Discrimination of Various Amine Vapors by a Triemissive Metal-Organic Framework Composite via the Combination of a Three-Dimensional Ratiometric Approach and a Confinement-Induced Enhancement Effect

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.

Dual-signal fluorescent test strips for spoilage sensing of packaged seafood: Visual monitoring of volatile basic nitrogens

With the food safety issues abounding, exploring reliable and efficient methods for evaluating food safety is crucial. Herein, a ratiometric test strip was proposed by using green-yellow fluorescent d-penicillamine capped silver nanocluster (DPA-AgNCs) as indicator and red-emitting bimetallic gold/silver nanoclusters (AuAgNCs) as an internal reference, providing a real-time and visual monitoring system for food freshness. Results showed that the as-prepared DPA-AgNCs displayed an excellent response and good sensitivity for volatile basic nitrogens (VBNs), with a limit of detection (LOD) of 0.51 μM and 0.08 ppm for spermidine and ammonia hydroxide, respectively. Subsequently, a ratiometric test strip was developed to visually monitor ammonia vapour, displaying an obvious fluorescence colour variation from mustard to deep-red. Moreover, the presented ratiometric test strip was successfully applied for non-contact and visual evaluating and monitoring VBNs in the shrimp sample, showing high potential for in-situ monitoring.

Conductometric Gas Sensor Based on MoO3 Nanoribbon Modified with rGO Nanosheets for Ethylenediamine Detection at Room Temperature

An ethylenediamine (EDA) gas sensor based on a composite of MoO3 nanoribbon and reduced graphene oxide (rGO) was fabricated in this work. MoO3 nanoribbon/rGO composites were synthesized using a hydrothermal process. The crystal structure, morphology, and elemental composition of MoO3/rGO were analyzed via XRD, FT-IR, Raman, TEM, SEM, XPS, and EPR characterization. The response value of MoO3/rGO to 100 ppm ethylenediamine was 843.7 at room temperature, 1.9 times higher than that of MoO3 nanoribbons. The MoO3/rGO sensor has a low detection limit (LOD) of 0.235 ppm, short response time (8 s), good selectivity, and long-term stability. The improved gas-sensitive performance of MoO3/rGO composites is mainly due to the excellent electron transport properties of graphene, the generation of heterojunctions, the higher content of oxygen vacancies, and the large specific surface area in the composites. This study presents a new approach to efficiently and selectively detect ethylenediamine vapor with low power.

Zn-MOFs composites loaded with silver nanoparticles are used for fluorescence sensing pesticides, Trp, EDA and photocatalytic degradation of organic dyes

The abuse of pesticides, antibiotics, organic solvents, etc., not only deteriorates the ecological environment, but even affects the normal development of organisms, posing a serious threat to global public health.Efficient and sensitive detection of pesticides, antibiotics, organic solvents and so on are very important, but also a challenge to scientists. By depositing Ag nanoparticles on the surface of Zn-MOF (1: {[Zn₂(bta)(bpy)(H₂O)₂]·2H₂O}_n), a new type of composite material (Ag@1) was successfully synthesized and analyzed by TEM, EDS, XPS, XRD, IR and other characterization methods. Ag@1 can serve as multi-response fluorescence sensor to detect pesticides (fluazinam (FLU) and emamectin benzoate (EMB)), Tryptophan (Trp) and Ethylenediamine (EDA). In particular, Ag@1 showed "turn-off" fluorescence sensing for FLU and EDA, and "turn-on" fluorescence sensing for EMB and Trp. It is worth mentioning that we further explored its analysis of FLU and Trp in real water samples and fetal bovine serum. The recoveries are satisfactory, 97.95 % - 102.39 % and 96.69 % - 101.85 %, respectively. In addition, the photocatalytic performance of Ag@1 was found to be excellent, the degradation rate of methylene blue (MB) reached 86 %, and its degradation mechanism was discussed.

Multi-emitter metal-organic frameworks as ratiometric luminescent sensors for food contamination and spoilage detection

Food contamination and spoilage is a worldwide concern considering its adverse effect on public health and food security. Real time monitoring food quality can reduce the risk of foodborne disease to consumers. Particularly, the emergence of multi-emitter luminescent metal-organic frameworks (LMOFs) as ratiometric sensory materials has provided the possibility for food quality and safety detection with high sensitivity and selectivity taking advantage of specific host-guest interactions, pre-concentrating and molecule-sieving effects of MOFs. Furthermore, the excellent sensing performance of multi-emitter MOF-based ratiometric sensors including self-calibration, multi-dimensional recognition and visual signal readout is able to meet the increasing rigor requirement of food safety evaluation. Multi-emitter MOF-based ratiometric sensors have become the focus of food safety detection. This review focuses on design strategies for different multiple emission sources assembly to construct multi-emitter MOFs materials based on at least two emitting centers. The design strategies for creating multi-emitter MOFs can be mainly classified into three categories: (1) multiple emission building blocks assembly in a single MOF phase; (2) single non-luminescent MOF or LMOF phase as a matrix for chromophore guest(s); (3) heterostructured hybrids of LMOF with other luminescent materials. In addition, the sensing signal output modes of multi-emitter MOF-based ratiometric sensors have critically discussed. Next, we highlight the recent progress for the development of multi-emitter MOF as ratiometric sensors in food contamination and spoilage detection. Their future improvement and advancing direction potential for their practical application is finally discussed.

Rapid and Specific Enhanced Luminescent Switch of Aniline Gas by MOFs Assembled from a Planar Binuclear Cadmium(II) Metalloligand

Due to the low vapor pressure of aniline, it is challenging to develop a specific rapid fluorescence detection material for low concentrations of aniline gas, which is suspected to result in carcinogenicity when people are exposed by ingestion, inhalation, and skin contact. Herein, the easy-preparing Schiff base ligands were employed to construct the binuclear cadmium(II) compounds featuring a good plane and fine luminescent property, and then, the end groups were changed, making the compounds metalloligands to further build the 3D metal-organic frameworks (MOFs), named MECS-2. It is found that MECS-2 can achieve specific luminescent enhancement response for aniline gas. Furthermore, a large-scale MECS-2a film could be easily prepared by electrospinning nanoMECS-2, which presents the highly efficient and visual detection for aniline gas with the luminescent enhancement effect up to 20 times and good repeatability. Our work provides a good example for the efficient construction of MOF-based films with the fluorescence detection function for organic aromatic gases.

Metal-Organic Framework-Based Selective Molecular Recognition of Organic Amines and Fixation of CO2 into Cyclic Carbonates

Synthesis and structural depiction of two new metal-organic frameworks (MOFs), namely, [{Zn(L)(oba)}·4H₂O]_α (Zn-MOF-1) and [{Cd_1/2(L)_1/2(nipa)_1/2(H₂O)_1/2}·(DMF)_1/2(H₂O)]_α (Cd-MOF-2) (where L = N²,N⁶-di(pyridin-4-yl)naphthalene-2,6-dicarboxamide, 4,4'-H₂oba = 4,4'-oxybisbenzoic acid, and 5-H₂nipa = 5-nitroisophthalic acid) are reported. Both Zn-MOF-1 and Cd-MOF-2 have been prepared by reacting ligand L and coligand 4,4'-H₂oba or 5-H₂nipa with the respective dihydrates of Zn(OAc)₂ and Cd(OAc)₂ (OAc = acetate). Crystal structure X-ray analysis discloses that Zn-MOF-1 displays an overall 2D → 3D interpenetrated framework structure. The topological analysis by ToposPro suggests a (4)-connected uninodal sql topology with a point symbol of {4⁴·6²} having 2D + 2D parallel polycatenation. However, crystal packing of Cd-MOF-2 adapted a porous framework architecture and was topologically simplified as (3,4)-connected binodal 2D net. In addition, both Zn-MOF-1 and Cd-MOF-2 were proved to be multifunctional materials for the recognition of organic amines and in the fixation of CO₂ to cyclic carbonates. Remarkably, Zn-MOF-1 and Cd-MOF-2 showed very good fluorescence stability in aqueous media and have shown 98 and 97% quenching efficiencies, respectively, for 4-aminobenzoic acid (4-ABA), among all of the researched amines. The mechanistic study of organic amines recognition proposed that fluorescence quenching happened mainly through hydrogen-bonding and π-π stacking interactions. Additionally, cycloaddition of CO₂ to epoxide in the presence of Zn-MOF-1 and Cd-MOF-2 afforded up to 96% of cyclic carbonate within 24 h. Both Zn-MOF-1 and Cd-MOF-2 exhibited recyclability for up to five cycles without noticing an appreciable loss in their sensing or catalytic efficiency.

Recent Progress of Fluorescence Sensors for Histamine in Foods

Biological amines are organic nitrogen compounds that can be produced by the decomposition of spoiled food. As an important biological amine, histamine has played an important role in food safety. Many methods have been used to detect histamine in foods. Compared with traditional analysis methods, fluorescence sensors as an adaptable detection tool for histamine in foods have the advantages of low cost, convenience, less operation, high sensitivity, and good visibility. In terms of food safety, fluorescence sensors have shown great utilization potential. In this review, we will introduce the applications and development of fluorescence sensors in food safety based on various types of materials. The performance and effectiveness of the fluorescence sensors are discussed in detail regarding their structure, luminescence mechanism, and recognition mechanism. This review may contribute to the exploration of the application of fluorescence sensors in food-related work.

Metal–organic frameworks (MOFs) as fluorescence sensors: principles, development and prospects

This review classifies the latest developments of MOF-based fluorescence sensors according to the analytes, and discusses the challenges faced by MOF-based fluorescence sensors and promotes some directions for future research.

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.

Luminescent monomeric and dimeric Ru(ii) acyclic carbene complexes as selective sensors for NH3/amine vapor and humidity

A new class of luminescent bis(bipyridyl) Ru(ii) pyridyl acyclic carbene complexes with environmentally-sensitive dimerization equilibrium have been developed. Owing to the involvement of the orbitals of the diaminocarbene ligand in the emissive excited state, the phosphorescence properties of these complexes are strongly affected by H-bonding interactions with various H-bonding donor/acceptor molecules. With the remarkable differences in the emission properties of the monomer, dimer, and H-bonded amine adducts together with the change of the dimerization equilibrium, these complexes can be used as luminescent gas sensors for humidity, ammonia, and amine vapors. With the responses to amines and humidity and the corresponding change in the luminescence properties, a proof-of-principle for binary optical data storage with a reversible concealment process has been described.

Naphthalimide Derivative-Functionalized Metal-Organic Framework for Highly Sensitive and Selective Determination of Aldehyde by Space Confinement-Induced Sensitivity Enhancement Effect

Facile and sensitive determination of formaldehyde (FA) in indoor environments still remains challenging. Herein, a fluorescent probe, termed PHN@MOF, was synthesized by embedding the fluorescent molecule of N-propyl-4-hydrazine-naphthalimide (PHN) into a metal-organic framework (MOF) for sensitive and visual monitoring of FA. The hydrazine group of PHN acts as the specific reaction group with FA based on the condensation reaction. The host of MOF (UiO-66-NH₂) offers the surrounding confinement space required for the reaction. Owing to the enrichment effect and molecular sieve selection of UiO-66-NH₂ to FA, PHN@MOF, compared with free PHN, exhibits very high sensitivity and selectivity based on space confinement-induced sensitivity enhancement (SCISE). Moreover, the fluorescence of UiO-66-NH₂ offers a reference signal for FA detection. Using this ratiometric fluorescent PHN@MOF probe, a colorimetric gel plate and test paper were developed and used to visually monitor FA in air.

A Fluorescent Metal-Organic Framework for Food Real-Time Visual Monitoring

Due to increasing food-safety issues, exploiting efficient approaches for food quality assessment and instrumentation has attracted concerns worldwide. Herein, a smart evaluation system based on a fluorescent metal-organic framework (MOF) is developed for real-time visual monitoring of food freshness. Via post-synthetic modification, a ratiometric fluorescent MOF probe is constructed by covalently coupling fluorescein 5-isothiocyanate (5-FITC) with NH₂ -rich lanthanide MOF. The probes exhibit a dual-emissive-responsive to biogenic amine, resulting in an increase in FITC emission along with a decrease in Eu³⁺ emission accompanied by a clear distinguishable color transition from orange red to green. After doping the probes on a flexible substrate, the obtained MOF composite film can be integrated with a smartphone-based portable platform easily. It is proved that this smart evaluation system can be used for on-site inspection of the freshness of raw fish samples. This work develops a fluorescent MOF-based smart evaluation system as a novel platform for application in food monitoring, which not only has enormous economic value but also holds great public health significance.

Hetero-Tetranuclear Cu2+ /Ca2+ /Ca2+ /Cu2+ Architectures Based On Malten Ligand: Scaffold for Anion Binding

The hetero-tetranuclear Cu²⁺ /Ca²⁺ /Ca²⁺ /Cu²⁺ complex obtained with the N,N'-bis((3-hydroxy-4-pyron-2-yl)methyl)-N,N'-dimethylethylendiamine (Malten) ligand has been studied in solid and solution states as scaffold to bind anions. Three crystal structures showing the same metal ions sequence have been examined; they display a tetracharged complex cation neutralized by four monocharged anions. The anions play two different roles: as coordinated (two ClO₄ ^- , Cl^- or NO₃ ^- ) or ancillary (two ClO₄ ^- ) guests. The tetranuclear scaffold hosts two anions also in aqueous and ethanol solutions. Spectrophotometric studies in ethanol allowed to determine the addition constant values for Cl^- and Br^- (Log K_1-2 =4.43(4), 4.39(3) for Cl^- , 3.80(3), 3.54(2) for Br^- ) while the others, although bound, showed lower affinity for the scaffold. Both the crystals and the solutions change their color depending on the added anion, namely pink, dark green or blue in the presence of ClO₄ ^- , Cl^- or NO₃ ^- , respectively, thus the presence of the different anions is visible to the naked eye. The hetero-tetranuclear Cu²⁺ /Ca²⁺ /Ca²⁺ /Cu²⁺ complex is a versatile architecture to be used as scaffold for anion binding.