High-Performance Turn-On Fluorescent Metal-Organic Framework for Detecting Trace Water in Organic Solvents Based on the Excited-State Intramolecular Proton Transfer Mechanism

Water-Responsive Fluorescence and Room-Temperature Phosphorescence Carbon Dots for Trace Water Detection in Ethylene Glycol and Multimodal Anticounterfeiting

Identifying trace water in ethylene glycol is essential for maintaining stringent quality control in chemical processes and ensuring product purity. However, the development of highly sensitive detection methods for aqueous impurities within this viscous solvent presents significant challenges, primarily arising from the strong intermolecular hydrogen bonding network within ethylene glycol, which not only masks the presence of water but also interferes with conventional analytical techniques. This work introduces a novel fluorescence-based detection method that combines simplicity, efficiency, and rapid response by leveraging water-responsive carbon dots (CDs). Specifically, we synthesized water-responsive carbon dots (WCDs) that exhibit enhanced fluorescence in anhydrous ethylene glycol. Notably, the introduction of water induces a concentration-dependent fluorescence enhancement at 394 nm, establishing a linear correlation within the 0-0.284% (v/v) water content range (detection limit: 0.017%, 3σ/S) with a remarkably low. By exploiting the hydrogen bonding between WCDs and cellulose paper matrices, the triplet excited states are effectively stabilized, thereby enabling green room-temperature phosphorescence (RTP) emission. This enables precise modulation of the WCDs' aggregation-dispersion states through controlled water/ethanol addition, a mechanism that drives stimulus-responsive transitions between fluorescence and RTP. These tunable optical properties not only validate the detection mechanism but also create new opportunities for developing dual-mode dynamic anticounterfeiting technologies.

Dual-mode luminescence and colorimetric sensing for Al3+ and Fe2+/Fe3+ ions in water using a zinc coordination polymer

A zinc(II) coordination polymer, [Zn(H2dhtp)(2,2'-bpy)(H2O)]n (1), has been utilized as a dual-mode luminescence-colorimetric sensor (H2dhtp2- = 2,5-dihydroxy terephthalate and 2,2'-bpy = 2,2'-bipyridine). The presence of hydroxyl groups in H2dhtp2- can promote excited-state intra- and intermolecular proton transfer (ESIPT) phenomena. Therefore, compound 1, which displays high stability in aqueous environments, exhibits a strong green-yellow photoluminescence. This luminescence signal can be considerably enhanced and blue-shifted upon the addition of Al3+ ions with a limit of detection (LOD) of 0.15 μM, and it demonstrates significant resistance to interference from several competing metal ions. To demonstrate a practical application, 1@paper strips were fabricated that can visually detect the Al3+ ion under a UV lamp. Moreover, 1 can detect either Fe2+ or Fe3+ ions in aqueous solutions by a visible color shift. Upon the incremental addition of Fe2+ or Fe3+ ions, the solution color changed from colorless to pink, exhibiting a pronounced absorption band at around 521 nm. The LODs were determined to be 1.55 and 0.34 μM for Fe2+ and Fe3+, respectively. Furthermore, compound 1 was used for the determination of Fe3+ ions in the real water samples, which can be evaluated on-site in real-time via a smartphone color-scanning application. The detection efficacy of 1 toward Al3+ and Fe2+/Fe3+ maintains significant luminescence stability and reusability.

Tunable-Emissive Zeolite-Confined Silver Nanoclusters as a Rapid and Highly Sensitive Photoluminescent Sensor for Trace Water Detection in Organic Solvents

Zeolite-confined silver nanoclusters (AgNCs/Zeolite) are highly promising luminescent materials. Nevertheless, the controlled synthesis of AgNCs/Zeolite composites with tunable luminescent responses remains a persistent challenge. In this study, highly luminescent AgNCs/Zeolite composites are developed through the counter-cations engineering strategy via confining AgNCs within the cages of FAUX with Na+ and Zn2+ as the counter cations (Ag-FAUX(Na/Zn)), which show water-triggered switchable yellow/green tunable-emissive properties with PLQY up to 90% and high tolerance to chemical reduction. Specifically, dehydration of Ag-FAUX(Na/Zn) increases its PLQY from 60% to 90% and shifts the emission peak from 560 to 520 nm, whereas the yellow emission of the sample without Zn2+ (Ag-FAUX(Na/Zn)) is almost completely quenched upon dehydration. The remarkably enhanced luminescence upon dehydration can be tentatively ascribed to the coordination of Zn2+ with framework oxygen (OF) of FAUX, thereby preventing AgNCs from direct interaction with OF of Ag-FAUX(Na/Zn), and finally forming the stable, tightly confined Ag4 interacting with nonframework oxygen and zinc atoms simultaneously. The Ag-FAUX(Na/Zn) composites demonstrated exceptional performance in fluorescence detection of trace water in organic solvents, achieving high sensitivity, a low detection limit of 0.015% v/v, rapid response times of less than 0.5 s, and reusability, thereby positioning it as a promising candidate for advanced fluorescence detection applications.

Water-Induced Turn-on of Lanthanide Photoluminescence Emission and Application in Colorimetric Sensing of Trace Water

To examine the water-induced photoluminescence turn-on and its potential application in trace water sensing, a new series of [LnIII(dmba)3(H2O)2]·2H2O, where LnIII = LaIII (I), PrIII (II), NdIII (III), SmIII (IV), EuIII (V), GdIII (VI), TbIII (VII), DyIII (VIII), HoIII (IX), and ErIII (X), were synthesized using dimethoxybenzoic acid (Hdmba). Their single-crystal structures and thermal and chemical robustness were investigated, and the effects of lanthanide contraction and noncovalent interactions were discussed. The photoluminescence and colorimetric properties of I-X were investigated. Their dependence on dehydration and rehydration was disclosed, from which the significant role of noncovalent interactions was proposed. Based on the dehydration-rehydration-dependent responses in the forms of photoluminescence emission and color, the turn-off (dehydration) and turn-on (rehydration) of the red emission of EuIII (V) were demonstrated. Using a mobile phone camera and freeware application, its use in the colorimetric sensing of trace water in polar organic solvents was successfully achieved. With respect to ethanol, acetonitrile, and acetone, linear correlations were established from 0 to 3-5% by volume of water with an R 2 of over 0.98. The detection and quantification limits were less than 0.5 and 1.5%, respectively. The percentage recoveries were 92 and 110%. The underlying mechanism was postulated.

Biomass-derived multiatom-doped carbon dots for water sensing based on excited state intraparticle proton transfer in organic solvents

The presence of trace water impurities in organic solvents can significantly influence chemical reactions and product quality; thus, the accurate detection of water content in these solvents is a critical requirement for industrial applications. Accordingly, an eco-friendly, effective, and economical sensor for detecting trace quantities of miscible water in organic solvents is required for industrial applications. In this study, we synthesized biomass-derived multi-atom-doped carbon dots (MACDs) as fluorescent probes and employed them for the detection of trace amounts of water impurities in several water-miscible organic solvents. The MACDs exhibited stable dual-color fluorescence emission under ultraviolet light irradiation and red and blue emissions in organic solvents and water. The fluorescence quantum yield was approximately 11 %, which indicates an excited intraparticle proton transfer response due to an increase in the water content within a wide response range from 0 % to 100 % (v/v) in organic solvents. The intensity of the red emission signal at 670 nm gradually decreased with an increase in the water content in the organic solvent. The MACDs could detect water with an instant response time of 55 s, a high sensitivity, and low limits of detection of 0.08 %, 1.36 %, 0.03 %, 0.04 %, 0.12 %, and 0.05 % (v/v) in ethanol, acetonitrile, dimethylformamide, methanol, isopropanol, and tetrahydrofuran, respectively. Hence, biomass-derived MACDs can serve as efficient and eco-friendly water sensors in organic solvents.

Rational construction of luminescent Eu-doped Y-MOF for ratiometric temperature sensing

Introducing lanthanide(iii) ions into a MOF structure is one of the most effective strategies to construct luminescent MOFs with multiple emission centers for fluorescent applications. In this work, a functionalized Eu3+-doped Y-MOF (Eu@SNNU-325) was constructed by using a cation exchange strategy. The photoluminescence result shows that Eu@SNNU-325 exhibits a unique emission spectrum, namely, the absence of the organic ligand peak and the very strong Y3+/Eu3+ characteristic peaks. Interestingly, the smart luminescent Eu@SNNU-325 as a ratiometric thermometer for temperature sensing has good self-calibrated ability and a high maximum relative sensitivity (S m) value (1.2% K-1 at 260 K). This work presents the construction of a smart Eu3+-functionalized Y-MOF thermometer through a cation exchange strategy, providing a good idea for the future development and design of Y-MOF thermometers.

Natural flavonols as probes for direct determination of borax: From conventional fluorescence analysis to paper-based smartphone sensing

Borax is strictly regulated in the food processing and pharmaceutical industry due to its physiological toxicity, and the development of a direct analytical method is essential for effectively monitoring the borax abuse. In this work, the fluorescence properties of flavonoids, including flavones, isoflavones and flavonols, were systematically investigated from aqueous to borax solutions, and it was found that the weak intrinsic fluorescence of flavonols could be pervasively sensitized by borax. A natural flavonol, morin, was subsequently chosen as a representative probe to develop a turn-on fluorescence sensing method for borax analysis, which achieved a linear response spanning four orders of magnitude with a detection limit of 1.07 μM (0.22 μg mL-1 in terms of Na2B4O7 content). Furthermore, a smartphone-assisted paper-based test device was designed and constructed by 3D printing technology. Using morin-impregnated test strips as the carrier, the borax could be visually detected by the RGB signals of the captured images, with a detection limit of 0.13 mM (27.05 μg mL-1 for Na2B4O7). Combining ion exchange treatment for food samples and sodium periodate oxidation for drug samples, the developed methods were successfully applied for the direct analysis of borax in various products with the recoveries of 86.9-106.3% for traditional fluorescence analysis and 82.7-108.8% for smartphone-assisted fluorescence sensing. The fluorescence property of the morin-borax system was studied using time-dependent density functional theory, and the sensing mechanism was discussed in conjunction with experimental research.

Anionic Hydrogen-Bonded Frameworks Showing Tautomerism and Colorful Luminescence for the Ultrasensitive Detection of Acetone

Tautomers coexisting in an equilibrium system have significant potential for regulating luminescent properties because of their structural differences. However, separating and stabilizing tautomers at room temperature is a considerable challenge. In this study, it is found that hydrogen-bonded organic frameworks (HOFs) composed of Br- anions can effectively separate and stabilize two proton-transfer tautomers of triarylformamidinium bromide: namely, the nitrogen cation (BA-N) and carbon cation (BA-C). The BA-N crystal consisting of a dense anionic HOF and parallelly aligned organic cations exhibits green thermally activated delayed fluorescence and red room-temperature phosphorescence (RTP). The BA-C crystal contains acetone molecules that induce an antiparallel arrangement of the organic cations to form a loose HOF, producing blue prompt fluorescence and green RTP. Interestingly, switching of the HOFs between BA-N and BA-C can be achieved through the uptake and release of acetone, thereby dynamically adjusting multiple luminescent properties. Consequently, the HOF crystals can be used for the highly sensitive and specific sensing of acetone with a detection limit of 66.74 ppm. This study not only stabilizes tautomeric luminescent materials at room temperature, but also provides a new method for constructing smart HOFs with a sensitive response to a stimulus.

Luminescence turn-on sensor for the selective detection of trace water and methanol based on a Zn(ii) coordination polymer with 2,5-dihydroxyterephthalate

A highly selective detection of trace water in organic solvents is urgently required for the chemical industry. In this work, the simple sonochemical method was used for producing a luminescent sensor, [Zn(H2dhtp)(2,2'-bpy)(H2O)]n (Zn-CP) (H2dhtp2- = 2,5-dihydroxyterephthalate and 2,2'-bpy = 2,2'-bipyridine). Zn-CP exhibits reversible thermally-induced and methanol-mediated structural transformation. Importantly, Zn-CP has exceptional water sensing performance in both dry methanol and dry ethanol, with high selectivity, wide linear ranges, and a low limit of detection (LOD) of 0.08% (v/v). Upon the incremental addition of water, the luminescent intensities enhanced and shifted, along with the emission color changing from green to greenish yellow. In addition, Zn-CP can detect methanol selectively through turn-on luminescence intensity with LODs of 0.28, 0.52, and 0.35% (v/v) in dry ethanol, dry n-propanol, and dry n-butanol, respectively. The excited-state proton transfer of linker H2dhtp2-via enol-keto tautomerism and collaboration with structural transformation could be attributed to the sensing mechanism.

Water detection in organic solvents using a copolymer membrane immobilised with a fluorescent intramolecular charge transfer-type dye: effects of intramolecular hydrogen bonds

Numerous fluorescent dye-based optical sensors have been developed to detect water in organic solvents. However, only a few such sensors can detect water in polar solvents such as methanol or dimethyl sulfoxide, and their detection range is generally narrow. Therefore, in this study, a copolymer membrane incorporated with a pyridinium betaine dye (denoted PB1), which exhibited intramolecular charge transfer (ICT) characteristics, was developed to realise simple water detection in organic solvents. The pyridinium betaine structure, comprising intramolecular hydrogen bonds between the oxygen in the maleimide moiety and the hydrogen in the pyridinium, was vital for achieving efficient fluorescence emission. The membrane was prepared by copolymerising PB1 with the N,N-dimethyl acrylamide/acrylamide monomer on a glass plate, and the fluorescence in water-mixed organic solvents was investigated (λabs = 490 nm, λfl = 630 nm). The fluorescence intensity of the dye-immobilised membrane decreased with increasing water content of the organic solvents. The detection ranges in tetrahydrofuran, ethanol, methanol, and dimethyl sulfoxide were approximately <40, <40, <40, and <60 vol% water, respectively. In contrast, membranes based on a quaternary pyridinium dye (without intramolecular hydrogen bonds) did not detect water in methanol and dimethyl sulfoxide, although it was more sensitive than PB1 in the narrow region of low water concentration in THF. Theoretical calculations corroborated the importance of the pyridinium betaine structure in detecting water in organic solvents, with the increase in polarity and the formation of intermolecular hydrogen bonds between PB1 and water found to induce molecular rotation and fluorescence quenching.

Multi-analyte fluorescence sensing based on a post-synthetically functionalized two-dimensional Zn-MOF nanosheets featuring excited-state proton transfer process

Covalent post-synthetic modification of metal-organic frameworks (MOFs) represents an underexplored but promising avenue for allowing the addition of specific fluorescent recognition elements to produce the novel MOF-based sensory materials with multiple-analyte detection capability. Here, an excited-state proton transfer (ESPT) active sensor 2D-Zn-NS-P was designed and constructed by covalent post-synthetic incorporation of the excited-state tautomeric 2-hydroxypyridine moiety into the ultrasonically exfoliated amino-tagged 2D Zn-MOF nanosheets (2D-Zn-NS). The water-mediated ESPT process facilitates the highly accessible active sites incorporated on the surface of 2D-Zn-NS-P to specifically respond to the presence of water in common organic solvents via fluorescence turn-on behavior, and accurate quantification of trace amount of water in acetonitrile, acetone and ethanol was established using the as-synthesized nanosheet sensor with the detection sensitivity (<0.01% v/v) superior to the conventional Karl Fischer titration. Upon exposure to Fe3+ or Cr2O72-, the intense blue emission of the aqueous colloidal dispersion of 2D-Zn-NS-P was selectively quenched even in the coexistence of common inorganic interferents. The prohibition of the water-mediated ESPT process and local emission, induced by the coordination of ESPT fluorophore with Fe3+ or by Cr2O72- competitively absorbs the excitation energy, was proposed to responsible for the fluorescence turn-off sensing of the respective analytes. The present study offers the attractive prospect to develop the ESPT-based fluorescent MOF nanosheets by covalent post-synthetic modification strategy as multi-functional sensors for detection of target analytes.

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.

Chemorobust dye-encapsulated framework as dual-emission self-calibrating ratiometric sensor for intelligent detection of toluene exposure biomarker in urine

By taking advantages of confinement effect can effectively prevent dye aggregation caused luminescent quenching, Eosin Y (EY) was encapsulated into a chemorobust porous CoMOF as secondary fluorescent signal to construct the dual-emitting sensor of EY@CoMOF. And the photo-induced electron transfer from CoMOF to EY molecules induced EY@CoMOF presenting a weak blue emission at 421 nm and a strong yellow emission at 565 nm. Those dual-emission features also endow EY@CoMOF itself great potentials as a self-calibrating ratiometric sensor in visually and efficiently monitoring hippuric acid (HA) in urine, with fast response, high sensitivity and selectivity, excellent recyclable, and low LOD (0.24 μg/mL). Furthermore, based on a tandem combinational logic gate, an intelligent detection system was designed to improve the practicability and convenience of HA detection in urine. To the best of our knowledge, this is the first example of dye@MOF based sensor for HA detection. And this work provides a promising approach for developing dye@MOF based sensors to intelligent detect bioactive molecules.