Characterization of multifunctional β-NaEuF4/NaGdF4 core-shell nanoparticles with narrow size distribution

Smart design of exquisite multidimensional multilayered sand-clock-like upconversion nanostructures with ultrabright luminescence as efficient luminescence probes for bioimaging application

A facile scalable approach is presented for the rational design of multidimensional, multilayered sand-clock-like UCNPs (denoted as UCCKs) bounded with high index facets, with a tunable Nd³⁺ content, and without a template or multiple complicated reaction steps. This was achieved using the seed-mediated growth and subsequent longitudinal direction epitaxial growth with the assistance of oleic acid and NH₄F. The as-formed UCCKs composed of an inner layer (NaYF₄:Yb,Er,Ca), an intermediate layer (NaYF₄:Yb,Ca), and an outer layer (NaNdF₄:Yb,Ca). The outer shell, enriched with Nd³⁺ sensitizer, augmented the near-infrared (NIR) photon absorption, whereas the intermediate shell, enriched with Yb³⁺, acted as a bridge for energy transfer from Nd³⁺ to Er³⁺ emitter in the inner core alongside with precluding any deleterious energy back-transfer from Er³⁺ or quenching effect from Nd³⁺. These unique structural and compositional properties of UCCKs endowed the UCL intensity of UCCKs by 22 and 10 times higher than that of hexagonal UCNP core (NaYF₄:Yb,Er,Ca) and hexagonal UCNP core-shell (NaYF₄:Yb,Er,Ca@NaYF₄:Yb,Ca), respectively. Intriguingly, the UCL intensity increased significantly with increasing the content of Nd³⁺ in the outer shell. The silica-coated UCCKs were used as excellent long-term luminescence probes for the in vitro bioimaging without any noteworthy cytotoxicity. The presented approach may pave the road for controlling the synthesis of multidimensional UCCKs for various applications. Graphical abstract We developed novel multidimensional multilayered sand-clock-like upconversion nanostructures composed of a spherical inner core (NaYF₄:Yb,Er,Ca), hexagonal intermediate shell (NaYF₄:Yb,Ca) and two up-down outer shell (NaNdF₄:Yb,Ca) with controllable Nd³⁺ as an efficient and safe probe for bioimaging applications without any quenching effect.

Tuning epitaxial growth on NaYbF4 upconversion nanoparticles by strain management

Core-shell structural engineering is a common strategy for tuning upconversion luminescence in lanthanide-doped nanoparticles. However, epitaxial growth on hexagonal phase NaYbF₄ nanoparticles typically suffers from incomplete shell coverage due to the large and anisotropic interfacial strain. Herein, we explore the effects of core particle size and morphology as well as reaction temperature on controlling the epitaxial growth of NaGdF₄ shells on NaYbF₄ nanoparticles with misfit parameters of f_a = 1.58% and f_l = 2.24% for axial and lateral growth, respectively. Rod-like core particles with a long length and a large diameter are found to promote shell growth with high surface coverage by facilitating the relaxation of lattice strains. Furthermore, the primary NaGdF₄ shell can serve as a transition layer to mediate the growth of additional NaNdF₄ coating layers that display an even larger lattice misfit with the core (f_a = 2.98%; f_l = 4.32%). The resultant NaYbF₄@Na(Gd/Nd)F₄ core-shell nanostructures simultaneously show strong multiphoton upconversion luminescence and superior magnetic resonance T₁ ionic relaxivity. Our findings are important for the rational design of core-shell upconversion nanoparticles with optimized properties and functionality for technological applications.

Energy transfer mechanism dominated by the doping location of activators in rare-earth upconversion nanoparticles

Research on the energy transfer mechanism of rare-earth-doped upconversion nanoparticles (UCNPs) has been an important area due to the increasing demand for tuning multicolor emission and enhancing the upconversion efficiency; however, because of large energy mismatch, many lanthanide activators, such as Eu3+, cannot realize highly efficient near infrared-to-visible upconversion by simple codoping of Yb3+. Therefore, introduction of other ions to assist the energy transfer process is required. Herein, we prepared core-shell nanoparticles with different doping locations to investigate the upconversion energy transfer mechanism. The upconversion luminescence (UCL) of core-shell nanoparticles was investigated by steady-state luminescence and time-resolved luminescence spectra. The UCL behaviors in these different multi-activator core-shell nanoparticles were observed. The results revealed different energy transfer channels influenced by the doping location of activators. This study may open up new avenues of structure design for fine-tuning of multicolor UCL for specific applications.

Mammalian cell defence mechanisms against the cytotoxicity of NaYF4:(Er,Yb,Gd) nanoparticles

Water-soluble upconversion nanoparticles (UCNPs), based on polyvinylpyrrolidone (PVP)-coated NaYF₄:Er³⁺,Yb³⁺,Gd³⁺, with various concentrations of Gd³⁺ ions and relatively high upconversion efficiencies, were synthesized. The internalization and cytotoxicity of the thus obtained UCNPs were evaluated in three cell lines (HeLa, HEK293 and astrocytes). No cytotoxicity was observed even at concentrations of UCNPs up to 50 μg ml^-1. The fate of the UCNPs within the cells was studied by examining their upconversion emission spectra with confocal microscopy and confirming these observations with transmission electron microscopy. It was found that the cellular uptake of the UCNPs occurred primarily by clathrin-mediated endocytosis, whereas they were secreted from the cells via lysosomal exocytosis. The results of this study, focused on the mechanisms of the cellular uptake, localization and secretion of UCNPs, demonstrate, for the first time, the co-localization of UCNPs within discrete cell organelles.

Lanthanide doped ultrafine hybrid nanostructures: multicolour luminescence, upconversion based energy transfer and luminescent solar collector applications

We herein demonstrate novel inorganic-organic hybrid nanoparticles (HNPs) composed of inorganic NPs, NaY_0.78Er_0.02Yb_0.2F₄, and an organic β-diketonate complex, Eu(TTA)₃Phen, for energy harvesting applications. Both the systems maintain their core integrity and remain entangled through weak interacting forces. HNPs incorporate the characteristic optical behaviour of both the systems i.e. they give an intense red emission under UV excitation, due to Eu³⁺ in organic complexes, and efficient green upconversion emission of Er³⁺ in inorganic NPs for NIR (980 nm) excitation. However, (i) an energy transfer from Er³⁺ (inorganic NPs) to Eu³⁺ (organic complex) under NIR excitation, and (ii) an increase in the decay time of ⁵D₀ → ⁷F₂ transition of Eu³⁺ for HNPs as compared to the Eu(TTA)₃Phen complex, under different excitation wavelengths, are added optical characteristics which point to an important role of the interface between both the systems. Herein, the ultra-small size (6-9 nm) and spherical shape of the inorganic NPs offer a large surface area, which improves the weak interaction force between both the systems. Furthermore, the HNPs dispersed in the PMMA polymer have been successfully utilized for luminescent solar collector (LSC) applications.