

It is generally accepted that a lanthanide ions based upconversion material follows an activator low doping strategy (normally <3 mol%), because of the restriction of the harmful concentration quenching effect. Here, we demonstrate that this limitation can be broken in nanostructures. Simply by using an inert shell coating strategy, the concentration quenching effect for the activator (Er³⁺) could be eliminated and highly efficient upconversion luminescence realized in the activator fully doped nanostructure, e.g. NaErF₄@NaYF₄. More importantly, this novel nanostructure achieves some long-cherished desires, such as multiple-band co-excitation (∼800 nm, ∼980 nm and ∼1530 nm) and monochromic red emission. Proof-of-concept experiments are presented of the potential benefit of this structure in solar cells and anti-counterfeiting. This nanostructure offers new possibilities in realizing high upconversion emission and novel functionalities of lanthanide based nanomaterials.

This special issue covers a series of cutting-edge works on exploring novel rare earth luminescent materials and their applications in lighting, display, information storage, sensing, and bioimaging as well as therapy.