Single-band upconversion emission in lanthanide-doped KMnF3 nanocrystals

Exploring the potential of lanthanide-doped oxyfluoride materials for bright green upconversion and their promising applications towards temperature sensing and drug delivery

The most efficient upconversion (UC) materials reported to date are based on fluoride hosts with low phonon energies, which reduce the amount of nonradiative transitions. In particular, NaYF4 doped with Yb3+ and Er3+ at appropriate ratios is known as one of the most efficient UC phosphors. However, its low thermal stability limits its use for certain applications. On the other hand, oxide hosts exhibit better thermal stability, yet they have higher phonon energies and are thus prone to lower UC efficiencies. As a result, developing host nanomaterials that combine the robustness of oxides with the high upconversion efficiencies of fluorides remains an intriguing prospect. Herein, we demonstrate the formation of ytrrium doped oxyfluoride (YOF:Yb3+,Er3+) particles, which are prepared by growing a NaYF4:Yb3+,Er3+ layer around SiO2 spherical particles and consecutively applying a high-temperature annealing step followed by the removal of SiO2 template. Our interest lies in employing these materials as Boltzmann type physiological range luminescence thermometers, but their weak green emission is a drawback. To overcome this issue, and engineer materials suitable for Boltzmann type thermometry, we have studied the effect of introducing different metal ion co-dopants (Gd3+, Li+ or Mn2+) into the YOF:Yb3+,Er3+ particles, focusing on the overall emission intensity, as well as the green to red ratio, upon 975 nm laser excitation. These materials are explored for their use as ratiometric thermometers, and further also as drug carriers, including their simultaneous use for these two applications. The investigation also includes examining their level of toxicity towards specific human cells - normal human dermal fibroblasts (NHDFs) - to evaluate their potential use for biological applications.

Enhanced Photocatalytic Degradation by the Preparation of a Stable La-Doped FeTiO3 Photocatalyst: Experimental and DFT Study

The rapid photocarrier recombination limits the photocatalytic activity of iron titanate (FeTiO3) to be further improved. Developing novel approaches to inhibit the rapid recombination rate of the FeTiO3 photocatalysts is crucial for efficiently degrading pollutants in wastewater. Rare earth ions, with unique electron dispositions and large ion radii, could effectively inhibit photocarrier recombination. Herein, novel lanthanum (La)-doped FeTiO3 photocatalysts were designed and successfully synthesized. The photocatalytic performance of the 12 mol % La/FeTiO3 photocatalyst was superior in degrading tetracycline hydrochloride (TCH), methylene blue (MB), and brilliant blue (BB). These degradation rate constants (k) were 0.12358, 0.01357, and 0.03064 L mg-1 min-1, respectively, which were 12.83, 1.61, and 7.78 times that of pure FeTiO3. The photoelectronic tests and density functional theory (DFT) calculations revealed that the La 4f orbital forms an impurity energy level in the conduction band of FeTiO3. This level narrows the bandgap and acts as an electron acceptor, capturing photoexcited electrons and inhibiting the rapid recombination of photoexcited electron-hole pairs in FeTiO3. This work enhances the potential of FeTiO3 in the photocatalysis field and provides important insights into the efficient degradation of organic pollutants in wastewater.

Dynamic modulation of multicolor upconversion luminescence of Er3+ via excitation pulse width

Lanthanide-doped upconversion (UC) luminescent materials display multicolor emissions, making them ideal for a variety of applications, such as multi-channel biological imaging, fluorescence encryption, anti-counterfeiting, and 3D display. Manipulating the UC emissions of the luminescent materials with a fixed composition is crucial for their applications. Herein, we propose a facile strategy to achieve pulse-width-dependent multicolor UC emissions in NaYF4:Yb/Er/Tm nanocrystals. Upon excitation with a 980 nm continuous-wave laser diode, Er3+ ions in NaYF4:20%Yb,15%Er,1%Tm nanocrystals exhibited UC emissions with a red-to-green (R/G) ratio of 11.3. Nevertheless, by employing a 980 nm pulse laser with pulse widths from 0.1 to 10 ms, the UC R/G ratio can be easily adjusted from 0.9 to 11.3, resulting in continuous and remarkable color transformation from green, yellow, orange, to red. By virtue of the dynamic luminescence color variation of these NaYF4:20%Yb,15%Er,1%Tm nanocrystals, we demonstrated their potential applications in the areas of anti-counterfeiting and information encryption. These findings provide deep insights into the excited-state dynamics and energy transfer of Er3+ in NaYF4:Yb/Er/Tm nanocrystals upon 980 nm pulse excitation, which may pave the way for designing multicolor UC materials toward versatile applications.

Advances in Engineered Nanoparticles for the Treatment of Ischemic Stroke by Enhancing Angiogenesis

Angiogenesis, or the formation of new blood vessels, is a natural defensive mechanism that aids in the restoration of oxygen and nutrition delivery to injured brain tissue after an ischemic stroke. Angiogenesis, by increasing vessel development, may maintain brain perfusion, enabling neuronal survival, brain plasticity, and neurologic recovery. Induction of angiogenesis and the formation of new vessels aid in neurorepair processes such as neurogenesis and synaptogenesis. Advanced nano drug delivery systems hold promise for treatment stroke by facilitating efficient transportation across the the blood-brain barrier and maintaining optimal drug concentrations. Nanoparticle has recently been shown to greatly boost angiogenesis and decrease vascular permeability, as well as improve neuroplasticity and neurological recovery after ischemic stroke. We describe current breakthroughs in the development of nanoparticle-based treatments for better angiogenesis therapy for ischemic stroke employing polymeric nanoparticles, liposomes, inorganic nanoparticles, and biomimetic nanoparticles in this study. We outline new nanoparticles in detail, review the hurdles and strategies for conveying nanoparticle to lesions, and demonstrate the most recent advances in nanoparticle in angiogenesis for stroke treatment.

Ultrapure single-band red upconversion luminescence in Er3+ doped sensitizer-rich ytterbium oxide transparent ceramics for solid-state lighting and temperature sensing

Achieving single-band upconversion (UC) is a challenging but rewarding approach to attain optimal performance in diverse applications. In this paper, we successfully achieved single-band red UC luminescence in Yb2O3: Er transparent ceramics (TCs) through the utilization of a sensitizer-rich design. The Yb2O3 host, which has a maximum host lattice occupancy by Yb3+ sensitizers, facilitates the utilization of excitation light and enhances energy transfer to activators, resulting in improved UC luminescence. Specifically, by shortening the ionic spacing between sensitizer and activator, the energy back transfer and the cross-relaxation process are promoted, resulting in weakening of green energy level 4S3/2 and 2H11/2 emission and enhancement of red energy level 4F9/2 emission. The prepared Yb2O3: Er TCs exhibited superior optical properties with in-line transmittance over 80% at 600 nm. Notably, in the 980nm-excited UC spectrum, green emission does not appear, thus Yb2O3: Er TCs exhibit ultra-pure single band red emission, with CIE coordinates of (0.72, 0.28) and color purity exceeding 99.9%. To the best of our knowledge, this is the first demonstration of pure red UC luminescence in TCs. Furthermore, the luminescent intensity ratio (LIR) technique was utilized to apply this pure red-emitting TCs for temperature sensing. The absolute sensitivity of Yb2O3: Er TCs was calculated to be 0.319% K-1 at 304 K, which is the highest level of optical thermometry based on 4F9/2 levels splitting of Er3+ known so far. The integration between pure red UC luminescence and temperature sensing performance opens up new possibilities for the development of multi-functional smart windows.

Potassium and Cesium Fluorinated β-Diketonates: Effect of a Cation and Terminal Substituent on Structural and Thermal Properties

As potential precursors for the synthesis of fluoroperovskites, a family of heavy alkali metal (MI = K, Cs) fluorinated β-diketonates were prepared and characterized by elemental analysis, IR, and powder-XRD. The crystal structures of the new six complexes, MI(β-dikF)(H2O)X, X = 0 or 1, were also determined. The structural diversity of this poorly explored class of complexes was discussed, including the preferred types of cation polyhedra and the ligand coordination modes, and the thermal properties of the metal β-diketonates were studied by TG-DTA in an inert (He) atmosphere. The data obtained allowed us to reveal the effect of the metal cation and the terminal substituent on the structural and thermal features of this family of complexes.

NIR-II Responsive Upconversion Nanoprobe with Simultaneously Enhanced Single-Band Red Luminescence and Phase/Size Control for Bioimaging and Photodynamic Therapy

Lanthanide based upconversion (UC) nanoprobes have emerged as promising agents for biological applications. Extending the excitation light to the second near-infrared (NIR-II), instead of the traditional 980/808 nm light, and realizing NIR-II responsive single-band red UC emission is highly demanded for bioimaging application, which has not yet been explored. Here, a new type of NIR-II (1532 nm) light responsive UC nanoparticles (UCNPs) with enhanced single-band red UC emission and controllable phase and size is designed by introducing Er³⁺ as sensitizer and utilizing Mn²⁺ as energy manipulator. Through tuning the content of Mn²⁺ in NaLnF₄ :Er/Mn, the crystal phase, size, and emitting color are readily controlled, and the red-to-green (R/G) ratio is significantly increased from ≈20 to ≈300, leading to NIR-II responsive single band red emission via efficient energy transfer between Er³⁺ and Mn²⁺ . In addition, the single band red emitting intensity can be further improved by coating shell to avoid the surface quenching effect. More importantly, NIR-II light activated red UC bioimaging and photodynamic therapy through loading photosensitizer of zinc phthalocyanine are successfully achieved for the first time. These findings provide a new strategy of designing NIR-II light responsive single-band red emissive UCNPs for biomedical applications.

Upconversion Luminescence through Cooperative and Energy-Transfer Mechanisms in Yb3+ -Metal-Organic Frameworks

Lanthanide-doped metal-organic frameworks (Ln-MOFs) have versatile luminescence properties, however it is challenging to achieve lanthanide-based upconversion luminescence in these materials. Here, 1,3,5-benzenetricarboxylic acid (BTC) and trivalent Yb³⁺ ions were used to generate crystalline Yb-BTC MOF 1D-microrods with upconversion luminescence under near infrared excitation via cooperative luminescence. Subsequently, the Yb-BTC MOFs were doped with a variety of different lanthanides to evaluate the potential for Yb³⁺ -based upconversion and energy transfer. Yb-BTC MOFs doped with Er³⁺ , Ho³⁺ , Tb³⁺ , and Eu³⁺ ions exhibit both the cooperative luminescence from Yb³⁺ and the characteristic emission bands of these ions under 980 nm irradiation. In contrast, only the 497 nm upconversion emission band from Yb³⁺ is observed in the MOFs doped with Tm³⁺ , Pr³⁺ , Sm³⁺ , and Dy³⁺ . The effects of different dopants on the efficiency of cooperative luminescence were established and will provide guidance for the exploitation of Ln-MOFs exhibiting upconversion.

Recent Development in Sensitizers for Lanthanide-Doped Upconversion Luminescence

The attractive features of lanthanide-doped upconversion luminescence (UCL), such as high photostability, nonphotobleaching or photoblinking, and large anti-Stokes shift, have shown great potentials in life science, information technology, and energy materials. Therefore, UCL modulation is highly demanded toward expected emission wavelength, lifetime, and relative intensity in order to satisfy stringent requirements raised from a wide variety of areas. Unfortunately, the majority of efforts have been devoted to either simple codoping of multiple activators or variation of hosts, while very little attention has been paid to the critical role that sensitizers have been playing. In fact, different sensitizers possess different excitation wavelengths and different energy transfer pathways (to different activators), which will lead to different UCL features. Thus, rational design of sensitizers shall provide extra opportunities for UCL tuning, particularly from the excitation side. In this review, we specifically focus on advances in sensitizers, including the current status, working mechanisms, design principles, as well as future challenges and endeavor directions.

Expanding the toolbox of photon upconversion for emerging frontier applications

Photon upconversion in lanthanide-based materials has recently shown compelling advantages in a wide range of fields due to their exceptional anti-Stokes luminescence performances and physicochemical properties. In particular, the latest breakthroughs in the optical manipulation of photon upconversion, such as the precise tuning of switchable emission profiles and lifetimes, open up new opportunities for diverse frontier applications from biological imaging to therapy, nanophotonics and three-dimensional displays. A summary and discussion on the recent progress can provide new insights into the fundamental understanding of luminescence mechanisms and also help to inspire new upconversion concepts and promote their frontier applications. Herein, we present a review on the state-of-the-art progress of lanthanide-based upconversion materials, focusing on the newly emerging approaches to the smart control of upconversion in aspects of light intensity, colors, and lifetimes, as well as new concepts. The emerging scientific and technological discoveries based on the well-designed upconversion materials are highlighted and discussed, along with the challenges and future perspectives. This review will contribute to the understanding of the fundamental research of photon upconversion and further promote the development of new classes of efficient upconversion materials towards diversities of frontier applications in the future.

Synthesis of core–shell nanoparticles based on interfacial energy transfer for red emission and highly sensitive temperature sensing

Highly efficient red up-conversion luminescence is achieved by constructing a core–shell structure of NaYF4:Er3+,Tm3+@NaYF4:Yb3+ based on the interfacial energy transfer process.

An overview of boosting lanthanide upconversion luminescence through chemical methods and physical strategies

Lanthanide-doped upconversion nanoparticles have attracted extensive research interest due to their promising applications in various fields.

Local Structure Engineering in Lanthanide-Doped Nanocrystals for Tunable Upconversion Emissions

Upconversion emissions from lanthanide-doped nanocrystals have sparked extensive research interests in nanophotonics, biomedicine, photovoltaics, photocatalysis, etc. Rational modulation of upconversion emissions is highly desirable to meet the requirements of specific applications. Among the diverse developed methods, local structure engineering is fundamentally feasible, through which the upconversion emission intensity, selectivity, wavelength shift, and lifetime can be tuned effectively. The underlying mechanism of the local-structure-dependent upconversion emissions lies in the degree of parity hybridization and energy level splitting of lanthanide ions as well as the interionic energy transfer efficiency. Over the past few years, there has been significant progress in local-structure-engineered upconversion emissions. In this Perspective, we first introduce the principles of upconversion emissions and typical characterization methods for local structure. Subsequently, we summarize recent achievements in tuning of upconversion emissions through local structure engineering, including host composition adjustment, external field regulation, and interfacial strain management. Finally, we propose a few perspectives that should tackle the current bottlenecks. This Perspective is expected to deepen the understanding of local-structure-dependent upconversion emissions and arouse adequate attention to the engineering of local structure for desired properties of inorganic nanocrystals.

Oxygen-Generating Cyanobacteria Powered by Upconversion-Nanoparticles-Converted Near-Infrared Light for Ischemic Stroke Treatment

Stroke is one of most common causes of death and disability. Most of neuroprotective agents fail to rescue neurons from cerebral ischemic insults, mainly because of targeting downstream cascading events, such as excitotoxicity, oxidative and nitrosative stress, and inflammation, rather than improving hypoxia that initially occurs. Here, we report a near-infrared light (NIR)-driven nanophotosynthesis biosystem capable of generating oxygen and absorbing carbon dioxide, thus rescuing neurons from ischemia toward treating stroke. Through cerebral delivery of S. elongatus that spontaneously photosynthesize and upconversion nanoparticles (UCNPs), NIR with excellent tissue penetrating capability is converted to visible light by UCNPs to activate S. elongatus generating oxygen in vivo, enhancing angiogenesis, reducing infarction, and facilitating repair of brain tissues, thus improving neuronal function recovery. The combination of cell-biological, biochemical, and animal-level behavioral data provides compelling evidence demonstrating that this oxygen-generating biosystem through jointly utilizing microorganism and nanotechnology represents a novel approach to stroke treatment.

Enhancing the static green up-conversion luminescence of NaY(MoO4)2:Yb/Er microcrystals via an annealing strategy for anti-counterfeiting applications

The majority of the fabrication procedures of lanthanide-doped materials involve thermal treatment that often results in crystallite regrowth, stabilizing the specific crystal structure and resulting in luminescence enhancement. The efficiency and intensity of up-conversion luminescence are closely related to the structure and synthesis process of the materials. Herein, well-crystallized and pure tetragonal NaY(MoO4)2 microcrystals with a uniform octahedral shape have been successfully synthesized via an environmentally friendly hydrothermal method, followed by annealing treatment. The phases, structures, morphologies, and compositions of the synthesized products annealed at 500-1000 °C remain unchanged, indicating high thermal stability. Furthermore, the NaY(MoO4)2:Yb3+/Er3+ microcrystals exhibit strong green emission when irradiated using infrared (980 nm) or ultraviolet (378 nm) wavelengths. Upon 980 nm excitation, up to 37-fold luminescence enhancement is achieved when the samples are annealed at about 700 °C. Interestingly, the high colour purity of the strong green emission is not only independent of the dopant concentration and heat treatment temperature, but it is also independent of the excitation conditions, including power and wavelength, and this makes it particularly suitable as a green safety signal light and luminescent security ink in paintings. As-prepared safety inks with NaY(MoO4)2:Yb3+/Er3+ microcrystals were used for visual fingerprint recognition printed on A4 paper with three-level fingerprint security features, significantly increasing the difficulty of illegal imitation and enhancing the levels of anti-counterfeiting.

Switching on near-infrared light in lanthanide-doped CsPbCl3 perovskite nanocrystals

The accessible emission spectral range of lead halide perovskite (LHP) CsPbX3 (X = Cl, Br, I) nanocrystals (NCs) has remained so far limited to wavelengths below 1 μm, corresponding to the emission line of Yb3+, whereas the direct sensitization of other near-infrared (NIR) emitting lanthanide ions is unviable. Herein, we present a general strategy to enable intense NIR emission from Er3+ at ∼1.5 μm, Ho3+ at ∼1.0 μm and Nd3+ at ∼1.06 μm through a Mn2+-mediated energy-transfer pathway. Steady-state and time-resolved photoluminescence studies show that energy-transfer efficiencies of about 39%, 35% and 70% from Mn2+ to Er3+, Ho3+ and Nd3+ are obtained, leading to photoluminescence quantum yields of ∼0.8%, ∼0.7% and ∼3%, respectively. This work provides guidance on constructing energy-transfer pathways in semiconductors and opens new perspectives for the development of lanthanide-functionalized LHPs as promising materials for optoelectronic devices operating in the NIR region.

Nano-Crystallization of Ln-Fluoride Crystals in Glass-Ceramics via Inducing of Yb3+ for Efficient Near-Infrared Upconversion Luminescence of Tm3

Transparent glass-ceramic composites embedded with Ln-fluoride nanocrystals are prepared in this work to enhance the upconversion luminescence of Tm³⁺. The crystalline phases, microstructures, and photoluminescence properties of samples are carefully investigated. KYb₃F₁₀ nanocrystals are proved to controllably precipitate in the glass-ceramics via the inducing of Yb³⁺ when the doping concentration varies from 0.5 to 1.5 mol%. Pure near-infrared upconversion emissions are observed and the emission intensities are enhanced in the glass-ceramics as compared to in the precursor glass due to the incorporation of Tm³⁺ into the KYb₃F₁₀ crystal structures via substitutions for Yb³⁺. Furthermore, KYb₂F₇ crystals are also nano-crystallized in the glass-ceramics when the Yb³⁺ concentration exceeds 2.0 mol%. The upconversion emission intensity of Tm³⁺ is further enhanced by seven times as Tm³⁺ enters the lattice sites of pure KYb₂F₇ nanocrystals. The designed glass ceramics provide efficient gain materials for optical applications in the biological transmission window. Moreover, the controllable nano-crystallization strategy induced by Yb³⁺ opens a new way for engineering a wide range of functional nanomaterials with effective incorporation of Ln³⁺ ions into fluoride crystal structures.

Room-Temperature Phosphorescent Organic-Doped Inorganic Frameworks Showing Wide-Range and Multicolor Long-Persistent Luminescence

Long-persistent luminescence based on purely inorganic and/or organic compounds has recently attracted much attention in a wide variety of fields including illumination, biological imaging, and information safety. However, simultaneously tuning the static and dynamic afterglow performance still presents a challenge. In this work, we put forward a new route of organic-doped inorganic framework to achieve wide-range and multicolor ultralong room-temperature phosphorescence (RTP). Through a facile hydrothermal method, phosphor (tetrafluoroterephthalic acid (TFTPA)) into the CdCO₃ (or Zn₂(OH)₂CO₃) host matrix exhibits an excitation-dependent colorful RTP due to the formation of diverse molecular aggregations with multicentral luminescence. The RTP lifetime of the doped organic/inorganic hybrids is greatly enhanced (313 times) compared to the pristine TFTPA. The high RTP quantum yield (43.9%) and good stability guarantee their easy visualization in both ambient and extreme conditions (such as acidic/basic solutions and an oxygen environment). Further codoped inorganic ions (Mn²⁺ and Pb²⁺) afford the hybrid materials with a novel time-resolved tunable afterglow emission, and the excitation-dependent RTP color is highly adjustable from dark blue to red, covering nearly the whole visible spectrum and outperforming the current state-of-the-art RTP materials. Therefore, this work not only describes a combined codoping and multicentral strategy to obtain statically and dynamically tunable long-persistent luminescence but also provides great opportunity for the use of organic-inorganic hybrid materials in multilevel anticounterfeiting and multicolor display applications.

Upconversion Nanoparticle-Mediated Optogenetics

Upconversion nanoparticle-mediated optogenetics enables remote delivery of upconverted visible light from a near-infrared light source to targeted neurons or areas, with the precision of a pulse of laser light in vivo for effective deep-tissue neuromodulation. Compared to conventional optogenetic tools, upconversion nanoparticle-based optogenetic techniques are less invasive and cause reduced inflammation with minimal levels of tissue damage. In addition to the optical stimulation, this design offers simultaneously temperature recording in proximity to the stimulated area. This chapter strives to provide life science researchers with an introduction to upconversion optogenetics, starting from the fundamental concept of photon upconversion and nanoparticle fabrication to the current state-of-the-art of surface engineering and device integration for minimally invasive neuromodulation.

Super Bright Red Upconversion in NaErF4:0.5%Tm@NaYF4:20%Yb Nanoparticles for Anti-counterfeit and Bioimaging Applications

Nanocrystals having single-band red emission under near-infrared (NIR) excitation through the upconversion process offer great advantages in terms of enhanced cellular imaging in in vitro and in vivo experiments in the biological window (600-900 nm), as a security ink, in photothermal therapy (PTT), in photodynamic therapy (PDT), and so forth but are challenging for materials scientists. In this work, we report for the first time the preparation of a super bright red emitter at 655 nm from monodispersed NaErF₄:0.5%Tm@NaYF₄:20%Yb nanocrystals (core@active shell). This phosphor exhibits 35 times stronger photoluminescence as compared to NaErF₄:0.5%Tm@NaYF₄ (core@inactive shell). Here, an Er³⁺-enriched host matrix works simultaneously as an activator and a sensitizer under NIR excitation. Upconversion red emission at 655 nm arises due to the electronic transition of Er³⁺ via the involvement of a three-photon absorption (expected to be a two-photon absorption), which has been confirmed via a power-dependent luminescence study. Tm³⁺ ions incorporated into the core with the active shell act as trapping centers, which promote the red band emission via the back-energy transfer process. Moreover, the active shell containing Yb³⁺ ions efficiently transfers the energy to the Er³⁺-enriched core, which suppresses the nonradiative channel rate, and Tm³⁺ ions act as trapping centers, which reduce the luminescence quenching via reduction of energy migration to the surface of the host lattice. Also, we have shown the potential applications of these nanocrystals: cellular imaging through downconversion and upconversion processes and security ink.

Local-structure-dependent luminescence in lanthanide-doped inorganic nanocrystals for biological applications

This feature article overviews the recent advances in the local-structure-dependent luminescence in lanthanide-doped inorganic nanocrystals for various biological applications.

Biodegradable Inorganic Upconversion Nanocrystals for In Vivo Applications

Lanthanide-doped inorganic upconversion nanocrystals (UCNCs) are promising as fluorescent diagnostic and therapeutic agents for in vivo applications ranging from biological imaging to disease theranostics. However, all currently available lanthanide-doped inorganic UCNCs are not biodegradable and thus cannot be harmlessly eliminated from the body of living organism during a reasonable period of time, making their clinical translations nearly impossible. Here, we report a class of red-emitting biodegradable UCNCs based on Yb³⁺/Er³⁺-doped inorganic potassium heptafluozirconate (K₃ZrF₇:Yb/Er) that features a dynamically "soft" crystal lattice containing water-soluble [ZrF₇]^3- cluster and a K⁺ cation. The red-emitting K₃ZrF₇:Yb/Er UCNCs exhibit a pH-dependent biodegradation capability upon exposure to water both in vitro and in vivo, and the rapid biodegradation rate, monitored using the intrinsic red upconversion luminescence, can be tuned particularly in a mild acidic tumor microenvironment (pH ∼5-6). More importantly, the final biodegradation products of K₃ZrF₇:Yb/Er UCNCs can be excreted from the body of mice in a short period of time with no evidence of toxicity, in stark contrast to the nondegradable β-NaYF₄:Yb/Er UCNCs that primarily accumulate in the main organs of mice. These findings described here unambiguously would benefit the future biomedical applications and clinical translations of lanthanide-doped inorganic UCNCs.

Metal Ions Doping for Boosting Luminescence of Lanthanide-Doped Nanocrystals

With the developing need for luminous materials with better performance, lanthanide-doped nanocrystals have been widely studied for their unique luminescence properties such as their narrow bandwidth emission, excellent chemical stability, and photostability, adjustable emission color, high signal-to-background ratio, deeper tissue penetration with less photo-damage, and low toxicity, etc., which triggered enthusiasm for research on the broad applications of lanthanide-doped nanocrystals in bioimaging, anti-counterfeiting, biosensing, and cancer diagnosis and treatment. Considerable progress has been made in the past few decades, but low upconversion luminescence efficiency has been a hindrance in achieving further progress. It is necessary to summarize the recently relevant literature and find solutions to improve the efficiency. The latest experimental and theoretical studies related to the deliberate design of rare earth luminescent nanocrystals have, however, shown the development of metal ion-doped approaches to enhance the luminescent intensity. Host lattice manipulation can enhance the luminescence through increasing the asymmetry, which improves the probability of electric dipole transition; and the energy transfer modulation offers a reduced cross-relaxation pathway to improve the efficiency of the energy transfer. Based on the mechanisms of host lattice manipulation and energy transfer modulation, a wide range of enhancements at all wavelengths or even within a particular wavelength have been accomplished with an enhancement of up to a hundred times. In this mini review, we present the strategy of metal ion-doped lanthanide nanocrystals to cope with the issue of enhancing luminescence, overview the advantages and tricky challenges in boosting the luminescence, and provide a potential trend of future study in this field.

Low threshold lasing emissions from a single upconversion nanocrystal

Cross-relaxation among neighboring emitters normally causes self-quenching and limits the brightness of luminescence. However, in nanomaterials, cross-relaxation could be well-controlled and employed for increasing the luminescence efficiency at specific wavelengths. Here we report that cross-relaxation can modulate both the brightness of single upconversion nanoparticles and the threshold to reach population inversion, and both are critical factors in producing the ultra-low threshold lasing emissions in a micro cavity laser. By homogenously coating a 5-μm cavity with a single layer of nanoparticles, we demonstrate that doping Tm³⁺ ions at 2% can facilitate the electron accumulation at the intermediate state of ³H₄ level and efficiently decrease the lasing threshold by more than one order of magnitude. As a result, we demonstrate up-converted lasing emissions with an ultralow threshold of continuous-wave excitation of ~150 W/cm² achieved at room temperature. A single nanoparticle can lase with a full width at half-maximum as narrow as ~0.45 nm.

Mechanisms and Applications of Steady-State Photoluminescence Spectroscopy in Two-Dimensional Transition-Metal Dichalcogenides

Two-dimensional (2D) transition-metal dichalcogenide (TMD) semiconductors exhibit many important structural and optoelectronic properties, such as strong light-matter interactions, direct bandgaps tunable from visible to near-infrared regions, flexibility and atomic thickness, quantum-confinement effects, valley polarization possibilities, and so on. Therefore, they are regarded as a very promising class of materials for next-generation state-of-the-art nano/micro optoelectronic devices. To explore different applications and device structures based on 2D TMDs, intrinsic material properties, their relationships, and evolutions with fabrication parameters need to be deeply understood, very often through a combination of various characterization techniques. Among them, steady-state photoluminescence (PL) spectroscopy has been extensively employed. This class of techniques is fast, contactless, and nondestructive and can provide very high spatial resolution. Therefore, it can be used to obtain optoelectronic properties from samples of various sizes (from microns to centimeters) during the fabrication process without complex sample preparation. In this article, the mechanism and applications of steady-state PL spectroscopy in 2D TMDs are reviewed. The first part of this review details the physics of PL phenomena in semiconductors and common techniques to acquire and analyze PL spectra. The second part introduces various applications of PL spectroscopy in 2D TMDs. Finally, a broader perspective is discussed to highlight some limitations and untapped opportunities of PL spectroscopy in characterizing 2D TMDs.

Nanomedicine and Early Cancer Diagnosis: Molecular Imaging using Fluorescence Nanoparticles

Incorporating nanotechnology into fluorescent imaging and magnetic resonance imaging (MRI) has shown promising potential for accurate diagnosis of cancer at an earlier stage than the conventional imaging modalities. Molecular imaging (MI) aims to quantitatively characterize, visualize, and measure the biological processes or living cells at molecular and genetic levels. MI modalities have been exploited in different applications including noninvasive determination and visualization of diseased tissues, cell trafficking visualization, early detection, treatment response monitoring, and in vivo visualization of living cells. High-affinity molecular probe and imaging modality to detect the probe are the two main requirements of MI. Recent advances in nanotechnology and allied modalities have facilitated the use of nanoparticles (NPs) as MI probes. Within the extensive group of NPs, fluorescent NPs play a prominent role in optical molecular imaging. The fluorescent NPs used in molecular and cellular imaging can be categorized into three main groups including quantum dots (QDs), upconversion, and dyedoped NPs. Fluorescent NPs have great potential in targeted theranostics including cancer imaging, immunoassay- based cells, proteins and bacteria detections, imaging-guided surgery, and therapy. Fluorescent NPs have shown promising potentials for drug and gene delivery, detection of the chromosomal abnormalities, labeling of DNA, and visualizing DNA replication dynamics. Multifunctional NPs have been successfully used in a single theranostic modality integrating diagnosis and therapy. The unique characteristics of multifunctional NPs make them potential theranostic agents that can be utilized concurrently for diagnosis and therapy. This review provides the state of the art of the applications of nanotechnologies in early cancer diagnosis focusing on fluorescent NPs, their synthesis methods, and perspectives in clinical theranostics.

Doping Lanthanide Nanocrystals With Non-lanthanide Ions to Simultaneously Enhance Up- and Down-Conversion Luminescence

The rare-earth nanocrystals containing Er³⁺ emitters offer very promising tools for imaging applications, as they can not only exhibit up-conversion luminescence but also down-conversion luminescence in the second near-infrared window (NIR II). Doping non-lanthanide cations into host matrix was demonstrated to be an effective measure for improving the luminescence efficiency of Er³⁺ ions, while still awaiting in-depth investigations on the effects of dopants especially those with high valence states on the optical properties of lanthanide nanocrystals. To address this issue, tetravalent Zr⁴⁺ doped hexagonal NaGdF₄:Yb,Er nanocrystals were prepared, and the enhancement effects of the Zr⁴⁺ doping level on both up-conversion luminescence in the visible window and down-conversion luminescence in NIR II window were investigated, with steady-state and transient luminescence spectroscopies. The key role of the local crystal field distortions around Er³⁺ emitters was elucidated in combination with the results based on both of Zr⁴⁺ and its lower valence counterparts, e.g., Sc³⁺, Mg²⁺, Mn²⁺. Univalent ions such as Li⁺ was utilized to substitute Na⁺ ion rather than Gd³⁺, and the synergistic effects of Zr⁴⁺ and Li⁺ ions by co-doping them into NaGdF₄:Yb,Er nanocrystals were investigated toward optimal enhancement. Upon optimization, the up-conversion emission of co-doped NaGdF₄:Yb,Er nanocrystals was enhanced by more than one order of magnitude compared with undoped nanocrystals. The current studies thus demonstrate that the local crystal field surrounding emitters is an effective parameter for manipulating the luminescence of lanthanide emitters.

Up-conversion Persistent Luminescence of a 980 nm Laser Activated Zn3Ga2(GexSn1-x)O8:Yb,Er,Cr Phosphors

Zn₃Ga₂(Ge_xSn_1-x)O₈:Yb,Er,Cr phosphors with different doping ratio of Ge/Sn were prepared by high temperature solid-state method. After excited at 270 nm with a xenon lamp for 15 min, the persistent luminescence (PL) of Zn₃Ga₂(Ge_xSn_1-x)O₈:Yb,Er,Cr phosphor can last more than 60 min. Under the excitation at 980 nm, Zn₃Ga₂(Ge_xSn_1-x)O₈:Yb,Er,Cr phosphors can generate effective up-conversion emissions from Er³⁺ ions at 410 nm, 525 nm, 550 nm, 660 nm and Cr³⁺ ions around 700 nm. Besides, after ceasing irradiation, Zn₃Ga₂(Ge_xSn_1-x)O₈:Yb,Er,Cr phosphors show up-conversion persistent luminescence (UCPL) from Cr³⁺ ions around 700 nm. Moreover, Zn₃Ga_1.995(Ge_0.3Sn_0.7) O₈:Yb³⁺,Er³⁺,Cr³⁺ phosphor is optimal with the best persistent luminescent and up-conversion persistent luminescent properties. In short, owing to the long afterglow time, strong up-conversion emissions and novel up-conversion persistent luminescence, the as-prepared Zn₃Ga₂(Ge_xSn_1-x)O₈:Yb,Er,Cr phosphors can be potentially applied in a wide range of fields, such as security materials, coating materials, infrared detection, fluorescent label, tracer and testing.

Optimized Multimetal Sensitized Phosphor for Enhanced Red Up-Conversion Luminescence by Machine Learning

In this research, machine learning including the genetic algorithm (GA) and support vector machine (SVM) algorithm is used to solve the "low up-conversion luminescence (UCL) intensity" problem in order to find the optimal phosphor with enhanced red UCL emission using multielement K/Li/Mn metal modulation. Compared with the first generation of phosphors, the best phosphors' fluorescence intensity occurs in the third generation optimized by the GA, with a stronger brightness (4.91-fold), a higher relative quantum yield (6.40-fold), and an enhanced tissue penetration depth (by 5 mm). The single and multiple dopants effect on the upconversion intensity of K+Li+Mn sensitizers is also studied: the intensity increases first and then decreases with the increase of Yb/Er/K+Li+Mn content, and the optimized K+Li+Mn concentration is 6.03%. In order to confirm the stability of the brightness optimization by the GA, a batch of phosphors was synthesized with the same element proportion, and the similarity of fluorescence intensity of two batches of phosphors was evaluated by the SVM algorithm with the classification accuracy index. Finally, the optimized phosphor was used for bioimaging and phosphor-LED.

Manganese-Mediated Growth of ZnS Shell on KMnF3:Yb,Er Cores toward Enhanced Up/Downconversion Luminescence

Epitaxially growing a semiconductor shell on the surface of upconversion nanocrystals to form a core/shell structure is believed to be a promising strategy to improve the luminescent efficiency of lanthanide ions doped in particle cores and, meanwhile, enriches the optical properties of the resulting nanocrystals. However, liquid-phase synthesis of such core/shell-structured nanocrystals comprised of a lanthanide ion-doped core and semiconductor shell remains challenging because of the chemical incompatibilities between lanthanides and the most intermediate gap semiconductors. In this context, the successful growth of ZnS shell on a KMnF₃ core codoped with Yb³⁺/Er³⁺ ions is reported to enhance the upconversion luminescence of Er³⁺ ions. The underlying core/shell formation mechanism is elucidated in detail combining the hard-soft acid-base theory with structural analysis of the resulting nanocrystals. Quite unexpectedly, Mn²⁺ diffusion across the core/shell interface occurs during ZnS shell growth, giving rise to Mn²⁺ emission from the ZnS shell. Thus, the resulting core/shell particles exhibited unique up/downconversion luminescence from doped lanthanide metal ions and transition-metal ions, respectively. By manipulating the ion diffusion and shell growth kinetics, the upconversion and downconversion luminescent performance of KMnF₃:Yb,Er@ZnS nanocrystals are further optimized and the related mechanisms are discussed. Further, temperature-dependent upconversion and downconversion photoluminescence properties of KMnF₃:Yb,Er@ZnS nanocrystals show potential for ratiometric luminescence temperature sensing.

Doping and ion substitution in colloidal metal halide perovskite nanocrystals

The past decade has witnessed tremendous advances in synthesis of metal halide perovskites and their use for a rich variety of optoelectronics applications. Metal halide perovskite has the general formula ABX3, where A is a monovalent cation (which can be either organic (e.g., CH3NH3+ (MA), CH(NH2)2+ (FA)) or inorganic (e.g., Cs+)), B is a divalent metal cation (usually Pb2+), and X is a halogen anion (Cl-, Br-, I-). Particularly, the photoluminescence (PL) properties of metal halide perovskites have garnered much attention due to the recent rapid development of perovskite nanocrystals. The introduction of capping ligands enables the synthesis of colloidal perovskite nanocrystals which offer new insight into dimension-dependent physical properties compared to their bulk counterparts. It is notable that doping and ion substitution represent effective strategies for tailoring the optoelectronic properties (e.g., absorption band gap, PL emission, and quantum yield (QY)) and stabilities of perovskite nanocrystals. The doping and ion substitution processes can be performed during or after the synthesis of colloidal nanocrystals by incorporating new A', B', or X' site ions into the A, B, or X sites of ABX3 perovskites. Interestingly, both isovalent and heterovalent doping and ion substitution can be conducted on colloidal perovskite nanocrystals. In this review, the general background of perovskite nanocrystals synthesis is first introduced. The effects of A-site, B-site, and X-site ionic doping and substitution on the optoelectronic properties and stabilities of colloidal metal halide perovskite nanocrystals are then detailed. Finally, possible applications and future research directions of doped and ion-substituted colloidal perovskite nanocrystals are also discussed.

Quaternary cerium(iv) containing fluorides exhibiting Ce3F16 sheets and Ce6F30 frameworks

A view of the size of trivalent cations M³⁺ plotted against the size of the tetravalent framework forming cations Th⁴⁺, U⁴⁺, and Ce⁴⁺ and a complex 3D framework structure of Na₃MCe₆F₃₀ (M = Al, Ga, Fe, and Cr) along the c-axes.

Rare-earth-containing perovskite nanomaterials: design, synthesis, properties and applications

As star material, perovskites have been widely used in the fields of optics, photovoltaics, electronics, magnetics, catalysis, sensing, etc. However, some inherent shortcomings, such as low efficiency (power conversion efficiency, external quantum efficiency, etc.) and poor stability (against water, oxygen, ultraviolet light, etc.), limit their practical applications. Downsizing the materials into nanostructures and incorporating rare earth (RE) ions are effective means to improve their properties and broaden their applications. This review will systematically summarize the key points in the design, synthesis, property improvements and application expansion of RE-containing (including both RE-based and RE-doped) halide and oxide perovskite nanomaterials (PNMs). The critical factors of incorporating RE elements into different perovskite structures and the rational design of functional materials will be discussed in detail. The advantages and disadvantages of different synthesis methods for PNMs will be reviewed. This paper will also summarize some practical experiences in selecting suitable RE elements and designing multi-functional materials according to the mechanisms and principles of REs promoting the properties of perovskites. At the end of this review, we will provide an outlook on the opportunities and challenges of RE-containing PNMs in various fields.

Solvothermal synthesis and modification of NaYF4:Yb/Er@NaLuF4:Yb for enhanced up-conversion luminescence for bioimaging

Water-soluble NaYF4:Yb/Er@NaLuF4:Yb up-converting nanoparticles (UCNPs) with a strong green emission were successfully prepared by a solvothermal method in a short period of time and at a low temperature. First, the hydrophobic UCNPs were prepared by a simple solvothermal method, then modified using a polyetherimide (PEI) surfactant or oxidation of the oleic acid ligands with the Lemieux-von Rudloff reagent. The modified UCNPs, having an average particle diameter of 60 ± 5 nm, showed a high dispersity. The oleic acid ligand on the sample surface was oxidized azelaic acid (HOOC(CH2)7COOH), identified from Fourier transform infrared (FTIR) spectroscopy, which results in the generation of free carboxylic acid, hence conferring a high solubility in water. The 3-4,5-dimethylthiazol-2-yl-2,5-diphenyl tetrazolium bromide (MTT) method and cell-targeted labeling proved that oleic acid-capped UCNPs after oxidation (UCNPs-OAO) have a higher biocompatibility than polyetherimide-capped UCNPs (UCNPs-PEI). Therefore, the UCNPs-OAO have a great potential in biomedical applications, such as multimodal imaging, targeted therapy, and gene therapy.

Facile synthesis of sub-10 nm-sized bright red-emitting upconversion nanophosphors via tetrahedral YOF:Yb,Er seed-mediated growth

Ultrasmall and uniform tetrahedral-shaped YOF:Yb,Er upconversion nanophosphors (UCNs) are synthesized and sub-10 nm YOF:Yb,Er/YOF core/shell UCNs are formed via YOF:Yb,Er seed-mediated synthesis. The ultrasmall YOF:Yb,Er/YOF core/shell UCNs realize intense red emission under near infrared light (λ_ex = 980 nm).

Tumor self-responsive upconversion nanomedicines for theranostic applications

To date, malignant tumors continue to be the most lethal disease, causing more than 8.2 million deaths worldwide each year. In recent years, nanostructures based on rare-earth upconversion luminescent nanoparticles have shown significant advantages in the integration of multimodal imaging and therapy. Compared with normal tissues, the tumor microenvironment (TME) exhibits unique characteristics including high interstitial fluid pressure, abnormal blood vessels, a hypoxic and slightly acidic environment, and high levels of glutathione (GSH) and hydrogen peroxide (H2O2). According to these characteristics, increasing attention in the antitumor field has been given to designing nanomedicines with specific responses to the TME based on rare-earth upconversion nanoparticles (UCNPs) and to achieving efficient tumor diagnosis and treatment under the premise of reducing side effects. Nevertheless, a review that systematically summarizes TME-responsive upconversion nanomedicines (UCNMs) for realizing tumor self-enhanced theranostics has not been published to date. In this review, we summarize the recent progress made in UCNP-based nanotherapeutics by highlighting the increasingly developing trend of TME-responsive UCNMs. The general characteristics of the TME are introduced in detail and their utilization in designing TME-responsive UCNMs is systematically discussed. Based on NIR light-excited optical imaging, we discuss the superiority of UCNMs when applied in tumor theranostics with an emphasis on how to use them to realize TME-mediated multimodal imaging-guided therapy.

Effect of Mn2+ on Upconversion Emission, Thermal Sensing and Optical Heater Behavior of Yb3+ - Er3+ Codoped NaGdF4 Nanophosphors

In thiswork, we investigate the influence of Mn2+ on the emission color, thermal sensing and optical heater behavior of NaGdF4: Yb/Er nanophosphors, which the nanoparticles were synthesized by a hydrothermal method using oleic acid as both a stabilizing and a chelating agent. The morphology and crystal size of upconversion nano particles (UCNPs) can be effectively controlled through the addition of Mn2+ dopant contents in NaGdF4: Yb/Er system. Moreover, an enhancement in overall UCL spectra of Mn2+ doped UCNPs for NaGdF4 host compared to the UCNPs is observed, which results from a closed back-energy transfer between Er3+ and Mn2+ ions (4S3/2 (Er3+) → 4T1 (Mn2+) → 4F9/2 (Er3+)). The temperature sensitivity of NaGdF4:Yb3+/Er3+ doping with Mn2+ based on thermally coupled levels (2H11/2 and 4S3/2) of Er3+ is similar to that particles without Mn2+ in the 303-548 K range. And the maximum sensitivity is 0.0043 K-1 at 523 K for NaGdF4:Yb3+/Er3+/Mn2+. Interestingly, the NaGdF4:Yb3+/Er3+/Mn2+ shows preferable optical heating behavior, which is reaching a large value of 50 K. These results indicate that inducing of Mn2+ ions in NaGdF4:Yb3+/Er3+ nanophosphors has potential in colorful display, temperature sensor.

KMnF₃:Yb3+,Er3+ Core-Active-Shell Nanoparticles with Broadband Down-Shifting Luminescence at 1.5 μm for Polymer-Based Waveguide Amplifiers

In this study, we prepared cubic-phase oleic-acid-coated KMnF₃: Yb³⁺,Er³⁺ nanoparticles (NPs) and NaYF₄:Yb³⁺,Er³⁺ NPs, which were about 23 nm. From the down-shifting emissions spectra of the two NPs obtained by 980 nm excitation, we observed the fact that the KMnF₃: 18%Yb³⁺,1%Er³⁺ NPs were a luminescent material with a broadband near-infrared emission of 1.5 μm, and full-width at half-maximum (FWHM) of 55 cm⁻¹, which was wider than that of the NaYF₄: 18%Yb³⁺,1% NPs. Therefore, we believe that the oleic-acid-coated KMnF₃:Yb³⁺,Er³⁺ NPs have great potential in fabricating broadband waveguide amplifiers. Through epitaxial growth of a KMnF₃: Yb³⁺ active-shell on the core NPs, we compounded KMnF₃:Yb³⁺,Er³⁺@KMnF₃:Yb³⁺ core-active-shell NPs whose 1.5-μm infrared emissions intensity was 3.4 times as strong as that of the core NPs. In addition, we manufactured waveguide amplifiers using KMnF₃:18%Yb³⁺,1%Er³⁺@KMnF₃:2%Yb³⁺ NPs as the core materials of the waveguide amplifiers. When the input signal power was 0.2 mW and the pump power was 200 mW, we achieved a relative gain of 0.6 dB at 1534 nm in a 10-mm long waveguide.

Revisiting the Enhanced Red Upconversion Emission from a Single β-NaYF4:Yb/Er Microcrystal By Doping with Mn2+ Ions

The presence of manganese ions (Mn2+) in Yb/Er-co-doped nanomaterials results in suppressing green (545 nm) and enhancing red (650 nm) upconversion (UC) emission, which can achieve single-red-band emission to enable applications in bioimaging and drug delivery. Here, we revisit the tunable multicolor UC emission in a single Mn2+-doped β-NaYF4:Yb/Er microcrystal which is synthesized by a simple one-pot hydrothermal method. Excited by a 980 nm continuous wave (CW) laser, the color of the single β-NaYF4:Yb/Er/Mn microrod can be tuned from green to red as the doping Mn2+ ions increase from 0 to 30 mol%. Notably, under a relatively high excitation intensity, a newly emerged emission band at 560 nm (2H9/2 → 4I13/2) becomes significant and further exceeds the traditional green (545 nm) emission. Therefore, the red-to-green (R/G) emission intensity ratio is subdivided into traditional (650 to 545 nm) and new (650 to 560 nm) R/G ones. As the doped Mn2+ ions increase, these two R/G ratios are in lockstep with the same tunable trends at low excitation intensity, but the tunable regions become different at high excitation intensity. Moreover, we demonstrate that the energy transfer (ET) between Mn2+ and Er3+ contributes to the adjustment of R/G ratio and leads to tunable multicolor of the single microrod. The spectroscopic properties and tunable color from the single microrod can be potentially utilized in color display and micro-optoelectronic devices.

Methylene Blue-Loaded Upconverting Hydrogel Nanocomposite: Potential Material for Near-Infrared Light-Triggered Photodynamic Therapy Application

The property of upconverting nanoparticles to convert the low-energy near-infrared (NIR) light into high-energy visible light has made them a potential candidate for various biomedical applications including photodynamic therapy (PDT). In this work, we show how a surface functionalization approach on the nanoparticle can be used to develop a nanocomposite hydrogel which can be of potential use for the PDT application. The upconverting hydrogel nanocomposite was synthesized by reacting 10-undecenoic acid-capped Yb3+/Er3+-doped NaYF4 nanoparticles with the thermosensitive N-isopropylacrylamide monomer. The formation of hydrogel was completed within 15 min and hydrogel nanocomposites showed strong enhancement in the visible light emission compared to the emission obtained from 10-undecenoic acid-capped Yb3+/Er3+-doped NaYF4 nanoparticles via the upconversion process (under 980 nm laser excitation). The upconverting hydrogel nanocomposites displayed high swelling behavior in water because of their porous nature. The porous structure ensured a higher loading of methylene blue dye (∼78% in 1 h) into the upconverting hydrogel, which was achieved via the swelling diffusion phenomenon. Upon excitation with the NIR light, the visible light emitted from the hydrogel activated the photosensitizer methylene blue which generated reactive oxygen species. Our results were able to show that the methylene blue-loaded composite hydrogel can be a potential platform for the future of NIR-triggered PDT in skin cancer treatment.