Encapsulation of Strongly Fluorescent Carbon Quantum Dots in Metal–Organic Frameworks for Enhancing Chemical Sensing

Metal-organic framework (MOF)/C-dots and covalent organic framework (COF)/C-dots hybrid nanocomposites: Fabrications and applications in sensing, medical, environmental, and energy sectors

Developing new hybrid materials is critical for addressing the current needs of the world in various fields, such as energy, sensing, health, hygiene, and others. C-dots are a member of the carbon nanomaterial family with numerous applications. Aggregation is one of the barriers to the performance of C-dots, which causes luminescence quenching, surface area decreases, etc. To improve the performance of C-dots, numerous matrices including metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), and polymers have been composited with C-dots. The porous crystalline structures, which are constituents of metal nodes and organic linkers (MOFs) or covalently attached organic units (COFs) provide privileged features such as high specific surface area, tunable structures, and pore diameters, modifiable surface, high thermal, mechanical, and chemical stabilities. Also, the MOFs and COFs protect the C-dots from the environment. Therefore, MOF/C-dots and COF/C-dots composites combine their features while retaining topological properties and improving performances. In this review, we first compare MOFs with COFs as matrices for C-dots. Then, the recent progress in developing hybrid MOFs/C-dots and COFs/C-dots composites has been discussed and their applications in various fields have been explained briefly.

The Application of Metal-Organic Frameworks in Water Treatment and Their Large-Scale Preparation: A Review

Over the last few decades, there has been a growing discourse surrounding environmental and health issues stemming from drinking water and the discharge of effluents into the environment. The rapid advancement of various sewage treatment methodologies has prompted a thorough exploration of promising materials to capitalize on their benefits. Metal-organic frameworks (MOFs), as porous materials, have garnered considerable attention from researchers in recent years. These materials boast exceptional properties: unparalleled porosity, expansive specific surface areas, unique electronic characteristics including semi-conductivity, and a versatile affinity for organic molecules. These attributes have fueled a spike in research activity. This paper reviews the current MOF-based wastewater removal technologies, including separation, catalysis, and related pollutant monitoring methods, and briefly introduces the basic mechanism of some methods. The scale production problems faced by MOF in water treatment applications are evaluated, and two pioneering methods for MOF mass production are highlighted. In closing, we propose targeted recommendations and future perspectives to navigate the challenges of MOF implementation in water purification, enhancing the efficiency of material synthesis for environmental stewardship.

Highly sensitive fluoroprobe for detecting Sudan dyes in paprika utilizing carbon dot-embedded zeolitic imidazolate framework-8

In this manuscript, we synthesized CDs@ZIF-8 through a one-step, in-situ method by integrating green-emitting carbon dots (CDs) with zeolitic imidazolate framework-8 (ZIF-8). The resulting CDs@ZIF-8 was utilized as an ultrasensitive probe for detection, leveraging the inner filter effect. The analysis demonstrated the capability to detect Sudan dyes. Sudan I, for example, could be detected within a concentration range spanning from 0.25 to 70 μM, achieving a remarkable detection limit of 76.56 nM. This established method was effectively employed for detecting Sudan I in paprika. Compared with CDs, CDs@ZIF-8 exhibited a 3.32-fold increase in sensitivity and a wider detection range. This enhanced performance was attributed to the porous ZIF-8, which allowed for the enrichment of targets around CDs and avoided the aggregation of CDs. Additionally, embedding the CDs in ZIF-8 improved their pH stability. Our study provides a new approach for using CDs under limited conditions by leveraging metal-organic frameworks.

Tailoring fluorescent ZIF-8 nanostructures through calix[4]pyrrole modification: tunable size and enhanced organic micropollutant removal capacity

Whitish-blue light emitting fluorescent ZIF-8 structures were synthesized by means of bis-carboxylate functional calix[4]pyrrole (BCCP) modification. The calix[4]pyrrole concentration was also manipulated to tune the sizes of the ZIF-8 structures. Moreover, the BCCP-modified ZIF-8 samples exhibited enhanced organic micropollutant removal capacity from aqueous solutions.

Detection mechanism and the outlook of metal-organic frameworks for the detection of hazardous substances in milk

Milk has a high nutritional value. However, milk is easily contaminated in the production, processing, and storage processes, which harms consumers' health. Therefore, the harmful substances' detection in milk is important. Metal-organic frameworks (MOFs) have proven high potential in food safety detection due to their unique porous structure, large effective surface area, large porosity, and structural tunability. This article systematically describes the detection mechanism of fluorescence, electrochemical, colorimetric, and enzyme-linked immunosorbent assay based on MOFs. The progress of the application of MOFs in the detection of antibiotics, harmful microorganisms and their toxins, harmful ions, and other harmful substances in milk in recent years is reviewed. The structural tunability of MOFs enables them to be functionalized, giving the ability to be applied to different detection methods or substances. Therefore, MOFs can be used as an advantageous sensing material for detecting harmful substances in the complex environment of milk.

Eutectic Media Open a Synthetic Route to Oligocitrazinic Acid Fluorophores of Purple Hue

Under isochoric and solvent-free conditions, the reaction between ammonium formate and citric acid results in a deeply purple reaction product with fluorescent properties. This brings this reaction in the realm of bio-based fluorophores and bottom-up carbon nanodots from citric acid. The reaction conditions are optimized in terms of UV-vis spectroscopic properties and, subsequently, the main reaction product is separated. While the structural analysis does not give any indication for carbon nanodots in a general sense, it points towards the formation of molecular fluorophores that consist of oligomerized citrazinic acid derivatives. Furthermore, EPR spectroscopy reveals the presence of stable free radicals in the product. We hypothesize that such open-shell structures may play a general role in molecular fluorophores from citric acid and are not yet sufficiently explored. Therefore, we believe that analysis of these newly discovered fluorophores may contribute to a better understanding of the properties of fluorophores and CND from citric acid in general.

Engineering of metal-organic frameworks (MOFs) for thermometry

Metal-organic frameworks (MOFs ) are excellent candidates for use in chemistry, material sciences and engineering thanks to their interesting qualitative features and potential applications. Quite interestingly, the luminescence of MOFs can be engineered by regulation of the ligand design, metal ion selection and encapsulation of guest molecules within the MOF cavity. Temperature is a very crucial physical parameter and the market share of temperature sensors is rapidly expanding with technology and medicinal advancement. Among the wide variety of available temperature sensors, recently MOFs have emerged as potential temperature sensors with the capacity to precisely measure the temperature. Lanthanide-based thermometry has advantages because of its ratiometric response ability, high quantum yield and photostability, and therefore lanthanide-based MOFs were initially focused on to construct MOF thermometers. As science and technology have gradually changed, it has been observed that with the inclusion of dye, quantum dots, etc. within the MOF cavity, it is possible to develop MOF-based thermometry. This review consolidates the recent advances of MOF-based ratiometric thermometers and their mechanism of energy transfer for determining the temperature (thermal sensitivity and temperature uncertainty). In addition, some fundamental points are also discussed, such as concepts for guiding the design of MOF ratiometric thermometers, thermometric performance and tuning the properties of MOF thermometers.

Lanthanide ternary complex as a fluorescent probe for highly sensitive and selective detection of copper ions based on selective recognition and photoinduced electron transfer

Copper ions have a very important role in human health, industrial and agricultural production. Herein, lanthanide ternary complex of 2,6-pyridinedicarboxylic acid (DPA)-Eu³⁺-polyethyleneimine (PEI) as a fluorescent probe was thus fabricated for highly sensitive and selective detection of copper ions. PEI itself is non-fluorescent, the PEI-Eu³⁺complex is also non-fluorescent, and PEI has specific recognition to copper ions due to its higher affinity ability to copper ion than other metal ions. It was found that Cu²⁺ ions cannot quench the characteristic fluorescence of Eu³⁺ in the DPA-Eu³⁺ system, while in the DPA-Eu³⁺-PEI system, Cu²⁺ ions can greatly quench the characteristic fluorescence of Eu³⁺ due to photoinduced electron transfer (PET). The luminescent and quenching mechanism was also discussed in detail. The DPA-Eu³⁺-PEI probe not only has high sensitivity and selectivity, but also has very rapid fluorescence response and the response time is only 1 min. A good linear relationship between the fluorescence ratios of F₀/F and the concentrations of Cu²⁺ was obtained in the range of 0.02 ∼ 10.0 μM (R² = 0.998), and the limit of detection (LOD) is 8.0 nM. The probe was successfully applied for the detection of Cu²⁺ ions in the lake and river water samples, wastewater and urine samples. This work may provide a new strategy for fabricating simple and effective fluorescence probe and a promising application for the rapid and on-site detection in environmental monitoring and biological fluids.

Luminescent Guests Encapsulated in Metal-Organic Frameworks for Portable Fluorescence Sensor and Visual Detection Applications: A Review

Metal-organic frameworks (MOFs) have excellent applicability in several fields and have significant structural advantages, due to their open pore structure, high porosity, large specific surface area, and easily modifiable and functionalized porous surface. In addition, a variety of luminescent guest (LG) species can be encapsulated in the pores of MOFs, giving MOFs a broader luminescent capability. The applications of a variety of LG@MOF sensors, constructed by doping MOFs with LGs such as lanthanide ions, carbon quantum dots, luminescent complexes, organic dyes, and metal nanoclusters, for fluorescence detection of various target analyses such as ions, biomarkers, pesticides, and preservatives are systematically introduced in this review. The development of these sensors for portable visual fluorescence sensing applications is then covered. Finally, the challenges that these sectors currently face, as well as the potential for future growth, are briefly discussed.

Metal organic frameworks as promising sensing tools for electrochemical detection of persistent heavy metal ions from water matrices: A concise review

Water bodies are being polluted rapidly by disposal of toxic chemicals with their huge entrance into drinking water supply chain. Among these pollutants, heavy metal ions (HMIs) are the most challenging one due to their non-biodegradability, toxicity, and ability to biologically hoard in ecological systems, thus posing a foremost danger to human health. This can be addressed by robust, sensitive, selective, and reliable sensing of metal ions which can be achieved by Metal organic frameworks (MOF) based electrochemical sensors. In the present era, MOFs have caught greater interest in a variety of applications including sensing of hazardous pollutants such as heavy metal ions. So, in this review article, types, synthesis and working mechanism of MOF based sensors is explained to give general overview with updated literature. First time, detailed study is done for sensing of metal ions such as chromium, mercury, zinc, copper, manganese, palladium, lead, iron, cadmium and lanthanide by MOFs based electrochemical sensors. The use of MOFs as electrochemical sensors has attractive success story along with some challenges of the area. Considering these challenges, we attempted to highlight the milestone achieved and shortcomings along with future prospective of the MOFs for employing it in electrochemical sensing devices for HMIs. Finally, challenges and future prospects have been discussed to promote the development of MOFs-based sensors in future.

A smartphone-assisted optosensing platform based on chromium-based metal-organic framework signal amplification for ultrasensitive and real-time determination of oxytetracycline

Ultrasensitive and onsite detection of oxytetracycline (OTC) is of vital significance for ensuring public health. Herein, a novel and versatile fluorescence biomimetic nanosensor, Mg,N-CDs@MIL-101@MIP, was elaborately tailored for the assay of OTC. MIL-101 with extraordinarily high surface area and porosity, as a favorable supporter, suppressed self-quenching of Mg,N-CDs and boosted mass transfer rate, realizing signal amplification. As an ultrasensitive signal transducer, high luminescent Mg,N-CDs yielded conspicuous fluorescence responses for OTC, enhancing the sensitivity of Mg,N-CDs@MIL-101@MIP. High-affinity imprinting sites further endowed Mg,N-CDs@MIL-101@MIP with superior anti-interference ability and reusability. Given prominent merits, Mg,N-CDs@MIL-101@MIP demonstrated a good linear range (0.05-40 μg mL^-1) with a lower limit of detection (16.8 ng mL^-1), supplying high accessibility to realize ultrasensitive and highly selective measurement of OTC in samples. Additionally, to attain precise onsite profiling of OTC, an intelligent sensing platform was developed by integrating Mg,N-CDs@MIL-101@MIP with a portable smartphone-assisted optical device. As both signal reader and analyzer, smartphone can instantly capture concentration-dependent fluorescent images and accurately digitize them, accomplishing quantitative analysis of OTC. More delightfully, the portable platform was utilized for visual determination of OTC in milk samples with satisfactory results, offering a promising tool for the high-performance onsite evaluation of food safety and environmental health.

Coordination polymer nanoprobe integrated carbon dot and phenol red for turn-on fluorescence detection of urease activity

The potential of coordination polymers (CPs) as a host of integrating multiple guest species to construct a fluorescence resonance energy transfer (FRET) nanoprobe was demonstrated. The ZnCPs built from zinc(II) and adenine was employed as a model of CPs to integrate carbon dot (CD) and phenol red (PR) for producing the FRET nanoprobe (CD/PR@ZnCPs). Benefiting from the confinement effect of ZnCPs, the integrated CD and PR can be brought in close proximity to favor the occurrence of FRET process from CD to PR, which leads to the quenching of CD fluorescence. However, the FRET process was disrupted upon the red-shift of PR absorption from 428 to 562 nm in alkaline medium, and consequently switches on the fluorescence of CD/PR@ZnCPs. Based on this finding, by utilizing urease to hydrolyze urea and mediate medium pH, a turn-on fluorescent method was established for the detection of urease activity. This fluorescent method has a linear response that covers 5 to 150 U/L urease with a detection limit of 0.74 U/L and exhibits an excellent selectivity over other enzymes. The successful determination of urease in saliva samples demonstrates the applicability of the fluorescent nanoprobe in complex biological matrix.

Metal-Organic-Framework-Based Chemosensor for Ultrafast and Ultrasensitive Detection of Pd2+ Ions in Water, Real Specimens, and Test Strips

A new ultrasensitive and ultrafast Al(III) metal-organic-framework (MOF)-based probe (1) was constructed to detect Pd²⁺ ions. Extremely selective recognition of Pd²⁺ ion was demonstrated by the guest-free compound 1 (called 1') using a fluorescence signal. The quenching in the fluorescence signal was observed due to the weak interaction between the linker alkyne-π bond and Pd²⁺. The mechanism of isophthalic alkyne-π and Pd²⁺ interaction was systematically examined with the help of isothermal titration calorimetry (ITC), X-ray photoelectron spectroscopy (XPS), and UV-vis spectroscopy. The response time of the MOF for sensing of Pd²⁺ was 30 s, which is the lowest response time for MOF-based Pd²⁺ sensing to date, with an ultralow detection limit (102 nM) and Stern-Volmer constant (4.39 × 10³ M^-1), evidencing the outstanding ability to sense Pd²⁺ ion by this probe. The Pd²⁺ detection limit falls among the lowest values. Activated MOF (1') also showed considerable recyclability up to five steps with a constant sensing ability. In different water resources (Milli-Q water, lake water, river water, and tap water), the probe also showed excellent sensing ability. A paper-strip device was developed for the applicability of our material for the real field sensing application of Pd²⁺. The relevance of 1' is not only up to Pd²⁺, but it could also sense palladium in other possible oxidation states.

Recent research progress in CDs@MOFs composites: fabrication, property modulation, and application

Carbon dots (CDs) have exhibited a promising application prospect in many fields because of their good fluorescence properties, biocompatibility, low toxicity, and easy functionalization. In order to improve their photoelectricity and stability, metal-organic frameworks (MOFs) can be used as host materials to provide ideal carriers for CDs to realize the multifunctional composites of CDs and MOFs (CDs@MOFs). At present, CDs@MOFs composites have shown tremendous application potential because they have various advantages of both CDs and MOFs. In this review, the synthesis methods of CDs@MOFs composites are firstly introduced. Then, the influence of the synergy between CDs and MOFs on the regulation of their structures and optical properties is highlighted. Furthermore, the recent application researches of CDs@MOFs composites in fluorescent probes, solid-state lighting, and photoelectrocatalysis are generalized. Finally, the critical issues, challenges, and solutions on their structure and property regulation and application are put forward, and their commercialization direction is also prospected.

Metal organic framework encapsulated tamavidin-Gluc reporter: application in COVID-19 spike antigen bioluminescent immunoassay

Enzyme linked immunosorbent assay (ELISA) is one of the most utilized serological methods to diagnose and identify etiologic agents of many infectious diseases and other physiologically important analytes. ELISA can be used either alone or adjunct to other diagnostic methods such as molecular arrays, and other serological techniques. Most ELISA assays utilize reagents that are proteinaceous in nature, which are not very stable and require cold-chain transport systems. Development of a desirable immunoassay requires stability of reagents used and its ability to be stored at room temperature without sacrificing the activity of the reagents or the protein of interest. Metal organic frameworks (MOFs) are a rapidly emerging and evolving class of porous polymeric materials used in a variety of biosensor applications. In this study, we introduce the use of MOFs to stabilize a universal reporter fusion protein, specifically, avidin-like protein (Tam-avidin2) and the small bioluminescent protein Gaussia luciferase (Gluc) forming the fusion reporter, tamavidin2-Gluc (TA2-Gluc). This fusion protein serves as a universal reporter for any assays that utilize biotin-avidin binding strategy. Using SARS-CoV2 S1 spike antigen as the model target antigen, we demonstrated that encapsulation of TA2-Gluc fusion protein using a nano-porous material, zeolitic imidazolate framework-8 (ZIF-8), allows us to store and preserve this reporter protein at room temperature for over 6 months and use it as a reporter for an ELISA assay. Our optimized assay was validated demonstrating a 0.26 μg mL-1 limit of detection, high reproducibility of assay over days, detection of spiked non-virulent SARS-COV2 pseudovirus in real sample matrix, and detection in real COVID-19 infected individuals. This result can lead to the utilization of our TA2-Gluc fusion protein reporter with other assays and potentially in diagnostic technologies in a point-of-care setting.

Capacitive Desalination and Disinfection of Water Using UiO-66 Metal-Organic Framework/Bamboo Carbon with Chitosan

The zirconium-based metal-organic framework (MOF) (UiO-66)/bamboo carbon (BC) composite with chitosan was prepared using hydrothermal and impregnation methods and used for capacitive desalination (CDI) and disinfection of water. The results showed that these composites had fast ion exchange and charge transfer properties. During the CDI process, these composites' electrodes exhibited good cycle stability, electrosorption capacity (4.25 mg/g) and excellent bactericidal effect. These carbon-based composites electrodes' bactericidal rate for Escherichia coli could reach 99.99% within 20 minutes; therefore, they had good performance and were a good choice for high-performance deionization applications.

Potential Development of N-Doped Carbon Dots and Metal-Oxide Carbon Dot Composites for Chemical and Biosensing

Among carbon-based nanomaterials, carbon dots (CDs) have received a surge of interest in recent years due to their attractive features such as tunable photoluminescence, cost effectiveness, nontoxic renewable resources, quick and direct reactions, chemical and superior water solubility, good cell-membrane permeability, and simple operation. CDs and their composites have a large potential for sensing contaminants present in physical systems such as water resources as well as biological systems. Tuning the properties of CDs is a very important subject. This review discusses in detail heteroatom doping (N-doped CDs, N-CDs) and the formation of metal-based CD nanocomposites using a combination of matrices, such as metals and metal oxides. The properties of N-CDs and metal-based CDs nanocomposites, their syntheses, and applications in both chemical sensing and biosensing are reviewed.

Sensing performance and mechanism of carbon dots encapsulated into metal-organic frameworks

Metal-organic frameworks (MOFs) can be combined with nanomaterials and the combined composites have excellent optical properties. Carbon dots (CDs) with tiny particle size, non-toxic and rich surface functional groups are novel fluorescent materials. Carbon dots@metal-organic frameworks (CDs@MOFs) are synthesized by encapsulating CDs into MOFs. CDs@MOFs are promising composites for the preparation of a new generation of fluorescence sensors, which combine the hybrid properties of MOFs and the special optical properties of CDs. Urged as such, we are encouraged to categorize according to the sensing mechanisms. These include fluorescence resonance energy transfer (FRET), aggregation-caused quenching (ACQ), static quenching, dynamic quenching, photo-induced electron transfer (PET), inner filter effect (IFE) and so on. Based on the above mechanisms, CDs@MOFs can specifically interact with target analytes to generate fluorescence quenching. This review covers the research progress of CDs@MOFs in recent five years (with 103 refs), synthetic design of CDs@MOFs and introduces the sensing mechanism. The current challenges and future research directions are discussed briefly. The sensing mechanism and applications of CDs@MOFs.

Recent Trends and Advances in Porous Metal-Organic Framework Nanostructures for the Electrochemical and Optical Sensing of Heavy Metals in Water

With the expansion and advancement in agricultural and chemical industries, various toxic heavy metals such as lead, cadmium, mercury, zinc, copper, arsenic etc. are continuously released into the environment. Intake of sources contaminated with such toxic metals leads to various health issues. Keeping the serious effects of these toxic metal ions in view, various organic-inorganic nanomaterials based sensors have been exploited for their detection via optical, electrochemical and colorimetric approaches. Since a chemical sensor works on the principle of interaction between the sensing layer and the analytes, a sensor material with large surface area is required to enable the largest possible interaction with the target molecules and hence the sensitivity of the chemical sensor. However, commonly employed materials such as metal oxides and conducting polymers tend to feature relatively low surface areas, and hence resulting in low sensitivity of the sensor. Metal-Organic Frameworks (MOFs) nanostructures are another category of organic-inorganic materials endowed with large surface area, ultra-high and tunable porosity, post-synthesis modification features, readily available active sites, catalytic activity, and chemical/thermal stability. These properties provide high sensitivity to the MOF based sensors due to the adsorption of large number of target analytes. The current review article focuses on MOFs based optical and electrochemical sensors for the detection of heavy metals.

Fabrication of a Reusable Carbon Dot/Gold Nanoparticle/Metal-Organic Framework Film for Fluorescence Detection of Lead Ions in Water

Solid-state sensing platforms are desirable for the development of reusable sensors to promote public health measures such as testing for drinking water contamination. A bioinspired metal-organic framework (MOF)-based material has been developed by imitating metal-protein interactions in biological systems to attain high sensitivity and selectivity to Pb²⁺ through fluorescence sensing. A zirconium terephthalate-type framework (also known as NH₂-UiO-66) was modified with both gold nanoparticles and thiol-functionalized carbon dots to give HS-C/Au(x)/UiO-66 composites with different Au content (x) and were subsequently adapted into films that show extraordinary sensitivity to Pb²⁺. The HS-C/Au(1.4)/UiO-66 film that consists of 1.4 wt % Au shows a quenching response with the limit of detection of 80 parts per trillion and sustained performance for five cycles. Moreover, the fluorescence response of the HS-C/Au(x)/UiO-66 film to Pb²⁺ can be reversed from emission quenching to enrichment of fluorescence by increasing the Au content. The performance of the HS-C/Au(x)/UiO-66 film as a solid-state sensor demonstrates its potential for application in reusable sensing devices to ensure public safety from Pb²⁺ contamination in drinking water.

Synthesis of Metal–Organic Frameworks Quantum Dots Composites as Sensors for Endocrine-Disrupting Chemicals

Hazardous chemical compounds such as endocrine-disrupting chemicals (EDCs) are widespread and part of the materials we use daily. Among these compounds, bisphenol A (BPA) is the most common endocrine-disrupting chemical and is prevalent due to the chemical raw materials used to manufacture thermoplastic polymers, rigid foams, and industrial coatings. General exposure to endocrine-disrupting chemicals constitutes a serious health hazard, especially to reproductive systems, and can lead to transgenerational diseases in adults due to exposure to these chemicals over several years. Thus, it is necessary to develop sensors for early detection of endocrine-disrupting chemicals. In recent years, the use of metal–organic frameworks (MOFs) as sensors for EDCs has been explored due to their distinctive characteristics, such as wide surface area, outstanding chemical fastness, structural tuneability, gas storage, molecular separation, proton conductivity, and catalyst activity, among others which can be modified to sense hazardous environmental pollutants such as EDCs. In order to improve the versatility of MOFs as sensors, semiconductor quantum dots have been introduced into the MOF pores to form metal–organic frameworks/quantum dots composites. These composites possess a large optical absorption coefficient, low toxicity, direct bandgap, formidable sensing capacity, high resistance to change under light and tunable visual qualities by varying the size and compositions, which make them useful for applications as sensors for probing of dangerous and risky environmental contaminants such as EDCs and more. In this review, we explore various synthetic strategies of (MOFs), quantum dots (QDs), and metal–organic framework quantum dots composites (MOFs@QDs) as efficient compounds for the sensing of ecological pollutants, contaminants, and toxicants such as EDCs. We also summarize various compounds or materials used in the detection of BPA as well as the sensing ability and capability of MOFs, QDs, and MOFs@QDs composites that can be used as sensors for EDCs and BPA.

A "bottle-around-ship" method to encapsulated carbon nitride and CdTe quantum dots in ZIF-8 as the dual emission fluorescent probe for detection of mercury (II) ion

A facile and efficient "bottle-around-ship" approach for preparing the ratiometric fluorescent probe has been developed by encapsulating the red-colored fluorescence CdTe quantum dots (QDs) and blue-colored fluorescence graphitic carbon nitride quantum dots (g-CNQDs) into the zeolitic imidazolate metal-organic frameworks (ZIF-8) in one step. At a single excitation of 360 nm, the obtained probe ZIF-8@g-CNQD/CdTe shows the dual-emission peaked at 450 and 633 nm, respectively. The red emission of CdTe QDs is selectively quenched by the Hg²⁺, whereas the blue fluorescence of g-CNQDs as an internal reference is insensitive, resulting in an apparent color transformation from pink to blue for special recognition of Hg²⁺. By this approach, the relative fluorescence intensity ratio (F₆₃₃/F₄₅₀) decreased linearly with increasing Hg²⁺ concentration in the 0.2-3.5 μM range with a low limit of detection (LOD) of ~ 46 nM. Therefore, we demonstrate that this "bottle-around-ship" process provides a new strategy for the construction of ratiometric fluorescent Hg²⁺ probes with good simplicity, high efficiency, and excellent stabilities. Moreover, the obtained Hg²⁺ fluorescent probe shows good results in the detection of actual samples.

Flower-like ZIF-8 enhance the peroxidase-like activity of nanoenzymes at neutral pH for detection of heparin and protamine

The peroxide-like catalytic activity of gold nanoclusters (Au-NCs) is very low under physiological conditions (pH 7.4), which greatly limits its biological detection applications. A new nanoenzyme platform was constructed by self-assembly of Au-NCs and ZIF-8/CQDs. It was found that heparin can significantly promote the peroxidase-like activity of Au-NCs on the nanoenzyme platform at pH 7.4. In the presence of H₂O₂, the catalytic activity of Au-NCs on the nanoenzyme platform for TMB increased nearly 50 times. Based on this phenomenon, a colorimetric method was developed to determine heparin in the range of 0.0185-9.25 U/mL, with a detection limit of 0.0027 U/mL. When protamine is introduced, heparin and protamine take the lead in specific binding due to antagonism, which makes heparin unable to adsorb on the surface of ZIF-8/CQDS, thus inhibiting the enhancement of the catalytic activity of Au-NCs. Based on this phenomenon, a colorimetric method was developed to determine protamine in the range of 0.01-0.5 μg/mL, with a detection limit of 0.003 μg/mL. Therefore, this method provides a new idea for the visual detection of heparin and protamine under physiological conditions.

Detection and Sorption of Heavy Metal Ions in Aqueous Media by a Fluorescent Zr(IV) Metal-Organic Framework Functionalized with 2-Picolylamine Receptor Groups

Increasing global environmental pollution due to heavy metal ions raises the importance of research on new multifunctional materials for simultaneous detection and removal of these contaminants from water resources. In this study, we report a microporous 8-connected Zr⁴⁺ metal-organic framework (MOF) based on a terephthalate ligand decorated with a chelating 2-picolylamine side group (dMOR-2), which shows highly efficient fluorescence sensing and sorption of heavy metal cations. We demonstrate by detailed fluorescence studies the ability of a water-dispersible composite of dMOR-2 with polyvinylpyrrolidone for real-time detection of Cu²⁺, Pb²⁺, and Hg²⁺ in aqueous media. The limits of detection were found to be below 2 ppb for these species, while the system's performance is not affected by the presence of other potentially competitive ions. In addition, sorption studies showed that a composite of dMOR-2 with calcium alginate (dMOR-2@CaA) is an excellent sorbent for Pb²⁺ and Cu²⁺ ions with capacities of 376 ± 15 and 117 ± 4 mg per gram of dMOR-2@CaA, respectively, while displaying the capability for simultaneous removal of various heavy metal ions in low initial concentrations and in the presence of large excesses of other cationic species. Structural and spectroscopic studies with model ligands analogous to our material's receptor unit showed chelation to the 2-picolylamine moiety to be the main binding mode of metal ions to dMOR-2. Overall, dMOR-2 is shown to represent a rare example of a MOF, which combines sensitive fluorescence detection and high sorption capacity for heavy metal ions.

Recent Advances in the Application and Mechanism of Carbon Dots/Metal-Organic Frameworks Hybrids in Photocatalysis and the Detection of Environmental Pollutants

Metal-organic frameworks (MOFs) are a class of crystalline porous materials with simple synthesis conditions, large specific surface area, structural diversity, and a wide range of interesting properties. The integration of MOFs with other materials can provide new multifunctional composites that exhibit both component properties and new characteristics. In recent years, the integration of carbon dots (CDs) into MOFs to form composites has shown improved optical properties and fascinating new characteristics. This review focuses on the design and synthesis strategies of CDs@MOFs composites (including pore-confined synthesis, in situ encapsulation, post-synthesis modification and impregnation method) and their recent research progress in photocatalysis and detection of environmental pollutants. Both the achievements and problems are evaluated and proposed, and the opportunities and challenges of CDs@MOF composite are discussed.

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.

Recent Progress in Fluorescent Probes For Metal Ion Detection

All forms of life have absolute request for metal elements, because metal elements are instrumental in various fundamental processes. Fluorescent probes have been widely used due to their ease of operation, good selectivity, high spatial and temporal resolution, and high sensitivity. In this paper, the research progress of various metal ion (Fe³⁺,Fe²⁺,Cu²⁺,Zn²⁺,Hg²⁺,Pb²⁺,Cd²⁺) fluorescent probes in recent years has been reviewed, and the fluorescence probes prepared with different structures and materials in different environments are introduced. It is of great significance to improve the sensing performance on metal ions. This research has a wide prospect in the application fields of fluorescence sensing, quantitative analysis, biomedicine and so on. This paper discusses about the development and applications of metal fluorescent probes in future.

Construction and Sensing Amplification of Raspberry-Shaped MOF@MOF

MOFs@MOFs (metal-organic frameworks, MOFs) possess precise customized functionalities and predesigned structures that enable the implementation of structure and property regulation for specific functions in comparison to traditional single MOFs. However, the synthesis and fluorescence properties of multilayer MOFs@MOFs are still worth improving. Herein, a fluorescent raspberry-shaped MOF@MOF was constructed via optimized seed-mediated synthesis by tuning the reaction time, reaction mode, and reaction concentration, involving the initial synthesis of the UiO-66-NH₂ core and then the coating of the UiO-67-bpy shell. The raspberry-shaped UiO-66@67-bpy showed stable fluorescence and desirable sensing selectivity for the Hg²⁺ ion under the interference of other ions; meanwhile, the raspberry-shaped UiO-66@67-bpy indicated amplified sensing performance than pure UiO-66-NH₂, mechanically mixed UiO-66-NH₂ + UiO-67-bpy, and UiO-66@UiO-67 counterpart due to the accumulation effect of outer UiO-67-bpy toward Hg²⁺. Density functional theory (DFT) calculations including adsorption energy calculations and electronic density difference analysis further showed that the enhanced fluorescence quenching was possibly attributed to the outer UiO-67-bpy enrichment promoting the charge transfer between Hg²⁺ and the ligands of fluorescent UiO-66@67-bpy. The synergistic effect of MOFs@MOFs unlocks more possibilities for the construction of enhanced sensors and other applications.

Machine Learning Models for Predicting Cytotoxicity of Nanomaterials

The wide application of nanomaterials in consumer and medical products has raised concerns about their potential adverse effects on human health. Thus, more and more biological assessments regarding the toxicity of nanomaterials have been performed. However, the different ways the evaluations were performed, such as the utilized assays, cell lines, and the differences of the produced nanoparticles, make it difficult for scientists to analyze and effectively compare toxicities of nanomaterials. Fortunately, machine learning has emerged as a powerful tool for the prediction of nanotoxicity based on the available data. Among different types of toxicity assessments, nanomaterial cytotoxicity was the focus here because of the high sensitivity of cytotoxicity assessment to different treatments without the need for complicated and time-consuming procedures. In this review, we summarized recent studies that focused on the development of machine learning models for prediction of cytotoxicity of nanomaterials. The goal was to provide insight into predicting potential nanomaterial toxicity and promoting the development of safe nanomaterials.

Multifunctional Anthracene-Based Ni-MOF with Encapsulated Fullerenes: Polarized Fluorescence Emission and Selective Separation of C70 from C60

We report an anthracene-based Ni-MOF [Ni(II) metal-organic framework, {[Ni(μ₂-L)₂Cl₂]·x(C₆H₆)·y(MeOH)}_n (1), L = anthracene-9,10-diylbis(methylene)diisonicotinate] whose crystal structure reveals the presence of hexagonal channels with a pore size of 1.4 nm that can accommodate guests such as C₆₀ and C₇₀. Both confocal fluorescence and Raman microscopy results are in agreement with a homogeneous distribution of fullerenes throughout the single crystals of 1. Efficient energy transfer from 1 to the fullerenes was observed, which emitted partially polarized fluorescence emission. Stronger binding between 1 and C₇₀ versus C₆₀ was confirmed from HPLC analysis of the dissolved material and provides a basis for the selective retention of C₇₀ in liquid chromatography columns packed with 1.

Recent advances in luminescent metal-organic frameworks and their photonic applications

In recent years, metal-organic frameworks (MOFs) have been attracting ever more interest owing to their fascinating structures and widespread applications. Among the optoelectronic materials, luminescent MOFs (LMOFs) have become one of the most attractive candidates in the fields of optics and photonics thanks to the unique characteristics of their frameworks. Luminescence from MOFs can originate from either the frameworks, mainly including organic linkers and metal ions, or the encapsulated guests, such as dyes, perovskites, and carbon dots. Here, we systematically review the recent progress in LMOFs, with an emphasis on the relationships between their structures and emission behaviour. On this basis, we comprehensively discuss the research progress and applications of multicolour emission from homogeneous and heterogeneous structures, host-guest hybrid lasers, and pure MOF lasers based on optically excited LMOFs in the field of micro/nanophotonics. We also highlight recent developments in other types of luminescence, such as electroluminescence and chemiluminescence, from LMOFs. Future perspectives and challenges for LMOFs are provided to give an outlook of this emerging field. We anticipate that this article will promote the development of MOF-based functional materials with desired performance towards robust optoelectronic applications.

Tetracycline antibiotics and NH4+ detection by Zn-organic framework fluorescent probe

A fluorescent probe based on single metal-organic framework material without additional fluorophores and active sites can significantly improve the stability of the probe for detection, and has very important application value in environmental analysis and detection. In this paper, a simple and rapid fluorescence detection method was established with Zn-MOF, which realized the highly sensitive detection of tetracycline antibiotics and NH₄⁺ in water. The prepared Zn-MOF has abundant pores and can exist stably in water. When tetracycline antibiotics are present in Zn-MOF aqueous solution, based on the unique coordination ability between Zn and N, tetracycline antibiotics rich in N will be adsorbed into the pore canals of MOF, and aggregation-induced luminescence will occur. The original non-fluorescent Zn-MOF will immediately produce yellow fluorescence, realizing the detection of tetracycline antibiotics in water, with the limit of detection reaching 0.017 μM in a linear range of 0.02-13 μM. Zn-MOF is further used for the detection of tetracycline antibiotics in actual samples of milk and honey. Oxytetracycline (OTC) with the best fluorescence response of tetracycline antibiotics was coated on Zn-MOF to synthesize OTC@Zn-MOF fluorescent probe. NH₄⁺ will replace the original ligand of Zn-MOF, which will disintegrate MOF and release OTC, resulting in a fluorescence decrease. Therefore, NH₄⁺ can be detected with low limit of detection (0.038 μM) in a linear range of 0 to 3 mM. The probe is expected to be able to detect ammonia in the environment.

A carbon-based fluorescent probe (N-CDs) encapsulated in a zeolite matrix (NaFZ) for ultrasensitive detection of Hg (II) in fish

In this work, a novel strategy was addressed to fabricate new sensing probe (N-CDs@NaFZ) from nitrogen doped carbon dots (N-CDs) confined in Al-free ferrisilicates zeolite (NaFZ) by hydrothermal/solvothermal method. The probe was systematically characterized by HR-TEM, FTIR, energy dispersive X-ray (EDX), powder X-ray diffraction, and UV-Vis absorption and fluorescence spectrophotometers. Characterization of the designed nanocomposite template N-CDs@NaFZ by fluorescence spectrum demonstrates a variety of important conducts as stability improvements, reasonable dispersibility in water, highly emission intensity enhancement at 435 nm when excited at 340 nm, excitation independent fluorescence behaviors, great quantum yield percentage of 91.2%, and narrow size distribution 12 nm, as a nano-space confinement effect of zeolite effectively increase the rigidity of N-CDs. Based on the fluorescence quenching mechanism, the designed approach exhibits an excellent selectivity and good sensitive response to the presence of Hg(II) ions under ambient temperature, with a wide linear range of 0.1-1500 nM and lower detection limits of 5.5 pM. Influences of variables pH and incubation time were optimized. The N-CDs@NaFZ sensor was effectively applied for the detection of Hg(II) ions in the farmed and wild rainbow trout fishes, and the results are in reasonable agreement when compared with that obtained by the cold vapor atomic absorption method.

Screening and Discrimination of Perfluoroalkyl Substances in Aqueous Solution Using a Luminescent Metal-Organic Framework Sensor Array

The extensive production and large-scale use of perfluoroalkyl substances (PFASs) have raised their presence in aquatic environments worldwide. Thus, the facile and reliable screening of PFASs in aqueous systems is of great significance. Herein, we designed a novel fluorescent sensor array for the rapid screening and discrimination of multiple PFASs in water. The sensor array comprised three highly stable zirconium porphyrinic luminescent metal-organic frameworks (i.e., PCNs) with different topological structures. The sensing mechanism was based on the static fluorescence quenching of PCNs by PFASs upon their adsorptive interactions. The fluorescence response patterns were characteristic for each PFAS because of their different adsorption affinities toward different PCNs. Through the interpretation of response patterns by statistical methods, the proposed PCN array successfully discriminated six different kinds of PFASs, each PFAS at different concentrations and PFAS mixtures at different molar ratios. The practicability of this array was further verified by effectively discriminating PFASs in two real water samples. Remarkably, the PCN sensors exhibited a very short response time toward PFASs (within 10 s) due to the ordered pore structure allowing fast PFAS diffusion. This study not only provides a facile method for rapid PFAS screening in waters but also broadens the application of luminescent metal-organic frameworks and array techniques in sensing fields.

Life Cycle Assessment on Different Synthetic Routes of ZIF-8 Nanomaterials

In the last twenty years, research activity around the environmental applications of metal–organic frameworks has bloomed due to their CO2 capture ability, tunable properties, porosity, and well-defined crystalline structure. Thus, hundreds of MOFs have been developed. However, the impact of their production on the environment has not been investigated as thoroughly as their potential applications. In this work, the environmental performance of various synthetic routes of MOF nanoparticles, in particular ZIF-8, is assessed through a life cycle assessment. For this purpose, five representative synthesis routes were considered, and synthesis data were obtained based on available literature. The synthesis included different solvents (de-ionized water, methanol, dimethylformamide) as well as different synthetic steps (i.e., hours of drying, stirring, precursor). The findings revealed that the main environmental weak points identified during production were: (a) the use of dimethylformamide (DMF) and methanol (MeOH) as substances impacting environmental sustainability, which accounted for more than 85% of the overall environmental impacts in those synthetic routes where they were utilized as solvents and as cleaning agents at the same time; (b) the electricity consumption, especially due to the Greek energy mix which is fossil-fuel dependent, and accounted for up to 13% of the overall environmental impacts in some synthetic routes. Nonetheless, for the optimization of the impacts provided by the energy use, suggestions are made based on the use of alternative, cleaner renewable energy sources, which (for the case of wind energy) will decrease the impacts by up to 2%.

Fluorescent enzyme-linked immunosorbent assay based on alkaline phosphatase-responsive coordination polymer composite

The fabrication of alkaline phosphatase (ALP)-responsive coordination polymer (CP) composite is demonstrated for establishing a fluorescent immunoassay. The CP composite (ThT@GMP/Eu) was synthesized by encapsulating thioflavin T (ThT) into the CP host that was composed of europium ion (Eu³⁺) and guanine monophosphate (GMP). The ThT@GMP/Eu composite shows a strong fluorescence in aqueous solution due to the confinement effect of GMP/Eu CPs, which restricts the conformational rotation of ThT. However, upon the addition of ALP, the structure of GMP/Eu CPs was disrupted to release ThT into solution. This results in the quenching of the fluorescence of ThT@GMP/Eu. The fluorescence of ThT@GMP/Eu has a linear response that covers 0.8 to 120 mU/mL ALP with a detection limit of 0.26 mU/mL and exhibits excellent specificity towards ALP against other enzymes. On this basis, inspired by the wide application of ALP as an enzyme label in enzyme-linked immunosorbent assay (ELISA), an ALP-based fluorescent immunoassay was further developed for the detection of mouse immunoglobulin G (mIgG). The developed immunoassay displays a linear fluorescent response towards mIgG from 0.8 to 100 ng/mL, and the detection limit is 0.16 ng/mL. The fluorescent immunoassay was successfully applied to the determination of mIgG in serum samples. Schematic of the responsivity of ThT@GMP/Eu to ALP that hydrolyzes GMP to release ThT, which leads to fluorescent quenching, and its application in the construction of a fluorescent immunoassay for mIgG determination.