Synthesis, Growth, and Er3+ Luminescence of Lanthanide Phosphate Nanoparticles

Template-free synthesis of mesoporous and amorphous transition metal phosphate materials

We present how mesoporosity can be engineered in transition metal phosphate (TMPs) materials in a template-free manner. The method involves the transformation of a precursor metal phosphate phase, called M-struvite (NH₄MPO₄·6H₂O, M = Mg²⁺, Ni²⁺, Co²⁺, Ni_xCo_1-x²⁺). It relies on the thermal decomposition of crystalline M-struvite precursors to an amorphous and simultaneously mesoporous phase, which forms during degassing of NH₃ and H₂O. The temporal evolution of mesoporous frameworks and the response of the metal coordination environment were followed by in situ and ex situ scattering and diffraction, as well as X-ray spectroscopy. Despite sharing the same precursor struvite structure, different amorphous and mesoporous structures were obtained depending on the involved transition metal. We highlight the systematic differences in absolute surface area, pore shape, pore size, and phase transitions depending on the metal cation present in the analogous M-struvites. The amorphous structures of thermally decomposed Mg-, Ni- and Ni_xCo_1-x-struvites exhibit high surface areas and pore volumes (240 m² g^-1 and 0.32 cm^-3 g^-1 for Mg and 90 m² g^-1 and 0.13 cm^-3 g^-1 for Ni). We propose that the low-cost, environmentally friendly M-struvites could be obtained as recycling products from industrial and agricultural wastewaters. These waste products could be then upcycled into mesoporous TMPs through a simple thermal treatment for further application, for instance in (electro)catalysis.

YAG:Ce3+ Phosphor: From Micron-Sized Workhorse for General Lighting to a Bright Future on the Nanoscale

The renowned yellow phosphor yttrium aluminum garnet (YAG) doped with trivalent cerium has found its way into applications in many forms: as powder of micron sized crystals, as a ceramic, and even as a single crystal. However, additional technological advancement requires providing this material in new form factors, especially in terms of particle size. Where many materials have been developed on the nanoscale with excellent optical properties (e.g., semiconductor quantum dots, perovskite nanocrystals, and rare earth doped phosphors), it is surprising that the development of nanocrystalline YAG:Ce is not as mature as for these other materials. Control over size and shape is still in its infancy, and optical properties are not yet at the same level as other materials on the nanoscale, even though YAG:Ce microcrystalline materials exceed the performance of most other materials. This review highlights developments in synthesis methods and mechanisms and gives an overview of the state of the art morphologies, particle sizes, and optical properties of YAG:Ce on the nanoscale.

High-efficient fabrication of core-shell-shell structured SiO2@GdPO4:Tb@SiO2 nanoparticles with improved luminescence

SiO₂@GdPO₄:Tb@SiO₂ nanoparticles with core-shell-shell structure were successfully synthesized by a cheap silane coupling agent grafting method at room temperature. This method not only homogeneously coated rare-earth phosphate nanoparticles on the surface of silica spheres but also saved the use of rare-earth resources. The obtained nanoparticles consisted of SiO₂ core with a diameter of approximately 210 nm, GdPO₄:Tb intermediate shell with thickness of approximately 7 nm, and SiO₂ outer shell with thickness of approximately 20 nm. This unique core-shell-shell structured nanoparticles exhibited strong luminescence properties compared with GdPO₄:Tb nanoparticles. The core-shell-shell structured nanoparticles can effectively quench the intrinsic fluorescence of bovine serum albumin through a static quenching mode. The as-synthesized nanoparticles show great potential in biological cell imaging and cancer treatment.

Eu3+ Sensitization via Nonradiative Interparticle Energy Transfer Using Inorganic Nanoparticles

Phosphors have been used successfully for both research and commercial applications for decades. Eu³⁺-doped materials are especially promising, because of their extremely stable, efficient, and narrow red emission lines. Although these emission properties are ideal for lighting applications, weak absorption in the blue spectral range has until now prevented the use of Eu³⁺-based phosphors in applications based on blue light-emitting diodes. Here, we demonstrate a sensitization mechanism of Eu³⁺ based on interparticle Förster resonance energy transfer (IFRET) between lanthanide-doped inorganic nanocrystals (NCs). Compared to co-doping different lanthanides in the same host crystal, IFRET allows an independent choice of host lattices for Eu³⁺ and its sensitizer while potentially greatly reducing metal-to-metal charge transfer quenching. We demonstrate IFRET between NCs, resulting in red Eu³⁺ emission upon blue excitation at 485 nm using LaPO₄:Tb/LaPO₄:Eu and LaPO₄:Tb/YVPO₄:Eu NC mixtures. These findings pave the way toward engineering blue-sensitized line emitters for solid-state lighting applications.

Colloidal LaPO4:Gd3+ nanocrystals: X-ray induced single line UV emission

Colloidal solutions of nearly monodisperse 5 nm LaPO₄:Gd³⁺ nanocrystals are shown to strongly emit UV radiation upon excitation with tungsten Kα radiation (59.3 keV) or vacuum UV radiation (160 nm). The UV emission of the particles consists mainly of a single line at 311 nm corresponding to the ⁶P_7/2-⁸S_7/2 transition of Gd³⁺. The highest emission intensity is observed for LaPO₄ nanocrystals with a Gd³⁺ concentration of 20%. Since the absorption cross section of biomaterials is low for X-rays but high for 311 nm radiation, the UV emission of particles embedded in the biological tissue can only affect the direct vicinity of the particles. Nanocrystals of LaPO₄:Gd³⁺ could, therefore, be interesting for biomedical applications such as strongly localized drug release by X-ray triggered UV uncaging reactions.

Lanthanide phosphate (LnPO4 ) rods as bio-probes: A systematic investigation on structural, optical, magnetic, and biological characteristics

The proposed work involves an exclusive study on the synthesis protocol, crystal structure analysis, and imaging contrast features of unique lanthanide phosphates (LnPO₄ ). XRD and Raman spectra affirmed the ability of the proposed synthesis technique to achieve unique LnPO₄ devoid of impurities. The crystal structure analysis confirms the P121/c1 space setting of NdPO₄ , EuPO₄ , GdPO₄ , and TbPO₄ that all uniformly crystallizes in monoclinic unit cell. In a similar manner, the tetragonal crystal setting of DyPO₄ , ErPO₄ , HoPO₄ , and YbPO₄ that unvaryingly possess the I41/amd space setting is confirmed. Under the same synthesis conditions, the monoclinic (Eu) and tetragonal (Ho) lanthanide phosphates displayed uniform rod-like morphologies. Absorption and luminescence properties of unique LnPO₄ were determined. In vitro biological studies demonstrated low toxicity levels of LnPO₄ and clearly distinguished fluorescence of TbPO₄ and EuPO₄ in Y79, retinoblastoma cell lines. The paramagnetic response of GdPO₄ , NdPO₄ , DyPO₄ , TbPO₄ , and HoPO₄ facilitated excellent magnetic resonance imaging (MRI) contrast features. Meanwhile, GdPO₄ , DyPO₄ , HoPO₄ , and YbPO₄ possessing higher X-ray absorption coefficient than clinical contrast Omnipaque™ exhibited high computed tomography (CT) efficiency. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 1372-1383, 2019.

EDTA-assisted phase conversion synthesis of (Gd0.95RE0.05)PO4 nanowires (RE = Eu, Tb) and investigation of photoluminescence

Hexagonal (Gd_0.95RE_0.05)PO₄·nH₂O nanowires ~300 nm in length and ~10 nm in diameter have been converted from (Gd_0.95RE_0.05)₂(OH)₅NO₃·nH₂O nanosheets (RE = Eu, Tb) in the presence of monoammonium phosphate (NH₄H₂PO₄) and ethylene diamine tetraacetic acid (EDTA). They were characterized by X-ray diffraction, thermogravimetry, electron microscopy, and Fourier transform infrared and photoluminescence spectroscopies. It is shown that EDTA played an essential role in the morphology development of the nanowires. The hydrothermal products obtained up to 180 °C are of a pure hexagonal phase, while monoclinic phosphate evolved as an impurity at 200 °C. The nanowires undergo hexagonal→monoclinic phase transformation upon calcination at ≥600 °C to yield a pure monoclinic phase at ~900 °C. The effects of calcination on morphology, excitation/emission, and fluorescence decay kinetics were investigated in detail with (Gd_0.95Eu_0.05)PO₄ as example. The abnormally strong ⁵D₀→⁷F₄ electric dipole Eu³⁺ emission in the hexagonal phosphates was ascribed to site distortion. The process of energy migration was also discussed for the optically active Gd³⁺ and Eu³⁺/Tb³⁺ ions.

Quick synthesis, functionalization and properties of uniform, luminescent LuPO4-based nanoparticles

Nanometric, luminescent Eu:LuPO₄ particles quickly (30 min) synthesised in the absence of organic additives and able to form stable colloidal suspensions in aqueous media.

Energy-transfer from Gd(III) to Tb(III) in (Gd,Yb,Tb)PO4 nanocrystals

The photoluminescence properties of (Gd,Yb,Tb)PO4 nanocrystals synthesized via a hydrothermal route at 150 °C are reported. Energy-transfer from Gd(3+) to Tb(3+) is witnessed by the detailed analyses of excited-state lifetimes, emission quantum yields, and emission and excitation spectra at room temperature, for Tb(3+) concentrations ranging from 0.5 to 5.0 mol%. Absolute-emission quantum yields up to 42% are obtained by exciting within the (6)I7/2-17/2 (Gd(3+)) manifold at 272 nm. The room temperature emission spectrum is dominated by the (5)D4 → (7)F5 (Tb(3+)) transition at 543 nm, with a long decay-time (3.95-6.25 ms) and exhibiting a rise-time component. The (5)D3 → (7)F6 (Tb(3+)) rise-time (0.078 ms) and the (6)P7/2 → (8)S7/2 (Gd(3+)) decay-time (0.103 ms) are of the same order, supporting the Gd(3+) to Tb(3+) energy-transfer process. A remarkably longer lifetime of 2.29 ms was measured at 11 K for the (6)P7/2 → (8)S7/2 (Gd(3+)) emission upon excitation at 272 nm, while the emission spectrum at 11 K is dominated by the (6)P7/2 → (8)S7/2 transition line, showing that the Gd(3+) to Tb(3+) energy-transfer process is mainly phonon-assisted with an efficiency of ~95% at room temperature. The Gd(3+) to Tb(3+) energy transfer is governed by the exchange mechanism with rates between 10(2) and 10(3) s(-1), depending on the energy mismatch conditions between the (6)I7/2 and (6)P7/2 levels of Gd(3+) and the Tb(3+ 5)I7, (5)F2,3 and (5)H5,6,7 manifolds and the radial overlap integral values.

(Gd,Yb,Tb)PO4 up-conversion nanocrystals for bimodal luminescence-MR imaging

Up-conversion (Gd,Yb,Tb)PO(4) materials and their potential for bimodal imaging have received little attention in the literature. Herein, we report the first study on the up-conversion emission of (Gd,Yb,Tb)PO(4) nanocrystals synthesized via a hydrothermal method at 150 °C. These materials exhibit ultraviolet, blue and green up-conversion emissions upon excitation with a 980 nm continuous wave laser diode. The intensity of the blue-emission band at 479 nm, ascribed to the cooperative up-conversion emission of a pair of excited Yb(3+) ions, depends on the Yb(3+)/Tb(3+) concentration ratio, calcination temperature and particle size. Strong green up-conversion emission of Tb(3+) is observed at 543 nm for the (5)D(4)→(7)F(5) transition. Relaxometry measurements reveal that the nanocrystals are efficient T(2)-weighted (negative) contrast agents which, combined with visible-light emission generated by infrared excitation, affords them considerable potential for being used in bimodal, photoluminescence-magnetic resonance, imaging.

Ultrasonic approach for formation of erbium oxide nanoparticles with variable geometries

Ultrasound (20 kHz, 29 W·cm(-2)) is employed to form three types of erbium oxide nanoparticles in the presence of multiwalled carbon nanotubes as a template material in water. The nanoparticles are (i) erbium carboxioxide nanoparticles deposited on the external walls of multiwalled carbon nanotubes and Er(2)O(3) in the bulk with (ii) hexagonal and (iii) spherical geometries. Each type of ultrasonically formed nanoparticle reveals Er(3+) photoluminescence from crystal lattice. The main advantage of the erbium carboxioxide nanoparticles on the carbon nanotubes is the electromagnetic emission in the visible region, which is new and not examined up to the present date. On the other hand, the photoluminescence of hexagonal erbium oxide nanoparticles is long-lived (μs) and enables the higher energy transition ((4)S(3/2)-(4)I(15/2)), which is not observed for spherical nanoparticles. Our work is unique because it combines for the first time spectroscopy of Er(3+) electronic transitions in the host crystal lattices of nanoparticles with the geometry established by ultrasound in aqueous solution of carbon nanotubes employed as a template material. The work can be of great interest for "green" chemistry synthesis of photoluminescent nanoparticles in water.

Functional coordination nanoparticles

Designing new objects in the perspective of creating useful functionalities at the nanoscale has been the subject of intense research efforts during the last 20 years. Coordination nanoparticles (CNPs) emerged less than 10 years ago, opening new possibilities for the design of bistable molecule-based objects where magnetism may be controlled or tuned by an external perturbation (light irradiation, temperature change, magnetic field, etc.). Magnetic cyanide-bridged networks have been shown to possess the potential to be shaped as nanoparticles, leading to new functionalities. Light- and temperature-induced bistable nanoparticles were thus discovered. Luminescent CNPs were also prepared, demonstrating the large potential of these objects.

Luminescent rare earth nanomaterials for bioprobe applications

Inorganic fluorescent nanoparticles (NPs) have initiated an extensive upsurge in biological application research. Just as quantum dots are regarded as a vigorous reinforcement of the organic dye family, rare earth (RE) fluorescent NPs, as another phosphors branch, also possess unique optical characteristics. The advantages of RE NPs in photostability and colorimetric purity make them suitable for bioprobe applications. Since the preparation technologies have been well developed, it is favourable to prompt the research in the interdisciplinary field of biology and material sciences. Herein, we summarize the synthesis and performance, together with bioprobe applications of RE oxide, sulfoxide, vanadate, phosphate, fluoride, and sodium RE fluoride nanomaterials. The prospects of these promising materials as applied in the biological field is described to draw readers' attention and to attract more research interest.

Facile, alternative synthesis of lanthanum phosphate nanocrystals by ultrasonication

Highly luminescent, rhabdophane (Ce(0.33)La(0.66))PO4.nH2O nanorods and nanoparticles were prepared in aqueous solutions by ultrasonication, at pH 1 and pH 12, respectively. Both nanorods (5 to 9 nm wide and several tens to several hundreds nm long) and nanoparticles (elongated, connected 5 nm particles) were as small and as uniform as products obtained from methods that utilize complexing agents or surfactants, only with no complexing agent. This method of synthesis by ultrasonication is a fast and simple method and it is expected to be applicable for the synthesis of other nanocrystalline lanthanide phosphates.

Epitaxial Synthesis of Uniform Cerium Phosphate One-Dimensional Nanocable Heterostructures with Improved Luminescence

Uniform CePO(4)@LaPO(4) and CePO(4):Tb(3+)@LaPO(4) one-dimensional single-crystalline nanocable heterostructures with highly enhanced photoluminescent emission have been synthesized via a mild and simple hydrothermal approach. The resulting one-dimensional single-crystalline nanocable heterostructures have smooth and uniform LaPO(4) sheaths, which is of great significance in effectively eliminating surface trap-states and suppressing the energy quenching in energy-transfer processes. The photoluminescence results for these one-dimensional nanocable heterostructures illustrate that the uniform LaPO(4) sheaths remarkably increase the luminescent efficiency.

Synthesis of Eu3+-Doped Core and Core/Shell Nanoparticles and Direct Spectroscopic Identification of Dopant Sites at the Surface and in the Interior of the Particles

A variety of redispersible Eu(3+)-doped LaPO(4) nanoparticles have been prepared in a high-boiling coordinating solvent mixture, and the Eu(3+) lattice sites of these materials have been investigated by luminescence line-narrowing measurements. In this spectroscopic method, Eu(3+) ions occupying different lattice sites are selectively excited with a tunable narrow-bandwidth laser system and distinguished by their luminescence spectra ("site-selective spectroscopy"). Depending on the concentration of the dopant, up to three different lattice sites could be identified in the interior of the LaPO(4) nanoparticles. These sites correspond to those known from bulk LaPO(4). In addition, a variety of surface sites is observed, which could be converted completely into bulk sites by overgrowing the nanoparticles with a shell of pure LaPO(4). The surface sites are identical to those obtained by reacting Eu(3+) with the surface of pure LaPO(4) nanoparticles. The spectroscopic properties of Eu(3+)-doped LaPO(4) nanoparticles differ from those of pure EuPO(4) nanoparticles, which were also investigated. Remarkably, the core/shell synthesis investigated in this paper allows one to prepare doped nanoparticles that contain no other dopant sites than those known from the corresponding bulk material.