Giant enhancement of upconversion emission in (NaYF₄:Nd³⁺/Yb³⁺/Ho³⁺)/(NaYF₄:Nd³⁺/Yb³⁺) core/shell nanoparticles excited at 808 nm

Highly efficient upconversion luminescence in narrow-bandgap Y2Mo4O15

Lanthanide-doped upconversion (UC) materials have been extensively investigated for their unique capability to convert low-energy excitation into high-energy emission. Contrary to previous reports suggesting that efficient UC luminescence (UCL) is exclusively observed in materials with a wide bandgap, we have discovered in this study that Y2Mo4O15:Yb3+/Tm3+ microcrystals, a narrowband material, exhibit highly efficient UC emission. Remarkably, these microcrystals do not display any four- or five-photon UC emission bands. This particular optical phenomenon is independent of the variation in doping ion concentration, temperature, phonon energy, and excitation power density. Combining theoretical calculations and experimental results, we attribute the vanishing emission bands to the strong interaction between the bandgap of the Y2Mo4O15 host matrix (3.37 eV) and the high-energy levels (1I6 and 1D2) of Tm3+ ions. This interaction can effectively catalyze the UC emission process of Tm3+ ions, which leads to Y2Mo4O15:Yb3+/Tm3+ microcrystals possessing very strong UCL intensity. The brightness of these microcrystals outshines commercial UC NaYF4:Yb3+,Er3+ green phosphors by a factor of 10 and is 1.4 times greater than that of UC NaYF4:Yb3+,Tm3+ blue phosphors. Ultimately, Y2Mo4O15:Yb3+/Tm3+ microcrystals, with their distinctive optical characteristics, are being tailored for sophisticated anti-counterfeiting and information encryption applications.

Integration of 3D Fluorescence Imaging and Luminescent Thermometry with Core-Shell Engineered NaYF4:Nd3+/Yb3+/Ho3+ Nanoparticles

The design of rare-earth-doped upconversion/downshifting nanoparticles (NPs) for theoretical use in nanomedicine has garnered considerable interest. Previous research has emphasized luminescent nanothermometry and photothermal therapy, while three-dimensional (3D) near-infrared (NIR) luminescent tracers have received less attention. Our study introduces Nd3+-, Yb3+-, and Ho3+-doped NaYF4 core-shell luminescent NPs as potential multiparametric nanothermometers and NIR imaging tracers. Nd3+ sensitizes at 804 nm, while Yb3+ bridges to activators Ho3+. We evaluated the photoluminescence properties of Nd3+-, Yb3+-, and Ho3+-doped core and core-shell NPs synthesized via polyol-mediated and thermal decomposition methods. The NaYF4:NdYbHo(7/15/3%)@NaYF4:Nd(15%) core-shell NPs demonstrate competitive nanothermometry capabilities. Specifically, the polyol-synthesized sample exhibits a sensitivity of 0.27% K-1 at 313 K (40 °C), whereas the thermally decomposed synthesized sample shows a significantly higher sensitivity of 0.55% K-1 at 313 K (40 °C) in the near-infrared range. Control samples indicate back energy transfer processes from both Yb and Ho to Nd, while Yb to Ho energy transfer enhances Ho3+-driven upconversion transitions in green and red wavelengths, suggesting promise for photodynamic therapy. Fluorescence molecular tomography confirms 3D NIR fluorescence nanoparticle localization in a biological media after injection, highlighting the potential of core-shell NPs as NIR luminescent tracers. The strategy's clinical impact lies in photothermal treatment planning, leveraging core-shell NPs for (pre)clinical applications, and enabling the easy addition of new functionalities through distinct ion doping.

The Coming of Age of Neodymium: Redefining Its Role in Rare Earth Doped Nanoparticles

Among luminescent nanostructures actively investigated in the last couple of decades, rare earth (RE³⁺) doped nanoparticles (RENPs) are some of the most reported family of materials. The development of RENPs in the biomedical framework is quickly making its transition to the ∼800 nm excitation pathway, beneficial for both in vitro and in vivo applications to eliminate heating and facilitate higher penetration in tissues. Therefore, reports and investigations on RENPs containing the neodymium ion (Nd³⁺) greatly increased in number as the focus on ∼800 nm radiation absorbing Nd³⁺ ion gained traction. In this review, we cover the basics behind the RE³⁺ luminescence, the most successful Nd³⁺-RENP architectures, and highlight application areas. Nd³⁺-RENPs, particularly Nd³⁺-sensitized RENPs, have been scrutinized by considering the division between their upconversion and downshifting emissions. Aside from their distinctive optical properties, significant attention is paid to the diverse applications of Nd³⁺-RENPs, notwithstanding the pitfalls that are still to be addressed. Overall, we aim to provide a comprehensive overview on Nd³⁺-RENPs, discussing their developmental and applicative successes as well as challenges. We also assess future research pathways and foreseeable obstacles ahead, in a field, which we believe will continue witnessing an effervescent progress in the years to come.

Photoswitching the injected energy flux via core-sensitized energy migration upconversion for emission-varying STED microscopy

Stimulated emission depletion (STED) microscopy achieved with lanthanide-doped upconversion nanoparticles (UCNPs) exhibits many outstanding advantages such as low-power illumination, near-infrared (NIR) excitation, and high photostability. However, the available types of UCNP-STED probes are very limited and rely greatly on the specific depletion mechanism. Here, by combining the STED and the energy migration upconversion processes, emissions of Tb³⁺, Eu³⁺, Dy³⁺, and Sm³⁺ distributed in the shell can all be depleted by interrupting the injected energy flux from the Tm³⁺-doped core nanoparticles. With the merit of the proposed strategy, new types of UCNP-STED probes are demonstrated to perform emission-varying STED imaging with one single, fixed pair of low-power NIR continuous wave lasers.

Excitation-Power-Dependent Upconversion Luminescence Competition in Single β-NaYbF4:Er Microcrystal Pumped at 808 nm

Controlling the upconversion luminescence (UCL) intensity ratio, especially pumped at 808 nm, is of fundamental importance in biological applications due to the water molecules exhibiting low absorption at this excitation wavelength. In this work, a series of β-NaYbF₄:Er microrods were synthesized by a simple one-pot hydrothermal method and their intense green (545 nm) and red (650 nm) UCL were experimentally investigated based on the single-particle level under the excitation of 808 nm continuous-wave (CW) laser. Interestingly, the competition between the green and red UCL can be observed in highly Yb³⁺-doped microcrystals as the excitation intensity gradually increases, which leads to the UCL color changing from green to orange. However, the microcrystals doped with low Yb³⁺ concentration keep green color which is independent of the excitation power. Further investigations demonstrate that the cross-relaxation (CR) processes between Yb³⁺ and Er³⁺ ions result in the UCL competition.

Hollow nanosphere arrays with a high-index contrast for enhanced scintillating light output from β-Ga2O3 crystals

β-Ga₂O₃ is a new type of fast scintillator with potential applications in medical imaging and nuclear radiation detection with high count-rate situations. Because of the severe total internal reflection with its high refractive index, the light extraction efficiency of β-Ga₂O₃ crystals is rather low, which would limit the performance of detection systems. In this paper, we use hollow nanosphere arrays with a high-index contrast to enhance the light extraction efficiency of β-Ga₂O₃ crystals. We can increase the transmission diffraction efficiency and reduce the reflection diffraction efficiency through controlling the refractive index and the thickness of the shell of the hollow nanospheres, which can lead to a significant increase in the light extraction efficiency. The relationships between the light extraction efficiency and the refractive index and thickness of the shell of the hollow nanospheres are investigated by both numerical simulations and experiments. It is found that when the refractive index of the shell of the hollow nanospheres is higher than that of β-Ga₂O₃, the light extraction efficiency is mainly determined by the diffraction efficiency of light transmitted from the surface with the hollow nanosphere arrays. When the refractive index of the shell is less than that of β-Ga₂O₃, the light extraction efficiency is determined by the ratio of the diffraction efficiency of the light transmitted from the surface with the hollow nanosphere arrays to the diffraction efficiency of the light that can escape from the lateral surface.

Temperature dependence of up-conversion luminescence and sensing properties of LaNbO4: Nd3+/Yb3+/Ho3+ phosphor under 808 nm excitation

LaNbO₄: Nd³⁺/Yb³⁺/Ho³⁺ phosphor was prepared by a conventional high temperature solid-state reaction method. The temperature dependence of up-conversion (UC) luminescence property of LaNbO₄: Nd³⁺/Yb³⁺/Ho³⁺ phosphor under 808 nm excitation and the potential application of exploiting the red-to-green UC emission intensity ratio (I_R/I_G) of Ho³⁺ in temperature sensing were studied. Two-photon processes were confirmed to be responsible for both the green and the red UC emissions at different temperatures by analyzing the excitation power density dependent UC luminescence spectra measured at different temperatures. The energy level diagram was drawn to analyze the UC luminescence mechanism of Ho³⁺. In addition, it was found that the ratio I_R/I_G of Ho³⁺ was independent of the excitation power density of 808 nm laser under the current experimental condition, but it was sensitive to the temperature. And the temperature dependent UC luminescence spectra displayed that the ratio I_R/I_G exhibited a good linear increasing tendency with temperature rising. The obtained temperature sensing sensitivity was 2.04 × 10^-3 K^-1 in the temperature range of 303-693 K. The results suggest that LaNbO₄: Nd³⁺/Yb³⁺/Ho³⁺ phosphor may be a good candidate for application in optical temperature sensors.

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.

Upconverting LuVO4:Nd3+/Yb3+/Er3+@SiO2@Cu2S Hollow Nanoplatforms for Self-monitored Photothermal Ablation

Self-monitored photothermal therapy (PTT) with minimal collateral damages has emerged as a challenging strategy for antibacterial and cancer treatments, which could be fulfilled via the rational integration of luminescent thermometry and photothermal ablation within a single upconverting (UC) nanoplatform. Herein, 808 nm light-driven dual-functional nanoplatforms LuVO₄:Nd³⁺/Yb³⁺/Er³⁺@SiO₂@Cu₂S were successfully developed using olivelike LuVO₄:Nd³⁺/Yb³⁺/Er³⁺ hollow nanoparticles as the thermal-sensing core and ultrasmall Cu₂S nanoparticles as the photothermal satellite. Irradiated by 808 nm laser, thermal-sensing behaviors of samples were evaluated based on the high-purity Er³⁺ green emissions, while the surface-attached Cu₂S exhibited superior photothermal effects due to the efficient absorption of incident laser and near-infrared emissions from the luminescent core. The feasibility of bifunctional samples acting as self-monitored photothermal agents in subtissues and antibacterial agents against drug-resistant bacteria was separately assessed. Results provide deeper insights into the desirable design of 808 nm-driven multifunctional nanoplatforms with intense UC emission, sensitive thermometry, and effective photothermal conversion toward self-monitored PTT with high therapeutic accuracy.

Tuning the sensitivity of lanthanide-activated NIR nanothermometers in the biological windows

Lanthanide-activated SrF₂ nanoparticles with a multishell architecture were investigated as optical thermometers in the biological windows. A ratiometric approach based on the relative changes in the intensities of different lanthanide (Nd³⁺ and Yb³⁺) NIR emissions was applied to investigate the thermometric properties of the nanoparticles. It was found that an appropriate doping with Er³⁺ ions can increase the thermometric properties of the Nd³⁺-Yb³⁺ coupled systems. In addition, a core containing Yb³⁺ and Tm³⁺ can generate light in the visible and UV regions upon near-infrared (NIR) laser excitation at 980 nm. The multishell structure combined with the rational choice of dopants proves to be particularly important to control and enhance the performance of nanoparticles as NIR nanothermometers.

Epitaxial growth of ultrathin layers on the surface of sub-10 nm nanoparticles: the case of β-NaGdF4:Yb/Er@NaDyF4 nanoparticles

Upconversion core–shell nanoparticles have attracted a large amount of attention due to their multifunctionality and specific applications. In this work, based on a NaGdF₄ sub-10 nm ultrasmall nanocore, a series of core–shell upconversion nanoparticles with uniform size doped with Yb³⁺, Er³⁺ and NaDyF₄ shells with different thicknesses were synthesized by a facile sequential growth process. NaDyF₄ coated upconversion luminescent nanoparticles showed an obvious fluorescence quenching under excitation at 980 nm as a result of energy resonance transfer between Yb³⁺, Er³⁺ and Dy³⁺. NaGdF₄:Yb,Er@NaDyF₄ core–shell nanoparticles with ultrathin layer shells exhibited a better T₁-weighted MR contrast.

In this work, a series of core–shell upconversion nanoparticles with uniform size doped with Yb³⁺, Er³⁺ and NaDyF₄ shells with different thicknesses were synthesized by a facile sequential growth process.

Ce3+ and Tb3+ doped Ca3Gd(AlO)3(BO3)4 phosphors: synthesis, tunable photoluminescence, thermal stability, and potential application in white LEDs

Novel blue-green-emitting Ca₃Gd(AlO)₃(BO₃)₄:Ce³⁺,Tb³⁺ phosphors were successfully synthesized via traditional high temperature solid reaction method. X-ray diffraction, luminescence spectroscopy, fluorescence decay time and fluorescent thermal stability tests have been used to characterize the as-prepared samples. The energy transfer from Ce³⁺ to Tb³⁺ ions in the Ca₃Gd(AlO)₃(BO₃)₄ host has been demonstrated to be by dipole–dipole interaction, and the energy transfer efficiency reached as high as 83.6% for Ca₃Gd_0.39(AlO)₃(BO₃)₄:0.01Ce³⁺,0.6Tb³⁺. The critical distance was calculated to be 9.44 Å according to the concentration quenching method. The emission colour of the obtained phosphors can be tuned appropriately from deep blue (0.169, 0.067) to green (0.347, 0.494) through increasing the doping concentrations of Tb³⁺. Moreover, the Ca₃Gd_0.39(AlO)₃(BO₃)₄:0.01Ce³⁺,0.6Tb³⁺ phosphor possessed excellent thermal stability at high temperature, and the emission intensity at 423 K was about 87% of that at 303 K. Finally, the fabricated prototype LED device with a BaMgAl₁₀O₇:Eu²⁺ blue phosphor, CaAlSiN₃:Eu²⁺ red phosphor, Ca₃Gd_0.39(AlO)₃(BO₃)₄:0.01Ce³⁺,0.6Tb³⁺ green phosphor and 365 nm-emitting InGaN chip exhibited bright warm white light. The current study shows that Ca₃Gd_0.39(AlO)₃(BO₃)₄:0.01Ce³⁺,0.6Tb³⁺ can be used as a potential green phosphor for white LEDs.

Novel thermal-stable blue-green-emitting Ca₃Gd(AlO)₃(BO₃)₄:Ce³⁺,Tb³⁺ phosphors were developed for near-ultraviolet-excited white LEDs.

Non-bleaching fluorescence emission difference microscopy using single 808-nm laser excited red upconversion emission

Optical super-resolution microscopy has become a powerful technique to help scientists to monitor the sample of interest at nanoscale. Fluorescence emission difference (FED) microscopy, a very facile super-resolution method, does not require high depleting laser intensity and is independent on the species of agents, which makes FED microscopy possess great potential. However, to date, the biomarkers applied in FED microscopy usually suffer from a photo-bleaching problem. In this work, by introducing Er³⁺ activated upconverting nanoparticles with red-color emission and non-photobleaching properties, we demonstrate nonbleaching super-resolution imaging with FED microscopy. The dopant neodymium ions (Nd³⁺) can work as highly efficient sensitizing ions and enable near infrared 808-nm CW laser excitation of relatively low power, which would potentially reduce high intensity/short-wavelength light induced tissue damage. Both simulations and experiments on monodispersed NaYF₄:Nd³⁺/Yb³⁺/Er³⁺@NaYF₄:Nd³⁺ UCNPs also indicate that the easy saturation of the multiphoton properties of these UCNPs is beneficial to resolution enhancement in FED microscopy.

808-nm-Light-Excited Lanthanide-Doped Nanoparticles: Rational Design, Luminescence Control and Theranostic Applications

808 nm-light-excited lanthanide (Ln³⁺ )-doped nanoparticles (LnNPs) hold great promise for a wide range of applications, including bioimaging diagnosis and anticancer therapy. This is due to their unique properties, including their minimized overheating effect, improved penetration depth, relatively high quantum yields, and other common features of LnNPs. In this review, the progress of 808 nm-excited LnNPs is reported, including their i) luminescence mechanism, ii) luminescence enhancement, iii) color tuning, iv) diagnostic and v) therapeutic applications. Finally, the future outlook and challenges of 808 nm-excited LnNPs are presented.

Highly Emissive Dye-Sensitized Upconversion Nanostructure for Dual-Photosensitizer Photodynamic Therapy and Bioimaging

Rare-earth-based upconversion nanotechnology has recently shown great promise for photodynamic therapy (PDT). However, the NIR-induced PDT is greatly restricted by overheating issues on normal bodies and low yields of reactive oxygen species (ROS, ¹O₂). Here, IR-808-sensitized upconversion nanoparticles (NaGdF₄:Yb,Er@NaGdF₄:Nd,Yb) were combined with mesoporous silica, which has Ce6 (red-light-excited photosensitizer) and MC540 (green-light-excited photosensitizer) loaded inside through covalent bond and electrostatic interaction, respectively. When irradiated by tissue-penetrable 808 nm light, the IR-808 greatly absorb 808 nm photons and then emit a broadband peak which overlaps perfectly with the absorption of Nd³⁺ and Yb³⁺. Thereafter, the Nd³⁺/Yb³⁺ incorporated shell synergistically captures the emitted NIR photons to illuminate NaGdF₄:Yb,Er zone and then radiate ultrabright green and red emissions. The visible emissions simultaneously activate the dual-photosensitizer to produce a large amount of ROS and, importantly, low heating effects. The in vitro and in vivo experiments indicate that the dual-photosensitizer nanostructure has trimodal (UCL/CT/MRI) imaging functions and high anticancer effectiveness, suggesting its potential clinical application as an imaging-guided PDT technique.

Ultrasmall-Superbright Neodymium-Upconversion Nanoparticles via Energy Migration Manipulation and Lattice Modification: 808 nm-Activated Drug Release

Nd³⁺-sensitized upconversion nanoparticles are among the most promising emerging fluorescent nanotransducers. They are activated by 808 nm irradiation, which features merits such as limited tissue overheating and deeper penetration depth, and hence are attractive for diagnostic and therapeutic applications. Recent studies indicate that ultrasmall nanoparticles (<10 nm) are potentially more suitable for clinical application due to their favorable biodistribution and safety profiles. However, upconversion nanoparticles in the sub-10 nm range suffer from poor luminescence due to their ultrasmall size and greater proportion of lattice defects. To reconcile these opposing traits, we adopt a combinatorial strategy of energy migration manipulation and crystal lattice modification, creating ultrasmall-superbright Nd³⁺-sensitized nanoparticles with 2 orders of magnitude enhancement in upconversion luminescence. Specifically, we configure a sandwich-type nanostructure with a Yb³⁺-enriched intermediate layer [Nd³⁺]-[Yb³⁺-Yb³⁺]-[Yb³⁺-Tm³⁺] to form a positively reinforced energy migration system, while introducing Ca²⁺ into the crystal lattice to reduce lattice defects. Furthermore, we apply the nanoparticles to 808 nm light-mediated drug release. The results indicate time-dependent cancer cells killing and better antitumor activities. These ultrasmall-superbright dots have unraveled more opportunities in upconversion photomedicine with the promise of potentially safer and more effective therapy.

High resolution fluorescence bio-imaging upconversion nanoparticles in insects

Imaging fluorescent markers with brightness, photostability, and continuous emission with auto fluorescence background suppression in biological samples has always been challenging due to limitations of available and economical techniques. Here we report a new approach, to achieve high contrast imaging inside small and difficult biological systems with special geometry such as fire ants, an important agricultural pest, using a homemade cost-effective optical system. Unlike the commonly used rare-earth doped fluoride nanoparticles, we utilized nanoparticles with a high upconversion efficiency in water. Specifically Y₂O₃:Er⁺³,Yb⁺³ nanoparticles (40-50 nm diameter) were fed to fire ants as food and then a simple illuminating experiment was conducted at 980 nm wavelength at relatively low pump intensity8 kW.cm^-2. The locations were further confirmed by X-ray tomography, where most particles aggregated inside the ant's mouth. High resolution, fast, and economical optical imaging system opens the door for studying more complex biological systems.

Tuning the morphology, luminescence and magnetic properties of hexagonal-phase NaGdF4: Yb, Er nanocrystals via altering the addition sequence of the precursors

Hexagonal-phase NaGdF₄: Yb, Er upconversion nanocrystals (UCNCs) with tunable morphology and properties were successfully prepared via a thermal decomposition method. The influences of the adding sequence of the precursors on the morphology, chemical composition, luminescence and magnetic properties were investigated by transmission electron microscopy (TEM), inductively coupled plasma-atomic emission spectrometry (ICP-AES), upconversion (UC) spectroscopy, and a vibrating sample magnetometer (VSM). It was found that the resulting nanocrystals, with different sizes ranging from 24 to 224 nm, are in the shape of spheres,  hexagonal plates and flakes; moreover, the composition percentage of Yb³⁺-Er³⁺ and Gd³⁺ ions was found to vary in a regular pattern with the adding sequence. Furthermore, the intensity ratios of emission colors (f _g/r, f _g/p), and the magnetic mass susceptibility of hexagonal-phase NaGdF₄: Yb, Er nanocrystals change along with the composition of the nanocrystals. A positive correlation between the susceptibility and f _g/r of NaGdF₄: Yb, Er was proposed. The decomposition processes of the precursors were investigated by a thermogravimetric (TG) analyzer. The result indicated that the decomposition of the resolved lanthanide trifluoroacetate is greatly different from lanthanide trifluoroacetate powder. It is of tremendous help to recognize the decomposition process of the precursors and to understand the related reaction mechanism.

Optical–magnetic bifunctional properties and mechanistic insights on upconversion of NaYF4:Yb,Ho,Tm@NaGdF4 with a tunable nanodumbbell morphology

Optical–magnetic bifunctional upconversion of core–shell particles of NaYF₄:Yb,Ho,Tm@NaGdF₄ with a nanodumbbell-shaped morphology.

Yb3+-Concentration dependent upconversion luminescence and temperature sensing behavior in Yb3+/Er3+ codoped Gd2MoO6 nanocrystals prepared by a facile citric-assisted sol–gel method

Er³⁺/Yb³⁺-Codoped Gd₂MoO₆ upconversion nanocrystals with high sensor sensitivity and wide operation range were demonstrated for non-contact optical thermometry.

Enhanced upconversion luminescence and modulated paramagnetic performance in NaGdF4:Yb, Er by Mg2+ tridoping

In this work, a new strategy to enhance upconversion emission has been realized for the first time, based on β-NaGdF₄:Yb³⁺, Er³⁺ nanocrystals (UCNC) using tridoping with magnesium (Mg²⁺) ions.

A bright yellow light from a Yb3+,Er3+-co-doped Y2SiO5upconversion luminescence material

A bright yellow light from Yb³⁺and Er³⁺doped Y₂SiO₅upconversion materials in particle and fiber form were prepared by a co-precipitation, and electrospinning method.

Understanding the effect of Mn2+ on Yb3+/Er3+ upconversion and obtaining a maximum upconversion fluorescence enhancement in inert-core/active-shell/inert-shell structures

NaYF₄@NaYF₄:Er³⁺/Yb³⁺/Mn²⁺@NaYF₄ (C/S_d/S) nanoparticles were synthesized which show an obvious efficiency enhancement of red upconversion emission.

Giant enhancement of upconversion emission in NaYF4:Er3+@NaYF4:Yb3+ active-core/active-shell nanoparticles

An effective method to enhance greatly infrared-visible upconversion emissions of Er³⁺ ions is demonstrated through coating an optimized active NaYF₄:Yb³⁺ shell around the NaYF₄:Er³⁺ core nanoparticles.