Highly Selective and Sensitive Turn-Off-On Fluorescent Probes for Sensing Al3+ Ions Designed by Regulating the Excited-State Intramolecular Proton Transfer Process in 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.

A Phototautomeric 3D Covalent Organic Framework for Ratiometric Fluorescence Humidity Sensing

Photoinduced proton transfer is an essential photochemical process for designing photocatalysts, white light emitters, bioimaging, and fluorescence sensing materials. However, deliberate control of the excited/ground states and meticulous manipulation of the excited state intramolecular proton transfer (ESIPT) pathway constitute a significant challenge in liquids and dense solids. Here, we present the integration of a hydronaphthoquinone fluorophore into a crystalline, porous, phototautomeric dynamic 3D covalent organic framework (COF) to show guest-induced fluorescence turn-on, emission redshift enhancement, and shortened lifetimes for ratiometric fluorescence humidity sensing. Theoretical and spectroscopic studies provide mechanistic insights into the conformational dynamics, charge transfer coupled with local excitation, and ground-state uphill regulation for the multiple tautomers. We illustrate the sensitive, rapid, steady, and self-calibrated ratiometric fluorescence sensing for a wide range of humidity benefiting from the architectural and chemical robustness and crystallinity of such a phototautomeric 3D COF. These findings provide molecular insights into the design of functional porous materials that integrate host-guest mutual recognition and photoelectronic response for multiplex molecular sensing for environmental monitoring and biomedical diagnostics applications.

Mixed-Bridging 2D Luminescent Coordination Polymer: Structure, Selective Sensing of Pollutants in Water, and Fabrication of Photoresponsive Electronic Device

Pyrazine (tppz) and 5-sulfosalicylic acid (H3SSA) mixed-bridging Cd(II)-CP, {[Cd2(HSSA)2(tppz)2]}n (1), is highly luminescent, and the emission has been quenched selectively by Al3+ in the presence of other cations, with a limit of detection (LOD) of 43.9 nM (1.18 ppb). The emission of 1 is also quenched by 4-nitroaniline (4-NA) (LOD, 49 nM) and dicloran (2,6-dichloro-4-nitroaniline, DCN), a pesticide (LOD, 42.8 nM), in an aqueous medium in the existence of 11 other nitroaromatic compounds (NACs). The structure of the probe, 1, shows the diverse bridging by pyridyl-N of tppz and carboxylate-O of HSSA2- to architect a 2D CP with a 16-member metallo-macrocycle, followed by S═O···Cd(II) (2.291(2) Å), π···π (3.892 Å), and C-H···π noncovalent interactions to form a 3D supramolecular structure and cavity. The phenolic-OH of HSSA2- remains uncoordinated in the cavity network. Upon interaction of Al3+/4-NA/DCN with Cd(II)-CP (1), the free phenolic-OH may induce easy dissociation, which may assist in energy transfer for quenching. The interaction has been explained through various experiments and theoretical calculations. The DFT computation determines the band gap as 3.42 eV (experimentally, 3.18 eV), which is motivated to measure the electrical conductivity. The conductivity, 3.53 × 10-5 S m-1 (dark), has been enhanced upon light irradiation to 7.66 × 10-5 S m-1.

Synthesis, Characterization and Application of Iron (III) Complex of Schiff Base Ligand as a Novel Enhancement Fluorescent Sensor for Al3+ Ion Detection

This study highlights the importance of developing sensitive and selective sensors for use in pharmaceutical applications for the first time. A novel iron(III)-complex, constructed from unsymmetrical tetradentate NNN'O type Schiff base ligand (E)-3-((6-aminopyridin-2-yl)imino)-1-phenyl butane-1-one (LH) and its structure of it characterized by using various spectroscopic techniques such as FT-IR, UV-Vis, elemental analysis, conductivity, magnetic susceptibility measurements and the TGA method. The correlation of all results revealed that the coordination of the (LH) with the metal ion in a molar ratio of 1:1 leads to the formation of an octahedral geometry around the metal ions. Conductivity data showed the electrolytic nature of the complex. Its fluorescence properties were thoroughly investigated by introducing aluminium ions in deionized water, which increased fluorescence intensity at 460 nm. The detection limit for Al3+ was optimized and found to be 1.5 µM. Notably, the fluorescent sensor successfully monitored Al³⁺ in pharmaceutical formulations. This fluorescence-based analytical method is an alternative to other methods due to its high selectivity, sensitivity, and speed. These results suggest the high potential of this system for pharmaceutical monitoring applications.

Multi-ion detection chemosensor based on rhodamine for turn-on fluorescence sensing and bioimaging of Fe3+, Al3+, Cr3+, and Hg2+ under different channels

A novel rhodamine-based multi-ion fluorescent sensor, RGN, was designed and synthesized for the highly selective detection of mercury ions (Hg2+) in ethanol and water systems, as well as trivalent cations (Fe3+, Al3+, and Cr3+) in acetonitrile and water systems using a two-step Schiff base reaction method. Nuclear magnetic titration experiments and theoretical calculations demonstrated that the sensor achieved the detection of the aforementioned metal ions through the fluorescence turn-on phenomenon induced by lactam ring-opening. Density functional theory (DFT) calculation results showed decreased HOMO-LUMO energy gaps and increased dipole moments, indicating the effective coordination of the sensor with the corresponding metal ions to form more stable complexes, thereby achieving detection objectives. Furthermore, the fluorescence turn-on sensor RGN exhibited relatively low detection limits, with limits of detection (LOD) for Fe3+, Al3+, Cr3+, and Hg2+ being 10.20 nM, 14.66 nM, 58.78 nM, and 73.33 nM, respectively. Finally, practical applications of sensor RGN in environmental water samples, L929 cells, and zebrafish were demonstrated, indicating its potential for detecting and tracking Fe3+, Al3+, Cr3+, and Hg2+ in environmental samples and biological systems, with prospects for biomedical applications in the diagnosis and treatment of heavy metal ion-induced diseases.

Selective Chromo-Fluorogenic Chemoprobe for nM Al3+ Recognition: Experimental and Living-Cell Applications

A rhodamine based chemoprobe BESN was engineered and employed as a selective ''OFF-ON'' chromo-fluorogenic sensor for Al3+ in H2O:MeOH (1:9, v:v). Notable changes in the absorption and emission spectra of BESN were clearly detectable upon the addition of Al3+. Sensitivity and binding mechanism studies demonstrated a good sensing performance of BESN with nanomolar detection limit (130 nM), and it was found to be highly selective towards interfering metal ions. Besides, the binding constant between BESN and Al3+ was found to be 3.19 × 103 M-1. Then, the validation study of BESN for Al3+ was performed based on significant analytical parameters and statistical tests. The binding of Al3+ with BESN (1:1) was probed via infrared, high-resolution mass and emission (Job's plot) spectroscopy measurements. The sensing performance of BESN could make it ideal chemosensor for real applications including vegetable, tuna fish and water samples, also for Smartphone and test-kit applications. The recovery values of the BESN to Al3+ were estimated within a range from 95.13% to 105.30% for water, 94.63% to 109.62% for tuna fish and 94.80% to 109.80% for vegetable samples. Additionally, the BESN has very low cytotoxicity and was triumphantly utilized for the recognition of Al3+ in living-cells.

A novel isophorone-based NIR fluorescent and colormetric probe for Al3+ sensing and its application for living cells and plants imaging

In this paper, an isophorone-based NIR fluorescent and colormetric probe BDDH for Al3+ was synthesized and characterized, it showed highly selectivity and sensitivity through significant fluorescence enhancement and visible color change towards Al3+. The job plot confirmed that the binding ratio of BDDH with Al3+ was 1:1. Furthermore, the limit of detection (LOD) of Al3+ was determined to be 4.01 × 10-8 M. Moreover, BDDH was successfully applicated in identification of Al3+ in the different water samples, cell imaging in alive MCF-7 cells and plant imaging in soybean roots.

"Turn-on" and pinhole-free ultrathin core-shell Au@SiO2 nanoparticle-based metal-enhanced fluorescent (MEF) chemodosimeter for Hg2

This study reports a metal-enhanced fluorescence chemodosimeter for highly sensitive detection of Hg2+ ions. Silica-coated Au nanoparticles (Au@SiO2 NPs) with a pinhole-free 4-5 nm shell were synthesized and functionalized with a monolayer of turn-on fluorescent probes. Compared to other organic fluorescent probes suffering from poor biocompatibility and detection limits, this design of a monolayer of turn-on fluorescent probes immobilized on the Au@SiO2 NPs with a pinhole-free 4-5 nm shell avoids fluorescence quenching and allows the fluorescent probe within the field of the inner Au NPs to experience metal-enhanced fluorescence. With this design, the chemodosimeter permits fluorescence emission in the presence of Hg2+ ions, because they trigger the ring-opening reaction of the fluorescent probe immobilized on the Au@SiO2 NPs. Additionally, the fluorescent probe is distanced by the thin SiO2 shell from directly attaching to the metallic Au NPs, which not only avoids fluorescence quenching but allows the fluorescent probe within the long-ranged field of the inner Au NPs to experience metal-enhanced fluorescence. As a result, the detection limit for the chemodosimeter can reach up to 5.0 × 10-11 M, nearly two orders of magnitude higher than that achieved for the free fluorescent probe. We also demonstrate the acquisition of images of Hg2+ in HTC116 cells and zebrafish using a simple fluorescence confocal imaging technique. The fluorescence response results for HTC116 cells and zebrafish show that the probes can permeate into cells and organisms. Considering the availability of the many organic fluorescent probes that have been designed, the current designed metal-enhanced fluorescence chemodosimeter holds great potential for fluorescence detection of diverse species and fluorescence imaging.

Advances in metal-organic frameworks for water remediation applications

Rapid industrialization and agricultural development have resulted in the accumulation of a variety of harmful contaminants in water resources. Thus, various approaches such as adsorption, photocatalytic degradation and methods for sensing water contaminants have been developed to solve the problem of water pollution. Metal-organic frameworks (MOFs) are a class of coordination networks comprising organic-inorganic hybrid porous materials having organic ligands attached to inorganic metal ions/clusters via coordination bonds. MOFs represent an emerging class of materials for application in water remediation owing to their versatile structural and chemical characteristics, such as well-ordered porous structures, large specific surface area, structural diversity, and tunable sites. The present review is focused on recent advances in various MOFs for application in water remediation via the adsorption and photocatalytic degradation of water contaminants. The sensing of water pollutants using MOFs via different approaches, such as luminescence, electrochemical, colorimetric, and surface-enhanced Raman spectroscopic techniques, is also discussed. The high porosity and chemical tunability of MOFs are the main driving forces for their widespread applications, which have huge potential for their commercial use.

Solvent- and pH-Stable Eu(III)-Based Metal-Organic Framework with Phosphate-Ratio Fluorescence Sensing and Significant Proton Conduction

Lanthanide-based metal-organic frameworks show good potential for applications due to their unique structures and functional properties. A highly thermally and acid-base stable Eu-MOF was synthesized by a solvothermal method with the molecular formula {[(CH3)2NH2]2[Eu2(NDDP)2(H2O)2]·H2O}n (Eu-MOF, H4NDDP = 5,5'-(naphthalene-2,6-diyl)diisophthalic acid). Eu-MOF takes a three-dimensional (4,4,8)-connected topology. The water molecules involved in the coordination, free water molecules, and [(CH3)2NH2]+ cations in the pore can be used as proton carriers. The proton conductivity attains 1.25 × 10-4 S cm-1 at room temperature and 2.42 × 10-3 S cm-1 at 70 °C and 98% relative humidity. Combined with the dual-emission properties from the ligands and Eu(III) ions enables Eu-MOF to be used as a ratiometric fluorescent sensor for phosphate efficiently and rapidly, with a limit of detection of 0.12 μM in the Tris-HCl buffer solution. These results provide a new approach for the construction of MOFs with high proton conductivity and a ratiometric fluorescence response.

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.

Construction and Application of Multipurpose metal-organic frameworks -based Hydrogen Sulfide Probe

Hydrogen sulfide (H2S) is a toxic gas derived from the sulfur industry and trace H2S in the environment can cause serious ecological damage while inhalation can cause serious damage and lead to disease. Therefore, the real-time and accurate detection of trace sulfur ions is of great significance for environmental protection and early disease detection. Considering the shortcoming of current H2S probes in terms of stability and sensitivity, the development of novel probes is necessary. Herein, a novel metal-organic frameworks (MOF)-based material, UiO-66-NH2@BDC, was designed and prepared for the visual detection of H2S with rapid response (< 6 s) and low detection limit of S2- (0.13 µM) by hydrogen bonding. Based on its good optical performance, the UiO-66-NH2@BDC probe can detect S2- in various water environments. More importantly, UiO-66-NH2@BDC probe realize imaging S2- in cells and live zebrafish.

Zinc(II)-Based Ring-and-Rod Coordination Layer as an Excitation-Wavelength-dependent Dual-Emissive Chemosensor for Discriminating Fe3+, Cr3+, and Al3+ in Water

The reactions of Zn(NO3)2, 3,6-bis(pyridin-3-yl)-9H-carbazole (bpycz), and 2,5-dihydroxyterephthalic acid (H4dhbdc) or 2-bromoterephthalic acid (Br-1,4-H2bdc) under hydro(solvo)thermal conditions yielded corresponding coordination polymers (CPs) {[Zn(H2dhbdc)(bpycz)]•0.5H2O}n (1) and [Zn(Br-1,4-bdc)(bpycz)]•2DMAc•H2O (2), respectively, with high thermal stability approaching 350 °C. CP 1 adopts a ring-and-rod layer structure, which is topologically described as a 4-connected net with the point symbol of 2•65. Two layers are interpenetrated in parallel interlocking mode to form a double 2D → 2D polyrotaxane entanglement with extra-framework void space of 19.6%. CP 2 has a non-interpenetrating ring-and-rod layer structure of 4-connected 2•65 net topology, with extra-framework void space of 16.6%. Thermally activated 1 and 2 revealed CO2 uptakes of 101.1 and 98.6 cm3 g-1, respectively, at P/P0 = 1 and 195 K. X-ray powder diffraction (XRPD) patterns confirmed that 1 and 2 both possessed high chemical stability in H2O, CH3OH, acetone, and DMF, and framework stability during gas adsorption-desorption. The H2O suspension of 1 displayed excitation-dependent dual-emissive properties, appearing at 432 nm upon excitation at 300 nm and at 528 nm upon excitation at 365 nm. Of note, 1 was capable of detection of Fe3+, Cr3+, and Al3+ ions in H2O, showing good anti-interference ability, excellent selectivity, and high sensitivity. More interesting, the dual-emissive properties make 1 to be an excellent luminescence chemosensor to screen Fe3+, Cr3+, and Al3+ from a pool of metal ions in H2O upon excitation at 300 nm via luminescence quenching effect and then discriminate Fe3+, Cr3+, and Al3+ upon excitation at 365 nm via luminescence quenching, unaltered, and enhancement responses, respectively. On the other hand, the H2O suspension of 2 demonstrated an excitation-independent emission appearing at around 430 nm, which could be utilized to sensitively detect Fe3+ and Cr3+ ions with good anti-interference ability and excellent selectivity via luminescence quenching effect. Further, 1 and 2 were recyclability and possessed cycling stability. The plausible sensing mechanisms for 1 and 2 toward Fe3+, Cr3+, and Al3+ were also explored in detail.

Triphenylamine-Functionalized Coordination Cage as a Supramolecular Fluorescence Sensor for Sequential Detection of Aluminum Ions and Nitrofurantoin

Coordination cages with a well-defined nanocavity are a class of promising supramolecular materials for molecular recognition and sensing. However, their applications in sequential sensing of multiple types of pollutants are highly desirable yet extremely limiting and challenging. Herein, we demonstrate a convenient strategy to develop a supramolecular fluorescence sensor for sequentially detecting environmental pollutants of aluminum ions and nitrofurantoin. A coordination cage (Ni-NTB), adopting an octahedral structure with triphenylamine chromophores occupying on the faces, is weakly emissive in solution due to the intramolecular rotations of the phenyl rings. Ni-NTB exhibits sensitive and selective fluorescence "off-on-off" processes during consecutive sensing of Al³⁺ and nitrofurantoin, an antibacterial drug. These sequential detection processes are highly interference-tolerant and visually observable with the naked eye. Mechanism studies reveal that the fluorescence switch is controllable by tuning the degree of intramolecular rotations of the phenyl rings and the pathway of intermolecular charge transfer, which is associated with the host-guest interaction. Moreover, the fabrication of Ni-NTB on test strips enabled a quick naked-eye sequential sensing of Al³⁺ and nitrofurantoin in seconds. Hence, this novel supramolecular fluorescence "off-on-off" sensing platform provides a new approach to developing supramolecular functional materials for monitoring environmental pollution.

A cyanobiphenyl-based fluorescent ''lighting-up'' sensor for highly selective and sensitive recognition of Al3+: Theoretical, practical and bioimaging studies

The recognition of toxic Al³⁺ in foods and biosystems has of great interest to researchers. Herein, a novel cyanobiphenyl-based chemosensor CATH (E)-N'-((4'-cyano-4-hydroxy-[1,1'-biphenyl]-3-yl)methylene)thiophene-2-carbohydrazide was fabricated and shown to recognize Al³⁺ in HEPES buffer:EtOH (90:10, v:v, pH = 7.4) by ''lighting-up'' fluorescence sensing. The CATH evidenced high sensitivity (LOD = 13.1 nM) and excellent selectivity to Al³⁺ over competing cations. The Job's plot, TOF-MS and theoretical computation studies were performed to probe the binding mechanism of Al³⁺ to CATH. Additionally; CATH was successfully utilized to practical applications and employed to recover of Al³⁺ from different food samples. More importantly, it was employed to intracellular Al³⁺ detection in living cells including THLE2 and HepG2.

A novel fluorescent molecule based on 1,2,3-triazole for determination of palladium (II) and hydrazine hydrate in aqueous system

In recent years, hydrazine hydrate has been widely used in various fields as fuel and chemical raw materials, etc. However, hydrazine hydrate is also a potential threat to living body and natural environment. The effective method is urgently needed to detect hydrazine hydrate in our living environment. Secondly, as a precious metal, palladium has attracted more and more attention because of its excellent properties in industrial manufacturing and chemical catalysis. However, its potential danger is also slowly approaching, so it is necessary to find an excellent way to detect palladium, too. Herein, a fluorescent molecule, 4,4',4'',4'''-(1,4-phenylenebis(2H-1,2,3-triazole-2,4,5-triyl)) tetrabenzoic acid (NAT), was synthesized. Firstly, NAT has very high selectivity and sensitivity for determination of Pd²⁺, because Pd²⁺ can coordinate well with carboxyl oxygen of NAT. The detection performance of Pd²⁺ is that the linear range is from 0.06 to 4.50 μM and the detection limit is 16.4 nM. Furthermore, the chelate (NAT-Pd²⁺) can continue to be used for quantitative determination of hydrazine hydrate with a linear range of 0.05-6.00 μM and the detection limit is 19.1 nM. The interaction time of NAT-Pd²⁺ and hydrazine hydrate is about 10 min. Of course, it also has good selectivity and strong anti-interference ability for many common metal ions, anions and amine like compounds. At last, the ability of NAT to quantitatively detect Pd²⁺ and hydrazine hydrate in actual samples has also been verified and the results are very satisfactory.

Numerical Recognition System and Ultrasensitive Fluorescence Sensing Platform for Al3+ and UO22+ Based on Ln (III)-Functionalized MOF-808 via Thiodiglycolic Acid Intermediates

Continuous accumulation of Al³⁺ in the human body and unintended leakage of UO₂²⁺ have posed a great threat to human health and the global environment; thus searching an efficient probe for the detection of Al³⁺ and UO₂²⁺ is of great importance. Herein, we designed and synthesized two hydrolytically stable Eu³⁺- and Tb³⁺-functionalized MOF materials Eu@MOF-808-TDA and Tb@MOF-808-TDA via thiodiglycolic acid (TDA) intermediates by the postsynthetic modification method. Among them, Tb@MOF-808-TDA was applied to construct numerical recognition systems of multiples of three and four by the combination of fluorescent signals, hierarchical cluster analysis, and logical gates. In addition, Tb@MOF-808-TDA exhibits good selectivity and sensitivity for the detection of Al³⁺ and UO₂²⁺. The detection limit is calculated to be 0.085 ppm for Al³⁺ and 0.082 ppm for UO₂²⁺ in aqueous solutions, which is lower than or close to that of latest reported Ln-MOFs. Moreover, the probe shows excellent hydrolytic stability and luminescence stability in the pH range of 4-11, further providing solid evidence for the practical application of Tb@MOF-808-TDA. More importantly, a mixed matrix hydrogel PVA-Tb@MOF-808-TDA was prepared to achieve the visual detection of Al³⁺, which broadens the potential in real-world sensing applications.

SCSC Transformation and Post-Synthesis Modification of MOFs with Proton Conduction and Ratiometric Fluorescence-Sensing Properties

The modification of metal-organic framework (MOF) materials to facilitate their practical applications is an extremely challenging and meaningful topic. In this work, two stepwise modification strategies for MOFs were conducted. First, we have demonstrated a single-crystal-to-single-crystal (SCSC) transformation from a microporous three-dimensional (3D) MOF to a two-dimensional (2D) coordination polymer (CP). The centrosymmetric [Cd(3-bpdb)(MeO-ip)]_n (1) transforms into a chiral [Cd₂(3-bpdb)(MeO-ip)₂(CH₃OH)₂]_n (2), which is triggered by the reaction time with methanol that acts as a structure-directing agent. The conversion relationship of 1 to 2 at different reaction times was studied in detail. Density functional theory (DFT) calculations clearly state that the irreversible formation of 2 is thermodynamically favorable. Intriguingly, 2 exhibits good proton conduction of 1.34 × 10^-3 S cm^-1 under 363 K and 98% relative humidity (RH) due to unique H-bond network characteristics. To the best of our knowledge, there are very few cases of 3D to 2D SCSC transformation stimulated by reaction time. The results have important implications for understanding the SCSC transformation mechanism and synthetic chemistry. On the other hand, the lanthanide³⁺-functionalized hybrids (Ln³⁺-MOF), Ln³⁺@1, were continuously prepared by incorporating luminescent Ln³⁺ ions into the structure of 1 through encapsulating post-synthesis modification (PSM). Tb³⁺@1 exhibits double emission in water and shows visual ratiometric fluorescence behavior for sensing glutamic acid (Glu), tryptophan (Trp), and Al³⁺, which is more reliable and accurate than single emission. Our work may not only provide new insights into the multiple modification of MOF materials but also promote the practical application of such materials.

Optical and quantitative detection of cobalt ion using graphitic carbon nitride-based chemosensor for hydrometallurgy of waste lithium-ion batteries

A hydrometallurgy is one of the most important techniques for recycling waste LIBs, where identifying the exact composition of the metal-leached solution is critical in controlling the metal extraction efficiency and the stoichiometry of the regenerated product. In this study, we report a simple and selective optical detection of high-concentrated Co²⁺ using a graphitic carbon nitride (g-CN)-based fluorescent chemosensor. g-CN is prepared by molten salt synthesis using dicyandiamide (DCDA) and LiI/KI. The mass ratio of LiI/KI to DCDA modifies the resulting g-CN (CNI) in terms of in-plane molecular distances of base sites including cyano functional groups (─CN) and fluorescent emission wavelength via nucleophilic substitution. The fluorescent sensing performance of CNIs is evaluated through photoluminescence (PL) emission spectroscopy in a broad Co²⁺ concentration range (10^-4-10⁰ M). The correlation between the surface exposure of hidden nitrogen pots (base sites) and PL intensity change is achieved where the linear relationship between the PL quenching and the logarithm of Co²⁺ concentration in the analyte solution is well established with the regression of 0.9959. This study will provide the design principle of the chemosensor suitable for the fast and accurate optical detection of Co²⁺ present in a broad concentration range for hydrometallurgy for the recycling of waste LIBs.

Identification of folic acid and sulfaquinoxaline using a heterometallic Zn-Eu MOF as a sensor

A d-f heterometallic MOF using the 2,2'-bipyridine-4,4'-dicarboxylic acid ligand (H₂LZ) was obtained by solvothermal synthesis, namely [EuZn(LZ)₂(HCOO)(H₂O)₃]_n (1). The structure analysis shows that compound 1 comprises heterometallic Zn²⁺ and Eu³⁺ ions, which are connected by LZ^2- and HCOO^- anions to form a three-dimensional framework. MOF 1 exhibited high stability of fluorescence intensity in the scope of pH 2-11 in an aqueous solution. Furthermore, MOF 1 served as an excellent selective sensing material for the detection of folic acid in the presence of some imitating materials of the human body and discerned sulfaquinoxaline in sulfonamide drugs with high sensitivity, selectivity, and reusability. Moreover, we designed and manufactured a sensor paper based on MOF 1 as a portable device for the visual detection of folic acid and sulfaquinoxaline. More crucially, this is the first example in which luminescent MOF is used to identify sulfaquinoxaline molecules in an aqueous solution. In addition, the luminescence sensing mechanisms of MOF 1 for the detection of the above analytes were explored in detail.

Fluorescence-Responsive Detection of Ag(I), Al(III), and Cr(III) Ions Using Cd(II) Based Pillared-Layer Frameworks

Two Cd(II) based coordination polymers, {Cd₃(btc)₂(BTD-bpy)₂]∙1.5MeOH∙4H₂O}_n (1) and [Cd₂(1,4-ndc)₂(BTD-bpy)₂]_n (2), where BTD-bpy = bis(pyridin-4-yl)benzothiadiazole, btc = benzene-1,3,5-tricarboxylate, and 1,4-ndc = naphthalene-1,4-dicarboxylate, were hydro(solvo)thermally synthesized. Compound 1 has a three-dimensional non-interpenetrating pillared-bilayer open framework with sufficient free voids of 25.1%, which is simplified to show a topological (4,6,8)-connected net with the point symbol of (3²4²56)(3⁴4⁴5⁴6²8)(3⁴4²6¹⁹7²8). Compound 2 has a three-dimensional two-fold interpenetrating bipillared-layer condense framework regarded as a 6-connected primitive cubic (pcu) net topology. Compounds 1 and 2 both exhibited good water stability and high thermal stability approaching 350 °C. Upon excitation, compounds 1 and 2 both emitted blue light fluorescence at 471 and 479 nm, respectively, in solid state and at 457 and 446 nm, respectively, in the suspension phase of H₂O. Moreover, compounds 1 and 2 in the suspension phase of H₂O both exhibited a fluorescence quenching effect in sensing Ag⁺, attributed to framework collapse, and a fluorescence enhancement response in sensing Al³⁺ and Cr³⁺, ascribed to weak ion-framework interactions, with high selectivity and sensitivity and low detection limit.

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

Simple, fast, and sensitive detection of trace water in organic solvents is an urgent requirement for chemical industries. Herein, combining the unusual excited-state intramolecular proton transfer (ESIPT) mechanism with the effective strategy of pore space partition, for the first time, we construct a powerful fluorescent metal-organic framework (SNNU-301) probe with excellent water stability. The SNNU-301 probe shows a remarkable performance for turn-on ESIPT-based fluorescence response to water in nine common organic solvents, exhibiting wide linear ranges, low limit of detection values, and ultrafast response, especially in dimethyl sulfoxide (0-5.2%; 0.011%, v/v; 110 s). The typical ESIPT-sensitive linker 2,5-dihydroxyterephthalate (DHBDC) imparts it with discriminative detection properties via enol-keto tautomerism, and light-responsive triangular tri(pyridin-4-yl)-amine (TPA) realizes pore space partition. The theoretical calculation gives an in-depth explanation about the proton transfer mechanism. Comparative experiments and GCMC simulation provide evidence that the synergy of the ESIPT process and TPA of the framework further boosts its performance effectively. Definitely, this work not only offers a promising candidate with fast detection speed, high sensitivity, excellent universality, and visual observation for the determination of water in organic solvents but also provides valuable guidance for the design of high-performance fluorescent probes.

Recent Advances of Anticancer Studies Based on Nano-Fluorescent Metal-Organic Frameworks

Nano-fluorescent metal-organic frameworks (NF-MOFs), a kind of newly emerged nano-scaled platform, can provide visual, rapid, and highly sensitive optical imaging of cancer lesions both in vitro and in vivo. Meanwhile, the excellent porosity, structural tunability, and chemical modifiability also enable NF-MOFs to achieve simultaneous loading of targeted molecules and therapeutic agents. These NF-MOFs not only possess excellent targeted imaging ability, but also can guide the carried cargos to perform precise therapy, drawing considerable attention in current framework of anticancer drug design. In this review, we outline the fluorescence types and response mechanisms of NF-MOFs, and highlight their applications in cancer diagnosis and therapy in recent years. Based on this panorama, we also discuss current issues and future trends of NF-MOFs in biomedical fields, attempting to clarify the potential value of fluorescence imaging guided anticancer investigations.

Reticular Chemistry with Art: A Case Study of Olympic Rings-Inspired Metal-Organic Frameworks

Herein, we demonstrate the successful utilization of reticular chemistry as an excellent designing strategy for the deliberate construction of a zirconium-tetracarboxylate metal-organic framework (MOF) inspired by the Olympic rings. HIAM-4017, with an unprecedented (4,8)-c underlying net topology termed jcs, was developed via insightful reconstruction of the rings and judicious design of a nonsymmetric organic linker. HIAM-4017 exhibits high porosity and excellent chemical and thermal stability. Furthermore, excited-state intramolecular proton transfer (ESIPT) was achieved in an isoreticular MOF, HIAM-4018, with a large Stokes shift of 155 nm as a result of introducing the hydroxyl group to the linker skeleton to induce OH···N interactions. Such interactions were analyzed thoroughly by employing the time-dependent density functional theory (TD-DFT). Because of their good thermal and chemical stability, and strong luminescence, nanosized HIAM-4017 and HIAM-4018 were fabricated and used for Cr₂O₇^2- detection. Both MOFs demonstrate excellent sensitivity and selectivity. This work represents a neat example of building structure- and property-specific MOFs guided by reticular chemistry.

A new NCDs@ZIF-90-based sensor: fluorescent "turn-on" detection of Al3+ ions with high selectivity and sensitivity

A new NCDs@ZIF-90 composite as fluorescence sensor was designed and prepared by encapsulation of N-doped carbon dots (NCDs) in metal-organic framework (MOF) ZIF-90 in one-pot synthesis. NCDs@ZIF-90 retained the crystal structure and high thermal stability of ZIF-90; meanwhile, it also displayed the good chemical stability. NCDs@ZIF-90 dispersion in ethanol exhibited selective "turn-on" fluorescence response towards Al³⁺. The other coexisting competing metal ions had no obvious influence on the sensing performance of NCDs@ZIF-90 for Al³⁺. The fluorescence intensity at 447 nm of NCDs@ZIF-90 dispersion in ethanol had good linear relation with the concentrations of Al³⁺ with a low detection limit of 3.196 μM. The fluorescence enhancement after the addition of Al³⁺ was attributed to the release of NCDs from the inside of ZIF-90 to ethanol solution. In addition, NCDs@ZIF-90 displayed good recovery in detection of Al³⁺ in water samples indicating its practical application capability. The high selectivity and sensitivity indicate that NCDs@ZIF-90 is a good candidate as a "turn-on" fluorescence chemosensor to identify and detect Al³⁺.

A Reversible Fluorescent Probe Based on a Redox-Switchable Excited-State Intramolecular Proton-Transfer Active Metal-Organic Framework for Detection and Imaging of Highly Reactive Oxygen Species in Live Cells

Detection and imaging of highly reactive oxygen species (hROS) in biological systems using fluorescent probes are critical for the study of physiological and pathological processes induced by hROS. Herein, we report a redox-active luminescent metal-organic framework (MOF), which incorporates a hydroquinone moiety that can undergo a reversible transformation from the hydroquinone to the quinone by hROS like •OH and ClO^-. Moreover, the intrinsic fluorescence originating from the excited-state intramolecular proton transfer (ESIPT) property of the organic linker can be finely regulated during this redox-switchable process. A reversible fluorescent probe for hROS is thus developed. The presented probe shows a sensitive, selective, and reversible response to hROS due to the integration of excellent structural characteristics and unique spectral properties of the MOF. The detection limits of •OH and ClO^- are 0.22 and 0.18 μM, respectively. Furthermore, with good photostability and super biocompatibility, this simple yet efficient fluorescent probe has been successfully applied to dynamic monitoring of endogenous and exogenous •OH and ClO^- in live cells.

Supramolecular Combination Cancer Therapy Based on Macrocyclic Supramolecular Materials

Supramolecular combination therapy adopts supramolecular materials to design intelligent drug delivery systems with different strategies for cancer treatments. Thereinto, macrocyclic supramolecular materials play a crucial role in encapsulating anticancer drugs to improve anticancer efficiency and decrease toxicity towards normal tissue by host-guest interaction. In general, chemotherapy is still common therapy for solid tumors in clinics. However, supramolecular combination therapy can overcome the limitations of the traditional single-drug chemotherapy in the laboratory findings. In this review, we summarized the combination chemotherapy, photothermal chemotherapy, and gene chemotherapy based on macrocyclic supramolecular materials. Finally, the application prospects in supramolecular combination therapy are discussed.

Multifunctional nanomaterials and nanocomposites for sensing and monitoring of environmentally hazardous heavy metal contaminants

The applications of conventional sensors are limited by the long response time, high cost, large detection limit, low sensitivity, complicated usage and low selectivity. These sensors are nowadays replaced by Nanocomposite-based modalities and nanomaterials which are known for their high selectivity and physical and chemical properties. These nanosensors effectively detect heavy metal contaminants in the environment as the discharge of heavy metals into natural water as a result of human activity has become a global epidemic. Exposure to these toxic metals might induce many health-related complications, including kidney failure, brain injury, immune disorders, muscle paleness, cardiac damage, nervous system impairment and limb paralysis. Therefore, designing and developing novel sensing systems for the detection and recognition of these harmful metals in various environmental matrices, particularly water, is of extremely important. Emerging nanotechnological approaches in the past two decades have played a key role in overcoming environmentally-related problems. Nanomaterial-based fabrication of chemical nanosensors has widely been applied as a powerful analytical tool for sensing heavy metals. Portability, high sensitivity, on-site detection capability, better device performance and selectivity are all advantages of these nanosensors. The detection and selectivity have been improved using molecular recognition probes for selective binding on different nanostructures. This study aims to evaluate the sensing properties of various nanomaterials such as metal-organic frameworks, fluorescent materials, metal-based nanoparticles, carbon-based nanomaterials and quantum dots and graphene-based nanomaterials and quantum dots for heavy metal ions recognition. All these nano-architectures are frequently served as effective fluorescence probes to directly (or by modification with some large or small biomolecules) sense heavy metal ions for improved selectivity. However, efforts are still needed for the simultaneous designing of multiple metal ion-based detection systems, exclusively in colorimetric or optical fluorescence nanosensors for heavy metal cations.

A new zinc-organic framework with 1D channel for constructing a ratiometric Al3+-selective sensor and four inputs INHIBIT logic gate

To develop Al³⁺ fluorescent sensor is significant because the abnormal levels of Al³⁺ in environment may pose great threat to human body. Herein, a novel metal-organic framework {Zn(Dpada)(Imdba)·H₂O}_n (Dpada = 3, 6-di(1H-imidazol-1-yl) pyridazine and Imdba = 2, 2'-iminodibenzoic acid), named Zn-MOF, has been architected with one-dimensional channel under hydrothermal conditions. Zn-MOF exhibits good thermal and solvent stability and can also keep structural integrity over the pH range of 5.0 - 9.0. Fluorescent experiments show that Zn-MOF has high selectivity and sensitivity towards Al³⁺ via ratiometric fluorescence signal changes (F_{470 nm}/F_{390 nm}) and the detection limit is evaluated to be 0.69 μM. In addition, Zn-MOF performs good recyclability in sensing of Al³⁺ with at least 5 cycles. Besides, an INHIBIT logic gate has been constructed with chemical ions (Al³⁺, Cr³⁺, Fe³⁺ and Hg²⁺) as input signals and emission ratio (F_{470 nm}/F_{390 nm}) as output signal. Significantly, Zn-MOF can be applied to tracing Al³⁺ using real water samples, presenting great potential in water quality monitoring application.

Tris(bipyridine)ruthenium(II)-functionalized metal-organic frameworks for the ratiometric fluorescence determination of aluminum ions

A novel ratiometric fluorescence probe was designed for the determination of Al³⁺ by self-assembling of NH₂-MIL-101(Fe) and [Ru(bpy)₃]²⁺. Under the excitation wavelength of 360 nm, the NH₂-MIL-101(Fe)@[Ru(bpy)₃]²⁺ presented a dual-emitting luminescent property at 440 and 605 nm, respectively. In the presence of Al³⁺, the blue fluorescence of NH₂-MIL-101(Fe)@[Ru(bpy)₃]²⁺ at 440 nm was enhanced remarkably, while the red emission at 605 nm was almost not influenced. Therefore, taking the fluorescence at 440 nm as the report signal and 605 nm as the reference signal, quantitative determination was achieved for Al³⁺ concentration in the ranges 0.2-25 μM and 25-250 μM. The limit of detection (LOD) and limit of quantification (LOQ) were calculated to be 73 nM and 244 nM, respectively. The sensing mechanisms were studied by theoretical calculation and optical spectra. The analysis of real food samples confirmed the suitability of the proposed method. More importantly, portable fluorescent test papers were successfully manufactured to provide a strategy for visual, rapid, and on-site detection of Al³⁺.

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

Luminescent metal-organic frameworks (LMOFs) are one of the most promising materials for being implemented as active layers in the fabrication of photonic devices such as luminescent sensors of harmful chemicals. It is highly desirable that these materials undergo quantifiable spectroscopic (absorption or emission) changes in the presence of vapors of those analytes, as in many industrial processes, these toxic compounds are in the gas phase. Although great progresses have been achieved in the field, in most of the examples reported hitherto, the detection of chemicals by LMOFs is attained in solution. Herein, we present a novel approach consisting of the encapsulation of proton transfer dyes (8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt, HPTS, and 3-hydroxyflavone, 3-HF) within the pores of two distinct MOFs. The trapped proton transfer dyes (PT-dyes) may exist as different structures (enol, anion, or zwitterion), each of these exhibiting unique optical properties. Indeed, our findings reveal that the dyes can be encapsulated as anionic or enol species. Remarkably, the PT-dye@MOF composites exhibit a high luminescence quantum yield (up to 30%), which is sensitive (showing shifting in the emission wavelengths with a concomitant quenching/enhancement of the intensity) in the presence of vapors of an acid (HCl) and a base (triethylamine). These results open a novel avenue for the development of smarter vapoluminescent MOF-based materials.

Tuning ESIPT-coupled luminescence by expanding π-conjugation of a proton acceptor moiety in ESIPT-capable zinc(II) complexes with 1-hydroxy-1H-imidazole-based ligands

The emission of ESIPT-fluorophores is known to be sensitive to various external and internal stimuli and can be fine-tuned through substitution in the proton-donating and proton-accepting groups. The incorporation of metal ions in the molecules of ESIPT fluorophores without their deprotonation is an emerging area of research in coordination chemistry which provides chemists with a new factor affecting the ESIPT reaction and ESIPT-coupled luminescence. In this paper we present 1-hydroxy-5-methyl-4-(pyridin-2-yl)-2-(quinolin-2-yl)-1H-imidazole (HLq) as a new ESIPT-capable ligand. Due to the spatial separation of metal binding and ESIPT sites this ligand can coordinate metal ions without being deprotonated. The reactions of ZnHal₂ with HLq afford ESIPT-capable [Zn(HLq)Hal2] (Hal = Cl, Br, I) complexes. In the solid state HLq and [Zn(HLq)Hal2] luminesce in the orange region (λ_max = 600-650 nm). The coordination of HLq by Zn²⁺ ions leads to the increase in the photoluminescence quantum yield due to the chelation-enhanced fluorescence effect. The ESIPT process is barrierless in the S₁ state, leading to the only possible fluorescence channel in the tautomeric form (T), S₁^T → S₀^T. The emission of [Zn(HLq)Hal2] in the solid state is blue-shifted as compared with HLq due to the stabilization of the ground state and destabilization of the excited state. In CH₂Cl₂ solutions, the compounds demonstrate dual emission in the UV (λ_max = 358 nm) and green (λ_max = 530 nm) regions. This dual emission is associated with two radiative deactivation channels in the normal (N) and tautomeric (T) forms, S₁^N → S₀^N and S₁^T → S₀^T, originating from two minima on the excited state potential energy surfaces. High energy barriers for the GSIPT process allow the trapping of molecules in the minimum of the tautomeric form, S₀^T, resulting in the possibility of the S₀^T → S₁^T photoexcitation and extraordinarily small Stokes shifts in the solid state. Finally, the π-system of quinolin-2-yl group facilitates the delocalization of the positive charge in the proton-accepting part of the molecule and promotes the ESIPT reaction.

A julolidine-chalcone-based fluorescent probe for detection of Al3+ in real water sample and cell imaging

A fluorescent probe 1 based on julolidine-chalcone derivative, which can specifically recognize aluminum ion with high selectivity and anti-interference, was developed. Probe 1 has good fluorescence stability and can detect Al³⁺ with turn-on fluorescence in a wide pH range of 4.0-9.0. The probe has good repeatability for the detection of Al³⁺ and fluorescence turn-on and off can be repeated with the alternate Al³⁺ and EDTA. The sensing mechanism is speculated that Al³⁺ will coordinate with hydroxyl oxygen and carbonyl oxygen on the probe through in situ ¹H NMR and HRMS combing with Job's plot. The probe can also detect Al³⁺ in actual water samples and applied to monitor Al³⁺ in biological system.

pH-Stable Luminescent Metal-Organic Frameworks for the Selective Detection of Aqueous-Phase FeIII and CrVI Ions

The development of chemically stable metal-organic framework (MOF)-based luminescent platforms for toxic ion detection in an aqueous medium is highly challenging because most of the classical MOFs are prone to water degradation, and that is the reason why most of the MOF-based luminescent sensors use a nonaqueous medium for sensing. In this contribution, we report two new water-stable luminescent MOFs (Zn-MOF-1 and Zn-MOF-2), assembled from a mixed-ligand synthesis approach. Because of the presence of a hydrophobic trifluoromethyl group to the backbone and stronger metal-N coordination, these MOFs exhibit excellent stability not only in water but also in acidic/alkaline aqueous solutions (pH = 3-10). Here, we report a green sensing approach by exploiting the significant reduction in photoluminescence of these MOFs in the presence of toxic ions. Fe³⁺ and CrO₄^2-/Cr₂O₇^2- ions could be traced with a detection limit (LOD) in the micromolar range (0.045 and 0.745/0.33 μM for Zn-MOF-1; 125.2 and 114.2/83.5 μM for Zn-MOF-2). The mechanistic study reveals that competitive absorption of the excitation energy coupled with fluorescent resonance energy transfer are responsible for the turn-off quenching. The anti-interference ability and recyclability along with the pH stability gave these MOFs high potential to be used as practical sensors toward Fe^III and Cr^VI ions in water as a greenest medium.

N-Hydroxy-N-oxide photoinduced tautomerization and excitation wavelength dependent luminescence of ESIPT-capable zinc(II) complexes with a rationally designed 1-hydroxy-2,4-di(pyridin-2-yl)-1H-imidazole ESIPT-ligand

The ability of 1-hydroxy-1H-imidazoles to undergo proton transfer processes and to exist in N-hydroxy and N-oxide tautomeric forms can be used in coordination chemistry for the design of ESIPT-capable complexes. A series of ESIPT-capable zinc(II) complexes [Zn(HL)Hal2] (Hal = Cl, Br, I) with a rationally designed ESIPT-ligand 1-hydroxy-5-methyl-2,4-di(pyridin-2-yl)-1H-imidazole (HL) featuring spatially separated metal binding and ESIPT sites have been synthesized and characterized. Crystals of these compounds consist of a mixture of two isomers of [Zn(HL)Hal2]. Only a major isomer has a short intramolecular hydrogen bond O-H⋯N as a pre-requisite for ESIPT. In the solid state, the complexes [Zn(HL)Hal2] demonstrate temperature- and excitation wavelength dependent fluorescence in the cyan region due to the interplay of two intraligand fluorescence channels with excited state lifetimes spanning from 0.2 to 4.3 ns. The coordination of HL by Zn²⁺ ions results in an increase in the photoluminescence efficiency, and the photoluminescence quantum yields (PLQYs) of the complexes reach 12% at λ_ex = 300 nm and 27% at λ_ex = 400 nm in comparison with the PLQY of free HL of ca. 2%. Quantum chemical calculations indicate that N-hydroxy-N-oxide phototautomerization is both thermodynamically and kinetically favourable in the S₁ state for [Zn(HL)Hal2]. The proton transfer induces considerable geometrical reorganizations and therefore results in large Stokes shifts of ca. 230 nm. In contrast, auxiliary ESIPT-incapable complexes [ZnL₂][Zn(OAc)₂]₂·2H₂O and [ZnL₂][ZnCl₂]₂·4H₂O with the deprotonated ligand exhibit excitation wavelength independent emission in the violet region with the Stokes shift reduced to ca. 130 nm.

Confinement of Luminescent Guests in Metal-Organic Frameworks: Understanding Pathways from Synthesis and Multimodal Characterization to Potential Applications of LG@MOF Systems

This review gives an authoritative, critical, and accessible overview of an emergent class of fluorescent materials termed "LG@MOF", engineered from the nanoscale confinement of luminescent guests (LG) in a metal-organic framework (MOF) host, realizing a myriad of unconventional materials with fascinating photophysical and photochemical properties. We begin by summarizing the synthetic methodologies and design guidelines for representative LG@MOF systems, where the major types of fluorescent guest encompass organic dyes, metal ions, metal complexes, metal nanoclusters, quantum dots, and hybrid perovskites. Subsequently, we discuss the methods for characterizing the resultant guest-host structures, guest loading, photophysical properties, and review local-scale techniques recently employed to elucidate guest positions. A special emphasis is paid to the pros and cons of the various methods in the context of LG@MOF. In the following section, we provide a brief tutorial on the basic guest-host phenomena, focusing on the excited state events and nanoscale confinement effects underpinning the exceptional behavior of LG@MOF systems. The review finally culminates in the most striking applications of LG@MOF materials, particularly the "turn-on" type fluorochromic chemo- and mechano-sensors, noninvasive thermometry and optical pH sensors, electroluminescence, and innovative security devices. This review offers a comprehensive coverage of general interest to the multidisciplinary materials community to stimulate frontier research in the vibrant sector of light-emitting MOF composite systems.

Dual-emitting metal-organic frameworks for ratiometric fluorescence detection of fluoride and Al3+ in sequence

Fluoride (F^-) and Al³⁺ are two common ions existing in drinking water and natural water bodies. Excessive intake of F^- can lead to serious health issues such as fluorosis and bone diseases while accumulated consumption of Al³⁺ may cause neurotoxicity-based diseases. Developing a fast, reliable, and sensitive sensor for visually detecting both F^- and Al³⁺ is of great significance. In the present work, a ratiometric fluorescence sensor was constructed by incorporating rhodamine B (RhB) in situ into a zirconium-based metal-organic framework, UiO-66-NH₂. The obtained nanocomposite UiO-66-NH₂@RhB exhibited similar octahedral structure to UiO-66-NH₂ with high BET surface area, and showed two emission peaks at 450 nm and 585 nm. The blue fluorescence from UiO-66-NH₂ was enhanced by the addition of F^- while subsequent Al³⁺ addition diminished the increased fluorescence intensity, and the red emission from RhB as the reference remained unchangeable to improve the detection precision. Under optimal conditions, detection of limits as low as 1.55 μM for F^- and 0.54 μM for Al³⁺ in aqueous solution were achieved with good selectivity. High recoveries in drinking water samples were also acquired, showing potential applications of this ratiometric fluorescence sensor for practical evaluation of F^- and Al³⁺.

Subtle Ligand Spacer Change in 2D Metal-Organic Framework Sheets for Dual Turn-On/Turn-Off Sensing of Acetylacetone and Turn-On Sensing of Water in Organic Solvents

Metal-organic framework (MOF)-based sensors for the detection of various analyte molecules has been a subject of absolute importance. However, most of these sensors rely on the turn-off (quenching) transduction response, while those reporting turn-on response are very rare. In this article, we have synthesized two new MOF-based sensors, {[Zn₂(oxdz)₂(tpbn)]·14H₂O}_n (1) and {[Zn₂(oxdz)₂(tpxn)]·10H₂O·2C₂H₅OH}_n (2), via the self-assembly of Zn(II) metal ions, a fluorogenic oxdz^2- linker, and bis(tridentate) ligands (tpbn and tpxn) under ambient conditions. Their formation from such a self-assembly process has been evaluated on the basis of the geometry around the five-coordinated Zn(II), preferential meridional binding of the bis(tridentate) ligands, and diverse binding of the carboxylate groups in oxdz^2-. Although 1 and 2 are isostructural, a difference in the transduction mechanism for the sensing of acetylacetone in organic solvents (turn-on for 1 and turn-off for 2) is observed and can be attributed to the spacer in the bis(tridentate) ligands. We have demonstrated the competing effect of the nonradiative interactions and photoinduced electron transfer toward the sensing mechanism. The results are well-supported by the Fourier transform infrared spectroscopy study, intensity versus concentration plots, spectral overlap measurements, time-resolved fluorescence studies, and MM2 and density functional theory calculations. Furthermore, we have showcased the utilization of 1 for the sensing of trace amounts of water in organic solvents.