Metal–organic frameworks of zeolitic imidazolate framework-7 and zeolitic imidazolate framework-60 for fast mercury and methylmercury speciation analysis

The design, synthesis and application of metal-organic framework-based fluorescence sensors

Fluorescence-based chemical sensors have garnered significant attention due to their rapid response, high sensitivity, cost-effectiveness and ease of operation. Recently, metal-organic frameworks (MOFs) have been extensively utilized as platforms for constructing fluorescence sensors, owing to their ultra-high porosity, flexible tunability, and excellent luminescent properties. This feature article summarizes the progress made mainly by our research group in recent years in the construction strategies, principles, and types of MOF sensors, as well as their applications in quantitative sensing, qualitative identification analysis, and multimodal/multifunctional analysis. In addition, the challenges and an outlook on the future progression of MOF-based sensors are discussed, highlighting how these studies can contribute to addressing these issues. Hopefully, this feature article can provide some valuable guidance for the construction and application of MOFs in fluorescence sensing, thereby broadening their practical applications.

Bifunctional materials based on poly(3-aminocarbazole) for efficient and highly selective detection and adsorption of Hg2+ in water

Herein, two poly(3-aminocarbazole) derivatives containing imidazole N-type acceptor were synthesized and reported, which are named PCPI and PCBI respectively. The fluorescence spectrum shows that PCPI (Em = 498 nm) and PCBI (Em = 398 nm) both have a strong fluorescence emission. It is worth noting that PCPI has a larger stokes shift of 153 nm, which is beneficial for improving the sensitivity of the sensor and enhancing its anti-interference ability. As expected, our experimental results indicate that both PCPI and PCBI can cause a specific response of "fluorescence OFF" to Hg2+ compared with other ions. And PCPI and PCBI both have excellent detection capabilities for Hg2+, with detection limits of 69.8 nM and 11.4 nM respectively. Moreover, PCBI exhibits excellent absorption of Hg2+ with a maximum absorption capacity of 477.8 mg/g at 20 °C. It indicates that PCBI can be used as a functional material for the detection and removal of Hg2+ in water.

Facile construction of copper-doped metal organic framework as a novel visible light-responsive photocatalyst for contaminant degradation

ABSTRACTMetal-organic frameworks (MOFs) with photocatalytic activity have garnered significant attentions in environmental remediation. Herein, copper-doped zeolitic imidazolate framework-7 (Cu-doped ZIF-7) was synthesized rapidly and easily using a microwave-assisted technique. Various analytical and spectroscopic methods were employed to access the framework, morphology, light absorption, photo-electrochemical and photocatalytic performance of the synthesized materials. Compared to ZIF-7, Cu/ZIF-7 (molar ratio of Cu2+ to Zn2+ is 1:1) demonstrates superior visible light absorption ability, narrower band gap, enhanced charge separation capability, and reduced electron-hole recombination performance. Under visible light irradiation, Cu/ZIF-7 serves as a Fenton-like catalyst and demonstrates exceptional activity for contaminant degradation, while virgin ZIF-7 remains inactive. With the addition of 9.8 mmol H2O2 and exposure to visible light for 30 min, 10 mg of Cu/ZIF-7 can completely decompose RhB solution (10 mg/L, 50 mL). The synergistic effect of the Cu/ZIF-7/H2O2/visible light system is attributed to visible light photocatalysis and Fenton-like reactions. Cu/ZIF-7 demonstrates excellent catalytic performance stability, with only a slight decrease in degradation efficiency from an initial 97.0% to 95.4% over four cycles. Additionally, spin-trapping ESR measurements and active species trapping experiments revealed that h+ and ·OH occupied a significant position for Rhodamine B (RhB) degradation. Degradation intermediate products of Rhodamine B have been identified using UPLC-MS, and the degradation pathways have been proposed and discussed. This work offers a facile and efficient technique for developing MOF-based visible light photocatalysts for water purification.

Sulfate Radical-Based Degradation of Organic Pollutants: A Review on Application of Metal-Organic Frameworks as Catalysts

The degradation of organic pollutants present in domestic and industrial effluents is a matter of concern because of their high persistence and ecotoxicity. Recently, advanced oxidation processes (AOPs) are being emphasized for organic pollutant removal from effluents, as they have shown higher degradation efficiencies when compared to conventional activated sludge processes. Sulfate radical-based methods are some of the AOPs, mainly carried out using persulfate (PS) and peroxymonosulfate (PMS), which have gained attention due to the ease of sulfate radical generation and the effective degradation of organic molecules. PMS is gaining more popularity because of its high reactivity and ability to generate excess sulfate radicals. PMS has been the major focus; therefore, its mechanism has been explained, and limitations have been elaborated. The involvement of metal-organic frameworks for PMS/PS activation applied to organic pollutant removal and recent advances in the application of biochar and hydrogel-assisted metal-organic frameworks have been discussed.

One-Pot Pretreatment Coupled to Microplasma Optical Emission Spectrometry for Field and Sensitive Determination of Inorganic Mercury and Methylmercury in Fish

Field and sensitive analysis of mercury species in seafood is helpful to assess the risk of human exposure to mercury, but the cumbersome pretreatment process is time-consuming and laborious. Herein, a simple one-pot pretreatment system is designed for extraction, separation, and enrichment of inorganic mercury (Hg(II)) and methylmercury (MeHg) in fish, and coupled to dielectric barrier discharge (DBD) microplasma optical emission spectrometry (OES). Both Hg(II) and MeHg species in fish can be effectively extracted by tetramethylammonium hydroxide under ultrasound, then separated from the fish matrix by vapor generation and photochemical vapor generation, and finally enriched on the activated carbon electrode tips. Mercury trapped on the activated carbon electrode tips can be rapidly released to produce OES under the DBD microplasma excitation for quantitative analysis. The pretreatment and analysis of a batch of 12 samples are completed within 50 min, and the extraction efficiency of total mercury is up to 90% for 100 mg of freeze-dried fish or 86% for 1 g of fresh fish. Under the optimized conditions, the detection limits are 2 μg kg^-1 for Hg(II) and 1.2 μg kg^-1 for MeHg in freeze-dried fish, and precisions are 3.2% for Hg(II) and 3.9% for MeHg. The present method is applied to the analysis of the certified reference material and real marine fishes, giving rise to spiked recoveries of 95-103%. The present system hardly leads to MeHg and Hg(II) transforming into each other during extraction, providing a simple, convenient, and low-cost analytical tool to evaluate the risk of mercury species in fish.

Aqueous Pb(II) Removal Using ZIF-60: Adsorption Studies, Response Surface Methodology and Machine Learning Predictions

Zeolitic imidazolate frameworks (ZIFs) are increasingly gaining attention in many application fields due to their outstanding porosity and thermal stability, among other exceptional characteristics. However, in the domain of water purification via adsorption, scientists have mainly focused on ZIF-8 and, to a lesser extent, ZIF-67. The performance of other ZIFs as water decontaminants is yet to be explored. Hence, this study applied ZIF-60 for the removal of lead from aqueous solutions; this is the first time ZIF-60 has been used in any water treatment adsorption study. The synthesized ZIF-60 was subjected to characterization using FTIR, XRD and TGA. A multivariate approach was used to investigate the effect of adsorption parameters on lead removal and the findings revealed that ZIF-60 dose and lead concentration are the most significant factors affecting the response (i.e., lead removal efficiency). Further, response surface methodology-based regression models were generated. To further explore the adsorption performance of ZIF-60 in removing lead from contaminated water samples, adsorption kinetics, isotherm and thermodynamic investigations were conducted. The findings revealed that the obtained data were well-fitted by the Avrami and pseudo-first-order kinetic models, suggesting that the process is complex. The maximum adsorption capacity (q_max) was predicted to be 1905 mg/g. Thermodynamic studies revealed an endothermic and spontaneous adsorption process. Finally, the experimental data were aggregated and used for machine learning predictions using several algorithms. The model generated by the random forest algorithm proved to be the most effective on the basis of its significant correlation coefficient and minimal root mean square error (RMSE).

Green Synthesis of Zeolitic Imidazolate Frameworks: A Review of Their Characterization and Industrial and Medical Applications

Metal organic frameworks (MOF) are a class of hybrid networks of supramolecular solid materials comprising a large number of inorganic and organic linkers, all bound to metal ions in a well-organized fashion. Zeolitic imidazolate frameworks (ZIFs) are a sub-group of MOFs with imidazole as an organic linker to metals; it is rich in carbon, nitrogen, and transition metals. ZIFs combine the classical zeolite characteristics of thermal and chemical stability with pore-size tunability and the rich topological diversity of MOFs. Due to the energy crisis and the existence of organic solvents that lead to environmental hazards, considerable research efforts have been devoted to devising clean and sustainable synthesis routes for ZIFs to reduce the environmental impact of their preparation. Green chemistry is the key to sustainable development, as it will lead to new solutions to existing problems. Moreover, it will present opportunities for new processes and products and, at its heart, is scientific and technological innovation. The green chemistry approach seeks to redesign the materials that make up the basis of our society and our economy, including the materials that generate, store, and transport our energy, in ways that are benign for humans and the environment and that possess intrinsic sustainability. This study covers the principles of green chemistry as used in designing strategies for synthesizing greener, less toxic ZIFs the consume less energy to produce. First, the necessity of green methods in today's society, their replacement of the usual non-green methods and their benefits are discussed; then, various methods for the green synthesis of ZIF compounds, such as hydrothermally, ionothermally, and by the electrospray technique, are considered. These methods use the least harmful and toxic substances, especially concerning organic solvents, and are also more economical. When a compound is synthesized by a green method, a question arises as to whether these compounds can replace the same compounds as synthesized by non-green methods. For example, is the thermal stability of these compounds (which is one of the most important features of ZIFs) preserved? Therefore, after studying the methods of identifying these compounds, in the last part, there is an in-depth discussion on the various applications of these green-synthesized compounds.

Two porous three-dimensional (3D) metal–organic frameworks based on diverse metal clusters: selective sensing of Fe3+ and Cr2O72−

Using a rigid 3,5-di(2′,5′-dicarboxylphenyl)benzoic acid, two porous 3D MOFs have been synthesized and characterized, and the luminescent properties have been studied.

Detection of hydrogen peroxide and glucose with a novel fluorescent probe by the enzymatic reaction of amino functionalized MOF nanosheets

Amino-functionalized two-dimensional (2D) MOFs have great potential in biosensors due to their excellent water solubility, high fluorescence, large specific surface area, good adsorption properties and good ability to enrich the target analytes. Fluorescence detection of hydrogen peroxide and glucose mostly relies on monitoring the single fluorescence intensity changes in a single excitation wavelength. Here, a ratiometric fluorescence sensor based on NH₂-MIL-53(Al) nanosheets to sensitively detect H₂O₂ and glucose through enzymatic reactions was developed. o-Phenylenediamine (OPD) was oxidized by H₂O₂ in the presence of horseradish peroxidase (HRP). Then, the oxidation product could be self-assembled on NH₂-MIL-53(Al) nanosheets by hydrogen bonding and π-π stacking. The orbital interaction or the fluorescence resonance energy transfer (FRET) between the nanosheets and the oxidation product could effectively quench the fluorescence of the nanosheets at 433 nm. At the same time, the oxidation product provided a new emission peak at 564 nm. The fluorescence ratio signal changes generated by this oxidation process were used to stably and sensitively detect H₂O₂ and glucose. Structural and mechanistic analysis was carried out by calculation methods such as AICD and ORCA to explore the π electron structure characteristics, the hole/electron orbitals and the quenching phenomenon. The detection limit was 26.9 nM for H₂O₂ and 0.041 μM for glucose. The detection of glucose in human serum has a satisfactory recovery of 97.4-102.8%. It is clear that the sensor has a good application prospect in real sample analysis.

Metal-organic framework application for mercury speciation using solid phase extraction followed by direct thermal release-electrothermal atomization atomic absorption spectrophotometric detection (ETA AAS)

Metal-organic frameworks (MOFs) are increasingly used in analytical chemistry for pre-concentration of trace elements followed by their determination using modern analytical techniques. However, there are a limited number of publications concerning the use of MOFs for speciation purposes, while their structural and functional features are perspective for the element species selective extraction and pre-concentration. It is known that mercury refers to the most hazardous elements which species demonstrate different toxicity, migration routes and bioavailability as well. Consequently the development of new approaches for mercury speciation in environments remains an actual objective of analytical chemistry. In present work a new methodology for inorganic and organic mercury speciation in water was proposed. This approach is based on pre-concentration using solid phase extraction (SPE) followed by their determination directly from the solid phase with the application of the thermal release - electrothermal-atomic-absorption technique (TR-ETA-AAS). An original SPE-procedure based on the use of UIO-66 [Zr₆O₄(OH)₄(bdc)₆] in two different modes (non-modified and modified with cysteine) as a sorbent was designed. As a result of SPE as well as TR-ETA-AAS optimization the detection limits (LOD) for all listed species at the level of 0.06 μg L^-1 have been achieved. It was also shown that the presence of the other elements (K, Na, Ca, Mg at the level of 100 μg L^-1, and Mn, Fe, Cr, Al, Zn, Cd, Pb - of 25 μg L^-1) does not affect the results obtained. The developed assay demonstrates a high efficiency, low LODs, wide linear range and admissible analysis duration. The reliability of the data obtained was confirmed by the standard addition approach and by a comparison with the results of independent analytical methods.

Inner-filter effect induced fluorescent sensor based on fusiform Al-MOF nanosheets for sensitive and visual detection of nitrofuran in milk

Developing highly sensitive and visual methods for rapid detection of antibiotics is significant to ensure food quality and safety. To meet the requirement of nitrofuran antibiotics detection, luminescent fusiform Al(III)-containing metal-organic frameworks (Al-MOF) nanosheets were successfully synthesized by one-step hydrothermal method. And then, the nanosheet served as a fluorescent probe to detect nitrofuran via the inner-filter effect mechanism. The developed sensor allowed sensitive and selective detection of nitrofuran with good linear relationships. And, the detection limit (LOD) values were estimated to be 0.53, 0.838 and 0.583 μM for nitrofurazone, nitrofurantoin and furazolidone detection, respectively. The practical application of the proposed system was verified by HPLC in spiked milk samples with satisfying recoveries ranging from 88.14 to 126.21% and low relative standard deviations of 2.85 ~ 8.13%. Moreover, we designed fluorescent test papers for semi-quantitative detection of nitrofuran via naked-eye colorimetric assay. The established method provides an alternative strategy for semiquantitative detection of nitrofuran.

MW Synthesis of ZIF-7. The Effect of Solvent on Particle Size and Hydrogen Sorption Properties

We report here fast (15 min) microwave-assisted solvothermal synthesis of zeolitic imidazolate framework material (ZIF-7). We have optimized solvent composition to achieve high porosity and hydrogen capacity and narrow particle size distribution. It was shown that synthesis in N,N-diethylformamide (DEF) results in a layered ZIF-7 III phase, while N,N-dimethylformamide (DMF) as solvent leads to a pure ZIF-7 phase in microwave conditions. A mixture of toluene with DMF allows the production of pure ZIF-7 material only with the triethylamine additive. Obtained materials were comprehensively characterized. We have pointed out that both X-ray diffraction and infrared spectroscopy could be used for the identification of ZIF-7 or ZIF-7 III phases. Although samples obtained in DMF, and in a mixture of DMF, toluene, and triethylamine were assigned to the pure ZIF-7 phase, solvent composition significantly affected the size of particles in the material and nitrogen and hydrogen adsorption process.

Four new metal–organic frameworks based on diverse secondary building units: sensing and magnetic properties

Four new metal–organic frameworks based on diverse secondary building units were assembled and 1 showed high selectivity in the detection of CrO₄²⁻, Cr₂O₇²⁻, MnO₄⁻ and Fe³⁺.

Seven luminescent metal–organic frameworks constructed from 5-(triazol-1-yl)nicotinic acid: luminescent sensors for CrVI and MnO4− ions in an aqueous medium

Seven new luminescent metal–organic frameworks were assembled and complex 2 shows high selectivity and sensitivity for CrO₄²⁻, Cr₂O₇²⁻ and MnO₄⁻ ions with low detection limits.

Highly Efficient Luminescent Metal-Organic Framework for the Simultaneous Detection and Removal of Heavy Metals from Water

We have designed and synthesized an isoreticular series of luminescent metal-organic frameworks (LMOFs) by incorporating a strongly emissive molecular fluorophore and functionally diverse colinkers into Zn-based structures. The three-dimensional porous networks of LMOF-261, -262, and -263 represent a unique/new type of nets, classified as a 2-nodal, (4,4)-c net (mot-e type) with 4-fold, class IIIa interpenetration. All compounds crystallize in a body-centered tetragonal crystal system (space group I4₁/a). A systematic study has been implemented to analyze their interactions with heavy metals. LMOF-263 exhibits impressive water stability, high porosity, and strong luminescence, making it an excellent candidate as a fluorescent chemical sensor and adsorbent for aqueous contaminants. It is extremely responsive to toxic heavy metals at a parts per billion level (3.3 ppb Hg²⁺, 19.7 ppb Pb²⁺) and demonstrates high selectivity for heavy metals over light metals, with detection ratios of 167.4 and 209.5 for Hg²⁺/Ca²⁺ and Hg²⁺/Mg²⁺, respectively. Mixed-metal adsorption experiments also show that LMOF-263 selectively adsorbs Hg²⁺ over other heavy metal ions in addition to light metals. The Pb²⁺ K_SV value for LMOF-263 (55,017 M^-1) is the highest among LMOFs reported to date, and the Hg²⁺ K_SV value is the second highest (459,446 M^-1). LMOF-263 exhibits a maximum adsorption capacity of 380 mg Hg²⁺/g. The Hg²⁺ adsorption process follows pseudo-second-order kinetics, removing 99.1% of the metal within 30 min. An in situ XPS study provides insight to help understand the interaction mechanism between Hg²⁺ and LMOF-263. No other MOFs have demonstrated such a high performance in both the detection and the capture of Hg²⁺ from aqueous solution.

Highly sensitive and selective fluorescent probes for Hg2+ in Ag(i)/Cu(ii) 3D supramolecular architectures based on noncovalent interactions

Three novel Ag(i)/Cu(ii) metal–organic assemblies displayed 3D supramolecular architectures with the help of noncovalent interactions. 1 performs as a highly sensitive and selective fluorescent probe for Hg²⁺, and 2 shows a unique semiconductive nature.

Four new lanthanide–organic frameworks: selective luminescent sensing and magnetic properties

Four new isostructural lanthanide–organic frameworks have been synthesized, which show 2D layer-by-layer structures, and exhibit highly selective and efficient luminescence sensing for Fe³⁺ and Cr⁶⁺ ions in aqueous solution.

Porphyrinic MOFs for reversible fluorescent and colorimetric sensing of mercury(ii) ions in aqueous phase

A simple, rapid and visual assay for Hg²⁺ detection is designed based on assembling Zr–O clusters and porphyrin ligands.