Developing efficient sensor materials with superior performance for selective, fast and sensitive detection of gases and volatile organic compounds (VOCs) is essential for human health and environmental protection, through monitoring indoor and outdoor air pollutions, managing industrial processes, controlling food quality and assisting early diagnosis of diseases. Metal–organic frameworks (MOFs) are a unique type of crystalline and porous solid material constructed from metal nodes (metal ions or clusters) and functional organic ligands. They have been investigated extensively for possible use as high performance sensors for the detection of many different gases and VOCs in recent years, due to their large surface area, tunable pore size, functionalizable sites and intriguing properties, such as electrical conductivity, magnetism, ferroelectricity, luminescence and chromism. The high porosity of MOFs allows them to interact strongly with various analytes, including gases and VOCs, thus resulting in easily measurable responses to different physicochemical parameters. Although much of the recent work on MOF-based luminescent sensors have been summarized in several excellent reviews (up to 2018), a comprehensive overview of these materials for sensing gases and VOCs based on chemiresistive, magnetic, ferroelectric, and colorimertic mechanisms is missing. In this review, we highlight the most recent progress in developing MOF sensing and switching materials with an emphasis on sensing mechanisms based on electricity, magnetism, ferroelectricity and chromism. We provide a comprehensive analysis on the MOF–analyte interactions in these processes, which play a key role in the sensing performance of the MOF-based sensors and switches. We discuss in detail possible applications of MOF-based sensing and switching materials in detecting oxygen, water vapor, toxic industrial gases (such as hydrogen sulfide, ammonia, sulfur dioxide, nitrous oxide, carbon oxides and carbon disulfide) and VOCs (such as aromatic and aliphatic hydrocarbons, ketones, alcohols, aldehydes, chlorinated hydrocarbons and N,N′-dimethylformamide). Overall, this review serves as a timely source of information and provides insight for the future development of advanced MOF materials as next-generation gas and VOC sensors.

Functional metal–organic frameworks as effective sensors of gases and volatile compounds

Due to their atomically precise structures and intriguing chemical/physical properties, metal nanoclusters are an emerging class of modular nanomaterials. Photo-luminescence (PL) is one of their most fascinating properties, due to the plethora of promising PL-based applications, such as chemical sensing, bio-imaging, cell labeling, phototherapy, drug delivery, and so on. However, the PL of most current nanoclusters is still unsatisfactory-the PL quantum yield (QY) is relatively low (generally lower than 20%), the emission lifetimes are generally in the nanosecond range, and the emitted color is always red (emission wavelengths of above 630 nm). To address these shortcomings, several strategies have been adopted, and are reviewed herein: capped-ligand engineering, metallic kernel alloying, aggregation-induced emission, self-assembly of nanocluster building blocks into cluster-based networks, and adjustments on external environment factors. We further review promising applications of these fluorescent nanoclusters, with particular focus on their potential to impact the fields of chemical sensing, bio-imaging, and bio-labeling. Finally, scope for improvements and future perspectives of these novel nanomaterials are highlighted as well. Our intended audience is the broader scientific community interested in the fluorescence of metal nanoclusters, and our review hopefully opens up new horizons for these scientists to manipulate PL properties of nanoclusters. This review is based on publications available up to December 2018.

Tailoring the photoluminescence of atomically precise nanoclusters.

Metal–organic frameworks (MOFs) have emerged as particularly exciting inorganic–organic hybrid porous materials which can be simply self-assembled from their corresponding inorganic metal ions/clusters with organic linkers. MOFs can combine the inherent physical and chemical properties of both inorganic and organic photonic units due to their inorganic–organic hybrid nature. Furthermore, the pores within MOFs can also be utilized to encapsulate a large number of guest species as photonic units. The vast combination possibilities, synergistic effects, as well as controllable and ordered arrangements of multiple photonic units (MPUs) have distinguished MOFs from other inorganic and organic photonic materials and enabled them to be a promising platform to realize novel photonic functional applications. In this review, we summarize the recent and important progress in the design and construction of photonic MOFs, as well as their various applications in luminescence sensing, white-light emission, photocatalysis, nonlinear optics, lasing devices, data storage, and biomedicine. In addition, we highlight the construction strategy and the synergistic effects of MOFs towards achieving high performance and novel photonic functions. Finally, we also outline the challenges in these fields and put forward the prospects and directions for future development.

Photonic functional metal-organic frameworks.

Toxic and hazardous chemical species are ubiquitous, predominantly emitted by anthropogenic activities, and pose serious risks to human health and the environment. Thus, the sensing and subsequent capture of these chemicals, especially in the gas or vapor phase, are of extreme importance. To this end, metal–organic frameworks have attracted significant interest, as their high porosity and wide tunability make them ideal for both applications. These tailorable framework materials are particularly promising for the specific sensing and capture of targeted chemicals, as they can be designed to fit a diverse range of required conditions. This review will discuss the advantages of metal–organic frameworks in the sensing and capture of harmful gases and vapors, as well as principles and strategies guiding the design of these materials. Recent progress in the luminescent detection of aromatic and aliphatic volatile organic compounds, toxic gases, and chemical warfare agents will be summarized, and the adsorptive removal of fluorocarbons/chlorofluorocarbons, volatile radioactive species, toxic industrial gases and chemical warfare agents will be discussed.

Sensing and capture of toxic and hazardous gases and vapors by metal–organic frameworks

The utility of electronically conductive metal-organic frameworks (EC-MOFs) in high-performance devices has been limited to date by a lack of high-quality thin film The controllable thin-film fabrication of an EC-MOF, Cu3 (HHTP)2 , (HHTP=2,3,6,7,10,11-hexahydroxytriphenylene), by a spray layer-by-layer liquid-phase epitaxial method is reported The Cu3 (HHTP)2 thin film can not only be precisely prepared with thickness increment of about 2 nm per growing cycle, but also shows a smooth surface, good crystallinity, and high orientation The chemiresistor gas sensor based on this high-quality thin film is one of the best room-temperature sensors for NH3 among all reported sensors based on various materials

Layer-by-Layer Assembled Conductive Metal-Organic Framework Nanofilms for Room-Temperature Chemiresistive Sensing.

Silver(i) chalcogenide/chalcogenolate clusters are promising photofunctional materials for sensing, optoelectronics and solar energy harvesting applications. However, their instability and poor room-temperature luminescent quantum yields have hampered more extensive study. Here, we graft such clusters to adaptable bridging ligands, enabling their interconnection and the formation of rigid metal–organic frameworks. By controlling the spatial separation and orientation of the clusters, they then exhibit enhanced stability (over one year) and quantum yield (12.1%). Ultrafast dual-function fluorescence switching (<1 s) is also achieved, with turn-off triggered by O2 and multicoloured turn-on by volatile organic compounds. Single-crystal X-ray diffraction of the inclusion materials, obtained by single-crystal-to-single-crystal transformation, enables precise determination of the position of the small molecules within the framework, elucidating the switching mechanism. The work enriches the cluster-based metal–organic framework portfolio, bridges the gap between silver chalcogenide/chalcogenolate clusters and metal–organic frameworks, and provides a foundation for further development of functional silver-cluster-based materials. The properties of discrete species can sometimes be improved by fixing them into extended materials. This strategy has now been applied to silver(I) chalcogenide/chalcogenolate clusters, resulting in a metal–organic framework with enhanced stability and fluorescent sensing capabilities. Crystallographic analysis allows precise structural determination of guest binding, which is responsible for both emission turn-off and multicoloured turn-on.

Hypersensitive dual-function luminescence switching of a silver-chalcogenolate cluster-based metal–organic framework

Utilizing aggregation-induced emission luminogens (AIEgens) as ligands has proven to be an effective strategy for constructing metal-organic frameworks (MOFs) with intense luminescent properties. However, highly luminescent AIEgen-based MOFs with adjustable emission properties are rarely achieved because of the rigid conformation of AIEgens in the crystalline state. Here, a dual-node 3D silver chalcogenolate cluster MOF (1) is designed and synthesized, where the AIE ligand shows relatively flexible and rotatable conformations. The conformations of AIE ligands in 1 are switchable by the absorption/desorption of guest molecules. As a result, 1 exhibited not only intense but also guest molecule switched luminescent properties. More importantly, the switching rate is tunable by using different guest molecules. 1 provides a unique visualized prototype to understand the mechanism of guest-triggered aggregation-induced emission in MOFs.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/advs.201801304

Guest-Triggered Aggregation-Induced Emission in Silver Chalcogenolate Cluster Metal-Organic Frameworks.

Developing highly luminescent and extensively stable silver cluster-assembled materials (SCAMs) from the inferior luminogens and unstable silver cluster is an important and challenging issue. Herein, a new luminescent three-dimensional SCAM (Ag12 CPPP, [Ag12 (StBu)6 (CF3 COO)6 (CPPP)2 (DMAc)12 ]n ; CPPP=2,5-bis(4-cyanophenyl)-1,4-bis(4-(pyridine-4-yl)-phenyl)-1,4-dihydropyrrolo[3,2-b]pyrrole, DMAc=dimethylacetamide) was designed and synthesized with a quadridentate rigid emission ligand (CPPP) and a silver-chalcogenolate cluster (SCC) containing 12 AgI atoms. The luminescence study indicates that CPPP is an aggregation-caused quenching (ACQ) molecule with twisted intramolecular charge transfer (TICT) character. Benefiting from the strong immobilization effect in the robust framework, the quantum yield of CPPP is greatly enhanced in Ag12 CPPP compared with that of CPPP in solution or in the solid state. As a result, Ag12 CPPP exhibits typical matrix coordination induced emission (MCIE) effect. Such efficient rigidifying methodology provides a promising approach for enhancing luminescence of ACQ molecules in an aggregated state and strengthening the silver cluster in an unstable state.

https://chemistry-europe.onlinelibrary.wiley.com/doi/pdf/10.1002/chem.201806413

Matrix Coordination Induced Emission in a Three‐Dimensional Silver Cluster‐Assembled Material

Crystalline cluster-assembled materials (CAMs) are emerging as types of exploratory fabrications due to their atomically precise structures and intriguing chemical/physical properties, as well as t...

https://www.chinesechemsoc.org/doi/pdf/10.31635/ccschem.019.20190024

Mesoporous Crystalline Silver-Chalcogenolate Cluster-Assembled Material with Tailored Photoluminescence Properties

Introduction: Functional disorder of the placenta is the principal cause of fetal growth restriction (FGR), usually cured with suitable clinical treatment and good nursing. However, some FGR mothers still give birth to small for gestational age (SGA) babies after treatment. The ineffectiveness of treatment in such a group of patients confused physicians of obstetrics and gynecology. Methods: In this study, we performed a microRNA-messenger RNA integrative analysis of gene expression profiles obtained from Gene Expression Omnibus. Differentially expressed genes were screened and checked using quantitative polymerase chain reaction. Target genes of significantly changed microRNA were screened and enriched for Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway analyses. Function of the obtained microRNA-messenger RNA was evaluated using HTR-8/SVneo trophoblast cells, human umbilical vein endothelial cells, and heterozygote male mice. Result: MiR-155-5p was upregulated (p = 0.001, fold-change = 2.275) in fetal-side placentals. Among the hub genes identified as key targets for miR-155-5p in fetal reprogramming, Smad2 was downregulated (p = 0.002, fold change = 0.426) and negatively correlated with miR-155-5p expression levels (r = −0.471, p < 1.0 E – 04) in fetal-side placental tissues. The miR-155-5p mimic blocks Smad2 expression and suppresses villous trophoblast cell and endothelial cell function (proliferation, migration, and invasion), indicating a close relationship with placental development. Luciferase assays further confirmed the targeting of miR-155-5p to Smad2. Furthermore, Smad2+/− heterozygote male mice were born small with low body weight (p = 0.0281) and fat composition (p = 0.013) in the fourth week post-natal. Discussion: We provide the first evidence of the role of the Smad2/miR-155-5p axis in the placental pathologies of FGR. Our findings elucidate the pathogenesis of FGR and provide new therapeutic targets.

/pdf/an-mirna-mrna-integrative-analysis-in-human-placentas-and-3bx0b2dv.pdf

Xiao-Hui Wu

Papers

Hypersensitive dual-function luminescence switching of a silver-chalcogenolate cluster-based metal–organic framework

Guest-Triggered Aggregation-Induced Emission in Silver Chalcogenolate Cluster Metal-Organic Frameworks.

Matrix Coordination Induced Emission in a Three‐Dimensional Silver Cluster‐Assembled Material

Mesoporous Crystalline Silver-Chalcogenolate Cluster-Assembled Material with Tailored Photoluminescence Properties

An miRNA-mRNA integrative analysis in human placentas and mice: role of the Smad2/miR-155-5p axis in the development of fetal growth restriction