Local Disorder and Tunable Luminescence in Sr1-x/2Al2-xSixO4 (0.2 ≤ x ≤ 0.5) Transparent Ceramics.

Eu-doped Sr1-x/2Al2-xSixO4 (x = 0.2, 0.4, and 0.5) transparent ceramics have been synthesized by full and congruent crystallization from glasses prepared by aerodynamic levitation and laser-heating method. Structural refinements from synchrotron and neutron powder diffraction data show that the ceramics adopt a 1 × 1 × 2 superstructure compared to the SrAl2O4 hexagonal polymorph. While the observed superstructure reflections indicate a long-range ordering of the Sr vacancies in the structure, 29Si and 27Al solid-state NMR measurements associated with DFT computations reveal a significant degree of disorder in the fully polymerized tetrahedral network. This is evidenced through the presence of Si-O-Si bonds, as well as Si(OAl)4 units at remote distances of the Sr vacancies and Al(OAl)4 units in the close vicinity of Sr vacancies departing from local charge compensation in the network. The transparent ceramics can be doped by europium to induce light emission arising from the volume under UV excitation. Luminescence measurements then reveal the coexistence of Eu2+ and Eu3+ in the samples, thereby allowing tuning the emission color depending on the excitation wavelength and suggesting possible applications such as solid state lighting.

Ligand-Free Synthesis of Tunable Size Ln:BaGdF5 (Ln = Eu³âº and Nd³âº) 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 and properties of multifunctional tetragonal Eu:GdPO4 nanocubes for optical and magnetic resonance imaging applications.

A simple and fast (7 min) procedure for synthesis of gadolinium phosphate nanocubes (edge = 75 nm) based on the microwave-assisted heating at 120 °C of gadolinium acetylacetonate and phosphoric acid solutions in buthylene glycol is reported. These nanocubes were highly crystalline and crystallized into a tetragonal structure, which has not been ever reported for pure gadolinium phosphate. Determination of such crystal structure has been carried out here for the first time in the literature by means of powder X-ray diffraction. The developed synthesis procedure was also successful for preparation of multifunctional europium(III)-doped the gadolinium phosphate nanocubes, which were nontoxic for cells and exhibited strong red luminescence under UV illumination and high transverse relaxivity (r(2)) values. These properties confer them potential applications as biolabels for in vitro optical imaging and as negative contrast agent for magnetic resonance imaging.

Crystal structures and photoluminescence across the La2Si2O7-Ho2Si2O7 system.

It is well-known that when an RE2Si2O7 matrix is doped with active lanthanide ions, it displays promising luminescent responses for optical applications. The crystalline structure adopted by the silicate matrix as well as the distribution of the dopants among the available RE crystallographic sites have important effects on the luminescent yields of these compounds. The present study is aimed at analyzing the structural behavior as well as the luminescent properties of Ho(3+)-substituted La2Si2O7. Several compositions across the La2Si2O7-Ho2Si2O7 system were synthesized using the sol-gel method followed by calcination at 1600 °C. The resulting powders were analyzed by means of X-ray and neutron diffraction to determine the phase stabilities across the system. The results indicated a solid solubility region of G-(La,Ho)2Si2O7 which extends to the La0.6Ho1.4Si2O7 composition. Compositions richer in Ho(3+) show a two-phase domain (G+δ), while δ-(La,Ho)2Si2O7 is the stable phase for Ho(3+) contents higher than 90% (La0.2Ho1.8Si2O7). Anomalous diffraction data interestingly indicated that the La(3+) for Ho(3+) substitution mechanism in the G-(La,Ho)2Si2O7 polymorph is not homogeneous, but a preferential occupation of Ho(3+) for the RE2 site is observed. The Ho(3+)-doped G-La2Si2O7 phosphors exhibited a strong green luminescence after excitation at 446 nm. Lifetime measurements indicated that the optimum phosphor was that with a Ho(3+) content of 10%.

Lanthanide vanadate-based trimodal probes for near-infrared luminescent bioimaging, high-field magnetic resonance imaging, and X-ray computed tomography.

We have developed a trimodal bioimaging probe for near-infrared luminescent imaging, high-field magnetic resonance imaging, and X-ray computed tomography using Dy3+ as the paramagnetic component and Nd3+ as the luminescent cation, both of them incorporated in a vanadate matrix. Among different essayed architectures (single phase and core-shell nanoparticles) the one showing the best luminescent properties is that consisting of uniform DyVO4 nanoparticles coated with a first uniform layer of LaVO4 and a second layer of Nd3+-doped LaVO4. The magnetic relaxivity (r2) at high field (9.4 T) of these nanoparticles was among the highest values ever reported for this kind of probes and their X-ray attenuation properties, due to the presence of lanthanide cations, were also better than those of a commercial contrast agent (iohexol) commonly used for X-ray computed tomography. In addition, they were chemically stable in a physiological medium in which they could be easily dispersed owing to their one-pot functionalization with polyacrylic acid, and, finally, they were non-toxic for human fibroblast cells. Such a probe is, therefore, an excellent multimodal contrast agent for near-infrared luminescent imaging, high-field magnetic resonance imaging, and X-ray computed tomography.

NaY(MoO4)2-based nanoparticles: synthesis, luminescence and photocatalytic properties.

We report on a novel synthesis method, which produces NaY(MoO4)2 nanoparticles having an almost spherical shape and hydrophilic character. The procedure is also suitable for the preparation of NaY(MoO4)2-based nanophosphors by doping this host with lanthanide cations (Eu3+, Tb3+ and Dy3+), which, under UV illumination, exhibit intense luminescence whose color is determined by the selected doping cation (red for Eu3+, green for Tb3+ and yellow for Dy3+). The effects of the cations doping level on the luminescent properties are analyzed in terms of emission intensities and luminescent lifetime, to find the optimum phosphors. Finally, the performance of these nanophosphors and that of the undoped system for the photocatalytic degradation of rhodamine B, used as a model compound, is also analyzed.

Highly Versatile Upconverting Oxyfluoride-Based Nanophosphor Films.

Fluoride-based compounds doped with rare-earth cations are the preferred choice of materials to achieve efficient upconversion, of interest for a plethora of applications ranging from bioimaging to energy harvesting. Herein, we demonstrate a simple route to fabricate bright upconverting films that are transparent, self-standing, flexible, and emit different colors. Starting from the solvothermal synthesis of uniform and colloidally stable yttrium fluoride nanoparticles doped with Yb3+ and Er3+, Ho3+, or Tm3+, we find the experimental conditions to process the nanophosphors as optical quality films of controlled thickness between few hundreds of nanometers and several micrometers. A thorough analysis of both structural and photophysical properties of films annealed at different temperatures reveals a tradeoff between the oxidation of the matrix, which transitions through an oxyfluoride crystal phase, and the efficiency of the upconversion photoluminescence process. It represents a significant step forward in the understanding of the fundamental properties of upconverting materials and can be leveraged for the optimization of upconversion systems in general. We prove bright multicolor upconversion photoluminescence in oxyfluoride-based phosphor transparent films upon excitation with a 980 nm laser for both rigid and flexible versions of the layers, being possible to use the latter to coat surfaces of arbitrary shape. Our results pave the way toward the development of upconverting coatings that can be conveniently integrated in applications that demand a large degree of versatility.

Synthesis, functionalization and properties of uniform europium-doped sodium lanthanum tungstate and molybdate (NaLa(XO4)2, X = Mo,W) probes for luminescent and X-ray computed tomography bioimaging.

A one-pot simple procedure for the synthesis of uniform, ellipsoidal Eu3+-doped sodium lanthanum tungstate and molybdate (NaLa(XO4)2, X  = W, Mo) nanophosphors, functionalized with carboxylate groups, is described. The method is based on a homogeneous precipitation process at 120 °C from appropriate Na+, Ln3+ and tungstate or molybdate precursors dissolved in ethylene glycol/water mixtures containing polyacrylic acid. A comparative study of the luminescent properties of both luminescent materials as a function of the Eu3+ doping level has been performed to find the optimum nanophosphor, whose efficiency as X-ray computed tomography contrast agent is also evaluated and compared with that of a commercial probe. Finally, the cell viability and colloidal stability in physiological pH medium of the optimum samples have also been studied to assess their suitability for biomedical applications.

Room temperature synthesis of water-dispersible Ln3+:CeF3 (Ln = Nd, Tb) nanoparticles with different morphology as bimodal probes for fluorescence and CT imaging.

The singular properties of lanthanide-based inorganic nanoparticles (NPs) has raised the attention of the scientific community in biotechnological applications. In particular, those systems with two or more functionalities are especially interesting. In this work, an effective and commercially attractive procedure has been developed that renders uniform, water-dispersible Ln3+:CeF3 (Ln = Tb, Nd) NPs with different shapes and size. The method consists of the homogeneous precipitation, in a mixture of polyol and water, of cations and anions using precursors that allow the controlled release of the latter. The advantages of the reported method are related to the absence of surfactants, dispersing agents or corrosive precursors as well as to the room temperature of the process. The obtained Tb:CeF3 NPs produce an intense emission after excitation through the Ce-Tb energy transfer band located in the UV spectral region, thus being potentially useful as phosphors for in-vitro imaging purposes. On the other hand, the synthesized Nd:CeF3 NPs are good candidates for in-vivo imaging because their excitation and emission wavelengths lie in the biological windows. Finally, the excellent X-ray attenuation efficacy of the Nd:CeF3 NPs is shown, which confers double functionality to this material as both luminescence bioprobe and contrast agent for X-ray computed-tomography.

Europium-doped NaGd(WO4)2 nanophosphors: synthesis, luminescence and their coating with fluorescein for pH sensing.

Uniform Eu-doped NaGd(WO4)2 nanophosphors with a spherical shape have been synthesized for the first time by using a wet chemistry method based on a homogeneous precipitation process at low temperature (120 °C) in ethylene glycol/water mixtures. The obtained nanoparticles crystallized into the tetragonal structure and presented polycrystalline character. The europium content in such phosphors has been optimized through the analysis of the luminescence dynamics (lifetime measurements). By coating the Eu3+-doped wolframate based nanoparticles with fluorescein through a layer-by-layer (LbL) approach, a wide range (4-10) ratiometric pH-sensitive sensor has been developed, which uses the pH insensitive emission of Eu3+ as a reference.