Novel Approach for Detecting Vapors of Acids and Bases with Proton-Transfer Luminescent Dyes Encapsulated within Metal-Organic Frameworks

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.

Highly Conjugated Imine-Linked Donor-Acceptor Covalent Organic Framework for Efficient HCl Sensing and Photocatalytic Oxidation of Benzylamine

Covalent organic frameworks (COFs) show potential as photocatalysts for harnessing solar energy, primarily because of their adjustable band gap, which enhances their ability to harvest light energy. However, their efficiency as photocatalysts is frequently hampered by inadequate charge transfer and fast charge recombination. In this work, we integrated a donor-acceptor pair into the 2D COF material to enhance charge transfer and reduce charge recombination, thereby improving the overall photoactivity. The 2D COF (OML-4) was synthesized through the polycondensation of a 4',4‴,4‴″,4‴‴'-(ethene-1,1,2,2-tetrayl)tetrakis(([1,1'-biphenyl]-4-carbaldehyde)) (ETBC) linker serving as the donor and a 4,4',4″-(1,3,5-triazine-2,4,6-triyl)trianiline (TAPT) linker acting as the acceptor. This imine-linked 2D COF (OML-4) material shows photocatalytic activity toward benzylic amine oxidation in the presence of visible light for the formation of aldehyde molecules in an aqueous medium. This COF was also investigated for HCl gas sensing behavior based on the protonation of the imine linkage and triazine unit present in the COF material. This exhibits exceptionally good sensitivity up to the ppm level and naked eye detection of HCl by changing in color from yellow to red and turning back to yellow in ammonia vapor, showing the reversible and recyclable nature of the COF.

Taking Advantage of a Luminescent ESIPT-Based Zr-MOF for Fluorochromic Detection of Multiple External Stimuli: Acid and Base Vapors, Mechanical Compression, and Temperature

Luminescent materials responsive to external stimuli have captivated great attention owing to their potential implementation in noninvasive photonic sensors. Luminescent metal-organic frameworks (LMOFs), a type of porous crystalline material, have emerged as one of the most promising candidates for these applications. Moreover, LMOFs constructed with organic linkers that undergo excited-state intramolecular proton-transfer (ESIPT) reactions are particularly relevant since changes in the surrounding environment induce modifications in their emission properties. Herein, an ESIPT-based LMOF, UiO-66-(OH)2, has been synthesized, spectroscopically and photodynamically characterized, and tested for detecting multiple external stimuli. First, the spectroscopic and photodynamic characterization of the organic linker (2,5-dihydroxyterephthalic acid (DHT)) and the UiO-66-(OH)2 MOF demonstrates that the emission properties are mainly governed by the enol → keto tautomerization, occurring in the organic linker via the ESIPT reaction. Afterward, the UiO-66-(OH)2 MOF proves for the first time to be a promising candidate to detect vapors of acid (HCl) and base (Et3N) toxic chemicals, changes in the mechanical compression (exercised pressure), and changes in the temperature. These results shed light on the potential of ESIPT-based LMOFs to be implemented in the development of advanced optical materials and luminescent sensors.

A Ratiometric Fluorescent Probe Based on RhB Functionalized Tb-MOFs for the Continuous Visual Detection of Fe3+ and AA

In this study, a red-green dual-emitting fluorescent composite (RhB@MOFs) was constructed by introducing the red-emitting organic fluorescent dye rhodamine B (RhB) into metal-organic frameworks (Tb-MOFs). The sample can be used as a ratiometric fluorescent probe, which not only avoids errors caused by instrument and environmental instability but also has multiple applications in detection. The results indicated that the RhB@MOFs exhibited a turned-off response toward Fe3+ and a turned-on response for the continuous detection of ascorbic acid (AA). This ratiometric fluorescent probe possessed high sensitivity and excellent selectivity in the continuous determination of Fe3+ and AA. It is worth mentioning that remarkable fluorescence change could be clearly observed by the naked eye under a UV lamp, which is more convenient in applications. In addition, the mechanisms of Fe3+- and AA-induced fluorescence quench and recovery are discussed in detail. This ratiometric probe displayed outstanding recognition of heavy metal ions and biomolecules, providing potential applications for water quality monitoring and biomolecule determination.

Biomimetic Metal-Organic Framework-Based Photonic Crystal Sensor for Highly Sensitive Visual Detection and Effective Discrimination of Benzene Vapor

Due to the large specific surface area and continuous pores in structures, metal-organic frameworks (MOFs) show great advantages in the adsorption of volatile organic compounds (VOCs). Photonic crystal (PC) sensors derived from MOFs are promising for the visual detection of VOC gases. However, they still have problems of low sensitivity and poor color saturation and tunability. Here, inspired by vapor-sensitive scales of Tmesisternus isabellae beetle and scattering light absorption of polydopamine, a porous one-dimensional PC sensor is constructed by combining ZIF-8 with TiO₂@PDA nanoparticles. The PC sensor shows significant color changes under different concentrations of benzene vapor and reaches a detection limit of 0.8 g/m³. It has a response time of less than 1 s and maintains stable optical performance after 100 times of reuse. Moreover, ZIF-67 and ZIF-7 are both incorporated into the PCs for comparison; it reveals that ZIF-8 shows superior benzene detecting property. Additionally, the synergistic adsorption of VOCs in inner and outer holes of the ZIF-8 layer is demonstrated by real-time mass monitoring with quartz crystal microbalance with dissipation. This study provides a valuable reference for the fabrication of high-quality MOF-based PC sensors and sensing mechanism study between microscopic molecular adsorption and macroscopic performance.

A Ratiometric Fluorescent Sensor Based on Dye/Tb (III) Functionalized UiO-66 for Highly Sensitive Detection of TDGA

Thiodiglycolic acid (TDGA) is a biomarker for monitoring vinyl chloride exposure. Exploring a facile, rapid and precise analysis technology to quantify TDGA is of great significance. In this research, we demonstrate a fluorescent sensor based on dual-emissive UiO-66 for TDGA detection. This ratiometric fluorescent material named C460@Tb-UiO-66-(COOH)₂ was designed and synthesized by introducing organic dye 7-diethylamino-4-methylcoumarin (C460) and Tb³⁺ into UiO-66-(COOH)₂. The as-obtained C460@Tb-UiO-66-(COOH)₂ samples showed highly selective recognition, excellent anti-interference and rapid response characteristics for the recognition of TDGA. The detection limit is 0.518 mg·mL^-1, which is much lower than the threshold of 20 mg·mL^-1 for a healthy person. In addition, the mechanism of TDGA-induced fluorescence quenching is discussed in detail. This sensor is expected to detect TDGA content in human urine.