BTEX Vapor Detection with a Flexible MOF and Functional Polymer by Means of a Composite Photonic Crystal

Photonic crystal gas sensors based on metal-organic frameworks and polymers

A photonic crystal (PC) is an optical microstructure with an adjustable dielectric constant. The PC sensor was deemed a powerful tool for gas molecule detection due to its excellent sensitivity, stability, online use and tailorable optical performance. The detection signals are generated by monitoring the changes of the photonic band gap when the sensing behavior occurs. Recently, many efforts have been devoted to improving the PC sensor's detection performance and reducing technical costs by selecting and refining functional materials. In this case, metal-organic frameworks (MOFs) with a large specific surface, tunable structural properties and polymers with unique swelling properties have attracted increasingly attention. In this review, a systematic review of PC gas sensors based on MOFs and polymers was carried out for the first time. Firstly, the optical properties and gas sensing mechanism of PCs were briefly summarized. Secondly, a detailed discussion of the structural properties and rapid preparation methods of distributed Bragg reflectors (DBRs), opals and inverse opals (IOPCs) was presented. Thirdly, the recent advances in MOF, polymer and MOF/polymer-based PC sensors over the past few years were summarized. It should be noted that the sensitivity and selectivity enhancement strategy by appropriate material species selection, organic ligand functionalization, metal-ion doping, diverse functional material arrays, and multi-component compounding were analyzed in detail. Finally, prospects on PC gas sensors are given in terms of preparation methods, material functionalization and future applications.

Harnessing the cation-π interactions of metalated gold monolayer-protected clusters to detect aromatic volatile organic compounds

The strong, non-covalent interactions between π-systems and cations have been the focus of numerous studies on biomolecule structure and catalysis. These interactions, however, have yet to be explored as a sensing mechanism for detecting trace levels of volatile organic compounds (VOCs). In this article, we provide evidence that cation-π interactions can be used to elicit sensitive and selective chemiresistor responses to aromatic VOCs. The chemiresistors are fitted with carboxylate-linked alkali metals bound to the surface of gold monolayer-protected clusters formulated on microfabricated interdigitated electrodes. Sensor responses to aromatic and non-aromatic VOCs are consistent with a model for cation-π interactions arising from association of electron-rich aromatic π-systems to metal ions with the relative strength of attraction following the order K⁺ > Na⁺ > Li⁺. The results point toward cation-π interactions as a promising research avenue to explore for developing aromatic VOC-selective sensors.

Monolithic MOF-Based Metal-Insulator-Metal Resonator for Filtering and Sensing

Metal-insulator-metal (MIM) configurations based on Fabry-Pérot resonators have advanced the development of color filtering through interactions between light and matter. However, dynamic color changes without breaking the structure of the MIM resonator upon environmental stimuli are still challenging. Here, we report monolithic metal-organic framework (MOF)-based MIM resonators with tunable bandwidth that can boost both dynamic optical filtering and active chemical sensing by laser-processing microwell arrays on the top metal layer. Programmable tuning of the reflection color of the MOF-based MIM resonator is achieved by controlling the MOF layer thicknesses, which is demonstrated by simulation of light-matter interactions on subwavelength scales. Laser-processed microwell arrays are used to boost sensing performance by extending the pathway for diffusion of external chemicals into nanopores of the MOFs. Both experiments and molecular dynamics simulations demonstrate that tailoring the period and height of the microwell array on the MIM resonator can advance the high detection sensitivity of chemicals.

Flexible TPU inverse opal fabrics for colorimetric detecting of VOCs†

Recently, responsive structure color fibers and fabrics have been designed and prepared for colorimetric detecting of volatile organic compounds (VOCs). Fabric substrates can offer greater flexibility and portability than flat and hard substrates such as glass, silicon wafers, etc. At present, one-dimensional photonic crystal (multilayer films) and three-dimensional dense photonic crystal layers are mainly constructed on fabrics to achieve the response to VOCs. However, the binding force between these structural color coatings and the fabrics was poor, and the dense structures inevitably hindered the diffusion of VOCs. Here, thermoplastic polyurethane (TPU) inverse opal (IOs) fabrics were prepared by sacrificing the SiO₂ photonic crystal templates to achieve colorimetric detecting of VOCs. The IOs layer of TPU was cured directly on the fabric surface, TPU infiltrated into the fabric yarns, and bonded the fabrics and IOs layer into a whole, which greatly improved the binding force, and the porous structure and large specific surface area of IOs were conducive to the diffusion of VOCs. The results showed that the TPU IOs fabrics have large reflection peak shifts to DMF, THF, toluene and chloroform vapors, and its concentration has a good linear relationship with the maximum reflection peak value of TPU IOs fabrics. The theoretical detection limits are 1.72, 0.89, 0.78 and 1.64 g m⁻³, respectively. The response times are 105, 62, 75 and 66 seconds, with good stability. Finally, it was calculated that the discoloration of the TPU IOs fabrics in VOCs was due to the joint-effects of lattice spacing and effective refractive index increase.

Thermoplastic polyurethane (TPU) inverse opal structural color fabrics for colorimetric detecting of volatile organic compounds (VOCs) vapor especially DMF, THF, toluene and chloroform.

Recent progress on MOF-based optical sensors for VOC sensing

The raising apprehension of volatile organic compound (VOC) exposures urges the exploration of advanced monitoring platforms. Metal-organic frameworks (MOFs) provide many attractive features including tailorable porosity, high surface areas, good chemical/thermal stability, and various host-guest interactions, making them appealing candidates for VOC capture and sensing. To comprehensively exploit the potential of MOFs as sensing materials, great efforts have been dedicated to the shaping and patterning of MOFs for next-level device integration. Among different types of sensors (chemiresistive sensors, gravimetric sensors, optical sensors, etc.), MOFs coupled with optical sensors feature distinctive strength. This review summarized the latest advancements in MOF-based optical sensors with a particular focus on VOC sensing. The subject is discussed by different mechanisms: colorimetry, luminescence, and sensors based on optical index modulations. Critical analysis for each system highlighting practical aspects was also deliberated.

Computational Investigations of Metal-Organic Frameworks as Sorbents for BTEX Removal

Sequestration of aromatic volatile organic compounds (VOCs) via metal-organic frameworks (MOFs) as sorbents is a viable means of environmental preservation. In this investigation, we shed light on the key features associated with MOFs that govern the selective uptake of a subclass of VOCs containing benzene, toluene, ethylbenzene, and xylenes (BTEX). We investigate, through a multistep computational framework including ab initio electronic structure and classical molecular dynamics simulations, the energetic and dynamical properties associated with BTEX capture in three MOFs: HKUST-1, ZIF-8, and MIL-53. Our work demonstrates the importance of considering both static and dynamical properties upon introduction of guest molecules in such computational investigations. We elucidate the key geometric factors associated with efficient capture of BTEX compounds and highlight possible postsynthetic modifications that can be used to produce next generation sorbents for BTEX capture.

High performance benzene vapor sensor based on three-dimensional photonic crystals of zeolitic imidazolate framework-8@graphene quantum dots

Superior sensitive, selective, and repeatable real-time detection of low concentrations of benzene vapor is vitally important for environmental protection and human health. A benzene vapor sensor using three-dimensional photonic crystals (3-D PCs) based on zeolitic imidazolate framework-8@graphene quantum dots (ZIF-8@GQDs) was proposed. The 3-D PCs were acquired by centrifuging ZIF-8@GQDs pseudo-solutions, which were prepared via hydrothermal methods. The application of the ZIF-8@GQDs 3-D PCs sensor for optical benzene vapor detection via the strong π-π stacking interactions and large specific surface area and abundant open-framework structure of the ZIF-8@GQDs was investigated. The ZIF-8@GQDs 3-D PCs sensor exhibits a more sensitive response to benzene vapor compared with the ZIF-8 3-D PCs sensor. The relationship between the wavelength shift and the benzene vapor concentration was demonstrated to be linear. Additionally, the ZIF-8@GQDs 3-D PCs sensor presents a fast optical response and recovery times of 1 s and 7 s for 200 ppm benzene vapor detection, the benzene vapor detection limit can reach 1 ppm, and the deviation of the reflected wavelength varied within 2 nm after 10 cycles. Moreover, the fabricated ZIF-8@GQDs 3-D PCs sensor exhibited reliability and exceptional thermal and long-time storage stability, demonstrating great potential for practical benzene vapor sensing applications.

Flexible luminescent non-lanthanide metal-organic frameworks as small molecules sensors

Toxic, carcinogenic, and hazardous materials are omnipresent, generally obtained by anthropogenic activities, industrial activities, aerobic and anaerobic degradation of waste materials and are harmful to human health and environment. Thus, sensing, colorimetric detection, and subsequent inclusion of these chemicals are of prime importance for human health and environment. In comparison to other classes of highly porous materials, luminescent metal-organic frameworks (LMOFs) have chromophoric organic ligands, high surface area, high degree of tunability and structural diversity. They have received scientific interest as sensory materials for device fabrication to detect and sense toxic small molecules. Especially, as soft-porous materials exhibiting a degree of flexibility or dynamic behaviour, flexible LMOFs are promising for selective detection and sensing, and for encapsulation of toxic and health hazardous molecules. Such flexible LMOFs offer a potential platform for selective adsorption/separation, molecular recognition, and sensing application. In this perspective, we highlight the advantages of flexibility of LMOFs for selective detection and sensing, and inclusion of toxic small molecules (solvents, anions, halobenzenes, aromatics, aromatic amines, nitro-explosives and acetylacetone). In addition, the principles and strategies guiding the design of these MOF based materials and recent progress in the luminescent detection of toxic small molecules are also discussed. In this perspective we limit our discussion on the 'non-lanthanide' based luminescent MOFs that have flexibility in the framework and show small molecule sensing applications.

Sensing Behavior of Metal-Free Porphyrin and Zinc Phthalocyanine Thin Film towards Xylene-Styrene and HCl Vapors in Planar Optical Waveguide

The sensing behavior of a thin film composed of metal-free 5, 10, 15, 20-tetrakis (p-hydroxy phenyl) porphyrin and zinc phthalocyanine complex towards m-xylene, styrene, and HCl vapors in a homemade planar optical waveguide (POWG), was studied at room temperature. The thin film was deposited on the surface of potassium ion-exchanged glass substrate, using vacuum spin-coating method, and a semiconductor laser light (532 nm) as the guiding light. Opto-chemical changes of the film exposing with hydrochloric gas, m-xylene, and styrene vapor, were analyzed firstly with UV-Vis spectroscopy. The fabricated POWG shows good correlation between gas exposure response and absorbance change within the gas concentration range 10-1500 ppm. The limit of detection calculated from the logarithmic calibration curve was proved to be 11.47, 21.08, and 14.07 ppm, for HCl gas, m-xylene, and styrene vapors, respectively. It is interesting to find that the film can be recovered to the initial state with trimethylamine vapors after m-xylene, styrene exposures as well as HCl exposure. The gas-film interaction mechanism was discussed considering protonation and π-π stacking with planar aromatic analyte molecules.

Form Birefringence in Resonant Transducers for the Selective Monitoring of VOCs under Ambient Conditions

In this work, we have developed a new kind of nanocolumnar birefringent Bragg microcavity (BBM) that, tailored by oblique angle deposition, behaves as a selective transducer of volatile organic compounds (VOCs). Unlike the atomic lattice origin of birefringence in anisotropic single crystals, in the BBM, it stems from an anisotropic self-organization at the nanoscale of the voids and structural elements of the layers. The optical adsorption isotherms recorded upon exposure of these nanostructured systems to water vapor and VOCs have revealed a rich yet unexplored phenomenology linked to their optical activity that provides both capacity for vapor identification and partial pressure determination. This photonic response has been reproduced with a theoretical model accounting for the evolution of the form birefringence of the individual layers upon vapor condensation in nanopores and internanocolumnar voids. BBMs that repel water vapor but are accessible to VOCs have been also developed through grafting of their internal surfaces with perfluorooctyltriethoxysilane molecules. These nanostructured photonic systems are proposed for the development of transducers that, operating under environmental conditions, may respond specifically to VOCs without any influence by the degree of humidity of the medium.

High-Efficiency Separation of n-Hexane by a Dynamic Metal-Organic Framework with Reduced Energy Consumption

The separation of n-alkanes from their branched isomers is vitally important to improve octane rating of gasoline. To facilitate mass transfer, adsorptive separation is usually operated under high temperatures in industry, which require considerable energy. Herein, we present a kind of dynamic pillar-layered MOF that exhibits self-adjustable structure and pore space, a behavior induced by guest molecules. A combination of the flexibility of the framework with the commensurate adsorption for n-hexane results in exceptional performance in separating hexane isomers. More significantly, lower temperature prompts the guest molecules to open the dynamic pores, which may provide a new perspective for optimized separation performance at lower temperatures with less energy consumption.

Eu3+ functionalized robust membranes based on the post-synthetic copolymerization of a metal-organic framework and ethyl methacrylate

Metal-organic frameworks (MOFs) are recognized as a class of promising crystalline materials. However, their subsequent processing and shaping still remain a challenge, and one emerging strategy is to hybridize MOFs with flexible polymers. Herein, by utilizing a simple and cost-effective post-synthetic polymerization method, under mild conditions, MOF particles with olefin bonds are covalently linked to polymer chains. Moreover, photoactive europium ions are also introduced into this system during the polymerization process. Importantly, the resulting MOF-based membrane (MOF1-Eu3+@PEMA) is uniform, showing great structural and fluorescence stability against strict conditions (aqueous solutions with pH 0.98-13.11). Besides, given its good luminescence properties, the membrane is employed for the identification of common volatile organic compounds and a selective response to toluene was achieved. This work accelerates the practical applications of MOF-based membranes and enriches the methods for MOF modification.