A brief review of characteristic luminescence properties of Eu3+ in mixed-anion compounds

Trivalent europium (Eu3+) ions show red luminescence with sharp spectral lines owing to the intraconfigurational 4f-4f transitions. Because of their characteristic luminescence properties, various Eu3+-doped inorganic compounds have been developed to meet the demands of optoelectronic devices. Regardless of shielding by the outer 5s and 5p orbitals, the properties of the Eu3+:4f-4f transition depend on the local environment, such as the shapes of the coordination polyhedra, site symmetry, nephelauxetic effects, crystal field effects, and bonding character. Mixed-anion coordination, where multiple types of anions surround a single Eu3+ ion, can directly affect the optical properties of Eu3+. We review the luminescence properties of Eu3+ ions in mixed-anion compounds of the oxynitride YSiO2N and oxyhalides YOX (X = Cl or Br). Oxynitride and oxyhalide coordination results in characteristic transition probabilities and branching ratios of the 5D0 → 7F0-6 transitions due to distorted structural environments and red-shifted charge transfer excitation bands due to an upward shift of the valence band. The expected and experimentally observed features of Eu3+ luminescence in mixed-anion compounds are outlined based on band and Judd-Ofelt theories. Future applications of the intense red luminescence at ∼620 nm under near-ultraviolet light illumination in Eu3+-doped mixed-anion compounds are introduced, and material design guidelines for new functional Eu3+-doped phosphors are presented.

Synthesis Optimization of BaGdF5:x%Tb3+ Nanophosphors for Tunable Particle Size

X-ray photodynamic therapy (XPDT) is aimed at the treatment of deep-located malignant tumors thanks to the high penetration depth of X-rays. In XPDT therapy, it is necessary to use materials that effectively absorb X-rays and convert them into visible radiation-nanophosphors. Rare-earth elements, fluorides, in particular, doped BaGdF₅, are known to serve as efficient nanophosphor. On the other hand, the particle size of nanophosphors has a crucial impact on biodistribution, cell uptake, and cytotoxicity. In this work, we investigated various Tb:Gd ratios in the range from 0.1 to 0.5 and optimized the terbium content to achieve the maximum luminescence under X-ray excitation. The effect of temperature, composition of the ethylene glycol/water solvent, and the synthesis technique (solvothermal and microwave) on the size of the nanophosphors was explored. It was found that the synthesis techniques and the solvent composition had the greatest influence on the averaged particle size. By varying these two parameters, it is possible to tune the size of the nanophosphor particles, which make them suitable for biomedical applications.

BaGdF5 Nanophosphors Doped with Different Concentrations of Eu3+ for Application in X-ray Photodynamic Therapy

X-ray photodynamic therapy (XPDT) has been recently considered as an efficient alternative to conventional radiotherapy of malignant tissues. Nanocomposites for XPDT typically consist of two components—a nanophosphor which re-emits X-rays into visible light that in turn is absorbed by the second component, a photosensitizer, for further generation of reactive oxygen species. In this study, BaGdF₅ nanophosphors doped with different Eu:Gd ratios in the range from 0.01 to 0.50 were synthesized by the microwave route. According to transmission electron microscopy (TEM), the average size of nanophosphors was ~12 nm. Furthermore, different coatings with amorphous SiO₂ and citrates were systematically studied. Micro-CT imaging demonstrated superior X-ray attenuation and sufficient contrast in the liver and the spleen after intravenous injection of citric acid-coated nanoparticles. In case of the SiO₂ surface, post-treatment core–shell morphology was verified via TEM and the possibility of tunable shell size was reported. Nitrogen adsorption/desorption analysis revealed mesoporous SiO₂ formation characterized by the slit-shaped type of pores that should be accessible for methylene blue photosensitizer molecules. It was shown that SiO₂ coating subsequently facilitates methylene blue conjugation and results in the formation of the BaGdF₅: 10% Eu³⁺@SiO₂@MB nanocomposite as a promising candidate for application in XPDT.

Ligand-Free Synthesis of Tunable Size Ln:BaGdF₅ (Ln = Eu³⁺ and Nd³⁺) Nanoparticles: Luminescence, Magnetic Properties, and Biocompatibility

Bifunctional and highly uniform Ln:BaGdF5 (Ln = Eu(3+) and Nd(3+)) nanoparticles have been successfully synthesized using a solvothermal method consisting of the aging at 120 °C of a glycerol solution containing the corresponding Lanthanide acetylacetonates and butylmethylimidazolium tetrafluoroborate. The absence of any surfactant in the synthesis process rendered hydrophilic nanospheres (with tunable diameter from 45 nm 85 nm, depending on the cations concentration of the starting solution) which are suitable for bioapplications. The particles are bifunctional because they showed both optical and magnetic properties due to the presence of the optically active lanthanides (Eu(3+) in the visible and Nd(3+) in the NIR regions of the electromagnetic spectrum) and the paramagnetic gadolinium ion, respectively. The luminescence decay curves of the nanospheres doped with different amounts of Eu(3+) and Nd(3+) have been recorded in order to determine the optimum dopant concentration in each case, which turned out to be 5% Eu(3+) and 0.5% Nd(3+). Likewise, proton relaxation times were measured at 1.5 T in water suspensions of the optimum particles found in the luminescence study. The values obtained suggested that both kinds of particles could be used as positive contrast agents for MRI. Finally, it was demonstrated that both the 5% Eu(3+) and 0.5% Nd(3+)-doped BaGdF5 nanospheres showed negligible cytotoxicity for VERO cells for concentrations up to 0.25 mg mL(-1).

Synthesis, morphology and spectroscopic properties of red-luminescent rhombohedral YOF: Yb3+, Er3+ powders

Rhombohedral YOF: Yb³⁺, Er³⁺ prepared by calcinating exhibit duality of morphology; efficient red light emission could be obtained when the powders are excited by 980 nm laser beam.

YOF nano/micro-crystals: morphology controlled hydrothermal synthesis and luminescence properties

SEM images and multicolored down-conversion and up-conversion luminescence photographs of the YOF: Ln³⁺ nano/micro-structures.

Fabrication and photoluminescence of chemically stable La2O3:Eu3+-La2Sn2O7 core-shell-structured nanoparticles

Core-shell-structured La2O3:Eu3+-La2Sn2O7 nanoparticles were fabricated through SnO2-coating of LaOF:Eu3+ in an aqueous solution and subsequent heat treatments at a higher temperature. The nanoparticles exhibited high chemical stability under an ambient atmosphere and intense red photoluminescence upon irradiation with ultraviolet light.

Fabrication of nanoparticulate porous LaOF films through film growth and thermal decomposition of ion-modified lanthanum diacetate hydroxide

This paper first reports fabrication of macro/nanotextured rare-earth oxyfluoride films. Usage of ion-modified lanthanum diacetate hydroxide (LDAH) as self-templates was successful in producing nanoparticulate lanthanum oxyfluoride (LaOF) films. LDAH template films were deposited on glass substrates through a chemical bath deposition in solutions composed of lanthanum acetate sesquihydrate, methanol, trifluoroacetic acid, and aqueous ammonia. The LDAH films had a unique, nestlike morphology owing to a two-dimensional hexagonal crystal growth. Modification of LDAH with trifluoroacetate ions led to formation of LaOF after pyrolyzing the template films at temperatures of 400-600 degrees C in air. The resultant LaOF films had a nanoparticulate porous microstructure, maintaining the morphology of the original LDAH template films. It was also successful to incorporate Eu3+ ions into LaOF through deposition of the LDAH film in a solution containing europium acetate tetrahydrate. The characteristic photoluminescence from Eu(3+) was observed with an ultraviolet-light excitation at 273 nm, indicating that Eu3+ was homogeneously distributed in LaOF host crystals. Thus the ion-modification of LDAH was also demonstrated to be a useful method for preparing nanostructured rare-earth oxyfluoride materials havingvarious cationic compositions.