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

Metal–organic frameworks (MOFs), with diverse framework architectures, have evolved as next-generation utility multifunctional hybrid materials. One of the key features of MOFs is their luminescence properties, which can be generated from the building ligands, emissive metal ions, guest ions, or molecules used to construct them as well as from their catalytic activities. MOFs with luminescent properties have been used as excellent platforms for designing luminescent sensors. Their chemically tailorable framework with specific host–guest interactions plays an important role in selectively sensing metal ions, small organic molecules, and biomolecules. This review intends to summarize the recent advances in the construction of MOF-based sensors for chemical sensing and biosensing. Specially, we focus on the fabrication strategies of MOF-based luminescent sensors, and summarize their sensing mechanisms in detail. Also, the major challenges and constraints for this research field are discussed.

Strategies to fabricate metal–organic framework (MOF)-based luminescent sensing platforms

Three-dimensional porous coordination polymers, which are known as metal–organic frameworks (MOFs), have drawn considerable attention owing to their properties, such as highly crystalline and ordered structures, inorganic–organic-hybrid nature, tunability of chemical functionality, high porosity and surface area and moderate-to-high stability. Due to these properties, MOFs are applied extensively as probes for the detection of large varieties of organic molecules. In this line, different MOF-based instrumental and material-based methods along with different strategies have been developed to study and extend our information about the capabilities of MOFs as sensing probes for the detection of organic molecules as well as improving and developing their detection limits, selectivity and sensitivity toward specific analytes. Considering these points, we have presented and classified the content of this review based on the chemical structures of organic analytes in three categories, including (I) nitroaromatic explosives and energetic materials, (II) small organic molecules, such as solvents and volatile organic compounds, and (III) organic amines. For each group of these analytes, different material and instrumental methods using MOFs have been explained, with illustrations of remarkable examples. Finally, we compare the methods for the detection of each group of analytes and discuss which instrumentation is more effective for each group of analytes.

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Sensing organic analytes by metal–organic frameworks: a new way of considering the topic

Ratiometric fluorescence sensing of metal-organic frameworks: Tactics and perspectives

Eu3+-activated Na2Gd(PO4)(MoO4): A novel high-brightness red-emitting phosphor with high color purity and quantum efficiency for white light-emitting diodes

A dual-emitting luminescent lanthanide/transition heterometal-organic frameworks of Eu/Zr-MOF was synthesized by incorporation of Eu 3+ ions into NH 2 -UiO-66 under microwave irradiation condition. The multiband fluorescence derived from the characteristic emission of Eu 3+ and linker-to-cluster (Eu- oxo or Zr- oxo) charge transfer (LCCT) transition was fabricated. By combination of the luminescent property with the intrinsic porosity and open sites of amine group to bind target analytes, the Eu/Zr-MOF exhibited small molecules-dependent luminescence enhancement and quench effects. Notably, a drastic enhancement of fluorescent at 465 nm induced by formaldehyde was observed. Thus, a ratiometric fluorescent sensing for formaldehyde was performed based on the intensity ratio of two emission bands at 465 and 615 nm for Eu/Zr-MOF. Under the excitation of 365 nm, the increase in intensity ratio of the two emission bands was nearly linearly proportional to the amount of formaldehyde. By this Eu/Zr - MOF sensor, the detection limit of formaldehyde was 0.2 mg/L. This sensing mechanism was ascribed to the binding interaction of free amino groups in Eu/Zr-MOF with the guest. An added electron transfer from amino group containing lone pair electrons to the positively charged formaldehyde leads to a drastic enhancement of luminescence at about 465 nm, while the characteristic emission of Eu 3+ at 615 nm enhances slightly. These studies demonstrate that the strategy of multiband emissive heterometal-MOFs can be served as a facile method to fabricate sensitive and specific fluorescent probes of polluting organic small molecules.

Chunmei Li

Papers

Dual-Emitting Fluorescence of Eu/Zr-MOF for Ratiometric Sensing Formaldehyde

Structure, energy transfer and tunable photoluminescence of Sr7Zr(PO4)6:Tb3+, Eu3+ phosphors for near-UV white LEDs

Synthesis, characterization and heterogeneous base catalysis of amino functionalized lanthanide metal-organic frameworks