Hollow nanoparticles synthesized via Ostwald ripening and their upconversion luminescence-mediated Boltzmann thermometry over a wide temperature range

Self-Template Evolution Toward Hierarchically Hollow Spherulites of Energetic Materials for Safety Control and Combustion Enhancement

The hierarchical hollow structure can endow functional materials with concerning performances, whereas its rational design remains challenging, especially for organic molecules. Herein, a novel strategy of self-template evolution is presented to construct hollow spherulites (HSs) for energetic organic materials with controllable safety and enhanced combustion, utilizing the mechanism of pseudomorphic replacement coupled with Ostwald ripening. Specifically, the spherulites of thermodynamically metastable β-2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (β-CL-20) as the self-template spontaneously evolves into the HSs of CL-20-based inclusion compounds in solution, during which the dissolution of β-CL-20 and the crystallization of CL-20-based inclusion compounds are spatiotemporally correlated, enabling pseudomorphic replacement preserving the spherulitic structure, and cavities are formed inside via Ostwald ripening. Moreover, the conversion rates and the mediating solvent cause different locations of cavities, with two cases exhibited. The hollow-core spherulites of CL-20-formic acid exhibit controllable safety through manipulating micro-/nanostructures, and the hollow-spoke spherulites of CL-20-CO2 can be the efficient carrier to composite with oxidants, enhancing combustion with increases of 23.2% in pressure duration effect, 9.8% in peak pressure, and 3.3% in pressurization rate. This work offers a novel route to construct hierarchical hollow energetic crystals with enhanced performance and puts insight into the design of hierarchical hollow crystals for broad material systems.

Optomagnetic Heater-Thermometer Nanoplatform for Tumor Magnetothermal Therapy with Operando Temperature Feedback

Real-time temperature feedback during hyperthermia enables precise lesion ablation with minimal damage to healthy tissue. However, the lack of on-demand theranostic agents eludes such applications. This study demonstrates in vivo operando temperature monitoring in tumor magnetothermal therapy using an optomagnetic heater-thermometer nanocluster agent. This agent was synthesized via a precise microemulsion-based assembly of lanthanide luminescent nanocrystals and superparamagnetic iron oxide nanoparticles. We show that the lanthanide luminescent nanocrystals within the agents provide temperature-sensitive luminescence lifetime and depth-sensitive luminescence intensity ratio in the second near-infrared biological window (NIR-II), enabling accurate 3D thermographic mapping of tissues with an uncertainty as low as 0.12 °C. Meanwhile, the iron oxide nanoparticles facilitate targeted accumulation in lesions under an oriented magnetic field and act as heating sources under oscillating magnetic fields without interfering with the operando temperature measurements of lanthanide nanothermometers. With spatiotemporally synchronized thermographic imaging, we achieved efficient, programmed, and noninvasive tumor ablation in mice through controlled mild hyperthermia by adjusting the direction and strength of the applied magnetic field. Our proof-of-concept results suggest that this optomagnetic heater-thermometer nanoplatform is promising for high accuracy in vivo magnetic hyperthermia applications.

Achieving Orthogonal Upconversion Luminescence of a Single Lanthanide Ion in Crystals for Optical Encryption

Optical encryption shows great potential in meeting the growing demand for advanced anti-counterfeiting in the information age. The development of upconversion luminescence (UCL) materials capable of emitting different colors of light in response to different external stimuli holds great promise in this field. However, the effective realization of multicolor UCL materials usually requires complex structural designs. In this work, orthogonal UCL is achieved in crystals with a simple structure simply by introducing modulator Tm3+ ions to control the photon transition processes between different energy levels of activator Er3+ ions. The obtained crystals emit red and green UCL when excited by 980 nm and 808 nm lasers, respectively. The orthogonal excitation-emission properties of crystals are shown to be very suitable for high-level optical encryption, which is important for information security and anti-counterfeiting. This work provides an effective strategy for obtaining orthogonal UCL in simple structural materials, which will encourage researchers to further explore novel orthogonal UCL materials and their applications, and has important implications for the development of the frontier photonic upconversion fields.

Constructing a Self-Referenced NIR-II Thermometer with Energy Tuning of Coordinating Water Molecules by a Minimalist Method

Fluorescence thermometry based on metal halide perovskites is increasingly becoming a hotspot due to its advantages of high detection sensitivity, noninvasiveness, and fast response time. However, it still presents certain technical challenges in practical applications, such as complex synthesis methods, the use of toxic solvents, and being currently mainly based on the visible/first near-infrared light with poor penetration and severe autofluorescence. In this study, we synthesize the second near-infrared (NIR-II) luminescent crystals based on Yb3+/Nd3+-doped zero-dimensional Cs2ScCl5·H2O by a simple "dissolve-dry" method. The whole synthesized process does not involve high temperatures or high pressures. Cs2ScCl5·H2O/Yb3+,Nd3+ has an optimum fluorescence performance when the Yb3+/Nd3+ doping amount is 15%/20%. The emission intensity ratio attributed to Yb3+ and Nd3+ varies with temperature, and this variation is exacerbated due to the fact that the 2F5/2 energy level of Yb3+ can be effectively aligned with the O-H bond stretching vibration energy of coordinating water molecules to facilitate energy transfer. Ultimately, the crystals can act as self-referenced ratiometric NIR-II luminescent thermometers with a maximum relative sensitivity of 1.66% K-1 at 323 K. This work highlights the advantages of NIR-II luminescent materials for temperature sensing, which is significant for advancement in the field of noncontact thermometers.

Postsynthetic Size Focusing via Digestive Ripening in HgTe Quantum Dots

Size uniformity in colloidal quantum dots (CQDs) critically influences their electronic properties. However, achieving precise size control remains challenging for mercury chalcogenide systems. We demonstrate that a postsynthetic reheating treatment with the addition of dodecanethiol (DDT) ligands, consistent with a digestive ripening mechanism, effectively narrows the size distribution of HgTe CQDs from 12% to 6%, accompanied by enhanced crystallinity and sharper absorption features. Significantly, this improved structural uniformity leads to a 10-fold reduction in intrinsic carrier density from 2.3 × 1018 to 1.8 × 1017 cm-3. Our theoretical analysis reveals that such carrier density reduction, combined with band-edge engineering, could enable quantum dot operation at significantly elevated temperatures up to 230 K with simple thermoelectric cooling. This work establishes a straightforward postsynthetic approach to refine mercury telluride CQDs, providing a valuable quality control strategy that could potentially enhance the yield and reliability in commercial IR CQD mass production.

Upconversion photoluminescence of Er-doped Bi4Ti3O12 ceramics enhanced by vacancy clusters revealed by positron annihilation spectroscopy

Doping of rare earth elements into Bi4Ti3O12 can significantly enhance the upconversion photoluminescence (UCPL) properties, but its structure–property relationship is still unclear. In this work, Er-doped bismuth titanate Bi4−x Er x Ti3O12 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) ceramics were synthesized via solid-state reaction method. The x-ray diffraction analysis confirmed the orthorhombic crystalline structure of the Bi4−x Er x Ti3O12 ceramics without any secondary phases. Experiments and calculations of positron annihilation spectroscopy were carried out to characterize their defect structure. The comparison between the experimental and calculated lifetime revealed that vacancy clusters were the main defects in the ceramics. The increase of the intensity of the second positron lifetime component (I 2) of Bi3.5Er0.5Ti3O12 ceramics indicated the presence of a high concentration of vacancy clusters. The UCPL spectra showed the sudden enhanced UCPL performance in Bi3.7Er0.3Ti3O12 and Bi3.5Er0.5Ti3O12 ceramics, which were consistent with the variation of the second positron lifetime component (I 2). These results indicate that the enhanced UCPL properties are influenced not only by the concentrations of rare earth ions but also by the concentration of vacancy clusters present within the ceramics.

Regulating superstructures of conjugated polymers towards enhanced and stable photocatalytic hydrogen evolution via covalent crosslinking and complementary supramolecular self-assembly

The modulation of microstructures in conjugated polymers represents a viable strategy for enhancing photocatalytic efficiency, albeit hampered by complex processing techniques. Here, we present an uncomplicated, template-free method to synthesize polymeric photocatalysts, namely BCN(x)@PPy, featuring a hollow nanotube-nanocluster core-shell superstructure. This configuration is realized through intramolecular covalent crosslinking and synergistic intermolecular donor-acceptor (D-A) interactions between phenylene pyrene (PPy, D) nanotubes and poly([1,1'-biphenyl]-3-carbonitrile) (PBCN, A) nanoclusters. Interestingly, the optimized BCN2@PPy composite demonstrates remarkably enhanced performance for photocatalytic hydrogen evolution, with an efficiency of 14.7-fold higher than that of unmodified PPy nanotubes. Experimental and density functional theory calculations revealed that BCN(x)@PPy composites are conducive to shortening photogenerated exciton migration, facilitating charge separation and transfer, reducing nanoclusters aggregation or re-stacking, and providing sufficient catalytically active sites, all contributing to the heightened efficiency in photocatalysis. These insights underscore the potential for precise molecular adjustments in conjugated polymers, advancing artificial photosynthesis.

Recent Advances in Hollow Nanostructures: Synthesis Methods, Structural Characteristics, and Applications in Food and Biomedicine

The development and investigation of innovative nanomaterials stand poised to advance technological progress and meet the contemporary demand for efficient, environmentally friendly, and intelligent products. Hollow nanostructures (HNS), characterized by their hollow architecture, exhibit diverse properties such as expansive specific surface area, low density, high drug-carrying capacity, and customizable structures. These elaborated structures, encompass nanospheres, nanoboxes, rings, cubes, and nanowires, have wide-ranging applications in biomedicine, materials chemistry, food industry, and environmental science. Herein, HNS and their cutting-edge synthesis methods, including solvothermal methods, liquid-interface assembly methods, and the self-templating methods are discussed in-depth. Meanwhile, the potential applications of HNS in food and biomedicine such as food packing, biosensor, and drug delivery over the past three years are summarized, together with a prospective view of future research directions and challenges. This review will offer new insights into designing next generation of hollow nanomaterials for food and biomedicine applications.

Near-Infrared-Responsive Rare Earth Nanoparticles for Optical Imaging and Wireless Phototherapy

Near-infrared (NIR) light is well-suited for the optical imaging and wireless phototherapy of malignant diseases because of its deep tissue penetration, low autofluorescence, weak tissue scattering, and non-invasiveness. Rare earth nanoparticles (RENPs) are promising NIR-responsive materials, owing to their excellent physical and chemical properties. The 4f electron subshell of lanthanides, the main group of rare earth elements, has rich energy-level structures. This facilitates broad-spectrum light-to-light conversion and the conversion of light to other forms of energy, such as thermal and chemical energies. In addition, the abundant loadable and modifiable sites on the surface offer favorable conditions for the functional expansion of RENPs. In this review, the authors systematically discuss the main processes and mechanisms underlying the response of RENPs to NIR light and summarize recent advances in their applications in optical imaging, photothermal therapy, photodynamic therapy, photoimmunotherapy, optogenetics, and light-responsive drug release. Finally, the challenges and opportunities for the application of RENPs in optical imaging and wireless phototherapy under NIR activation are considered.

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.

Anomalous thermal activation of green upconversion luminescence in Yb/Er/ZnGdO self-assembled microflowers for high-sensitivity temperature detection

Non-contact optical temperature detection has shown a great promise in biological systems and microfluidics because of its outstanding spatial resolution, superior accuracy, and non-invasive nature. However, the thermal quenching of photoluminescence significantly hinders the practical applications of optical temperature probes. Herein, we report thermally enhanced green upconversion luminescence in Yb/Er/ZnGdO microflowers by a defect-assisted thermal distribution mechanism. A 1.6-fold enhancement in green emission was demonstrated as the temperature increased from 298 K to 558 K. Experimental results and dynamic analysis demonstrated that this behavior of thermally activating green upconversion luminescence originates from the emission loss compensation, which is attributed to thermally-induced energy transfer from defect levels to the green emitting level. In addition, the Yb/Er/ZnGdO microflowers can act as self-referenced radiometric optical thermometers. The ultrahigh absolute sensitivity of 1.61% K-1 and an excellent relative sensitivity of 15.5% K-1 based on the 4F9/2/2H11/2(2) level pair were synchronously achieved at room temperature. These findings provide a novel strategy for surmounting the thermal quenching luminescence, thereby greatly promoting the application of non-contact sensitive radiometric thermometers.

Suppression of Cation Intermixing Highly Boosts the Performance of Core-Shell Lanthanide Upconversion Nanoparticles

Lanthanide upconversion nanoparticles (UCNPs) have been extensively explored as biomarkers, energy transducers, and information carriers in wide-ranging applications in areas from healthcare and energy to information technology. In promoting the brightness and enriching the functionalities of UCNPs, core-shell structural engineering has been well-established as an important approach. Despite its importance, a strong limiting issue has been identified, namely, cation intermixing in the interfacial region of the synthesized core-shell nanoparticles. Currently, there still exists confusion regarding this destructive phenomenon and there is a lack of facile means to reach a delicate control of it. By means of a new set of experiments, we identify and provide in this work a comprehensive picture for the major physical mechanism of cation intermixing occurring in synthesis of core-shell UCNPs, i.e., partial or substantial core nanoparticle dissolution followed by epitaxial growth of the outer layer and ripening of the entire particle. Based on this picture, we provide an easy but effective approach to tackle this issue that enables us to produce UCNPs with highly boosted optical properties.

New Strategy: Molten Salt-Assisted Synthesis to Enhance Lanthanide Upconversion Luminescence

Lanthanide-doped upconversion luminescent materials (LUCMs) have attracted much attention in diverse practical applications because of their superior features. However, the relatively weak luminescence intensity and low efficiency of LUCMs are the bottleneck problems that seriously limit their development. Unfortunately, most of the current major strategies of luminescence enhancement have some inherent shortcomings in their implementation. Here, a new and simple strategy of molten salt-assisted synthesis is proposed to enhance lanthanide upconversion luminescence for the first time. As a proof-of-concept, a series of rare earth oxides with obvious luminescence enhancement are prepared by a one-step method, utilizing molten NaCl as the high-temperature reaction media and rare earth chlorides as the precursors. The enhancement factors at different reaction temperatures are systematically investigated by taking Yb3+ /Er3+ co-doped Y2 O3 as an example, which can be enhanced up to more than six times. In addition, the molten salts are extended to all alkali chlorides, indicating that it is a universal strategy. Finally, the potential application of obtained UCL materials is demonstrated in near-infrared excited upconversion white light-emitting diodes (WLEDs) and other monochromatic LEDs.

Targeted high-precision up-converting thermometer platform over multiple temperature zones with Er3

Ratiometric luminescence thermometry based on trivalent erbium ions is a noninvasive remote sensing technique with high spatial and temporal resolution. The thermal coupling between two adjacent energy levels follows the Boltzmann statistics, whose effective range is related to the energy gap between the multi-excited states. However, the limitations of different thermally coupled levels (TCLs) in Er-based thermometers are rarely mentioned. Here, a type of targeted high-precision luminescence thermometer was designed using a lead-free double perovskite platform by selecting multiple TCLs of the Er³⁺ ion. According to the selection of different TCLs in a single system platform, more precise temperature resolution can be obtained in different temperature regions from 100 K to almost 880 K. This work provides a quantitative guideline that may pave the way for the development of the next generation of temperature sensor based on trivalent erbium ions.

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.

Thermally activated upconversion luminescence and ratiometric temperature sensing under 1064 nm/808 nm excitation

In this research, a thermally activated upconversion luminescence (UCL) probe with ratiometric temperature sensing under 1064 nm and 808 nm excitation was designed. Especially, Nd³⁺, Tm³⁺and Ce³⁺were doped in rare earth nanoparticles (RENPs) as UCL modulators. By optimizing the elements and ratios, the excitation wavelength is successfully modulated to 1064 nm excitation with UCL intensity enhanced. Additionally, the prepared RENPs have a significant temperature response at 1064 nm excitation and can be used for thermochromic coatings. The intensity ratio of three-photon UCL (1064 nm excitation) to two-photon UCL (808 nm excitation) as an exponential function of temperature can be used as a ratiometric temperature detector. Therefore, this designed thermochromic coatings may enable new applications in optoelectronic device and industrial sensor.

Semiconductor Plasmon Enhanced Upconversion toward a Flexible Temperature Sensor

Noninvasive and sensitive thermometry is crucial to human health monitoring and applications in disease diagnosis. Despite recent advances in optical temperature detection, the construction of sensitive wearable temperature sensors remains a considerable challenge. Here, a flexible and biocompatible optical temperature sensor is developed by combining plasmonic semiconductor W₁₈O₄₉ enhanced upconversion emission (UCNPs/WO) with flexible poly(lactic acid) (PLA)-based optical fibers. The UCNPs/WO offers highly thermal-sensitive and obviously enhanced dual-wavelength emissions for ratiometric temperature sensing. The PLA polymer endows the sensor with excellent light-transmitting ability for laser excitation and emission collection and high biocompatibility. The fabricated UCNPs/WO-PLA sensor exhibits stable and rapid temperature response in the range 298-368 K, with a high relative sensitivity of 1.53% K^-1 and detection limit as low as ±0.4 K. More importantly, this proposed sensor is demonstrated to possess dual function on real-time detection for physiological thermal changes and heat release, exhibiting great potential in wearable health monitoring and biotherapy applications.

Energy Gap Linear Superposition of Thermally Coupled Levels toward Enhanced Relative Sensitivity of Ratiometric Thermometry

Ratiometric luminescence thermometry (RLT) has attracted considerable attention for its non-invasive, fast response, and strong electromagnetic interference resistance; however, improving relative sensitivity (S_R) is of great significance. Herein, we propose a design principle to promote S_R by linearly superposing the energy gaps of thermally coupled levels (TCLs) subordinated to luminescence centers. A new fluorescence intensity ratio (FIR') is derived from multiplying the previous FIRs of multi-pair TCLs. Then, a new S_R' is significantly enhanced and proves to be the sum of the original S_R values. The feasibility of this approach is proclaimed by applying to several materials [Na_0.5La_0.5TiO₃:Yb/Nd, Y₂O₃:Yb/Er, and (LiMg)₂Mo₃O₁₂:Yb/Er] with improved S_R for RLT. Finally, a flexible film is fabricated for temperature measurement of actual scenes and manifests the superiority of the energy gap linear superposition method as ratiometric thermometry.

Multi-Mode Lanthanide-Doped Ratiometric Luminescent Nanothermometer for Near-Infrared Imaging within Biological Windows

Owing to its high reliability and accuracy, the ratiometric luminescent thermometer can provide non-contact and fast temperature measurements. In particular, the nanomaterials doped with lanthanide ions can achieve multi-mode luminescence and temperature measurement by modifying the type of doped ions and excitation light source. The better penetration of the near-infrared (NIR) photons can assist bio-imaging and replace thermal vision cameras for photothermal imaging. In this work, we prepared core-shell cubic phase nanomaterials doped with lanthanide ions, with Ba₂LuF₇ doped with Er³⁺/Yb³⁺/Nd³⁺ as the core and Ba₂LaF₇ as the coating shell. The nanoparticles were designed according to the passivation layer to reduce the surface energy loss and enhance the emission intensity. Green upconversion luminescence can be observed under both 980 nm and 808 nm excitation. A single and strong emission band can be obtained under 980 nm excitation, while abundant and weak emission bands appear under 808 nm excitation. Meanwhile, multi-mode ratiometric optical thermometers were achieved by selecting different emission peaks in the NIR window under 808 nm excitation for non-contact temperature measurement at different tissue depths. The results suggest that our core-shell NIR nanoparticles can be used to assist bio-imaging and record temperature for biomedicine.

Excitation Pulse Duration Response of Upconversion Nanoparticles and Its Applications

Lanthanide-doped upconversion nanoparticles (UCNPs) have rich photophysics exhibiting complex luminescence kinetics. In this work, we thoroughly investigated the luminescence response of UCNPs to excitation pulse durations. Analyzing this response opens new opportunities in optical encoding/decoding and the assignment of transitions to emission peaks and provides advantages in applications of UCNPs, e.g., for better optical sectioning and improved luminescence nanothermometry. Our work shows that monitoring the UCNP luminescence response to excitation pulse durations (while keeping the duty cycle constant) by recording the average luminescence intensity using a low-time resolution detector such as a spectrometer offers a powerful approach for significantly extending the utility of UCNPs.

Special Issue: Rare earth luminescent materials

This special issue covers a series of cutting-edge works on exploring novel rare earth luminescent materials and their applications in lighting, display, information storage, sensing, and bioimaging as well as therapy. [Image: see text]

NaBiF4-based hollow upconversion nanoparticles for temperature sensing

Hollow upconversion nanoparticles with tunable central cavity size can be used as self-referenced luminescent thermometers over a wide temperature range.