Highly sensitive and selective fluoride detection in water through fluorophore release from a metal-organic framework

Highly Selective and Scalable Molecular Fluoride Sensor for Naked-Eye Detection

Fluoride is widely present in nature, and human exposure to it is generally regarded as inevitable. High levels of fluoride intake induce acute and chronic illnesses. To reduce potential harm to the general public, it is essential to create selective fluoride detectors capable of providing a colorimetric response for naked-eye detection without the need for sophisticated equipment. Here, we report a one-pot synthesis of four different diaminomaleonitrile-derived Schiff base sensors. The terephthalaldehyde adduct provided a strong color change visible to the naked eye at a F- concentration level as low as 2 ppm. From the evaluation against other anions, such as CN-, I-, Br-, Cl-, NO3-, PO43-, OAc-, and HSO4-, the molecular sensor displayed a visible color change exclusively upon exposure to fluoride, underscoring exceptional selectivity. As a key intermediate for understanding the mechanism, HF2- was confirmed by 19F nuclear magnetic resonance. Theoretical calculations suggested a deprotonation-triggered bathochromic shift brought about by the unique electronic structure of the sensor. Furthermore, the simple synthetic protocol from economically accessible materials allowed for the preparation of the compound on a large scale, rendering it a highly practical visual fluoride sensor.

A MOF-on-MOF composite encapsulating sensitized Tb(III) as a built-in self-calibrating fluorescent platform for selective sensing of F ions

The establishment of simple and sensitive detection methods for fluoride ion (F^-) is of great importance for its effective prevention and control, and metal-organic framework (MOF) has attracted much attention for sensing applications due to its high surface areas and tunable structures. Herein, we successfully synthesized a fluorescent probe for ratiometric sensing of F^- by encapsulating sensitized Tb³⁺ in a MOF-on-MOF material (UIO66/MOF801, with the formula of C₄₈H₂₈O₃₂Zr₆ and C₂₄H₂O₃₂Zr₆, respectively). We found that Tb³⁺@UIO66/MOF801 can be used as a built-in fluorescent probe for fluorescence-enhanced sensing of F^-. Interestingly, the two fluorescence emission peaks of Tb³⁺@UIO66/MOF801 at 375 nm and 544 nm exhibit different fluorescence responses to F^- under excitation at 300 nm. The 544 nm peak is sensitive to F^-, while the 375 nm peak is insensitive to it. Photophysical analysis indicated that the photosensitive substance was formed, which promotes the absorption of 300 nm excitation light by the system. Self-calibrating fluorescent detection of F^- was achieved due to the unequal energy transfer toward the two different emission centers. The detection limit of Tb³⁺@UIO66/MOF801 for F^- was 4.029 μM, which is far lower than the WHO guideline for drinking water. Moreover, the ratiometric fluorescence strategy showed a high concentration tolerance of interference, because of its inner-reference effect. This work highlights the high potential of lanthanide ion encapsulated MOF-on-MOF as environmental sensors, and offers a scalable way for construction of the ratiometric fluorescence sensing systems.

Amino-Functionalized Single-Lanthanide Metal-Organic Framework as a Ratiometric Fluorescent Sensor for Quantitative Visual Detection of Fluoride Ions

Excessive content of fluoride ions (F^-) in water will lead to water pollution and endanger human health, so the research on the method of low-cost, rapid, and efficient detection of F^- is of particular significance. In this work, an amino-functionalized ligand with an appropriate triplet energy excited state, 2'-amino-[1,1':4',1″-terphenyl]-3,3″,5,5″-tetracarboxylic acid (H₄TPTC-NH₂), was selected to construct a luminescent single-lanthanide metal-organic framework, EuTPTC-NH₂, with uncoordinated amino groups for the detection of F^-. Based on host-guest interactions, that is, hydrogen bonds formed between the free amino groups and F^- ions, EuTPTC-NH₂ was developed as a ratiometric fluorescence probe for F^- detection with good anti-interference ability, low detection limit, high water stability, and selectivity. It was found that EuTPTC-NH₂ has an excellent linear response to F^- in the concentration range of 0-80 μM with high sensitivity and a low detection limit of 11.26 μM. A hydrogel membrane based on the combination of EuTPTC-NH₂ and agarose was also prepared for the quantitative visual detection of F^- in water.

Facile fabrication of amino-functionalized MIL-68(Al) metal-organic framework for effective adsorption of arsenate (As(V))

An amino-functionalized MIL-68(Al) metal–organic framework (amino-MIL-68(Al) MOF) was synthesized by solvothermal method and then characterized by FESEM, XRD, FTIR, EDX-mapping, and BET-BJH techniques. In order to predict arsenate (As(V)) removal, a robust quadratic model (R² > 0.99, F-value = 2389.17 and p value < 0.0001) was developed by the central composite design (CCD) method and then the genetic algorithm (GA) was utilized to optimize the system response and four independent variables. The results showed that As(V) adsorption on MOF was affected by solution pH, adsorbent dose, As(V) concentration and reaction time, respectively. Predicted and experimental As(V) removal efficiencies under optimal conditions were 99.45 and 99.87%, respectively. The fitting of experimental data showed that As(V) adsorption on MOF is well described by the nonlinear form of the Langmuir isotherm and pseudo-second-order kinetic. At optimum pH 3, the maximum As(V) adsorption capacity was 74.29 mg/g. Thermodynamic studies in the temperature range of 25 to 50 °C showed that As(V) adsorption is a spontaneous endothermic process. The reusability of MOF in ten adsorption/regeneration cycles was studied and the results showed high reusability of this adsorbent. The highest interventional effect in inhibiting As(V) adsorption was related to phosphate anion. The results of this study showed that amino-MIL-68(Al) can be used as an effective MOF with a high surface area (> 1000 m²/g) and high reusability for As(V)-contaminated water.

Microwell Fluoride Screen for Chemical, Enzymatic, and Cellular Reactions Reveals Latent Microbial Defluorination Capacity for -CF3 Groups

The capacity to defluorinate polyfluorinated organic compounds is a rare phenotype in microbes but is increasingly considered important for maintaining the environment. New discoveries will be greatly facilitated by the ability to screen many natural and engineered microbes in a combinatorial manner against large numbers of fluorinated compounds simultaneously. Here, we describe a low-volume, high-throughput screening method to determine defluorination capacity of microbes and their enzymes. The method is based on selective binding of fluoride to a lanthanum chelate complex that gives a purple-colored product. It was miniaturized to determine biodefluorination in 96-well microtiter plates by visual inspection or robotic handling and spectrophotometry. Chemicals commonly used in microbiological studies were examined to define usable buffers and reagents. Base-catalyzed, purified enzyme and whole-cell defluorination reactions were demonstrated with fluoroatrazine and showed correspondence between the microtiter assay and a fluoride electrode. For discovering new defluorination reactions and mechanisms, a chemical library of 63 fluorinated compounds was screened in vivo with Pseudomonas putida F1 in microtiter well plates. These data were also calibrated against a fluoride electrode. Our new method revealed 21 new compounds undergoing defluorination. A compound with four fluorine substituents, 4-fluorobenzotrifluoride, was shown to undergo defluorination to the greatest extent. The mechanism of its defluorination was studied to reveal a latent microbial propensity to defluorinate trifluoromethylphenyl groups, a moiety that is commonly incorporated into numerous pharmaceutical and agricultural chemicals. IMPORTANCE Thousands of organofluorine chemicals are known, and a number are considered to be persistent and toxic environmental pollutants. Environmental bioremediation methods are avidly being sought, but few bacteria biodegrade fluorinated chemicals. To find new organofluoride biodegradation, a rapid screening method was developed. The method is versatile, monitoring chemical, enzymatic, and whole-cell biodegradation. Biodegradation of organofluorine compounds invariably releases fluoride anions, which was sensitively detected. Our method uncovered 21 new microbial defluorination reactions. A general mechanism was delineated for the biodegradation of trifluoromethylphenyl groups that are increasingly being used in drugs and pesticides.

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⁺.

Determination of Arsenic Content in Water Using a Silver Coordination Polymer

In this report, we describe a practical method for the colorimetric determination of dissolved inorganic arsenic content in water samples, using a silver coordination polymer as the sensing material. We demonstrate that a crystalline polymer framework can be used to stabilize silver(I) ions, greatly reducing both photosensitivity and water solubility, while still affording sufficient reactivity to detect arsenic in water samples at low parts-per-billion (ppb) levels. Test strips fabricated with the silver-based polymer are shown to be effective for field tests of groundwater under real-world operating conditions and display performance that is competitive with commercially available mercury-based test strips. Spectroscopic methods are also used to probe the reaction products formed, in order to better understand the sensing mechanism. Thus, our work provides the foundation for an improved field test that could be deployed to help manage groundwater usage in regions where arsenic contamination is problematic but sophisticated lab testing is not readily available.

Construction of Pyrazine-Appended 1D and 3D Cobalt(II) Succinate Coordination Polymers: Influence of Solvent on Architectures and Applications in Gas Adsorption and NAC Detection

An underexplored reaction of pyrazine (rigid and linear) and succinic acid (flexible) with Co(NO₃)₂·6H₂O afforded four new coordination polymers (CPs): [Co(H₂O)(pyz)(suc)] (1), [Co(H₂O)₂(pyz)(suc)] (2), [Co(H₂O)₄(pyz)](suc) (3) and [Co₂(H₂O)₂(pyz)(suc)₂] (4), as well as [Co(HCO₂)₂(pyz)] (5) being lately reported along with well-known 6 and 7. The CPs were obtained as stable crystalline materials and characterized by conventional solid-state techniques, including X-ray crystallography. Hydrothermally produced compounds 1 and 2 were both 3D CPs. While 3 and 4 obtained under ambient/solvothermal conditions in DMSO generated 1D and 3D structures, 5 isolated from DMF under solvothermal conditions had a 3D structure. The topologies of the coordination polymers 1-7 were described by underlying nets 3D 5-c fet, 3D 4-c cds, 1D 2-c 2C1, 3D 5-c bnn, 3D 6-c rob, 1D 2-c 2C1, and 3D 6-c pcu, respectively. The plot of χ_M ^-1 versus T was essentially linear in the entire temperature range following the Curie-Weiss law with a Curie constant (C) of 2.525 and a negative Weiss constant (ϕ) of -46.24 K, suggesting weak antiferromagnetic (AF) exchange interactions. CO₂ and N₂ adsorption studies of 1-5 featured type III isotherms. 1 was found to show remarkably higher quenching efficiencies for nitrophenols (η = 98% for o-NP) over other NACs. The Stern-Volmer plot exhibited deviation in linearity with K _sv values about 200 times greater than that for the simplest nitroaromatic compound (NB), signifying its exclusive quenching ability toward 1. The LOD for p-NP addition to 1 was found to be 0.995 ppm.

Versatile Covalent Postsynthetic Modification of Metal Organic Frameworks via Thermal Condensation for Fluoride Sensing in Waters

Having access to safe drinking water is one of the 17 sustainable development goals defined by the United Nations (UN). However, many settlements around the globe have limited access to drinkable water due to non-anthropogenic pollution of the water sources. One of those pollutants is fluoride, which can induce major health problems. In this manuscript, we report on a post synthetic functionalization of metal organic frameworks for the sensing of fluoride in water. The proposed thermal condensation methodology allows for a high yield of functionalization using few steps, reducing reagent costs and generating minimal by-products. We identified a Rhodamine B functionalized Al-BDC-NH2 metal organic framework as one particularly suitable for fluoride detection in water.

A switchable sensor and scavenger: detection and removal of fluorinated chemical species by a luminescent metal-organic framework

Fluorosis has been regarded as a worldwide disease that seriously diminishes the quality of life through skeletal embrittlement and hepatic damage. Effective detection and removal of fluorinated chemical species such as fluoride ions (F⁻) and perfluorooctanoic acid (PFOA) from drinking water are of great importance for the sake of human health. Aiming to develop water-stable, highly selective and sensitive fluorine sensors, we have designed a new luminescent MOF In(tcpp) using a chromophore ligand 2,3,5,6-tetrakis(4-carboxyphenyl)pyrazine (H₄tcpp). In(tcpp) exhibits high sensitivity and selectivity for turn-on detection of F⁻ and turn-off detection of PFOA with a detection limit of 1.3 μg L⁻¹ and 19 μg L⁻¹, respectively. In(tcpp) also shows high recyclability and can be reused multiple times for F⁻ detection. The mechanisms of interaction between In(tcpp) and the analytes are investigated by several experiments and DFT calculations. These studies reveal insightful information concerning the nature of F⁻ and PFOA binding within the MOF structure. In addition, In(tcpp) also acts as an efficient adsorbent for the removal of F⁻ (36.7 mg g⁻¹) and PFOA (980.0 mg g⁻¹). It is the first material that is not only capable of switchable sensing of F⁻ and PFOA but also competent for removing the pollutants via different functional groups.

A robust In-MOF, In(tcpp), demonstrates sensitive detection of the fluorinated chemical species F⁻ and PFOA via distinctly different luminescence signal change, and effective adsorption and removal of both species from aqueous solution.

Enhanced Fungicidal Efficacy by Co-Delivery of Azoxystrobin and Diniconazole with Cauliflower-Like Metal-Organic Frameworks NH2-Al-MIL-101

Long-term use of a single fungicide increases the resistance risk and causes adverse effects on natural ecosystems. Controlled release formulations of dual fungicides with different modes of action can afford a new dimension for addressing the current issues. Based on adjustable aperture and superhigh surface area, metal-organic frameworks (MOFs) are ideal candidates as pesticide release carriers. This study used Al3+ as the metal node and 2-aminoterephthalic acid as the organic chain to prepare aluminum-based metal-organic framework material (NH2-Al-MIL-101) with "cauliflower-like" structure and high surface area of 2359.0 m2/g. Fungicides of azoxystrobin (AZOX) and diniconazole (Dini) were simultaneously encapsulated into NH2-Al-MIL-101 with the loading content of 6.71% and 29.72%, respectively. Dual fungicide delivery system of AZOX@Dini@NH2-Al-MIL-101 demonstrated sustained and pH responsive release profiles. When the maximum cumulative release rate of AZOX and Dini both reached about 90%, the release time was 46 and 136 h, respectively. Furthermore, EC50 values as well as the percentage of inhibition revealed that AZOX@Dini@NH2-Al-MIL-101 had enhanced germicidal efficacy against rice sheath blight (Rhizoctonia solani), evidenced by the synergistic ratio of 1.83. The present study demonstrates a potential application prospect in sustainable plant protection through co-delivery fungicides with MOFs as a platform.

Designed multifunctional ratiometric fluorescent probe for directly detecting fluoride ion/ dichromate and indirectly monitoring urea

UiO-66-NH₂@eosin Y composite was obtained by confining eosin Y (EY) into the cavities of Zr-MOF and could emit two fluorescence peaks at 453 and 543 nm at an excitation wavelength of 355 nm. This multi-responsive and multifunctional ratiometric fluorescent nanoprobe not only enable directly distinct detection of F^-/Cr₂O₇^2- with ultra-high selectivity and sensitivity, but also could indirectly monitor the concentration of urea based on unique enzymatic hydrolysis reaction. The multifunctional probe was utilized for fluorescence labeling F-/Cr₂O₇^2- in sweat latent fingerprint through an environmentally friendly powder strategy and exhibited obvious luminescence visualization changes. Notably, the corresponding portable on-line test strips of probe for detection of F^- and Cr₂O₇^2- were made for monitoring the levels of F^- and Cr₂O₇^2-. Furthermore, the probe was applied to evaluate the degrees of F^-/Cr₂O₇^2- in HepG-2 cell and urea in serum with superior results,which indicate the potential application of the as-synthesized UiO-66-NH₂@EY as multifunctional probe for the detection of F^-, Cr₂O₇^2- and urea in biological samples. Finally, in order to extend the device-based applications of probe, an AND-OR-coupled molecular logic gate was put on agenda.

Highly Sensitive Colorimetric Naked-Eye Detection of HgII Using a Sacrificial Metal-Organic Framework

This study has developed a specific, easy, and novel approach to designing a sacrificial metal-organic framework (MOF) that can detect and measure the amount of Hg²⁺ in aqueous and nonaqueous solutions using the naked eye. The functionalized [Zn(oba)(RL3)_0.5]_n·1.5DMF (TMU-59) provides the ability of simple visual assessment or colorimetric readout without sophisticated analytical equipment. Because of the special interaction with Hg²⁺, degradation of the structure of this unique MOF causes the solution to change color from colorless to a pink that is easily recognizable to the naked eye. The presence of a methyl group plays a major role in naked-eye detection by a qualitative sensor. Furthermore, this qualitative sensor data for the production of a simple, instant, and portable red, green, and blue (RGB)-based quantitative sensor were used to determine the concentration of Hg²⁺ in different specimens. As a turn-off fluorescence sensor, this unique structure is also capable of detecting Hg²⁺ at very low concentrations (the limit of detection is 0.16 ppb). To the best of our knowledge, TMU-59 is the first MOF-based naked-eye sensor that can successfully and specifically display the presence of Hg²⁺ through a major color change.

Recently Developed Adsorbing Materials for Fluoride Removal from Water and Fluoride Analytical Determination Techniques: A Review

In recent years, there has been an increase in public perception of the detrimental side-effects of fluoride to human health due to its effects on teeth and bones. Today, there is a plethora of techniques available for the removal of fluoride from drinking water. Among them, adsorption is a very prospective method because of its handy operation, cost efficiency, and high selectivity. Along with efforts to assist fluoride removal from drinking waters, extensive attention has been also paid to the accurate measurement of fluoride in water. Currently, the analytical methods that are used for fluoride determination can be classified into chromatographic methods (e.g., ionic chromatography), electrochemical methods (e.g., voltammetry, potentiometry, and polarography), spectroscopic methods (e.g., molecular absorption spectrometry), microfluidic analysis (e.g., flow injection analysis and sequential injection analysis), titration, and sensors. In this review article, we discuss the available techniques and the ongoing effort for achieving enhanced fluoride removal by applying novel adsorbents such as carbon-based materials (i.e., activated carbon, graphene oxide, and carbon nanotubes) and nanostructured materials, combining metals and their oxides or hydroxides as well as natural materials. Emphasis has been given to the use of lanthanum (La) in the modification of materials, both activated carbon and hybrid materials (i.e., La/Mg/Si-AC, La/MA, LaFeO3 NPs), and in the use of MgO nanostructures, which are found to exhibit an adsorption capacity of up to 29,131 mg g−1. The existing analytical methodologies and the current trends in analytical chemistry for fluoride determination in drinking water are also discussed.

Selective and Fast Detection of Fluoride-Contaminated Water Based on a Novel Salen-Co-MOF Chemosensor

The development of selective and fast optical sensitive chemosensors for the detection and recognition of different cations and anions in a domain is still a challenge in biological, industrial, and environmental fields. Herein, we report a novel approach for the detection and determination of fluoride ion (F-) sensing based on a salen-cobalt metal-organic framework (Co(II)-MOF). By a simple method, the Co(II)-MOF was synthesized and characterized using several tools to elucidate the structure and morphology. The photoluminescence (PL) spectrum of the Co(II)-MOF (100.0 nM/L) was examined versus different ionic species like F-, Br-, Cl-, I-, SO4 2-, and NO3 - and some cationic species like Mg2+, Ca2+, Na+, and K+. In the case of F- ions, the PL intensity of the Co(II)-MOF was scientifically enhanced with a remarkable red shift. With the increase of F- concentration, the Co(II)-MOF PL emission spectrum was also professionally enhanced. The limit of detection (LOD) for the Co(II)-MOF chemosensor was 0.24 μg/L, while the limit of quantification (LOQ) was 0.72 μg/L. Moreover, a comparison of the Co(II)-MOF optical approach with other published reports was studied, and the mechanism of interaction was also investigated. Additionally, the applicability of the current Co(II)-MOF approach in different real water samples, such as tap water, drinking water, Nile River water, and wastewater, was extended. This easy-to-use future sensor provides reliable detection of F- in everyday applications for nonexpert users, especially in remote rural areas.

A chromo-fluorogenic HMT sensor for Ag+ and the resultant HMT-Ag complex turn-off probe for F- in DMSO: experimental and theoretical studies

The photophysical properties of Hexamethylenetetramine (HMT) were investigated through physical methods and spectroscopically in dimethyl sulfoxide (DMSO) at ambient temperature. Evidently, HMT turned out as a sensor, selective and sensitive to silver ion (Ag⁺) only, among other cations, through colorimetric and fluorometric activities (observable by naked eye) and spectrally, both by UV-Vis and fluorescence spectroscopy. The resulting complex pedant (HMT-Ag) is highly responsive to the presence of fluoride ion (F⁻) in aqueous soluble DMSO, evidenced by changes in absorption spectra as well as fluorescence quenching, upon addition of the respective ions. Consequently, spectral changes induced by the addition of these ions, were consistently concomitant with colour changes, from colourless to light brown (HMT-Ag) to dark brown (HMT-Ag-F) in daylight condition, while bright light blue colour (HMT) to dark blue brownish (HMT-Ag) under UV-light conditions. The experimental results were complimented by theoretical studies, which are well within agreement of one another.

Optical Anion Receptors with Urea/Thiourea Subunits on a TentaGel Support

Two simple chemosensors with urea (L1) and thiourea (L2) groups were synthesized and studied by different spectroscopic techniques. Both receptors can sense acetate (Ac^-), dihydrogen phosphate (H₂PO₄ ^-), and fluoride (F^-) anions, accompanied by changes in UV-vis and ¹H NMR spectra, and an optical response is observed as a color change of the solutions due to deprotonation and hydrogen-bonding processes. Also, L1 and L2 were supported on TentaGel resins (R1 and R2), and their fluoride-sensing properties in DMSO and water solutions were studied. Interestingly, R2 can sense fluoride ions in sample solutions of 100% water.

Presence of fluoride in water diminishes fast the SPR peak of silver nanocrystals showing large red shift with quick sedimentation - A fast sensing and fast removal case

The medicinal use of fluoride is beneficial up to a low ppm level but regular ingestion at high doses show many adverse effects in human. Fluoride may get ingested through drinking water specially through ground water near the hilly regions, where fluoride content is huge. Hence, fluoride sensing and removal of it from water is very important as fluoride contaminated water is transparent and not easily detectable. Here, we have studied colorimetric and spectrophotometric techniques for sensing of fluoride along with its fast removal from water by using prepared saponin capped silver nanoparticles (AgNPs). Colorimetric study has confirmed the presence of fluoride ions in water samples above the colorimetric detection limit of 10 ppm. But spectroscopic sensing further provides more lower limit of sensing with the total removal of fluoride ions up to 1.2 ppm level. On repeating this this study 15 times, sensing and removal of fluoride is found to be well reproducible. The interaction of fluoride ions with silver nanoparticles has resulted in sedimentation of fluoride ions in the form of fluoride-nanoparticles complex and that interaction has been confirmed by FTIR spectral study of the sedimented part. EDX analysis has also provided the information of easy removal of the fluoride ions from water as presence of fluoride peak has been found in EDX spectra.

Lowest aqueous picomolar fluoride ions and in vivo aluminum toxicity detection by an aluminum(iii) binding chemosensor

Aluminum toxicity in biological systems is a well-known issue yet remains as a prevalent and unsolvable problem due to the lack of proper molecular tools that can detect free aluminum(iii) or Al(iii) ions in vivo. Herein, we report a water-soluble photo-induced electron transfer (PET)-based turn-ON/OFF fluorometric chemosensor for the dual detection of Al(iii) and fluoride ions in aqueous media with a nanomolar (∼1.7 × 10^-9 M) and picomolar (∼2 × 10^-12 M, lowest ever detection so far) detection limit, respectively. Fluoride ions in sea water could be detected as well as the recognition of non-contamination in drinking water. In addition, using live-cell microscopy, Al(iii) ions were detected in live biological samples in vivo to aid establishing the aluminum-toxicity effect.

Fluoride sensing performance of fluorescent NH2-MIL-53(Al): 2D nanosheets vs. 3D bulk

Due to their ultra-thin morphology, larger specific surface area and more exposed active sites, two-dimensional (2D) metal-organic framework (MOF) nanosheets can break the limitations of three-dimensional (3D) MOFs in sensitivity, response speed and the limit of detection for sensing applications. In this work, fluorescent NH2-MIL-53(Al) nanosheets were developed as a fluoride detection sensor compared with the 3D bulk counterpart. The morphological and structural characteristics of the obtained products were systematically characterized, and the favourable chemical and fluorescence stability of the NH2-MIL-53(Al) nanosheets were explored. The fluorescent NH2-MIL-53(Al) nanosheets showed high sensitivity, fast response speed (as short as 10 seconds), low limit of detection (15.2 ppb), and wide linear detection range (5-250 μM), and all performances were better than those of their bulk counterpart. In addition, the sensing mechanism was investigated to be based on the transformation of the NH2-MIL-53(Al) framework that induced the release of fluorescent ligands, resulting in an exceptionally enhanced fluorescence. This work highlights the advantages of 2D MOF nanosheets in fluorescence sensing applications.

Metal-Organic Framework Nanoparticles Induce Pyroptosis in Cells Controlled by the Extracellular pH

Ion homeostasis is essential for cellular survival, and elevated concentrations of specific ions are used to start distinct forms of programmed cell death. However, investigating the influence of certain ions on cells in a controlled way has been hampered due to the tight regulation of ion import by cells. Here, it is shown that lipid-coated iron-based metal-organic framework nanoparticles are able to deliver and release high amounts of iron ions into cells. While high concentrations of iron often trigger ferroptosis, here, the released iron induces pyroptosis, a form of cell death involving the immune system. The iron release occurs only in slightly acidic extracellular environments restricting cell death to cells in acidic microenvironments and allowing for external control. The release mechanism is based on endocytosis facilitated by the lipid-coating followed by degradation of the nanoparticle in the lysosome via cysteine-mediated reduction, which is enhanced in slightly acidic extracellular environment. Thus, a new functionality of hybrid nanoparticles is demonstrated, which uses their nanoarchitecture to facilitate controlled ion delivery into cells. Based on the selectivity for acidic microenvironments, the described nanoparticles may also be used for immunotherapy: the nanoparticles may directly affect the primary tumor and the induced pyroptosis activates the immune system.

Quinoline appended pillar[5]arene (QPA) as Fe3+ sensor and complex of Fe3+ (FeQPA) as a selective sensor for F-, arginine and lysine in the aqueous medium

A quinoline functionalized pillar[5]arene, QPA has been prepared and its interaction with biologically relevant ions and molecules in aqueous solution has been demonstrated. The sensor molecule, QPA has shown selectivity towards Fe³⁺ among eleven metal ions studied. The Fe³⁺ complex of QPA (FeQPA) selectively interacts with F^- among halides by ∼4 fold fluorescence enhancement. Further, FeQPA has shown selectivity towards arginine and lysine among twenty naturally occurring amino acids. The binding of QPA with Fe³⁺ has been confirmed by MALDI-TOF and ¹H NMR titrations.

Luminescent metal organic frameworks-based chemiluminescence resonance energy transfer platform for turn-on detection of fluoride ion

A luminescent metal organic frameworks (MOFs)-based chemiluminescence resonance energy transfer (CRET) platform was constructed for turn-on detection of fluoride ion. A hybrid MOFs was prepared by encapsulating strong fluorescence 2',7'-dichlorofluorescein (DCF) into the frames of NH₂-MIL-101(Al) MOFs, which led to a significant suppression of fluorescence signal of DCF. In the presence of fluoride ion, it destroyed the structure of the hybrid MOFs and released DCF molecules from the frames due to the formation of more stable aluminum hexafluoride complex ions [AlF₆^3-] between fluoride ion and aluminum ion. The released DCF molecules accepted the energy originating from the chemical reaction of bis(2,4,6-trichlorophenyl)oxalate (TCPO) with hydrogen peroxide (H₂O₂), producing a strong chemiluminescence (CL) emission. The CL signal was strong dependent on the concentration of fluoride ion presented and showed a linear response in the range of 0.5-80.0 μmol L^-1 (9.5 μg L^-1-1.52 mg L^-1). The detection limit was 0.05 μmol L^-1 (about 0.95 μg L^-1) fluoride ion and the relative standard deviations was 2.3% for 40.0 μmol L^-1 fluoride ion solution (n = 11). This MOFs-based CRET method was successfully applied to the determination of fluoride ion in drinking water samples, demonstrating its potential application in analysis of real samples.

Fluorescent "Turn-on" Sensing Based on Metal-Organic Frameworks (MOFs)

Metal-organic frameworks (MOFs) have evolved as an exciting class of materials in the domain of porous materials. The unique features of these materials arise from the combined properties of metal ions/clusters and organic struts which form the building blocks of these fascinating architectures. Among other multifarious applications, MOFs have shown tremendous applications as sensory materials for a wide variety of species. The signal transduction induced mechanism in these confined nanospaces generate optical output in response to a particular analyte which can be detected by wide variety of detection techniques. Fluorometric methods of sensing is one of widely studied method over past few decades. MOF-based fluorometric detection is a key research theme developed over the past few years. In this review, we give a brief overview of the recent developments of MOFs as "turn-on" sensors for a wide range of analytes (viz. cations, anions, volatile organic compounds (VOCs), etc.).

β-carboline-based turn-on fluorescence chemosensor for quantitative detection of fluoride at PPB level

A novel β-carboline-based chemosensor, having an acidic NH proton that leads to fluoride-induced deprotonation involving a vivid color change from colorless to yellow is described. The absorption spectrum of the chemosensor in acetonitrile has a peak at 375 nm, which changes to 428 nm with the gradual addition of only fluoride in the solution with a clear isosbestic points at 357 nm and 392 nm. More interestingly, the chemosensor gives a turn-on type of fluorescence at 554 nm in the presence of fluoride. Further it was found that the sensor is highly selective towards fluoride over other anions including chloride, bromide, iodide, nitrate, borate, perchlorate and can quantitatively detect fluoride at ppb level with a limit of detection of 0.02 mg/ L or 20 ppb. The chemosensor was successfully demonstrated to assess the fluoride concentration in the tap water.

A simple urea-based multianalyte and multichannel chemosensor for the selective detection of F−, Hg2+ and Cu2+ in solution and cells and the extraction of Hg2+ and Cu2+ from real water sources: a logic gate mimic ensemble

Herein, a hydrazine-based chromogenic, fluorogenic and electrochemical chemosensor BCC [1,5-bis(4-cyanophenyl) carbonohydrazide] was reported.

A dye@MOF crystalline probe serving as a platform for ratiometric sensing of trichloroacetic acid (TCA), a carcinogen metabolite in human urine

Novel microporous dual-emitting dye@MOF FS@1 hybrid has been designed and prepared to effectively detect TCA, the biomarker for carcinogenic TCE in human urine.

A Novel Tb@Sr-MOF as Self-Calibrating Luminescent Sensor for Nutritional Antioxidant

Sesamol, is well-known antioxidant and can reduce the rate of oxidation and prolong expiration date. It is also potentially antimutagenic and antihepatotoxic, the detection of sesamol is important and remains a huge challenge. Herein, a new 3D alkaline earth Sr metal organic framework [Sr(BDC)DMACH₂O]n (BDC = benzene-1,4-dicarboxylate; DMAC = N,N-dimethylacetamide) is synthesized and a probe based on Tb3+ functionalized Sr-MOF. The Tb(3+)@Sr-MOF showed good luminescence and thermal property. Due to the energy competition between sesamol and ligand, the luminescence intensity of sesamol increases meantime luminescence intensity of Tb3+ decreases, the ratio of the emission intensities (I344/I545) linearly increases with sesamol in concentrations ranging from 1 × 10-7 to 8 × 10-4 M. Furthermore, the fluorescence-detected circular test shows that the composite Tb(3+)@Sr-MOF can serve as ratiometric sensor for sensing of sesamol. This is the first example for self-calibrated detecting sesamol based on metal-organic framework (MOF).

Extraordinary sensitivity for H2S and Fe(iii) sensing in aqueous medium by Al-MIL-53-N3 metal-organic framework: in vitro and in vivo applications of H2S sensing

An Al(iii) metal-organic framework (MOF) called Al-MIL-53-N₃ (1) was synthesized under solvothermal reaction conditions using Al(NO₃)₃·9H₂O and H₂BDC-N₃ (H₂BDC-N₃ = 2-azido-1,4-benzenedicarboxylic acid) ligand in a DMF/water (DMF = N,N-dimethylformamide) mixture. Phase purity was checked by performing X-ray powder diffraction, infrared spectroscopy and thermogravimetric analysis. Thermogravimetric analysis suggests that 1 is highly stable up to 300 °C under air atmosphere. The activated 1 (called 1') showed a very fast fluorescence response to H₂S (turn-on) and Fe(iii) ions (turn-off) in an aqueous medium with excellent sensitivity and selectivity even in the presence of other potentially intrusive analytes. In the presence of H₂S, the conversion of the azide moiety to amine is responsible for the fluorescence turn-on properties. On the other hand, the partial replacement of framework Al(iii) ions by Fe(iii) can be assigned for the selective detection behavior to Fe(iii) ions. The detection limits (90.47 nM for H₂S and 0.03 μM for Fe(iii) ions in water) of 1' are lower than those of the formerly reported MOF type of fluorescent sensors. The 1'-loaded J774A.1 macrophage cells are healthy and respond to intracellular H₂S to exhibit strong blue fluorescence, confirming its suitability to detect H₂S inside the cells. In addition, 1' can detect H₂S in human blood plasma (HBP) and sulfide ions in real water samples. These features make 1' a very promising candidate for the on-site sensing of Fe(iii) ions and the detection of intracellular and extracellular H₂S.