Design of Lanthanide-Doped Colloidal Nanocrystals: Applications as Phosphors, Sensors, and Photocatalysts

Preparation and Characterization of Uniform and Controlled Silica Encapsulating on Lithium Yttrium Fluoride-Based Upconversion Nanoparticles

In this work, we present an advancement in the encapsulation of lithium yttrium fluoride-based (YLiF4:Yb,Er) upconversion nanocrystals (UCNPs) with silica (SiO2) shells through a reverse microemulsion technique, achieving UCNPs@SiO2 core/shell structures. Key parameters of this approach were optimized to eliminate the occurrence of core-free silica particles and ensure a controlled silica shell thickness growth on the UCNPs. The optimal conditions for this method were using 6 mg of UCNPs, 1.5 mL of Igepal CO-520, 0.25 mL of ammonia, and 50 μL of tetraethyl orthosilicate (TEOS), resulting in a uniform silica shell around UCNPs with a thickness of 8 nm. The optical characteristics of the silica-encased UCNPs were examined, confirming the retention of their intrinsic upconversion luminescence (UC). Furthermore, we developed a reliable strategy to avoid the coencapsulation of multiple UCNPs within a single silica shell. This approach led to a tenfold increase in the UC luminescence of the annealed particles compared to their nonannealed counterparts, under identical silica shell thickness and excitation conditions. This significant improvement addresses a critical challenge and amplifies the applicability of the resulting UCNPs@SiO2 core/shell structures in various fields.

Nucleation and Growth of Bipyramidal Yb:LiYF4 Nanocrystals-Growing Up in a Hot Environment

Lanthanide-doped LiYF₄ (Ln:YLF) is commonly used for a broad variety of optical applications, such as lasing, photon upconversion and optical refrigeration. When synthesized as nanocrystals (NCs), this material is also of interest for biological applications and fundamental physical studies. Until now, it was unclear how Ln:YLF NCs grow from their ionic precursors into tetragonal NCs with a well-defined, bipyramidal shape and uniform dopant distribution. Here, we study the nucleation and growth of ytterbium-doped LiYF₄ (Yb:YLF), as a template for general Ln:YLF NC syntheses. We show that the formation of bipyramidal Yb:YLF NCs is a multistep process starting with the formation of amorphous Yb:YLF spheres. Over time, these spheres grow via Ostwald ripening and crystallize, resulting in bipyramidal Yb:YLF NCs. We further show that prolonged heating of the NCs results in the degradation of the NCs, observed by the presence of large LiF cubes and small, irregular Yb:YLF NCs. Due to the similarity in chemical nature of all lanthanide ions our work sheds light on the formation stages of Ln:YLF NCs in general.

Urea Glass Route as a Way to Optimize YAGG:Ce3+,Cr3+,Pr3+ Nanocrystals for Persistent Luminescence Applications

A new approach for the synthesis of Y₃Al₂Ga₃O₁₂ (YAGG) nanophosphors allowing the preparation of crystallites with sizes starting from 45 nm is presented. The controllability of the energy and trap density of the resulting material samples by annealing temperature was confirmed by thermoluminescence (TL) measurements. It has been shown that the annealing of samples at temperatures up to 1300 °C does not cause any substantial growth of crystallites, still remaining below 100 nm, but leads to changes in the activation energy of the persistent luminescence (PersL) process. On the other hand, annealing above 1400 °C results in grain growth on the submicron scale, which was confirmed by X-ray powder diffraction (XRPD) and electron transmission microscopy (TEM) measurements. In addition, with an increase in the molar ratio of urea to the total amount of metals used (R), qualitative changes are observed in the PersL process occurring from the excited states of Cr³⁺ and Pr³⁺ ions. This proves the influence of the synthesis process, in particular of the metal complexation at its initial stage, on the final structure ordering in the annealed materials. These observations are linked to previously reported defects in the YAGG structure, leading to PersL.

Bis[3-(anthracen-9-yl)pentane-2,4-dionato-κ2 O,O′](N,N-dimethylformamide-κO)[tris(pyrazol-1-yl-κN 2)hydroborato]europium(III)

The title Eu complex, featuring both Anthracac and tris­pyrazolyl­hydro­borato ligands, exhibits an octa­vertex square-pyramidal coordination environment.

The title compound, [Eu(C₉H₁₀BN₆)(C₁₉H₁₅O₂)₂(C₃H₇NO)] or [TpEu(Anthracac)₂(DMF)], was synthesized by reaction of a tris­(pyrazol­yl)borate (Tp⁻) Eu³⁺ complex with 3-(anthracen-9-yl)pentane-2,4-dione (HAnthracac) in the presence of tri­ethyl­amine. In the title compound, Eu³⁺ is located in an octa­vertex square-pyramidal coordination environment. In the two Anthracac⁻ ligands, the anthracene and nearly planar acetyl­acetonate fragments are almost orthogonal. Neighboring mol­ecules of TpEu(Anthracac)₂(DMF) are connected in the crystal by inter­molecular van der Waals inter­actions, while π-stacking inter­actions are limited to the edges of two anthracene rings.

Eu(III)-Chelated Polymeric Hybrid Nanoplatform for Luminescence Resonance Energy Transfer (LRET)-Based Real-Time Monitoring of Organic Cargo Release

The real-time monitoring of specific guest release from nanoscale assemblies has been of great interest for the potential application in nanomedicine. Herein, we present a facile one-pot strategy to achieve a metal-chelated nanoscale platform that enables a highly efficient luminescence resonance energy transfer (LRET) for the monitoring of hydrophobic cargo release. To this end, Eu(III) as a lanthanide luminophore was employed to induce the metal-mediated self-assembly of chelating block copolymers in the presence of fluorescent Nile Blue (NB) as an organic cargo, which can then produce a nanoscale assembly containing a hybrid polyionic complex (HPIC) of Eu(III) and NB as LRET pairs. Exploiting this Eu(III)-chelated, NB-incorporated polymeric assembly as a luminescent platform that allows for the intermolecular distance-sensitive LRET, we further demonstrate that the facile monitoring of NB release from the carriers was made possible upon the addition of serum albumin as a protein reservoir for the released hydrophobic guest molecules.

Luminescence sensitization of terbium-loaded supramolecular gels by hydroxybenzoic acids and used for salicylates sensing

The luminescent terbium (Tb³⁺)-loaded supramolecular gels were facilely prepared through the self-assembly of Fmoc-diphenylalanine (FmocPhePhe) at room temperature. Hydroxybenzoic acid (HA, the isomers are denoted as 2-HA, 3-HA, and 4-HA depending upon the positions of hydroxyl groups) was used as a sensitizer to Tb³⁺. The luminescence sensitization of Tb³⁺ in the gels was realized by the coordination with hydroxybenzoic acids. The spectra of luminescence, UV-vis, FT-IR, and ¹H NMR verified that this sensitization was attributed to the energy transfer from hydroxybenzoic acids to Tb³⁺. The results of XRD, SEM, and phase transfer temperature further indicated that the initial molecule arrangement of the gels was significantly changed by 2-HA, resulting in more ordered and more compact morphology of the gels. 2-HA exhibited more effective sensitization to Tb³⁺ in the gels than 3-HA and 4-HA. It was also found that 2-HA did not affect the self-assembly of FmocPhePhe. Due to the effective fluorescence sensitization by 2-HA, the as-prepared gels can be used for salicylic acid sensing with 6.8 μM of the detection limit. This strategy has been successfully used for the detection of salicylates in pharmaceuticals and cosmetics.

Luminescent Nanomaterials (I)

From molecular probes, also known as fluorophores (typically emitting a longer wavelength than the absorbing wavelength), to inorganic nanoparticles, various light-emitting materials have been actively studied and developed for various applications in life science owing to their superior imaging and sensing ability. Especially after the breakthrough development of quantum dots (QDs), studies have pursued the development of the optical properties and biological applications of luminescent inorganic nanoparticles such as upconversion nanoparticles (UCNPs), metal nanoclusters, carbon dots, and so on. In this review, we first provide a brief explanation about the theoretical background and traditional concepts of molecular fluorophores. Then, currently developed luminescent nanoparticles are described as sensing and imaging platforms from general aspects to technical views.

Ligand-Sensitised LaF3 :Eu3+ and SrF2 :Eu3+ Nanoparticles and in Vitro Haemocompatiblity Studies

Luminescent Ln³⁺ -doped nanoparticles (NPs) functionalised with the desired organic ligand molecules for haemocompatibility studies were obtained in a one-pot synthesis. Chelated aromatic organic ligands such as isophthalic acid, terephthalic acid, ibuprofen, aspirin, 1,2,4,5-benzenetetracarboxylic acid, 2,6-pyridine dicarboxylic acid and adenosine were applied for surface functionalisation. The modification of the nanoparticles is based on the donor-acceptor character of the ligand-nanoparticle system, which is an alternative to covalent functionalisation by peptide bonding as presented in our recent report. The aromatic groups of selected ligands absorb UV light and transfer their excited-state energy to the dopant Eu³⁺ ions in LaF₃ and SrF₂ NPs. Herein, we discuss the structural and spectroscopic characterisation of the NPs and the results of haemocompatibility studies. Flow cytometry analysis of the nanoparticles' membrane-binding is also presented.

Influence of Stabilizing Ion Content on the Structure, Photoluminescence and Biological Properties of Zr1–xEuxO2–0.5x Nanoparticles

Quasi-spherical nanoparticles of ZrO2 containing EuO1.5 from 2 to 15 mol.% were synthesized from the chlorides of the corresponding metals under hydrothermal conditions. The structural changes of Zr1–xEuxO2–0.5x (x = 0.02 ÷ 0.15) nanoparticles depending on the content of europium (III) ions were studied using the complementary methods (X-ray diffraction, electron microdiffraction, Raman and photoluminescence spectroscopy). It was shown that increasing the Eu3+ concentration in the Zr1–xEuxO2–0.5x nanoparticles leads to a transition from the equilibrium monoclinic zirconia phase to metastable tetragonal and cubic polymorphic modifications. In this case, the size of the nanoparticles decreases from 11.5 nm to 9 nm; the specific surface area grows from 80.2 to 111.3 m2/g, and the electrokinetic potential increases monotonously from −8.7 to 16.3 mV. The evolution of the phase composition of Zr1–xEuxO2-0.5x nanoparticles from monoclinic to tetragonal/cubic allomorphs with an increase in the molar fraction of stabilizer ions was correlated with changes in the sublevel structure of 5D0 → 7F2 and 5D0 → 7F4 optical transitions for Eu3+ in the luminescence spectra. Besides, for the nanoparticles obtained by hydrothermal synthesis from chlorides, the quantum efficiency does not exceed 3%. According to the M.T.T. assay, as a result of three-day human fibroblast cultivation in the aqueous dispersion of Zr1–xEuxO2–0.5x (x = 0.02 ÷ 0.15) nanoparticles, the proliferation activity of the cells is maintained, indicating that they do not have cytotoxic properties. Such nanoparticles can be used in organic–inorganic composites for medical applications in order to strengthen the polymer scaffolds and visualize changes in their structure within time.

Design and synthesis of a vanadate-based ratiometric fluorescent probe for sequential recognition of Cu2+ ions and biothiols

YVO₄:Eu³⁺@CDs core–shell nanomaterial was synthesized through a simple self-assembly of carbon dots (CDs) with YVO₄:Eu³⁺, since the high affinity of oxygen-containing groups such as –COOH or –OH of CDs to the metal ions on the surface of YVO₄:Eu³⁺.