Real-Time In Situ Volatile Organic Compound Sensing by a Dual-Emissive Polynuclear Ln-MOF with Pronounced LnIII Luminescence Response

Lanthanide Antenna Amplifier Multiplies the Optical Sensing Efficiency in Phototautomeric Metal-Organic Frameworks

Metal-organic frameworks (MOFs) incorporating phototautomeric ligands have shown significant potential for new-generation optical sensing devices. It is challenging to enlarge the energy difference of tautomers, which is crucial for improving the sensing efficiency. Herein, we report a novel tactic to amplify the excited-state intramolecular proton transfer (ESIPT) effect via the "antenna effect" in lanthanide sensitization. Specifically, by leveraging the enol (E)-keto (K) phototautomerization process, we achieve precise manipulation of the energy gap between the lowest ligand triplet state and the Eu(III) emitting level through fine-tuning. As a result, the small wavelength shift (10 nm) between the E* and K* emissions is amplified into an approximately 65-fold intensity change in Eu(III) emission. This enhancement is accompanied by a significantly lowered limit of detection (LOD: 0.53 μg/L) and improved sensitivity (0.6955% RH-1) in real-time humidity monitoring across a broad linear detection range (0-95.0% RH). Computational simulations and single-crystal analyses demonstrate two key mechanisms: (1) the appropriate hydrophilic/hydrophobic distribution in the MOF cavity facilitates rapid hydration/dehydration and (2) H2O desorption/adsorption-dependent ESIPT switching governs the deactivation/activation of the Eu(III) 4f excited state. This work presents an optimization approach to enhance energy utilization in MOF-based optical sensing.

Smart Lanthanide Metal-Organic Frameworks with Multicolor Luminescence Switching Induced by the Dynamic Adaptive Antenna Effect of Molecular Rotors

In this work, dynamic molecular rotors are used to construct smart lanthanide metal-organic frameworks (Ln-MOFs) emitters with adaptive antenna effects for the first time. The movement or distortion of the molecular rotors can be easily regulated by temperature changes, thereby inducing a dynamically changing antenna effect that can automatically match different lanthanide ions, achieving cyclic multicolor luminescence switching behavior and extremely complex multiple encryption anti-counterfeiting technology. In addition, by regulating the doping ratios of Gd(III) and Tb(III) with Eu(III) within the Ln-MOFs, differentiated energy transfer pathways are discovered, and red light emission very close to the BT.2020 color gamut standard is obtained. Gd0.99Eu0.01-MOF containing only 1% Eu(III) can show bright red luminescence, and in the range of 1-9% Eu(III) content, the characteristic emission intensity of Eu(III) ions and the content show an excellent linear relationship with a slope k as high as 2299. This can be used to identify the content of Eu(III) ions impurities in gadolinium salts from different manufacturers. Eu/Tb-MOF showed highly sensitive and visualized smart photoresponse behaviors to specific antibiotics and amino acids, respectively, with detection limits of 3.2/2.7 nM (tetracycline), 1.7/15.5 nM (oxytetracycline), 0.13/0.97 nM (aspartic acid), and 0.26/1.16 nM (glutamic acid).

Fluorescent distinguishing flavonoid glycosides against aglycones based on the selective recognization of boric acid-functional Eu(III)-organic framework

Flavonoid glycosides are formed by dehydration condensation of aglycones and sugar molecules. Therefore, discrimination of flavonoid glycosides from their corresponding aglycones is a challenging task because they contain the same aglycone part in their molecular structures. Herein, boric acid-functional Eu(III)-organic framework (BA-Eu-MOF) was applied to discriminate flavonoid glycosides including baicalin (Bai), wogonoside (Wog), rutin (Rut), puerarin (Pue), quercitrin (Que) and astragalin (Ast) from their corresponding aglycones for the first time. Besides as organic ligand to sensitize the luminescence of Eu3+ through "antenna" effect, 5-boronobenzene-1,3 dicarboxylic acid provided recognition site for flavonoid glycosides. Infrared, fluorescence, UV-vis, and mass spectra were used to investigate the recognition reaction between BA-Eu-MOF and flavonoid glycosides. The data indicated that the cis-diols of flavonoid glycosides from sugars covalently bonded to boric acid group to form cyclic boronic esters, which quenched the fluorescence of BA-Eu-MOF at 620 nm through decreasing the intersystem efficiency, inner filter effect and photoelectron transfer. In contrast, aglycones could not alter the fluorescence of BA-Eu-MOF because of no covalent bond between them. This probe exhibited high sensitivity towards flavonoid glycosides with the low detection limits of 3.3 nM, 3.5 nM, 33 nM, 56 nM, 5.1 nM and 5.5 nM for Bai, Que, Wog, Ast, Pue and Rut, respectively. The unique recognition ability of boric acid group enables selective and sensitive detection of flavonoid glycosides without the interference of their corresponding aglycones.

Synergistic Aggregation-Induced Emissive Linkers in Metal-Organic Frameworks for Ultrasensitive and Quantitative Visual Sensing

Luminescent metal-organic frameworks (MOFs) represent an emerging class of materials for visual analyte detection. In this study, we present a strategy that integrates two synergistic aggregation-induced emissive (AIE) linkers into a MOF, significantly enhancing sensing sensitivity, selectivity, and quantification capabilities for practical applications. The dual AIE linkers simultaneously optimize porosity and amplify emission intensity. The tailored pore structure precisely matches the molecular dimensions of the pesticide 2,6-dichloro-4-nitroaniline (DCN), while Förster resonance energy transfer between the linkers achieves an exceptional fluorescence quantum yield of 92.6%. This design enables ultrasensitive DCN detection in water, with an unprecedented detection limit at the ppb level, along with superior selectivity, rapid response time, high quantification accuracy, recyclability, and strong resistance to interference. A comprehensive investigation using UV-vis, fluorescence, transient absorption, X-ray photoelectron, and Raman spectroscopies, supported by theoretical calculations, attributes the efficient fluorescence quenching to photoinduced energy transfer. Additionally, we demonstrate instant, naked-eye detection of DCN residues on fruit surfaces and contaminated soil by applying MOF solutions and illuminating under UV light. Quantitative analysis of DCN residues on fruits was further achieved using computer vision and a custom script, providing a practical, on-site method for rapid and precise detection of pesticide residues.

Antenna effect-modulated luminescent lanthanide complexes for biological sensing

With the discovery and further exploitation of the antenna effect, the optical properties of luminescent lanthanide complexes (LLCs) have been greatly improved. Antenna effect-modulated LLCs exhibit long luminescence lifetimes, large Stokes shifts, narrow emission spectra, pure chromaticity, and high photostability. Meanwhile, LLCs have garnered considerable attention in recent years and are widely used as biosensors in the fields of food safety, environmental monitoring, clinical diagnosis, and drug analysis. In this review, we first systematically review the design of antenna effect-modulated LLC sensors, including the construction principle of antenna effect in LLCs and the selection of antenna ligands. Secondly, the classification of antenna ligands was discussed in detail. Thirdly, biological sensing applications of antenna effect-modulated LLCs in the past three years are described in terms of the role of LLCs in fluorescence sensors and electrochemiluminescence sensors. Finally, we also discussed the challenges and emerging opportunities of antenna effect-modulated LLCs in future sensing applications.

Electrically Modulated Fluorescence in Single Rare-Earth Particles

Rare earth (RE)-based complexes, due to their unique electronic structures, exhibit excellent fluorescence properties with high quantum yield and a long lifetime. From an application perspective, exploring RE-based complexes in luminescent optoelectronic devices asks for effective modulation approaches that control the luminescent properties. Here we report an electrically modulated fluorescence phenomenon in an RE-based complex, namely Eu16(μ4-F)6(μ3-F)12(tBuCOO)18[N(CH2CH2O)3]4 (EuFC-16) particles, which effectively controls the optical behavior of individual particles. Frequency-dependent measurements and theoretical analysis reveal a charging mechanism on particles that rationalizes the voltage-modulated fluorescence. This charging-induced fluorescence modulation enables the localized capacitance mapping of individual RE particles at the single-pixel level. Moreover, modulation heterogeneity is observed within a single EuFC-16 particle, highlighting the importance of charge-distribution-controlled optical properties at the subparticle level. Our finding may offer a potential approach for controlling the luminescence of RE-based complexes with spatiotemporal controllability and potential scaling capability, which may enrich potential applications of RE-based electro-optical devices.

Smart Luminescent Materials for Emerging Sensors: Fundamentals and Advances

Smart luminescent materials have drawn a significant attention owing to their unique optical properties and versatility in sensor applications. These materials, encompassing a broad spectrum of organic, inorganic, and hybrid systems including quantum dots, organic dyes, and metal-organic frameworks (MOFs), offer tunable emission characteristics that can be engineered at the molecular or nanoscale level to respond to specific stimuli, such as temperature, pH, and chemical presence. This adaptability makes them crucial in developing advanced sensor technologies for environmental monitoring, biomedical diagnostics, and industrial applications with the help of the luminescence mechanisms, such as fluorescence, phosphorescence, and upconversion. Recent advancements have been driven by the integration of nanotechnology, which enhances the sensitivity and selectivity of luminescent materials in sensor platforms. The development of photoluminescent and electrochemiluminescent sensors, for instance, has enabled real-time detection and quantification of target analytes with high accuracy. Additionally, the incorporation of these materials into portable, user-friendly devices, such as smartphone-based sensors, broadens their applicability and accessibility. Despite their potential, challenges remain in optimizing the stability, efficiency, and biocompatibility of these materials under different conditions. This review provides a comprehensive overview of the fundamental principles of smart luminescent materials, discusses recent innovations in their use for sensor applications, and explores future directions aimed at overcoming current limitations and expanding their capabilities in meeting the growing demand for rapid and cost-effective sensing solutions.

A novel Ln3+/Al3+ metallacrown multifunctional material for latent fingerprint detection, luminescent thermometers and luminescent sensors

Lanthanide luminescent complexes are active and thriving in various research fields due to their unique optical properties, while optical materials across a wide spectral range and with multiple functions in one were rarely reported. In this work, a new class of Ln3+/Al3+ metallacrowns (MCs) were constructed with excellent luminescence properties in both the visible and near-infrared regions, and the elaborate luminescence modulation can be achieved by doping with different Ln3+ ions. Strikingly, the powder of LnMC was developed as a luminescent nanomaterial for the detection of latent fingerprints (LFPs), and even the third level details of fingerprints can be clearly recognized, which provides a reference for the identification of fingerprints in the field of criminal investigation. More importantly, TbMC and Tb0.1Sm0.9MC can be successfully used as luminescent thermometers with sensitivities of 2.51% °C-1 and 2.33% °C-1, respectively, higher than most reported values. Meanwhile, TbMC was developed as a luminescent probe for Fe3+ and 2,6-pyridinedicarboxylic acid (DPA) with low limits of detection (LOD) of 0.51 μM and 4.26 μM, respectively, representing the first example of MC with luminescence sensing. Also of note is that SmMC, Tb0.1Sm0.9MC and TbMC can be functionalized as luminescent inks and films due to their clear recognizable colours in the visible range, suggesting a new strategy for high-level anti-counterfeiting. In short, the LnMC luminescent material has wide application prospects in many fields, especially rare for multifunctional applications of small-molecule complexes with non-metal-organic frameworks.

Lanthanide-Based Metal-Organic Frameworks Offering Hydrogen Bonding Cavities: Luminescent Characteristics and Sensing Applications

This work presents the synthesis and characterization of three isomorphous lanthanide-based metal-organic frameworks (Ln-MOFs) (Ln3+=Eu (1), Tb (2), and Sm (3)) supported by a pyridine-2,6-dicarboxamide-based linker offering appended arylcarboxylate groups. Single crystal X-ray diffraction studies highlight that these Ln-MOFs present three-dimensional porous architectures offering large cavities decorated with hydrogen bonding (H-bonding) groups. These Ln-MOFs display noteworthy luminescent characteristics. The mixed-metal strategy affords a series of Ln-MOFs exhibiting color-tunable emissions. The Eu-MOF was utilized for the nanomolar sensing of both nitrobenzene and 4-nitrophenol. The critical role of H-bonding in detecting these analytes is validated through multiple spectroscopic, ξ potential, and molecular docking studies. The Eu-MOF illustrated notable anticounterfeiting as well as practical sensing applications.

Hydrogen-Bond-Regulated Tb3+-Centered Emission in a ZnII-TbIII Heterometallic Compound for Water Sensing in Ethanol and Gasoline

Luminescent lanthanide compounds stand out for their distinctive characteristics including narrow emission bands, substantial Stokes shifts, high quantum yields, and unique luminescent colors. However, Ln3+ is highly susceptible to vibrational quenching from X-H (X = O/N) high-energy oscillators in the embedded organic antenna, resulting in significant nonradiative energy dissipation of the 5D excited states of Ln3+. Herein, we introduce a strategy based on supramolecular interactions to modulate the nonradiative transitions in a new ZnII-TbIII heterometallic compound, [ZnTb(HL)2(NO3)Cl2]·2CH3CN·H2O (ZnTb), based on a phenyl-substituted pyrazolinone-modified salicylamide-imide ligand (H2L). The regulation mechanisms are explored in detail both experimentally and theoretically. With the N,N'-dimethylformamide (DMF)-boosted Tb3+ luminescence, ZnTb⊃DMF (1 mg of ZnTb + 2 mL of DMF) realizes a rapid (3s) and sensitive detection of water in DMF with a detection limit of 0.021%. Further, ZnTb⊃DMF can detect trace amounts of water in ethanol and ethanol gasoline with a low detection limit of 0.023% and 0.048%. In addition, portable paper strips of ZnTb⊃DMF are prepared to improve its practicability, which can afford real-time and faster (1 s) in situ visual sensing of trace amounts of water in common organic solvents and ethanol gasoline with sensitivity comparable to the titration results. This study provides a new idea for the lanthanide luminescence modulation and application in the field of fluorescence sensing.

Polarity Sensor Based on Multivariate Lanthanide Metal-Organic Framework for Constructing Biosensing Platform

It is significant but challenging to develop polarity sensors that can measure multiscenario polarity in a modular, customized, sensitive, and accurate manner. In this work, we proposed a polarity sensor based on multivariate lanthanide metal-organic framework (Ln-MOF) nanoclusters through the modular programming design of ligands. This multivariate Ln-MOF combines the advantages of modularity, ease of design, high flexibility and low cost, and can be precisely customized for different polarity systems. The MOF Eu0.1Tb0.9-isophthalic acid (IPA) and Eu0.3Tb0.7-o-phthalic acid (OPA) are suitable for the detection of trace water in dimethylsulfoxide (DMSO) and hyaluronidase activity, respectively. Especially, Eu0.3Tb0.7-OPA can achieve high-sensitivity detection of hyaluronidase activity within 8 min, with the limit of detection as low as 0.016 U/L. The results enable us to break through our previous understanding of polarity parameter, allowing us to develop more polarity-related biosensing platforms. Ln-MOFs are believed to utilize their adjustable polar intermolecular interactions to achieve the optimal compatibility and high sensitivity in polarity sensing systems, which is supported by experiments and density functional theory calculations. These polarity sensors based on multivariate Ln-MOF nanoclusters offer significant potential in biosensing and medical diagnostics, overcoming traditional biosensor limitations in synthesis and customization.

Magnetism-Functionalized Lanthanide MOF-on-MOF with Plasmonic Differential Signal Amplification for Ultrasensitive Fluorescence Immunoassays

The successful application of fluorescence immunoassays for clinical diagnosis requires stable photoluminescent materials and highly efficient signal amplification strategies. In this work, the magnetism-functionalized lanthanide MOF-on-MOF (Fe3O4@SiO2@MOF-on-MOF) was synthesized through intermolecular (van der Waals) interaction-assisted growth and further homogeneous epitaxial growth, which significantly improved the fluorescence performances and uncovered the underlying mechanism. The quantum chemical theory calculation and experimental studies revealed that the introduced magnetic Fe3O4@SiO2 not only endowed magnetic separation capability but also promoted fluorescence performances, which increased the energy transfer of the intersystem crossing process and suppressed the luminescence of ligands and aggregation-induced quenching. Furthermore, the plasmonic Ag/Au nanocages were developed as highly efficient fluorescence quenchers to improve the sensitivity of the fluorescence immunoassay. On the basis of the proposed differential signal amplification (DSA) strategy, the immunoassay displayed superior detection ability, with a limit of detection of 0.13 pg·mL-1 for severe acute respiratory syndrome coronavirus 2 nucleocapsid protein. The designed magnetic lanthanide MOF-on-MOF and proposed DSA strategy give new insights into ultrasensitive fluorescence immunoassays.

A Multimode Optical Sensor for Highly Selective and Sensitive Detection of Hypochlorous Acid in Water and Body Fluid

Hypochlorous acid (HClO), as an important reactive oxygen species (ROS), plays a crucial role in our daily life and in biological systems, and its convenient and accurate detection is significant and imperative. In this work, a self-calibrated multimode optical sensor for convenient and accurate HClO detection was elaborately fabricated based on a multifunctional metal-organic framework platform with catalytic active metal nodes, fluorescent responsive bridging ligands, and intrinsic pores for functional molecule accommodation. The sensor shows not only turn-on and ratiometric fluorescence response but also color change in response to HClO. The detection limits are as low as 16.9, 17.3, 66.5, and 63.2 nM for ratiometric fluorometry, absorbance-based colorimetry, and smartphone-based fluorescenceand color analysis, respectively. The accuracy and practicability of this sensor were also demonstrated by the detection of hypochlorous acid in actual water and body fluid samples, and the recovery rates ranged from 97.8 to 103.8%.

Imaging dendrite growth in solid-state sodium batteries using fluorescence tomography technology

Dendrite growth in solid-state sodium batteries (SSBs) is one of the most concerned issues that critically affect the battery efficiency and cycling performance. Here, by designing a fluorescent Eu3+-doped Na3Zr2Si2PO12 solid electrolyte (SE) to facilitate three-dimensional (3D) optical imaging on a confocal laser scanning microscopy, a fluorescence tomography (FT) method is developed for observing the sodium dendrite growth during charge/discharge cycles of the SSBs in a 3D view. It is quantitatively revealed that small-size sodium islands appear after several cycles, and with the cycles increasing, large-size dendrites in tens of micrometers gradually form until a critical sodium dendrite volume arrives where a short circuit or severe performance deterioration occurs. Furthermore, by regulating the Eu3+ doping ratio, a record-high sodium plating/stripping cycling stability for more than 1 year (487.5 days) is achieved at 25°C. This work demonstrates an FT method observing sodium dendrite growth in SSBs and will promote the functional design of high-performance SEs.

Boric acid functionalized Fe3O4@CeO2/Tb-MOF as a luminescent nanozyme for fluorescence detection and degradation of caffeic acid

Caffeic acid (CA) is a natural polyphenol that can have various positive effects on human health. However, its extraction and processing can cause significant ecological issues. Therefore, it is crucial to detect and degrade CA effectively in the environment. In this study, we have developed a multifunctional magnetic luminescent nanozyme, Fe3O4@CeO2/Tb-MOF, which combines peroxidase activity to detect and degrade CA. The fluorescence of the nanozyme was significantly attenuated due to the specific nucleophilic reaction between its boronic acid moiety and the o-diphenol hydroxyl group of CA, energy competition absorption and photo-induced electron transfer (PET) effect. This nanozyme demonstrates a linear detection range from 50 nM to 500 μM and an exceptionally low detection limit of 18.9 nM, along with remarkable selectivity and stability. Moreover, the synergistic catalysis of Fe3O4 and CeO2 within Fe3O4@CeO2/Tb-MOF fosters peroxidase activity, leading to the generation of substantial free radicals catalyzed by H2O2, which ensures the efficient degradation of CA (∼95%). The superparamagnetic property of Fe3O4 further enables the efficient reuse and recycling of the nanozyme. This research provides a novel approach for the concurrent detection and remediation of environmental contaminants.

Eu3+-Doped Mixed-Ligand UiO-66-Type Metal-Organic Framework for Ratiometric Fluorescence Sensing Fluoride Ions with Ultralow Detection Limit

Metal-organic frameworks (MOFs) have emerged as a highly promising platform for various sensing applications due to their tunable structures and functionalities. In the present work, a Eu3+-doped mixed-ligand MOF, namely, the Eu3+@UiO-66-IPA, exhibited excellent luminescent properties and high fluorescence stability in aqueous media, displaying dual-emission peaks under 395 and 615 nm excitation that were readily visible to the naked eye. Importantly, the presence of fluoride ions (F-) promoted the "antenna effect" between the ligand and the Eu3+ centers, which significantly enhanced the emission intensity of the Eu3+ characteristic peak. In addition, the addition of F- also inhibited the quenching effect of high-energy O-H bonds existing in the aqueous environment. Notably, Eu3+@UiO-66-IPA demonstrated exceptional selectivity for F- over a range of competing anions, with a remarkable limit of detection as low as 0.22 μM. The developed Eu3+-doped mixed-ligand MOF system offers a highly promising strategy for the simple and accurate sensing of F- in practical applications.

Leap-Type Response of Redox/Photo-Active Lanthanide-Based Metal-Organic Frameworks for Early and Accurate Screening of Prostate Cancer

The development of high-accuracy technologies to distinguish the quite tiny concentration change of tumor markers between negative and positive is of vital significance for early screening and diagnosis of cancers, but is still a great challenge for the conventional biosensors because of their "gradual" detection mode. Herein, a unique "leap-type" responsive lanthanide MOF-based biosensor (designated as Tb-CeMOF-X) with defect-mediated redox-/photo-activities is developed for precisely identifying acid phosphatase (ACP), an early pathological marker of prostate cancer (PCa) in serum. The engineered Tb-CeMOF-X probe achieves a bursting switch-on luminescence at the critical concentration of ACP (9 U ⋅ L-1), while keeping silent below this threshold, undergoing a qualitative signal change from "zero" to "one" between negative and positive indicators and thus significantly improving the identification precision. Significantly, such "leap-type" response performance can be further edited and amplified by rational defect engineering in the crystal structure to improve the accessibility of active centers, consequently maximizing the detection sensitivity toward ACP in the complex biological media. This study proposes the first paradigm for the development of "leap-type" biosensors with ultra-sensitive differentiation capability between negative and positive, and provides a potentially valuable tool for early and accurate screening of PCa.

Two Stable Pillar-Layered Zn-LMOFs for Highly Fluorescence Sensing of Inorganic Pollutants and Nitro Aromatic Compounds in Water

Luminescent metal-organic frameworks (LMOFs) are a potential class of functional materials for the photoluminescent detection of a wide range of analytes as well as for the detection of pollutants in wastewater. Herein, by using the pillar-layered strategy, two new luminescence Zn-LMOFs (JLU-MOF222 and JLU-MOF223) were successfully solvothermal synthesized. The 2D layers are both consisting of Zn2+ and TPHC [TPHC = (1,1':2',1″-terphenyl)-3,3″,4,4',4″,5'-hexacarboxylic acid] ligands and then pillared by the different N-donor ligands to form the 3D Zn-LMOFs with fsh topology. Benefiting from the uncoordinated carboxylate sites, uncoordinated N atom, or -NH2 group in the pillaring ligands and excellent stability in pH = 2-13 aqueous phase, JLU-MOF222 and JLU-MOF223 not only can sensitively detect trace amounts of inorganic pollutants (Fe3+, Cr2O72-) and nitro aromatic compounds TNP and 2,4-DNP (TNP = 2,4,6- trinitrophenol, 2,4-DNP = 2,4-dinitrophenol) through luminescence quenching but also exhibit high selectivity of other anti-interference competing analytes. The two new Zn-LMOFs can be used as potential luminescent sensors for pollutant detection in water due to their high KSV and low limit of detection (LOD).

Stepwise-Induced Synthesis and Excellent Corrosion Protection of Ce/Eu Codoped ZnO Solid Solution Materials

Under purely inorganic conditions, a synthesis route was devised wherein elements were introduced stepwise via coprecipitation based on differences in compound solubility. This synthesis method can change the microscopic morphology of the material without relying on a templating agent, resulting in the formation of the multilayered lamellar Ce/Eu codoped zinc oxide solid solution (ZCEOSS) with a self-assembled nested imbrication structure. The study improves the critical matter of corrosion by focusing on the electron and energy transfer mechanisms. By introduction of the bandgap modulator cerium element and fluorescence enhancer europium element into the ZnO material, the anticorrosion material has been successfully endowed with both photocathodic protection and luminescent initiative/stress dual corrosion defense functions. Due to the energy level staircase protection mechanism synergistically generated by the 4f electron shell of rare-earth elements in concert with semiconductor zinc oxide, the energy band positions were modulated to progressively guide the direction of electron flow, thereby suppressing corrosion reactions. In particular, the ZCEOSS material synthesized by doping 1% cerium and 7% europium and adding rare-earth elements at pH 7 exhibited the best corrosion inhibition performance. After immersion in simulated seawater for 96 h, Tafel polarization test results showed that compared to epoxy resin and ZnO anticorrosion systems, the ZCEOSS anticorrosion system exhibited significantly improved corrosion inhibition efficiency with enhancements of 1028.3 and 402.9%, respectively. This study provides new insights into the development of highly efficient inorganic anticorrosion materials.

Flexible Metal-Organic Frameworks: From Local Structural Design to Functional Realization

ConspectusFlexible metal-organic frameworks (MOFs), also known as soft porous crystals, exhibit dynamic behaviors in response to external physical and chemical stimuli such as light, heat, electric or magnetic field, or the presence of particular matters, on the premise of maintaining their crystalline state. The reversible structural transformation of flexible MOFs, a unique characteristic seldomly found in other types of known solid-state materials, affords them distinct properties in the realms of molecule separation, optoelectronic devices, chemical sensing, information storage, biomedicine applications, and so on. The mechanisms underlying their dynamic behaviors can be comprehensively investigated at the molecular level by means of in situ single-crystal or powder X-ray diffraction as well as other in situ spectroscopic techniques due to the high regularity of these crystalline materials during stimuli-responsive phase transitions. Through the introduction of specific stimuli-responsive groups/moieties into the well-defined and ordered molecular arrays, targeted applications can be achieved, and the performance of flexible MOFs can also be further improved via rational structural design.In this Account, we summarize our progress on the design, synthesis, and applications of flexible MOFs over the past few years. First, we highlight the construction principle of flexible MOFs, emphasizing the pivotal role of local structural design. Using an F-modified ligand, a flexible MOF with remarkable structural transformations can be obtained; the regulation of the metal coordination environment and interpenetrating frameworks is also crucial for achieving flexible MOFs. We also propose a strong correlation strategy based on the supramolecular interactions between the guest molecules and the framework, which realizes the temperature-responsive dynamic spatial "open-closed" regulation. Mechanisms of the dynamic behaviors investigated by the in situ techniques were also presented for the obtained materials. Second, some representative specific applications of the newly developed dynamic coordination systems were reviewed. The gas molecule responsive flexible MOFs show efficient short-chain alkane separation properties with discriminatory sorption behavior toward similar gaseous substrates. Smart sensing of temperature, pressure, and volatile organic compounds was achieved by several novel flexible fluorescent MOFs, with optimization potential through state-of-the-art chemical design. Furthermore, multiferroic materials with multiple bistable states and high working temperatures were also obtained based on flexible MOFs.Finally, we provide a discussion of the challenges of flexible MOFs in future research, including precise and efficient synthesis, in-depth structure-property relationship investigation, performance optimization, and industrialization. We hope that this Account will stimulate further research interest in developing next-generation smart materials based on flexible MOFs for applications in challenging chemical separation, extreme environmental sensing, massive information storage, and beyond.

Manipulating AIE ligands into layers of pillar-layered MOFs for enhanced emission

Four pillar-layered AIEgen-based MOFs exhibit higher thermal stability, tunable emission colors and improved QYs compared with that of non-pillar-layered AIEgen-based MOFs by confining the AIE ligands into layers. These results reveal that rationally manipulating AIE ligands into layers of pillar-layered MOFs is an effective strategy for the design and construction of tunable luminescent MOF systems.

Vibration-Dependent Dual-Phosphorescent Cu4 Nanocluster with Remarkable Piezochromic Behavior

The dual emission (DE) characteristics of atomically precise copper nanoclusters (Cu NCs) are of significant theoretical and practical interest. Despite this, the underlying mechanism driving DE in Cu NCs remains elusive, primarily due to the complexities of excited state processes. Herein, a novel [Cu4(PPh3)4(C≡C-p-NH2C6H4)3]PF6 (Cu4) NC, shielded by alkynyl and exhibiting DE, was synthesized. Hydrostatic pressure was applied to Cu4, for the first time, to investigate the mechanism of DE. With increasing pressure, the higher-energy emission peak of Cu4 gradually disappeared, leaving the lower-energy emission peak as the dominant emission. Additionally, the Cu4 crystal exhibited notable piezochromism transitioning from cyan to orange. Angle-dispersive synchrotron X-ray diffraction results revealed that the reduced inter-cluster distances under pressure brought the peripheral ligands closer, leading to the formation of new C-H⋅⋅⋅N and N-H⋅⋅⋅N hydrogen bonds in Cu4. It is proposed that these strengthened hydrogen bond interactions limit the ligands' vibration, resulting in the vanishing of the higher-energy peak. In situ high-pressure Raman and vibrationally resolved emission spectra demonstrated that the benzene ring C=C stretching vibration is the structural source of the DE in Cu4.

A Multimode Optical Sensor for Selective and Sensitive Detection of Harmful Heavy Metal Cr(VI) in Fresh Water and Sea Water

Water pollution originating from heavy metals has shown great impacts on the ecological environment and human health due to their extremely low biodegradability. Hexavalent chromium Cr(VI), as one harmful heavy metal with strong oxidation, high biological permeability, and high carcinogenicity, is becoming an increasingly serious threat to human health. Therefore, conveniently but accurately, monitoring the Cr(VI) level in water to maintain its normal level and ensuring the stability of the ecosystem and human health become very valuable. However, most of these heavy metal sensors reported are turn-off type single-emission sensors. In this work, a ratiometric fluorescence/colorimetry/smartphone triple-mode turn-on optical sensor for Cr(VI) was developed based on a multifunctional metal-organic framework platform. The detection limits for these three mutual verification modes were only 1.28, 4.89, and 68.4 nM, respectively. Additionally, the color changes of the detection system under sunlight can also be observed directly by the naked eye. The accuracy and practicability of this multimode sensor were further proved by the detection of Cr(VI) in actual water and seawater samples, and the recovery rate ranged from 97.308 to 104.041%.

Recent advancements in modifications of metal-organic frameworks-based materials for enhanced water purification and contaminant detection

Metal-organic frameworks (MOFs) have emerged as a key focus in water treatment and monitoring due to their unique structural features, including extensive surface area, customizable porosity, reversible adsorption, and high catalytic efficiency. While numerous reviews have discussed MOFs in environmental remediation, this review specifically addresses recent advancements in modifying MOFs to enhance their effectiveness in water purification and monitoring. It underscores their roles as adsorbents, photocatalysts, and in luminescent and electrochemical sensing. Advancements such as pore modification, defect engineering, and functionalization, combined synergistically with advanced materials, have led to the development of recyclable MOF-based nano-adsorbents, Z-scheme photocatalytic systems, nanocomposites, and hybrid materials. These innovations have broadened the spectrum of removable contaminants and improved material recyclability. Additionally, this review delves into the creation of multifunctional MOF materials, the development of robust MOF variants, and the simplification of synthesis methods, marking significant progress in MOF sensor technology. Furthermore, the review addresses current challenges in this field and proposes potential future research directions and practical applications. The growing research interest in MOFs underscores the need for an updated synthesis of knowledge in this area, focusing on both current challenges and future opportunities in water remediation.

Construction of a Zn(II)-Eu(III) Nanoring with Temperature-Dependent Luminescence for the Qualitative and Quantitative Detection of Neopterin as an Inflammatory Marker

A nine-metal Zn(II)-Eu(III) nanoring 1 with a diameter of about 2.3 nm was constructed by the use of a long-chain Schiff base ligand. It shows a luminescence response to neopterin (Neo) through the enhancement of lanthanide emission with high selectivity and sensitivity, which can be used to quantitatively analyze the concentrations of Neo in fetal calf serum and urine. The luminescence sensing of 1 to Neo is temperature-dependent, and it displays more obvious response behavior at lower temperatures. Filter paper strips bearing 1 can be used to qualitatively detect Neo by the color change from chartreuse to red under a UV lamp. The limit of detection is as low as 3.77 × 10-2 nM.

Aggregation induced emission dynamic chiral europium(III) complexes with excellent circularly polarized luminescence and smart sensors

The synthesis of dynamic chiral lanthanide complex emitters has always been difficult. Herein, we report three pairs of dynamic chiral EuIII complex emitters (R/S-Eu-R-1, R = Et/Me; R/S-Eu-Et-2) with aggregation-induced emission. In the molecular state, these EuIII complexes have almost no obvious emission, while in the aggregate state, they greatly enhance the EuIII emission through restriction of intramolecular rotation and restriction of intramolecular vibration. The asymmetry factor and the circularly polarized luminescence brightness are as high as 0.64 (5D0 → 7F1) and 2429 M-1cm-1 of R-Eu-Et-1, achieving a rare double improvement. R-Eu-Et-1/2 exhibit excellent sensing properties for low concentrations of CuII ions, and their detection limits are as low as 2.55 and 4.44 nM, respectively. Dynamic EuIII complexes are constructed by using chiral ligands with rotor structures or vibration units, an approach that opens a door for the construction of dynamic chiral luminescent materials.

Reversible single-crystal-to-single-crystal transition in Gd(III) metal-organic frameworks induced by heat and solvents with a significant magnetocaloric effect

The design and synthesis of a Gd(III) metal-organic framework with the formula [Gd4(BTDI)3(DMF)4]n (JXUST-40, H4BTDI = 5,5'-(benzo[c][1,2,5]thiadiazole-4,7-diyl)diisophthalic acid) are reported hererin. Interestingly, a reversible single-crystal-to-single-crystal transition between JXUST-40 and {[Gd4(BTDI)3(H2O)4]·6H2O}n (JXUST-40a) was achieved under the stimulation of heat and solvents. Both JXUST-40 and JXUST-40a exhibited good stability when soaked in common solvents and aqueous solutions with pH values of 1-12. Magnetic studies showed that JXUST-40a has a larger magnetocaloric effect with -ΔSmaxm = 26.65 J kg-1 K-1 at 2 K and 7 T than JXUST-40 due to its larger magnetic density. Structural analyses indicated that the coordinated solvent molecules play a crucial role in the coordination environment around the Gd(III) ions and the change in the framework, ultimately leading to the changes in the pore size and magnetism between JXUST-40 and JXUST-40a. In addition, both isomorphic [Dy4(BTDI)3(DMF)4]n (JXUST-41) and {[Dy4(BTDI)3(H2O)4]·6H2O}n (JXUST-41a) displayed slow magnetic relaxation behaviour.

Multi-stimuli-responsive luminescence enabled by crown ether anchored chiral antimony halide phosphors

Stimuli-responsive optical materials have provided a powerful impetus for the development of intelligent optoelectronic devices. The family of organic-inorganic hybrid metal halides, distinguished by their structural diversity, presents a prospective platform for the advancement of stimuli-responsive optical materials. Here, we have employed a crown ether to anchor the A-site cation of a chiral antimony halide, enabling convenient control and modulation of its photophysical properties. The chirality-dependent asymmetric lattice distortion of inorganic skeletons assisted by a crown ether promotes the formation of self-trapped excitons (STEs), leading to a high photoluminescence quantum yield of over 85%, concomitant with the effective circularly polarized luminescence. The antimony halide enantiomers showcase highly sensitive stimuli-responsive luminescent behaviours towards excitation wavelength and temperature simultaneously, exhibiting a versatile reversible colour switching capability from blue to white and further to orange. In situ temperature-dependent luminescence spectra, time-resolved luminescence spectra and theoretical calculations reveal that the multi-stimuli-responsive luminescent behaviours stem from distinct STEs within zero-dimensional lattices. By virtue of the inherent flexibility and adaptability, these chiral antimony chlorides have promising prospects for future applications in cutting-edge fields such as multifunctional illumination technologies and intelligent sensing devices.

Construction of Multifunctional Luminescent Lanthanide MOFs for Luminescent Sensing of Temperature, Trifluoroacetic Acid Vapor and Explosives

Metal-organic frameworks (MOFs) with multifunctional and tunable optical properties have unique advantages in the field of sensing, and the structure and properties of MOFs are significantly influenced by the ligands. In this study, a Y-type tricarboxylic acid ligand containing amide bonds was synthesized through functional guidance, and three isomorphic and heterogeneous three-dimensional MOFs (Eu-MOF, Tb-MOF, and Gd-MOF) were obtained by solvothermal reaction. Further studies revealed that both the Tb-MOF and Eu-MOF could selectively detect picric acid (PA). The luminescence quenching of the two MOFs by PA was attributed to competing absorption and photoelectron energy transfer mechanisms. In addition, due to the energy transfer between Tb and Rhodamine B, Rhodamine B was encapsulated into Tb-MOF. The obtained material exhibited a linear relationship between the temperature parameters I544/I584 and temperature within the range of 280-400 K, the correlation coefficient (R2) reached an impressive value of 0.999, and the absolute sensitivity of the sample used for temperature sensing was 1.534% K-1. What is more, the material exhibited a good response to trifluoroacetic acid vapor, which suggests the potential of the material for temperature sensing and detection of trifluoroacetic acid vapor. The designed and investigated strategy can also serve as a reference for further research on excellent multifunctional sensors.

Fluorescence sensing and device fabrication with luminescent metal-organic frameworks

Metal-organic frameworks (MOFs) are a novel class of hybrid porous multi-functional materials consisting of metal ions/clusters and organic ligands. MOFs have exclusive benefits due to their tunable structure and diverse properties. Luminescent MOFs (LMOFs) exhibit both porosity and light emission. They display abundant host and guest responses, making them conducive to sensing. Currently, LMOF sensing research is gaining more depth, with attention given to their device and practical applications. This work reviews recent advancements and device applications of LMOFs as chemical sensors toward ions, volatile organic compounds, biomolecules, and environmental toxins. Furthermore, the detection mechanism and the correlation between material properties and structure are elaborated. This analysis serves as a valuable reference for the preparation and efficient application of targeted LMOFs.

Excitation Wavelength-Dependent Fluorescence of a Lanthanide Organic Metal Halide Cluster for Anti-Counterfeiting Applications

The achievement of significant photoluminescence (PL) in lanthanide ions (Ln3+ ) has primarily relied on host sensitization, where energy is transferred from the excited host material to the Ln3+ ions. However, this luminous mechanism involves only one optical antenna, namely the host material, which limits the accessibility of excitation wavelength-dependent (Ex-De) PL. Consequently, the wider application of Ln3+ ions in light-emitting devices is hindered. In this study, we present an organic-inorganic compound, (DMA)4 LnCl7 (DMA+ =[CH3 NH2 CH3 ]+ , Ln3+ =Ce3+ , Tb3+ ), which serves as an independent host lattice material for efficient Ex-De emission by doping it with trivalent antimony (Sb3+ ). The pristine (DMA)4 LnCl7 compounds exhibit high luminescence, maintaining the characteristic sharp emission bands of Ln3+ and demonstrating a high PL quantum yield of 90-100 %. Upon Sb3+ doping, the compound exhibits noticeable Ex-De emission with switchable colors. Through a detailed spectral study, we observe that the prominent energy transfer process observed in traditional host-sensitized systems is absent in these materials. Instead, they exhibit two independent emission centers from Ln3+ and Sb3+ , each displaying distinct features in luminous color and radiative lifetime. These findings open up new possibilities for designing Ex-De emitters based on Ln3+ ions.

Boosted ability of ZIF-8 for early-stage adsorption and degradation of chemical warfare agent simulants

Efficient adsorption of hazardous substances from the environment is crucial owing to the considerable risks they pose to both humans and ecosystems. Consequently, the development of porous materials with strong adsorption capabilities for hazardous substances, such as chemical warfare agents (CWAs), is pivotal for safeguarding human lives. Specifically, the early-stage adsorption proficiency of the adsorbents plays a vital role in determining their effectiveness as ideal adsorbents. Herein, we report the efficient adsorption of CWA simulants using thermally treated ZIF-8 (T-ZIF-8). The T-ZIF-8 samples were prepared by subjecting ZIF-8 to a simple thermal treatment, which resulted in a more positive surface charge with extra open metal sites. Although the pore volume of T-ZIF-8 decreased after thermal treatment, the positive surface charge of T-ZIF-8 proved advantageous for the adsorption of the CWA simulants. As a result, the adsorption capacity of T-ZIF-8 for the CWA simulants improved compared to that of pure ZIF-8. Notably, T-ZIF-8 exhibited a remarkably enhanced adsorption ability in the early stage of exposure to the CWA simulants, possibly due to the effective polar interactions between T-ZIF-8 and the simulants via the electron-rich components within the CWA simulants. Moreover, the enhanced adsorption capacity of T-ZIF-8 led to the fast degradation of simulant compared to pure ZIF-8. T-ZIF-8 also demonstrated excellent stability over three adsorption cycles. These findings highlight that T-ZIF-8 is an outstanding material for the early-stage adsorption and degradation of CWA simulants, offering high effectiveness and stability.

AIE Ligand-Based Luminescent Ln-MOFs for Rapid and Selective Sensing of Tetracycline

The design of highly stable and dual-emission lanthanide metal-organic frameworks (Ln-MOFs) is promising for practical chemical sensor applications. Rational design and synthesis of photoresponsive organic ligands provide a feasible approach to achieving highly fluorescent dual-emission Ln-MOFs. In this study, a tetraphenylpyrazine-based AIE ligand, H4L, was synthesized and combined with lanthanide ions (including Sm3+, Eu3+, Gd3+, and Tb3+) to fabricate a series of Ln-MOFs named Ln-L. The single-crystal analysis revealed that all Ln-L belonged to the tetragonal space group P4212 and featured a 2-fold interpenetrated 3D structure. Leveraging rational design, Eu-L exhibited a sensitive response to tetracycline, making it a promising fluorescence sensor for tetracycline detection. The experiments demonstrated that Eu-L could rapidly and quantitatively detect tetracycline and its analogs within 30 s. The lowest detection limits for tetracycline, oxytetracycline, and chlortetracycline were 0.43, 0.92, and 0.81 μM, respectively. Additionally, the probe displayed excellent reusability and exceptional selectivity. A plausible sensing mechanism was proposed, supported by both experimental and theoretical analyses. Furthermore, the study discovered that on-site and real-time determination of TCs in aqueous solutions could be achieved by using luminescence test papers and composite films derived from Eu-L.

Tetranuclear Cluster-Based Eu(III)-Metal-Organic Framework: Ratiometric Platform Design and Ultrasensitive Phenylglyoxylic Acid Detection

Phenylglyoxalic acid (PGA) is a typical metabolite produced by the invasion of styrene into the human body. The detection of PGA can not only reflect the health status of the human body but also assess the level of styrene contamination in the environment. Herein, a novel Eu(III)-MOF (Eu-ttpd) with excellent fluorescence properties was designed by employing the tetrazole-based ligand of 5-((4'-(tetrazol-5'-yl)benzyl)oxy) isophthalic acid (H2ttpd), which successfully used a fluorescent sensor for PGA. The as-synthesized Eu-ttpd features the unique 10-connected tetranuclear cluster [Eu4(μ3-O)2(COO)8]4+ and exhibits a novel (3,10)-connected topological. Benefiting from the perfectly matched excited-state energy levels of the employed H2ttpd ligand with PGA, rapid photoinduced electron transfer (PET) and Dexter-ET can occur, which entitle Eu-ttpd a fast fluorescence quenching response to PGA with a remarkable LOD of 0.269 μM. More importantly, by integrating Eu-ttpd and Mg,N-CDs into the polyacrylamide hydrogel, we optimized Eu-ttpd into a hydrogel sensor which exhibited enhanced detection ability (LOD = 0.052 μM) accompanied by a distinguished color transformation (red-to-blue) and realized ultrasensitive and visual detection of PGA. This work offers an indication for the development of smart sensing materials for human health and environmental safety.

Lanthanide Metal-Organic Framework Isomers with Novel Water-Boosting Lanthanide Luminescence Behaviors

Lanthanide metal-organic frameworks (Ln-MOFs) with exceptional optical performance and structural diversity offer a unique platform for the development of luminescent materials. However, Ln-MOFs often suffer from luminescence quenching by high-vibrating oscillators, especially in aqueous solution. Thus, multiple strategies have been adopted to improve the luminescence of Ln3+. Anomalous research about water-induced lanthanide luminescence enhancement of Ln-MOFs is in the primary stage. Here, two Eu-based metal-organic framework (Eu-MOF) isomers named QXBA-Eu-1 and QXBA-Eu-2 were constructed by using the same ligand under different solvent thermal conditions, which exhibited distinctive water- and methanol-boosting emission behaviors. As for QXBA-Eu-1, water and methanol molecules replaced the free N,N-dimethylacetamide (DMA) molecules in the framework, repressed the rotation or libration suppression of the QXBA linker, and formed hydrogen bonds with the coordinated water molecules, which suppressed the O-H high-energy vibrations, reduced nonradiative transitions, stabilized the T1 state, and facilitated the intersystem crossing (ISC) process. For QXBA-Eu-2, water molecules tended to replace the coordinated DMA ligands, which altered the S1 and T1 energy levels of the ligand and facilitated the ligand-to-metal energy transfer (LMET) process and strengthened the luminescence of Eu3+. Importantly, free solvent molecules and the hydroxylation of Eu3+ centers also restrained the rotation or libration of the QXBA linker, by which the nonradiative transition was further inhibited and the lanthanide luminescence enhanced. Thus, this work not only opened an unprecedented path to enhance lanthanide luminescence in aqueous solution but also expanded its application scope.

Highly Luminescent Lanthanide Metal-Organic Frameworks with Tunable Color for Nanomolar Detection of Iron(III), Ofloxacin and Gossypol and Anti-counterfeiting Applications

Solvothermal reaction of 5,5'-(pyridine-2,6-diylbis(oxy))diisophthalic acid (H4 L) with europium(III) or terbium(III) nitrates in acetonitrile-water (1 : 1) at 120 °C gave rise to isostructural 2D coordination polymers, [Ln(HL)(H2 O)3 ]∞ (NIIC-1-Eu and NIIC-1-Tb), the layers of which are composed by eight-coordinated lanthanide(III) ions interconnected by triply deprotonated ligands HL3- . The layers are packed in the crystal without any specific intermolecular interactions between them, allowing the facile preparation of stable water suspensions, in which NIIC-1-Tb exhibited top-performing sensing properties through luminescence quenching effect with exceptionally low detection limits towards Fe3+ (LOD 8.62 nM), ofloxacin (OFX) antibiotic (LOD 3.91 nM) and cotton phytotoxicant gossypol (LOD 2.27 nM). In addition to low detection limit and high selectivity, NIIC-1-Tb features fast sensing response (within 60-90 seconds), making it superior to other MOF-based sensors for metal cations and organic toxicants. The photoluminescence quantum yield of NIIC-1-Tb was 93 %, one of the highest among lanthanide MOFs. Mixed-metal coordination polymers NIIC-1-Eux Tb1-x demonstrated efficient photoluminescence, the color of which could be modulated by the excitation wavelength and time delay for emission monitoring (within 1 millisecond). Furthermore, an original 2D QR-coding scheme was designed for anti-counterfeiting labeling of goods based on unique and tunable emission spectra of NIIC-1-Ln coordination polymers.

Stable Europium(III) Metal-Organic Framework Demonstrating High Proton Conductivity and Fluorescence Detection of Tetracyclines

A europium(III) metal-organic framework (MOF), namely, {[[(CH₃)₂NH₂]₃Eu₂(DTTP-2OH)₂(HCOO)(H₂O)]·4H₂O}_n (Eu-MOF, H₄DTTP-2OH = 2',5'-dihydroxy-[1,1':4',1″-terphenyl]-3,3″,5,5″-tetracarboxylic acid) has been assembled through solvothermal method. The Eu-MOF is a three-dimensional (3D) (4,4,8)-connected topological framework with binuclear Eu(III) clusters as secondary building units, in which a richly ordered hydrogen bonding network formed among the free H₂O molecules, dimethylamine cations, and phenolic hydroxyl groups provides a potential pathway for proton conduction. The proton conductivity reaches the category of superionic conductors (σ > 10^-4 S cm^-1) at room temperature with a maximum conductivity of 1.91 × 10^-3 S cm^-1 at 60 °C and 98% RH. Moreover, it also can be used as a fluorescence sensor in aqueous solution with detection limits of 0.14 μM for tetracycline, 0.13 μM for oxytetracycline and 0.11 μM for doxycycline. These results pave new methods for constructing MOFs with high proton conductivity and responsive fluorescence.