Triple-Mode upconversion emission for dynamic multicolor luminescent anti-Counterfeiting

Lanthanide (Ln³⁺) luminescent materials play a crucial role in information security and data storage owing to their excellent and unique optical properties. The advances in dynamic colorful luminescent anti-counterfeiting nanomaterials enable the generation of a high-level information encryption. In this work, a superior thermal, optical wavelength and excitation power triple-mode stimuli-responsive emission color modulation is demonstrated in a lanthanide-doped nanostructured luminescent material. The plentiful emission colors are manipulated by modulating the composition of a fluoride core-shell nanostructure with different Ln³⁺ at different doping concentrations. The nanomaterials display remarkable excitation wavelength/power-dependent color change, along with temperature-dependent color variation in the range from 298 K to 437 K, with a good relative sensitivity S_r of 1.1387% K^-1 at 398 K. The universal optical modulation, combined with the excellent optical and structural stability of the luminescent nanoparticles, renders many advantages for the anti-counterfeiting application. This work explores a universal strategy for the manipulation of triple-mode stimuli-responsive dynamic luminescence and demonstrates its good potential for anti-counterfeiting application.

Lanthanide-Ion-Doping Effect on the Morphology and the Structure of NaYF4:Ln3+ Nanoparticles

Two series of β-NaYF₄:Ln³⁺ nanoparticles (Ln = La-Nd, Sm-Lu) containing 20 at. % and 40 at. % of Ln³⁺ with well-defined morphology and size were synthesized via a facile citric-acid-assisted hydrothermal method using rare-earth chlorides as the precursors. The materials were composed from the particles that have a shape of uniform hexagonal prisms with an approximate size of 80-1100 nm. The mean diameter of NaYF₄:Ln³⁺ crystals non-monotonically depended on the lanthanide atomic number and the minimum size was observed for Gd³⁺-doped materials. At the same time, the unit cell parameters decreased from La to Lu according to XRD data analysis. The diameter-to-length ratio increased from La to Lu in both studied series. The effect of the doping lanthanide(III) ion nature on particle size and shape was explained in terms of crystal growth dynamics. This study reports the correlation between the nanoparticle morphologies and the type and content of doping lanthanide ions. The obtained results shed light on the understanding of intrinsic factors' effect on structural features of the nanocrystalline materials.

Gd3+-Doping Effect on Upconversion Emission of NaYF4: Yb3+, Er3+/Tm3+ Microparticles

β-NaYF₄ microcrystals co-doped with Yb³⁺, Er³⁺/Tm³⁺, and Gd³⁺ ions were synthesized via a hydrothermal method using rare-earth chlorides as the precursors. The SEM and XRD data show that the doped β-NaYF₄ form uniform hexagonal prisms with an approximate size of 600-800 nm. The partial substitution of Y by Gd results in size reduction of microcrystals. Upconversion luminescence spectra of microcrystals upon 980 nm excitation contain characteristic intra-configurational ff bands of Er³⁺/Tm³⁺ ions. An addition of Gd³⁺ ions leads to a significant enhancement of upconversion luminescence intensity with maxima at 5 mol % of dopant.

An efficient down conversion luminescencent probe based on a NaGdF4:Eu3+/Ce3+ nanophosphor for chemical sensing of heavy metal ions (Cd2+, Pb2+ and Cr3+) in waste water

Eu³⁺ doped and Eu³⁺/Ce³⁺ co-doped NaGdF₄ nanophosphors are fabricated via a facile hydrothermal route. The codoped counterpart is demonstrated for efficient photoluminescence sensing of heavy metal ions (Cd²⁺, Pb²⁺ and Cr³⁺) present in industrial effluents.

A down converting serine-functionalised NaYF4:Ce3+/Gd3+/Eu3+@NaGdF4:Tb3+ photoluminescent probe for chemical sensing of explosive nitroaromatic compounds

In this contribution, we explored a novel serine-functionalised NaYF₄:Ce³⁺/Gd³⁺/Eu³⁺@NaGdF₄:Tb³⁺ core–shell nanophosphor as a down-converting photoluminescent probe for efficient sensing of nitroaromatic explosives.

Core-mediated synthesis, growth mechanism and near-infrared luminescence enhancement of α-NaGdF4@β-NaLuF4:Nd3+ core–shell nanocrystals

High-performance α-NaGdF₄@β-NaLuF₄:Nd³⁺ nanocrystals have been constructed based on a core-mediated method. Their near-infrared emission intensity was eventually enhanced by more than 2 times.

Growth of β-NaYF4:Eu3+ Crystals by the Solvothermal Method with the Aid of Oleic Acid and Their Photoluminescence Properties

Lanthanide-doped hexagonal β-NaYF₄ crystals have received much attention in recent years due to their excellent photoluminescence properties. However, lanthanide-doped β-NaYF₄ crystals with micron and submicron scales as well as uniform morphology have received less attention. In this study, Eu³⁺-doped β-NaYF₄ (β-NaYF₄:Eu³⁺) crystals of micron and submicron size scales were synthesized using the solvothermal method with ethylene glycol as the solvent. The β-NaYF₄:Eu³⁺ crystals were highly crystallized. A comparison of the characteristics of the β-NaYF₄:Eu³⁺ crystals synthesized with and without the use of oleic acid as a surfactant was conducted. It was found that the utilization of oleic acid as a surfactant during their synthesis greatly decreased their particle size from micron to submicron scale, while adding a small amount of ethanol further reduced their particle size. In addition, they exhibited much smoother surfaces and more uniform morphologies, which were hexagonal prism bipyramids. The microstructural characteristics and photoluminescence properties of the β-NaYF₄:Eu³⁺ crystals were studied in detail. Results showed that β-NaYF₄:Eu³⁺ crystals prepared with the aid of oleic acid as a surfactant during their synthesis exhibited stronger photoluminescence.

Upconversion luminescence properties of BaMoO4: Yb3+, Ln3+ (Ln3+ = Er3+/Tm3+, Er3+/Tm3+/Ho3+) micro-octahedrons

Yb³⁺, Ln³⁺ (Ln³⁺ = Er³⁺/Tm³⁺, Er³⁺/Tm³⁺/Ho³⁺) doped BaMoO₄ micro-octahedrons were synthesized by a hydrothermal process. The as-prepared phosphors were characterized by x-ray powder diffraction, field emission scanning electron microscopy, elemental mapping, energy-dispersive x-ray spectroscopy, x-ray photoelectron spectroscopy, and UV-vis diffuse reflectance spectroscopy. The upconversion luminescence properties of the samples were investigated under 980 nm near infrared excitation. The different concentrations of Er³⁺, Tm³⁺, and Ho³⁺ were used for tuning the multicolor (blue, green, and red) emissions. The multicolor emissions were investigated by Commission Internationale de l'Elcairage chromaticity and decay lifetime. The photon process as well as the energy transfer mechanism between the Yb³⁺ to Er³⁺, Tm³⁺, and Ho³⁺ were described.

Controlling upconversion luminescence patterns in space with red emission enhancement from a single fluoride microcrystal by tuning the excitation mode

The ability to control the upconversion (UC) luminescence patterns in space from lanthanide-doped UC luminescence materials is very important for many applications including three-dimensional color displays, optical waveguides and optical communication. In this work, the fascinating UC luminescence patterns could be adjusted from the blue or green flower-like emission pattern to the red flame-like irradiation pattern with a red luminescence enhancement from a single β-NaYF₄:Yb³⁺/(Tm³⁺ or Er³⁺) microcrystal (MC) by varying the excitation position. The red-to-blue (R/B) and the red-to-green (R/G) emission intensity ratios from the single MC particle that the focal point position of excitation NIR laser is on the side lace are much stronger than that in the case of the focused laser beam on the center. We think that the physical mechanism from the changes in the luminescence pattern and the emission intensity ratio is explained by the optical waveguide effect based on the total reflection effect. These results provide a new strategy for facilitating fundamental investigations of the UC micro/nano-materials, which will lead to promising applications in three-dimensional color display, optical waveguides and optical communication.

The UC luminescence patterns can be controlled from the flower-like emission pattern to the red flame-like irradiation pattern with a red luminescence enhancement for a single lanthanide-doped fluoride material by varying the excitation position.

Preparation, growth mechanism, size manipulation and near-infrared luminescence enhancement of β-NaYF4:Nd3+ microcrystals via Ca2+ doping

The near-infrared emission intensity of NaYF₄:3% Nd³⁺ doped with 20 mol% Ca²⁺ is 3 times that of the Ca²⁺-free samples.

Enhancing Upconversion Luminescence Efficiency via Chiral β-NaYF4:Er3+/Yb3+ Microcrystals Based on Mesoscale Regulation

Chirality, universal characteristics of nature, introduces asymmetry in synthetic materials. Revealing the microscopic asymmetry of macroscopically symmetric materials is the key to control the growth of chiral materials and give full play to their application potential. Materials for photon upconversion (UC) are of great interest for many applications owing to their anti-Stoke luminescence process, especially for chiral UC materials. For the preparation of UC materials, a tiny change in reaction parameters will lead to variations in morphology, phase components, and fluorescence intensity, as well as its chirality. Because of the strict reaction conditions for the formation of chiral UC materials, there are no reports of the successful synthesis of chiral UC materials. Therefore, a facile method for the controllable synthesis of chiral UC materials is highly desired. Herein, chiral-assembled hexagonal prisms of β-NaYF₄:Er³⁺/Yb³⁺ microcrystals were synthesized to realize the smart manipulation of their morphology as well as a great improvement of the fluorescence efficiency. We proposed a three-stage doped β-NaYF₄ crystal growth mechanism on mesoscale regulation, where the fluorescence enhancement principle of chirality was revealed. The enhancement of fluorescence efficiency of chiral UC materials endows their promising application in luminescent displays.

Simple preparation of potassium sulfate nanoparticles

Potassium sulfate nanoparticles were prepared by anti-solvent precipitation, and the particle size could be controlled within the range of 10–100 nm by adjusting the amount of polyacrylic acid. The obtained nanoparticles should be suitable as sacrificial template materials for preparing nanoporous materials, hollow nanomaterials, and other nanoparticles.

Light-Emitting Photon-Upconversion Nanoparticles in the Generation of Transdermal Reactive-Oxygen Species

Common photosensitizers used in photodynamic therapy do not penetrate the skin effectively. In addition, the visible blue and red lights used to excite such photosensitizers have shallow penetration depths through tissue. To overcome these limitations, we have synthesized ultraviolet- and visible-light-emitting, energy-transfer-based upconversion nanoparticles and coencapsulated them inside PLGA-PEG (methoxy poly(ethylene glycol)-b-poly(lactic-co-glycolic acid)) nanoparticles with the photosensitizer protoporphyrin IX. Nd³⁺ has been introduced as a sensitizer in the upconversion nanostructure to allow its excitation at 808 nm. The subcytotoxic doses of the hybrid nanoparticles have been evaluated on different cell lines (i.e., fibroblasts, HaCaT, THP-1 monocytic cell line, U251MG (glioblastoma cell line), and mMSCs (murine mesenchymal stem cells). Upon NIR (near infrared)-light excitation, the upconversion nanoparticles emitted UV and VIS light, which consequently activated the generation of reactive-oxygen species (ROS). In addition, after irradiating at 808 nm, the resulting hybrid nanoparticles containing both upconversion nanoparticles and protoporphyrin IX generated 3.4 times more ROS than PLGA-PEG nanoparticles containing just the same dose of protoporphyrin IX. Their photodynamic effect was also assayed on different cell cultures, demonstrating their efficacy in selectively killing treated and irradiated cells. Compared to the topical application of the free photosensitizer, enhanced skin permeation and penetration were observed for the nanoparticulate formulation, using an ex vivo human-skin-permeation experiment. Whereas free protoporphyrin IX remained located at the outer layer of the skin, nanoparticle-encapsulated protoporphyrin IX was able to penetrate through the epidermal layer slightly into the dermis.

Simultaneous morphology manipulation and upconversion luminescence enhancement of β-NaYF4:Yb3+/Er3+ microcrystals by simply tuning the KF dosage

A strategy has been adopted for simultaneous morphology manipulation and upconversion luminescence enhancement of β-NaYF₄:Yb³⁺/Er³⁺ microcrystals by simply tuning the KF dosage. X-ray power diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and photoluminescence spectra (PL) were used to characterize the samples. The influence of molar ratio of KF to Y³⁺ on the crystal phase and morphology has been systematically investigated and discussed. It is found that the molar ratio of KF to Y³⁺ can strongly control the morphology of the as-synthesized β-NaYF₄ samples because of the different capping effect of F⁻ ions on the different crystal faces. The possible formation mechanism has been proposed on the basis of a series of time-dependent experiments. More importantly, the upconversion luminescence of β-NaYF₄:Yb³⁺/Er³⁺ was greatly enhanced by increasing the molar ratio of KF to RE³⁺ (RE = Y, Yb, Er), which is attributed to the distortion of local crystal field symmetry around lanthanide ions through K⁺ ions doping. This synthetic methodology is expected to provide a new strategy for simultaneous morphology control and remarkable upconversion luminescence enhancement of yttrium fluorides, which may be applicable for other rare earth fluorides.

A novel synthetic route towards monodisperse β-NaYF4:Ln(3+) micro/nanocrystals from layered rare-earth hydroxides at ultra low temperature

Monodisperse β-NaYF4:Ln(3+) (Ln = Yb/Er, Yb/Tm) single micro/nanocrystals were first one-pot fabricated at 50 °C through a novel synthetic route containing two pivotal processes: (a) synthesis of a β-NaYF4 {0001} unit layer through a novel style of ion-exchange from Y2(OH)5NO3·nH2O (LYH) and (b) the subsequent oriented-assembly process.

Tuning of structure and enhancement of upconversion luminescence in NaLuF4:Yb3+,Ho3+ crystals

The formation processes of NaLuF₄ nano/micro-crystals prepared under different experimental conditions.

Visible and near-infrared upconversion photoluminescence in lanthanide-doped KLu3F10 nanoparticles

Upconversion (UC) photoluminescent nanocrystals of Ln³⁺-doped KLu₃F₁₀ have been synthesized via a facile and environmentally-friendly hydrothermal route.

Synthesis of hollow rare-earth compound nanoparticles by a universal sacrificial template method

A universal sacrificial-template method to synthesize hollow structured materials with fluorescence properties.

Unusual upconversion emission from single NaYF4:Yb3+/Ho3+ microrods under NIR excitation

Rare earth ion-doped upconversion materials show great potential applications in optical and optoelectronic devices due to their novel optical properties.

Salt-assisted rapid transformation of NaYF4:Yb3+,Er3+ nanocrystals from cubic to hexagonal

High-quality hexagonal nanocrystals (β-NaYF₄:Yb³⁺,Er³⁺) were prepared through one-pot mild solvothermal synthesis. The crystal structure of NaYF₄:Yb³⁺,Er³⁺ can be controlled by changing the molar ratio of phosphate to Ln³⁺ (Ln³⁺ represents the total amount of Y³⁺ and the doped rare earth elements such as Yb³⁺, Er³⁺).