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

Metal-organic framework-based hybrids with photon upconversion

Upconversion materials (UCMs) featuring an anti-Stokes type emission establish them as an important category of photoluminescent materials. Metal-organic frameworks (MOFs) are rapidly gaining prominence as a class of versatile materials with favourable physical and chemical properties, including high porosity, controllable pore size, flexible design, and diverse functional sites. To endow MOFs with upconversion capability and improve the properties and performance of UCMs, the hybrids integrating UCMs and MOFs are proven to be successful. This review focuses on the research advancements of upconverting MOF-based hybrids, encompassing classifications, luminescence mechanisms, designs, properties, and applications in energy, catalysis, and biomedical fields. The analyses on the functions of upconversion and MOFs, as well as the advantages and disadvantages of various upconverting MOF-based hybrids, are included. Future research directions spanning from properties and performance to applications are explored. This review will be valuable in highlighting the research accomplishments, inspiring more ideas, facilitating deeper investigations in diverse avenues, and further advancing the research field.

Upconverting Luminescent MOF for Highly Sensitive Dual-Mode Recognition of Synthetic Dyes

Lanthanide-based luminescent materials hold promise in sensing applications due to their distinct optical properties. Though advancements in lanthanide-based metal-organic frameworks (MOFs) have enhanced downshifting luminescence, achieving upconversion remains challenging. In this effort, we prepared upconverting ytterbium-doped europium MOFs (x%Yb3+-EuMOFs; x = 10, 20, and 30) via the solvothermal method using 2,6-naphthalenedicarboxylic acid (NDC) as an organic linker. Upconversion (UC) luminescence studies revealed that 30%Yb3+-EuMOF (MOF-3) rapidly detects malachite green (MG, a textile dye) and brilliant green (BG, a food colorant) with excellent sensitivity (λex = 980 nm). Notably, UC luminescence offers a lower detection limit (MG: 36.33 nM, BG: 287.9 nM) compared to the downconverting sensing approach (λex 270 nm), while the related dual-mode luminescence minimizes the risk of false positives from interfering ultraviolet-visible (UV-vis) light-absorbing substances. Upconversion quenching has been linked to the FRET process, with its luminescence assay accurately detecting MG in fish, water, and soil samples under 980 nm excitation (98-105% recovery).

Defect Regulation Strategy of Porous Persistent Phosphors for Multiple and Dynamic Information Encryption

Optical encryption technologies based on persistent luminescence material have currently drawn increasing attention due to the distinctive and long-lived optical properties, which enable multi-dimensional and dynamic optical information encryption to improve the security level. However, the controlled synthesis of persistent phosphors remains largely unexplored and it is still a great challenge to regulate the structure for optical properties optimization, which inevitably sets significant limitations on the practical application of persistent luminescent materials. Herein, a controlled synthesis method is proposed based on defect structure regulation and a series of porous persistent phosphors is obtained with different luminous intensities, lifetime, and wavelengths. By simply using diverse templates during the sol-gel process, the oxygen vacancy defects structures are successfully regulated to improve the optical properties. Additionally, the obtained series of porous Al2O3 are utilized for multi-color and dynamic optical information encryption to increase the security level. Overall, the proposed defect regulation strategy in this work is expected to provide a general and facile method for optimizing the optical properties of persistent luminescent materials, paving new ways for broadening their applications in multi-dimensional and dynamic information encryption.

Water-Vapor-Triggered Dual-Mode Optical Responses in Rare-Earth-Doped Hollow Nanospheres

Multimode responsive optical materials are garnering ever-increasing attention due to their diverse applications. This work showcases a film assembled with rare-earth-doped CaF2 hollow nanospheres that exhibit water-vapor-triggered dual-mode optical responses. Upon exposure to flowing water vapor, the film rapidly (less than 1.5 s for a 7.7 μm thickness) transitions to a transparent state and simultaneously undergoes a sharp decrease in the photoluminescence intensity. Both of these changes fully reverse upon water evaporation, demonstrating an impressive reversibility over at least 200 cycles. The water-vapor-induced dual-mode responses are attributed to the altered incident light propagation path stemming from the similar refractive indices between CaF2 and water, coupled with the water-induced energy loss of the rare-earth ions. The fabrication of encryption patterns displaying separate signals in multiple channels, as well as the demonstration of noncontact sensing for water vapor distribution, underscore the promising application potential of this dual-mode responsive system.

Distortion Engineering: A Strategy to Modulate Molecular Upconversion with Molecular Cluster-Aggregates

The rational engineering of molecules is a powerful chemistry tool of pivotal importance in the fields of molecular magnetism and luminescence. Hence, systems that can be modulated via molecular engineering and composition control are expected to present extra versatility regarding the tunability of their properties. This is the case with molecular cluster aggregates (MCAs), high nuclearity molecular compounds. Herein, we demonstrate how the union of both strategies, namely, composition control and molecular engineering, can be employed to enhance molecular upconversion in MCAs. This was achieved by doping a {Gd8Er2Yb10} MCA with CeIII ions. By replacement of the optically silent GdIII ions with CeIII, the upconversion mechanism is modified due to CeIII-mediated cross-relaxation. In addition to this effect, we could also engineer the degree of metal site distortion due to the larger size of CeIII ions, relaxing the selection rules and impacting the upconversion quantum yield and luminescent thermometry. Opto-structural correlations demonstrate that the presented molecular engineering strategy can be used to enhance the performance of molecular upconverters.

Regulation of the Metal Center in Lanthanide Nanoparticles to Achieve Multifunctional Sensing

Development of a multifunctional sensor is highly desirable. In this work, traces of a carcinoid cancer biomarker of 5-hydroxyindole-3-acetic acid (5-HIAA) in real human urine can be detected by lanthanide nanoparticle Eu-CFC (CFC = 7-chloro-1-cyclopropyl-6-fluoro-4-oxo-1,4-dihydroquinoline-3-carboxylic acid) and the sensing devices of the test paper and agarose gel, achieving an ultralow LOD of 0.8 × 10-3 ppm within a sensing time of 2.0 min. Interestingly, by metal center regulation of Tb and Eu codoping, nanoparticle TbEu2-CFC shows high-sensitivity and low-LOD (0.019% v/v) sensing of water in ethanol. The sensing mechanisms are revealed by both experiments and quantum chemical studies.

Excitation power-dependent multicolor upconversion in NaLnF4:Er3+ under 1532 nm irradiation for anti-counterfeiting application

Upconversion (UC) materials are renowned for their ability to convert low-energy photons into high-energy ones. The manipulation of parameters allows for the observation of multicolored UC luminescence (UCL) within a single material system. While modulation of multicolored UCL commonly relies on excitation at approximately 980 nm, investigation into multicolored UC materials activated by a 1532 nm excitation source remains comparatively scarce. In this work, we introduce NaLnF4:Er3+ as a novel class of smart luminescent materials. When the power density of a 1532 nm laser increases from 0.5 to 20.0 W/cm2, the emission peak positions remain unchanged, but the red-to-green (R/G) ratio decreases significantly from 18.82 to 1.48, inducing a color shift from red to yellow and ultimately to green. In contrast, no color variation is observed when NaLnF4:Er3+ is excited with a 980 nm laser at different power densities. This power-dependent multicolored UCL of NaLnF4:Er3+ excited at 1532 nm can be attributed to the competitive processes of upward pumping and downward relaxation of electrons on the 4I9/2 level of Er3+. By utilizing the unique UC characteristics of NaLnF4:Er3+, its potential utility in anti-counterfeiting applications is demonstrated. Our research highlights the distinctive optical properties of NaLnF4:Er3+ and provides novel insights into the use of luminescent materials in optical anti-counterfeiting technologies.

Hydro-Photo-Thermo-Responsive Multicolor Luminescence Switching of a Ternary MOF Hybrid for Advanced Information Anticounterfeiting

Developing smart materials capable of solid-state multicolor photoluminescence (PL) switching in response to multistimuli is highly desirable for advanced anticounterfeiting. Here, a ternary MOF hybrid showing hydro-photo-thermo-responsive multicolor PL switching in the solid state is presented. This hybrid is constructed by co-immobilizing Eu3+ and methyl viologen (MV) cations within an anionic MOF via the cation-exchange approach. The confined guest cations are well arranged in the framework channels, facilitating the synergistic realization of stimuli-responsive multiple PL color-switching through intermolecular coupling. The hybrid undergoes a rapid and reversible PL color-switching from red to blue upon water simulation, which is achieved by activating the blue emission of the framework linker while simultaneously quenching the Eu3+ emission. Furthermore, the hybrid displays photo-thermo-responsive PL switching from red to dark. UV-light irradiation or heating triggers the chromic conversion of MV to its colored radical form, which exhibits perfect spectral overlap with Eu3+, thus activating Förster resonance energy transfer (FRET) from Eu3+ to MV radicals and quenching the Eu3+ emission. Inspired by these results, PL morse patterns are designed and fabricated using a novel triple-level encryption strategy, showcasing the exciting potential of this hybrid in advanced anticounterfeiting applications.

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.

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.

Over 104 -fold amplified upconversion luminescence of lanthanide nanocrystals through optical oscillator-like system

Recently, lanthanide (Ln) luminescent nanocrystals have attracted increasing attention in various fields such as biomedical imaging, lasers, and anticounterfeiting. However, due to the forbidden 4f-4f transition of lanthanide ions, the absorption cross-section and luminescence brightness of lanthanide nanocrystals are limited. To address the challenge, we constructed an optical oscillator-like system to repeatedly simulate lanthanide nanocrystals to enhance the absorption efficiency of lanthanide ions on excitation photons. In this optical system, the upconversion luminescence (UCL) of Tm3+ emission of ~450 nm excited by a 980 nm laser can be amplified by a factor beyond 104 . The corresponding downshifting luminescence of Tm3+ at 1460 nm was enhanced by three orders of magnitude. We also demonstrated that the significant luminescence enhancement in the designed optical oscillator-like system was general for various lanthanide nanocrystals including NaYF4 :Yb3+ /Ln3+ , NaErF4 @NaYF4 and NaYF4 :Yb3+ /Ln3+ @NaYF4 :Yb3+ @NaYF4 (Ln = Er, Tm, Ho) regardless of the wavelengths of excitation sources (808 and 980 nm). The mechanism study revealed that both elevated laser power in the optical system and multiple excitations on lanthanide nanocrystals were the main reason for the luminescence amplification. Our findings may benefit the future development of low-threshold upconversion and downshifting luminescence of lanthanide nanocrystals and expand their applications.

Er3+-Sensitized Upconversion/Down-Shifting Luminescence in Metal-Organic Frameworks

Photoluminescent metal-organic frameworks (MOFs) are used as optical materials with excellent properties, of which the lanthanide-doped MOFs are able to emit in a broad region from visible to near-infrared due to their unique 4f-orbital electronic structure. Herein, Er3+ and Y3+ ions are selected as the metal centers of the MOFs and Er3+ is used as a sensitizer to absorb 980 nm excitation light. At the same time, Er3+ ions also act as activators that emit upconverting visible light and down-shifting near-infrared light. In addition, Tm3+, Ho3+, and Eu3+ ions were individually doped into the Er3+-doped MOFs to investigate the variation of energy-transfer paths in the presence of different lanthanide activators. Finally, the pathway of energy transfer in these Er3+-sensitized luminescent-MOFs was summarized. This work provides new insights for further development of both upconversion and down-shifting luminescence of MOFs.

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.