Activating Ultrahigh Thermoresponsive Upconversion in an Erbium Sublattice for Nanothermometry and Information Security

Manipulating energy migration in nanoparticles toward tunable photochromic upconversion

Smart control of energy interactions plays a key role in manipulating upconversion dynamics and tuning emission colors for lanthanide-doped materials. However, quantifying the energy flux in particular energy migration in the representative sensitizer-activator coupled upconversion system has remained a challenge. Here we report a conceptual model to examine the energy flux in a single nanoparticle by designing an interfacial energy transfer mediated nanostructure. We show that energy migration indeed occurs simultaneously with energy transfer in a sensitizer-activator system and the competition between them can be quantified by proposing a characteristic ratio parameter. Moreover, this model is also able to realize the color-switchable photochromic upconversion by temporal control of up-transition processes. These findings offer a deep insight into the understanding of upconversion dynamics and provide a versatile approach to manipulating the energy flux in nanostructures with tunable emission colors, showing great promise in applications of logic operation and information security.

Unveiling the Antithermal Quenching Behavior in 0D Inorganic Metal Halide Cs2InCl5(H2O) Mediated by Upconversion Emission

Inorganic metal halides (IMHs) often suffer from severe fluorescence thermal quenching, limiting their application at elevated temperatures. Therefore, the exploration of IMHs exhibiting antithermal quenching (ATQ) behavior is of great importance. In this study, we developed a green synthetic route using a solvent evaporation method to successfully synthesize the 0D IMHs Cs2InCl5(H2O). By precise control over the doping ratios of Sb3+, Yb3+, and Er3+, unique dual-mode emission properties are obtained. As the temperature increases, the compound exhibited downconversion and upconversion luminescence, with relative sensitivity SR-max values of 7.11% K-1 and 1.21% K-1, respectively. Particularly anomalous is the compound's manifestation of an unconventional ATQ behavior during the upconversion process. In situ structural analysis confirmed that under high-temperature conditions, the 0D Cs2InCl5(H2O) metal halide undergoes structural evolution, transitioning through a Cs3In2Cl9 phase, which is responsible for the ATQ. This study provides experimental evidence for the abnormal ATQ of 0D metal halides, offering new inspiration for the multifunctionalization of 0D metal halides in high-temperature temperature sensing and dual-mode luminescence.

Tunable Tri‐Channel Orthogonal Full‐Color Luminescence in Nanostructure toward Anticounterfeiting and Information Security

Tunable orthogonal full‐color luminescence has emerged as a new class of smart luminescence phenomenon with wide applications ranging from photonics to biomedicine. However, the current research is focused on complex multilayer core‐shell nanostructures (e.g., 5–8 shell layers) with a single upconversion mode, greatly limiting their synthesis and practical application. Herein, this work proposes a simple core‐shell structure to integrate upconversion and downshifting dual‐mode luminescence based on Gd3+‐mediated interfacial energy transfer and Ce3+‐assisted cross relaxation. This design is able to suppress cross‐talk of multiple emissions and simplify the sample structure by removing the conventionally required intermediate isolation layer. Importantly, it further enables the arbitrarily controllable multicolor output at a single nanoparticle level by adopting the tri‐channel selective excitation wavelengths (980/808/254 nm), greatly expanding the conventional red‐green‐blue (RGB) color gamut. Moreover, the use of these nanoparticles promotes the information security level and the complexity of anti‐counterfeiting modes by adopting a pre‐set logic Morse information encryption and decryption strategy. These results provide effective guidance for the rational nanostructure design of novel orthogonal trichromatic emissive materials for a variety of frontier applications such as advanced anticounterfeiting and information security.

Excitation-Power Dependence of Lanthanide-Based Ratiometric Luminescent Nanothermometry

Ratiometric luminescent nanothermometry has emerged as a promising tool for remote thermal mapping at the nanoscale, yet its dependence on excitation power has been largely overlooked. Herein, we investigate the excitation power dependence of lanthanide-based ratiometric luminescent nanothermometers by examining two nonlinear pumping processes of Tm3+, where the differing slope factors of two emissions introduce significant intrinsic deviations in the luminescence intensity ratio (LIR) under varying excitation power densities. The robustness of the observed exponential relationship between excitation power density and LIR across different temperatures enables the derivation of a new calibration curve, applicable to any excitation power density. Additionally, analyzing the effect of excitation power on thermometric performance reveals that the absolute thermal sensitivity will change with the excitation power density, while the relative thermal sensitivity remains constant. This study provides valuable insights for optimizing ratiometric luminescent nanothermometry, offering a pathway to more accurate and reliable temperature measurements.

Ascorbic acid mediated fluorescence emission of MnO2 modified upconversion nanoparticles for anti-counterfeiting

Anti-counterfeiting ink can prevent important documents from being forged or tampered with. We reported a strategy to improve upconversion luminescence intensity of NaYF4:18%Yb3+,0.5%Tm3+ core nanoparticles (NPs) by coating the NaYF4 shell. We synthesized NaYF4:18%Yb3+,0.5%Tm3+ core NPs and NaYF4:18%Yb3+,0.5%Tm3+/NaYF4 core-shell NPs by high temperature thermal decomposition method. In comparison with the core NPs, the upconversion luminescence intensity of the core-shell NPs was enhanced by 2.3 times in the wavelength range of 445 nm to 495 nm. We designed composite nanomaterials based on NaYF4:Yb3+,Tm3+/NaYF4 core-shell NPs and MnO2, and synthesized NaYF4:Yb3+,Tm3+/NaYF4@MnO2 composite NPs by physical doping method. Here, MnO2 acts as a quencher to quench the upconversion fluorescence of Tm3+ ions of the core-shell NPs. Afterwards, we used the prepared product for document anti-counterfeiting. And then reducing agent (AA) can destroy the structure of MnO2 to restore the upconversion luminescence of Tm3+ ions. We use NaYF4:18%Yb3+,0.5%Tm3+/NaYF4@MnO2 composite NPs as anti-counterfeiting ink to write the letter "L". Under ambient conditions or the irradiation of 980 nm continuous light, "L" does not emit light. After AA is evenly applied on the letter "L", "L" can emit blue fluorescence under the irradiation of 980 nm continuous light. These results showed that NaYF4:Yb3+,Tm3+/NaYF4@MnO2 composite NPs can be used in important document anti-counterfeiting tasks to enhance information security.

Upconversion luminescence through dynamically regulating the depletion of Ho3+: 5I6 level for speed sensing

In this work, a strategy for dynamically adjusting the upconversion luminescence (UCL) color of NaGdF4:Yb3+/Ho3+/Ce3+/Sc3+ is reported based on a phosphor wheel. It has been demonstrated that the rotation-dependent UCL mainly originated from the regulation of depletion mode for the Ho3+: 5I6 level. Due to the dominant linear decay, a high-pure red UCL is observed under the steady-state excitation. However, as the proportion of the steady-state excitation decreases, the green-red emission intensity ratio gradually increases, followed by the color conversion from red to green. An approximate physical model is proposed to understand the dependence of IG/IR on rotation speed. We not only report a UCL material that shows potential application in velocity sensing but also provide new insights into wheel-based dynamic UCL regulation.

The Art of Nanoparticle Design: Unconventional Morphologies for Advancing Luminescent Technologies

The advanced design of rare-earth-doped (RE-doped) fluoride nanoparticles has expanded their applications ranging from anticounterfeiting luminescence and contactless temperature measurement to photodynamic therapy. Several recent studies have focused on developing rare morphologies of RE-doped nanoparticles. Distinct physical morphologies of RE-doped fluoride materials set them apart from contemporary nanoparticles. Every unusual structure holds the potential to dramatically improve the physical performance of nanoparticles, resulting in a remarkable revolution and a wide range of applications. This comprehensive review serves as a guide offering insights into various uniquely structured nanoparticles, including hollow, dumbbell-shaped, and peasecod-like forms. It aims to cater to both novices and experts interested in exploring the morphological transformations of nanoparticles. Discovering new energy transfer pathways and enhancing the optical application performance have been long-term challenges for which new solutions can be found in old papers. In the future, nanoparticle morphology design is expected to involve more refined microphysical methods and chemically-induced syntheses. Targeted modification of nanoparticle morphology and the aggregation of nanoparticles of various shapes can provide the advantages of different structures and enhance the universality of nanoparticles.

Amplifying Photon Upconversion in Alloyed Nanoparticles for a Near-Infrared Photodetector

Photon upconverison has attracted a substantial amount of interest in diverse fields due to its characteristic anti-Stokes emissions. However, obtaining intense emission under low-power laser irradiation has remained a challenge. Here we report a mechanistic design of activator-sensitizer alloyed nanoparticles to achieve bright upconversion under weak infrared irradiation. This design allows a nearest sensitizer-activator separation to facilitate efficient energy transfer that results in remarkably enhanced upconversion (>2 orders of magnitude) under 0.26 W cm-2 irradiation compared to that of the Er sublattice, and the upconversion quantum yield also shows a 20-fold increase. Interestingly, the alloyed nanoparticles exhibit a gradual change in emission color with an increase in Yb3+ content, and moreover, their emission colors can be dynamically controlled by simply modulating the excitation laser power and pulse widths. Such alloyed nanoparticles show great promise for application in a near-infrared photodetector.

Thermally Prolonged NIR-II Luminescence Lifetimes by Cross-Relaxation

Temperature regulates nonradiative processes in luminescent materials, fundamental to luminescence nanothermometry. However, elevated temperatures often suppress the radiative process, limiting the sensitivity of thermometers. Here, we introduce an approach to populating the excited state of lanthanides at elevated temperatures, resulting in a sizable lifetime lengthening and intensity increase of the near-infrared (NIR)-II emission. The key is to create a five-energy-level system and use a pair of lanthanides to leverage the cross-relaxation process. We observed the lifetime of NIR-II emission of Er3+ has been remarkably increased from 3.85 to 7.54 ms by codoping only 0.5 mol % Ce3+ at 20 °C and further increased to 7.80 ms when increasing the temperature to 40 °C. Moreover, this concept is universal across four ion pairs and remains stable within aqueous nanoparticles. Our findings emphasize the need to design energy transfer systems that overcome the constraint of thermal quenching, enabling efficient imaging and sensing.

Quantitative Fluorescent Lateral Flow Strip Sensor for Myocardial Infarction Using Purity-Color Upconversion Nanoparticles

Acute myocardial infarction is a serious cardiovascular disease and poses significant risks to human health. Its early diagnosis and real-time detection are of great importance. Herein, we design a low-cost device that has a high sensitivity of cTnT and cTnI detection. Dual-color upconversion nanoparticles (UCNPs) are prepared as probes, which not only have high-purity red upconversion luminescence (UCL) under 980 or 808 nm excitation but also achieve good temperature sensing. Temperature-dependent multicolor emission excitation is obtained, and the color turns from white to orange and red with increasing temperature. In particular, the maximum SR and SA values based on nonthermally coupled levels are 4.76% K-1 and 8.6% K-1, which are higher than those based on thermally coupled levels. With the UCNPs-based lateral flow strip (LFS), the specific detection of cTnI and cTnT antigens in samples is achieved with a detection limit of 0.001 ng/mL, which is 1 order of magnitude lower than that of their clinical cutoff. The UCNPs-LFS device has a low-cost laser diode and a simplified laser and permits a mobile-phone camera to collect the results, which has an important influence on the field of biomarker sensing.

Spatiotemporal control of photochromic upconversion through interfacial energy transfer

Dynamic control of multi-photon upconversion with rich and tunable emission colors is stimulating extensive interest in both fundamental research and frontier applications of lanthanide based materials. However, manipulating photochromic upconversion towards color-switchable emissions of a single lanthanide emitter is still challenging. Here, we report a conceptual model to realize the spatiotemporal control of upconversion dynamics and photochromic evolution of Er3+ through interfacial energy transfer (IET) in a core-shell nanostructure. The design of Yb sublattice sensitization interlayer, instead of regular Yb3+ doping, is able to raise the absorption capability of excitation energy and enhance the upconversion. We find that a nanoscale spatial manipulation of interfacial interactions between Er and Yb sublattices can further contribute to upconversion. Moreover, the red/green color-switchable upconversion of Er3+ is achieved through using the temporal modulation ways of non-steady-state excitation and time-gating technique. Our results allow for versatile designs and dynamic management of emission colors from luminescent materials and provide more chances for their frontier photonic applications such as optical anti-counterfeiting and speed monitoring.

Pressure-Triggered Fluorescence Intensity Ratio Variations of YNbO4:Bi3+/Ln3+ (Ln = Eu or Sm) for High-Sensitivity Optical Pressure Sensing

Optical pressure sensing by phosphors is a growing area of research. However, the main pressure measurement methods rely on the movement of the central peak position, which has significant drawbacks for practical applications. This paper demonstrates the feasibility of using the fluorescence intensity ratio (FIR) of different emission peaks for pressure sensing. The FIR (IBi3+/ILn3+) values of the synthesized YNbO4:Bi3+/Ln3+ (Ln = Eu or Sm) phosphors are all first-order exponentially related to pressure, and YNbO4:Bi3+/Ln3+ (Ln = Eu or Sm) phosphors have high pressure-sensing sensitivities (Sp and Spr), which are 6 times higher than those from our previously reported work. In addition, the changes in FIR values during the decompression process were also calculated, and the trend was similar to that during the compression process. The YNbO4:Bi3+,Eu3+ phosphor has better pressure recovery performance. In summary, the YNbO4:Bi3+/Ln3+ (Ln = Eu or Sm) phosphors reported in this paper are expected to be applied in the field of optical pressure sensing, and this study provides a new approach and perspective for designing new phosphors for pressure measurement.

Luminescence Thermometry Beyond the Biological Realm

As the field of luminescence thermometry has matured, practical applications of luminescence thermometry techniques have grown in both frequency and scope. Due to the biocompatibility of most luminescent thermometers, many of these applications fall within the realm of biology. However, luminescence thermometry is increasingly employed beyond the biological realm, with expanding applications in areas such as thermal characterization of microelectronics, catalysis, and plasmonics. Here, we review the motivations, methodologies, and advances linked to nonbiological applications of luminescence thermometry. We begin with a brief overview of luminescence thermometry probes and techniques, focusing on those most commonly used for nonbiological applications. We then address measurement capabilities that are particularly relevant for these applications and provide a detailed survey of results across various application categories. Throughout the review, we highlight measurement challenges and requirements that are distinct from those of biological applications. Finally, we discuss emerging areas and future directions that present opportunities for continued research.

Ultrasensitive Colorimetric Luminescence Thermometry by Progressive Phase Transition

Luminescent materials that display quick spectral responses to thermal stimuli have attracted pervasive attention in sensing technologies. Herein, a programmable luminescence color switching in lanthanide-doped LiYO2 under thermal stimuli, based on deliberate control of the monoclinic (β) to tetragonal (α) phase transition in the crystal lattice, is reported. Specifically, a lanthanide-doping (Ln3+ ) approach to fine-tune the phase-transition temperature in a wide range from 294 to 359 K is developed. Accordingly, an array of Ln3+ -doped LiYO2 crystals that exhibit progressive phase transition, and thus sequential color switching at gradually increasing temperatures, is constructed. The tunable optical response to thermal stimuli is harnessed for colorimetric temperature indication and quantitative detection, demonstrating superior sensitivity and temperature resolution (Sr = 26.1% K-1 , δT = 0.008 K). The advances in controlling the phase-transition behavior of luminescent materials also offer exciting opportunities for high-performance personalized health monitoring.

Swallowed Embedding of Nanopetal-Rich Microflowers in Flexible Photocatalytic and Thermoresponsive Functional Composite Fibers

Developing efficient catalysts to degrade pollutants in water is a very important way to alleviate water pollution. However, it is crucial but challenging to broaden the functions of conventional photocatalysts and improve their environmental adaptability. In this paper, Bi(Er3+/Yb3+)OBr/polyacrylonitrile (BOB-EY/PAN) composite fibers with a swallowed-embedded structure assembled with nanopetal-rich microflowers were designed and fabricated, integrating photocatalytic and temperature-monitoring functions simultaneously. Their unique structure brings a large specific surface area, and the doping of rare earth ions improves the separation efficiency of electron-hole pairs, which enhances the photocatalytic efficiency and endows the fibers with a temperature-monitoring function at the same time. Under simulated sunlight irradiation, the nanofibers show a maximum degradation efficiency of 99.2% for tetracycline hydrochloride (TC) with a degradation constant of K as high as 0.078 min-1. Based on the fluorescence intensity ratio (FIR), the two thermally coupled levels of Er3+ in the nanofibers, 2H11/2 and 4S3/2, provide real-time temperature feedback, displaying a maximum relative sensitivity as high as 0.0215 K-1 at 303 K. Dual-functional BOB-EY/PAN composite nanofibers show great potential for industrial wastewater disposition, providing solutions for wastewater purification in special scenarios.

Tandem Photon Avalanches for Various Nanoscale Emitters with Optical Nonlinearity up to 41st-Order through Interfacial Energy Transfer

Photon avalanche has received continuous attention owing to its superior nonlinear dynamics and promising advanced applications. However, its impact is limited due to the intrinsic energy levels as well as the harsh requirements for the composites and sizes of doped materials. Here, with a universal mechanism named tandem photon avalanche (TPA), giant optical nonlinear response up to 41st-order in erbium ions, one of the most important lanthanide emitters, has been achieved on the nanoscale through interfacial energy transfer process. After capturing energy directly from the avalanched energy state 3 H4 of Tm3+ (800-nm emission), erbium ions also exhibit bright green and red PA emissions with intensities comparable to that of Tm3+ at a low excitation threshold (7.1 kWcm-2 ). Using the same strategy, effective PA looping cycles are successfully activated in Ce3+ and Ho3+ . Additionally, Yb3+ -mediated networks are constructed to further propagate PA effects to lowly-doped Tm3+ , enabling 475-nm PA emission. The newly proposed TPA strategy provides a facile route for generating photon avalanche not only from erbium ions but also from various emitters in multilayered core-shell nanoparticles.

Cross Relaxation Enables Spatiotemporal Color-Switchable Upconversion in a Single Sandwich Nanoparticle for Information Security

Smart control of ionic interaction dynamics offers new possibilities for tuning and editing luminescence properties of lanthanide-based materials. However, it remains a daunting challenge to achieve the dynamic control of cross relaxation mediated photon upconversion, and in particular the involved intrinsic photophysics is still unclear. Herein, this work reports a conceptual model to realize the color-switchable upconversion of Tm3+ through spatiotemporal control of cross relaxation in the design of NaYF4 :Gd@NaYbF4 :Tm@NaYF4 sandwich nanostructure. It shows that cross relaxation plays a key role in modulating upconversion dynamics and tuning emission colors of Tm3+ . Interestingly, it is found that there is a short temporal delay for the occurrence of cross relaxation in contrast to the spontaneous emission as a result of the slight energy mismatch between relevant energy levels. This further enables a fine emission color tuning upon non-steady state excitation. Moreover, a characteristic quenching time is proposed to describe the temporal evolution of cross relaxation quantitatively. These findings present a deep insight into the physics of ionic interactions in heavy doping systems, and also show great promise in frontier applications including information security, anti-counterfeiting and nanophotonics.

Highly sensitive response of luminescence chromaticity to laser power in Lu2Mo4O15:Yb3+/Ho3+ upconverting materials

Power-based upconversion luminescence color regulation (PUCR) is especially suitable for developing dynamic luminescence anti-counterfeiting owing to its straightforward usage. However, it remains a challenge to achieve visually remarkable Ho3+-based PUCR. Herein, favorable PUCR behavior is achieved by codoping Yb3+ and Ho3+ into the Lu2Mo4O15 lattice. It has been demonstrated that the ultrashort lifetime of the Ho3+:5I6 level and the anomalous three-photon nature of green emission are essential. The former causes high-purity red emission at low power, while the latter enables power-responsive tuning from red to green. Compared with Er2Mo4O15:4% Tm3+ we recently reported that Lu2Mo4O15:90% Yb3+/1% Ho3+, thanks to the high solubility of Yb3+ ions, showed a ∼25-fold enhancement in emission intensity. This new material is potentially applicable in dynamic luminescence anti-counterfeiting.

A General Strategy for Developing Ultrasensitive "Transistor-Like" Thermochromic Fluorescent Materials for Multilevel Information Encryption

Thermochromic fluorescent materials (TFMs) exhibit great potential in information encryption applications but are limited by low thermosensitivity, poor color tunability, and a wide temperature-responsive range. Herein, a novel strategy for constructing highly sensitive TFMs with tunable emission (450-650 nm) toward multilevel information encryption is proposed, which employs polarity-sensitive fluorophores with donor-acceptor-donor (D-A-D) type structures as emitters and long-chain alkanes as thermosensitive loading matrixes. The structure-function relationships between the performance of TFMs and the structures of both fluorescent emitters and phase-change molecules are systematically studied. Benefiting from the above design, the obtained TFMs exhibit over 9500-fold fluorescence enhancement toward the temperature change, as well as ultrahigh relative temperature sensitivity up to 80% K-1 , which are first confirmed. Thanks to the superior transducing performance, the above-prepared TFMs can be further developed as information-storage platforms within a relatively narrow interval of temperature variation, including temperature-dominated multicolored information display and multilevel information encryption. This work will not only provide a novel perspective for designing superior TFMs for information encryption but also bring inspiration to the design and preparation of other response-switching-type fluorescent probes with ultrahigh conversion efficiency.

Spotlight on Luminescence Thermometry: Basics, Challenges, and Cutting-Edge Applications

Luminescence (nano)thermometry is a remote sensing technique that relies on the temperature dependency of the luminescence features (e.g., bandshape, peak energy or intensity, and excited state lifetimes and risetimes) of a phosphor to measure temperature. This technique provides precise thermal readouts with superior spatial resolution in short acquisition times. Although luminescence thermometry is just starting to become a more mature subject, it exhibits enormous potential in several areas, e.g., optoelectronics, photonics, micro- and nanofluidics, and nanomedicine. This work reviews the latest trends in the field, including the establishment of a comprehensive theoretical background and standardized practices. The reliability, repeatability, and reproducibility of the technique are also discussed, along with the use of multiparametric analysis and artificial-intelligence algorithms to enhance thermal readouts. In addition, examples are provided to underscore the challenges that luminescence thermometry faces, alongside the need for a continuous search and design of new materials, experimental techniques, and analysis procedures to improve the competitiveness, accessibility, and popularity of the technology.

Superenhancement Photon Upconversion Nanoparticles for Photoactivated Nanocryometer

Highly doped lanthanide luminescent nanoparticles exhibit unique optical properties, providing exciting opportunities for many ground-breaking applications, such as super-resolution microscopy, deep-tissue bioimaging, confidentiality, and anticounterfeiting. However, the concentration-quenching effect compromises their luminescence efficiency/brightness, hindering their wide range of applications. Herein, we developed a low-temperature suppression cross-relaxation strategy, which drastically enhanced upconversion luminescence (up to 2150-fold of green emission) in Er³⁺-rich nanosystems. The cryogenic field opens the energy transport channel of Er³⁺ multiphoton upconversion by further suppressing phonon-assisted cross-relaxation. Our results provide direct evidence for understanding the energy loss mechanism of photon upconversion, deepening a fundamental understanding of the upconversion process in highly doped nanosystems. Furthermore, it also suggests the potential applications of upconversion nanoparticles for extreme ambient-temperature detection and anticounterfeiting.

Selectively Manipulating Interactions between Lanthanide Sublattices in Nanostructure toward Orthogonal Upconversion

Smart control of ionic interactions is a key factor to manipulate the luminescence dynamics of lanthanides and tune their emission colors. However, it remains challenging to gain a deep insight into the physics involving the interactions between heavily doped lanthanide ions and in particular between the lanthanide sublattices for luminescent materials. Here we report a conceptual model to selectively manipulate the spatial interactions between erbium and ytterbium sublattices by designing a multilayer core-shell nanostructure. The interfacial cross-relaxation is found to be a leading process to quench the green emission of Er³⁺, and red-to-green color-switchable upconversion is realized by fine manipulation of the interfacial energy transfer on the nanoscale. Moreover, the temporal control of up-transition dynamics can also lead to an observation of green emission due to its fast rise time. Our results demonstrate a new strategy to achieve orthogonal upconversion, showing great promise in frontier photonic applications.

Shell Engineering on Thermal Sensitivity of Lifetime-Based NIR Nanothermometers for Accurate Temperature Measurement in Murine Internal Liver Organ

Lifetime-based NIR luminescent nanothermometry is ideally suited for temperature detection in living cells and in vivo, but the thermal sensitivity (S_r) modulation remains elusive. Herein, a thorough investigation is performed to unveil the shell effect on lifetime-based S_r by finely controlling the shell thickness of lanthanide-doped core-shell-shell nanoparticles. Owing to the space-dependent energy transfer and back energy transfer between Nd³⁺ and Yb³⁺ as well as the energy migration to surface quenchers, both active and inert shells can regulate the thermal-dependent nonradiative decays and NIR luminescence lifetime of Yb³⁺, which in turn modulates the S_r from 0.56% to 1.54% °C^-1. After poly(acrylic acid) modification of the optimal architecture, the tiny nanoprobes possess robust stability to fluctuations in the microenvironment, which enables accurate temperature mapping of inflammation in the internal liver organ of living mouse. This work will provide new insights for optimizing S_r and guidance for precise temperature measurements in vivo.

Red upconversion emission of Er³⁺ enhanced by building NaErF₄@ NaYbF₄:2%Er³⁺ core-shell structure

Building core-shell structures are widely used to enhance and regulate the luminescence properties of rare-earth-doped micro/nano materials. In this work, a variety of different NaErF₄ core-shell and core-shell-shell nanocrystals are successfully constructed based on high temperature co-precipitation method by epitaxial growth technology. The upconversion red emission intensities of Er³⁺ ions in different core-shell structures are effectively enhanced by regulating their structures and doping ions. The experimental structures show that the constructed core-shell nanocrystals each have a hexagonal phase structure, and core-shell structure of about 40 nm. In the near infrared 980 nm laser excitation, the NaErF₄ core-shell nanocrystal shows a strong single-band red emission. And the single-band red emission intensity of Er³⁺ ions is enhanced through constructing the NaErF₄@NaYbF₄:2%Er³⁺ core-shell structure. The experimental results show that red emission intensity of Er³⁺ ions is about 1.4 times higher than that of the NaErF₄@NaYbF₄ core-shell structure by constructing the NaErF₄@NaYbF₄:2%Er³⁺ core-shell structures under 980 nm excitation, and its red/green emission intensity ratio increases from 5.4 to 6.5. Meanwhile, when NaErF₄@NaYbF₄:2%Er³⁺ core-shell structure recoats the NaYF₄ inert shell and is added with a small quantity of Tm³⁺ ions, their red emission intensities of Er³⁺ ions are 23.2 times and 40.3 times that of NaErF₄@NaYbF₄ core-shell structures, and their red/green emission intensity ratios reach 7.5 and 10.2, respectively. The red emission enhancement of Er³⁺ ions is mainly caused by bidirectional energy transfer process of high excitation energy of Yb³⁺ ions and energy trapping center of Tm³⁺ ions which effectively change the density of population of luminescent energy levels of Er³⁺ ions. Furthermore, the coated NaYF₄ inert shell also effectively weakens the surface quenching effect of nanocrystals. The mechanisms of red enhancement in different core-shell structures are discussed based on the spectral properties, the process of interion energy transfer, and luminescence kinetics. The constructed NaErF₄@NaYbF₄:2%Er³⁺@NaYF₄ core-shell structures with high-efficiency red emission in this work have great potential applications in the fields of colorful anti-counterfeiting, display and biological imaging.

Multichannel Control of PersL/Upconversion/Down-Shifting Luminescence in a Single Core-Shell Nanoparticle for Information Encryption

Persistent luminescence (PersL) has been attracting substantial attention in diverse frontier applications such as optical information security and in vivo bioimaging. However, most of the reported PersL emissions are based on the dopants instead of the host matrix, which also plays an important role. In addition, there are few works on the PersL-based multifunctional nanoplatform in nanosized materials. Here, we report a class of novel nanostructure designs with PersL, upconversion, and down-shifting luminescence to realize the fine-tuning of emission colors under different excitation modes including steady-state irradiation, time-gating, and PersL generation. Blue, orange, and green emissions were easily achieved in such a single nanoparticle under suitable excitation modes. Moreover, the physical origin of the PersL of the CaF₂ matrix was discussed by simulating the energy band structure with Ca_xF_y defects. Our results provide new opportunities for the design of a new class of multifunctional materials, showing great promise in the field of information encryption security and multilevel anticounterfeiting.

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.