Small Alkaline-Earth-based Core/Shell Nanoparticles for Efficient Upconversion

Abstract

The optical efficiency of lanthanide-based upconversion is intricately related to the crystalline host lattice. Different crystal fields interacting with the electron clouds of the lanthanides can significantly affect transition probabilities between the energy levels. Here, we investigate six distinct alkaline-earth rare-earth fluoride host materials (M1-xLnxF2+x, MLnF) for infrared-to-visible upconversion, focusing on nanoparticles of CaYF, CaLuF, SrYF, SrLuF, BaYF, and BaLuF doped with Yb3+ and Er3+. We first synthesize ~5 nm upconverting cores of each material via a thermal decomposition method. Then we introduce a dropwise hot-injection method to grow optically inert MYF shell layers around the active cores. Five distinct shell thicknesses are considered for each host material, resulting in 36 unique, monodisperse upconverting nanomaterials each with size below ~15nm. The upconversion quantum yield (UCQY) is measured for all core/shell nanoparticles as function of shell thickness and compared with hexagonal (β-phase) NaGdF4, a traditional upconverting host lattice. While the UCQY of core nanoparticles is below the detection limit (<10−5%), it increases by four to five orders of magnitude as the shell thickness approaches 4–6 nm. The UCQY values of our cubic MLnF nanoparticles meet or exceed the β-NaGdF4 reference sample. Across all core/shell samples, SrLuF nanoparticles are the most efficient, with UCQY values of 0.53% at 80 W/cm2 for cubic nanoparticles with ~11 nm edge length. This efficiency is 5 times higher than our β-NaGdF4 reference material with comparable core size and shell thickness. Our work demonstrates efficient and bright upconversion in ultra-small alkaline-earth-based nanoparticles, with applications spanning biological imaging and optical sensing.

Doping Lanthanide Nanocrystals With Non-lanthanide Ions to Simultaneously Enhance Up- and Down-Conversion Luminescence

“…Different from these two approaches, doping non-Ln ions to enhance the luminescence through modulating the local chemical environment of the emitting centers doesn't necessarily increase the particle size apart from forming more stable structures (Huang et al, 2019 ). Because of the interplay between the 4f electrons of Er 3+ and the crystal field of the host doped with non-Ln ions, the probability of radiative transitions within Er 3+ can hopefully be enhanced by relaxing the selection rules (Fischer et al, 2019 ). For example, metal ions such as Li + , Ca 2+ , Mn 2+ , and Fe 3+ , were adopted as non-Ln dopants to enhance the up-conversion luminescence (Cheng et al, 2012 ; Zeng et al, 2014 ; Tang et al, 2020b ; Verma et al, 2020 ).…”

Highly colloidally stable trimodal 125I-radiolabeled PEG-neridronate-coated upconversion/magnetic bioimaging nanoprobes

“…Despite huge progress in the synthesis and understanding of the properties of UCNPs during recent years 26,27 , a number of limitations still have to be solved before these unique particles are translated into clinical praxis. One of the main challenges is their poor colloidal stability in water, and more importantly, in physiological buffers (e.g., PBS).…”

Bright Infrared‐to‐Ultraviolet/Visible Upconversion in Small Alkaline Earth‐Based Nanoparticles with Biocompatible CaF2 Shells

“…[46] We use Lu 3+ as the rare-earth component of the host materials based on previous studies that have shown higher upconversion efficiencies for MLuF UCNPs. [46,50] Efficient blue upconversion in the b-NaLnF 4 system is generally achieved with 20 %Y b 3+ and % 0.5 %T m 3+ . [51] Based on the average distance between active ions in our alkaline earth host lattices,wedope the MLnF UCNPs with 28 %Yb 3+ and 0.5 %T m 3+ (see Supporting Information).…”

“…[44,45] We also demonstrated the potential of alkaline-earth rare-earth fluoride (M 1Àx Ln x F 2+x, MLnF) host lattices for small (sub-20 nm) lanthanide-based NIR to visible Yb 3+ -Er 3+ upconversion. [46] Theh igh UCQY of these particles encouraged us to consider this new alkaline-earth host for UV and visible emission with Yb 3+ -Tm 3+ upconversion.…”