CuO-ZIF-8 modified electrode surface as a new electrochemical sensing platform for detection of free chlorine in aqueous solution

This work has applied metal-organic frameworks (MOFs) with high adsorbability and catalytic activity to develop electrochemical sensors to determine free chlorine (free-Cl) concentrations in aqueous media. A zeolitic imidazolate frameworks, Zn(Hmim)2 (ZIF-8) has been synthesized and incorporated with CuO nanosheets to decorate a glassy carbon electrode (GCE) and provide a new sensor for free-Cl determination. The as-prepared ZIF-8 and CuO-ZIF-8 composites have been characterized by FESEM, EDX, XRD, and FT-IR analyses. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) utilized to characterize the CuO-ZIF-8/GC modified electrode electrochemically, demonstrated the ability of the sensor to measure free-Cl concentration. Using differential pulse voltammetry (DPV) and under the optimal conditions, the prepared CuO-ZIF-8/GC modified electrode showed a linear response in the 0.25-60 ppm range with a 12 ppb detection limit (LOD) for free-Cl concentration. Finally, the fabricated sensor was applied to analyze free-Cl from actual swimming pool water samples with promising 97.5 to 103.0% recoveries.

Construction of Indium(III)-Organic Framework Based on a Flexible Cyclotriphosphazene-Derived Hexacarboxylate as a Reusable Green Catalyst for the Synthesis of Bioactive Aza-Heterocycles

The constant demand for eco-friendly methods of synthesizing complex organic compounds inspired researchers to design and develop modern, highly efficient heterogeneous catalytic systems. Herein, In-HCPCP metal-organic framework (SRMIST-1), a heterogeneous Lewis acid catalyst containing less toxic indium and eco-friendly robust cyclotriphosphazene and exhibiting notable chemical and thermal stability, durable catalytic activity, and exceptional reusability was produced through the reaction between indium(III) nitrate hydrate and hexakis(4-carboxylatophenoxy)-cyclotriphosphazene. In the SRMIST-1 structure, secondary building units {InO7} are assembled by a connection of η2- and η1-carboxylic oxo atoms from different HCPCP ligands, forming a three-dimensional network. The occurrence of regularly distributed In(III) sites in SRMIST-1 confers superior reactivity on the catalyst toward the synthesis of 2,3-dihydroquinazolin-4(1H)-ones and 3,4-dihydro-2H-1,2,4-benzothiadiazine-1,1-dioxides by the cyclization reaction of 2-aminobenzamides and 2-aminobenzenesulphonamides with aldehydes under optimized reaction conditions, respectively. The notable features of this method include broad functional group compatibility, low catalyst loading (1-5 mol %), mild reaction conditions, easy workup procedures, good to excellent reaction yields, ethanol as a green solvent, reusability of the catalyst (five cycles), and economic attractiveness, which is mainly due to sustainability of SRMIST-1 as a reusable green catalyst. Our findings demonstrate that the highly reactive and reusable green catalyst finds widespread applications in medicinal chemistry.

High-Selectivity Laminated Gas Sensor Based on Characteristic Peak under Temperature Modulation

Aiming at the bottleneck problem of insufficient selectivity of metal oxide gas sensors, a reliable scheme to improve selectivity is proposed, that is, a laminated sensor structure of a gas-sensitive membrane plus catalytic membrane combined with the temperature modulation technology. It is presented as a highly selective ethanol sensor as an example for verification. The laminated gas sensor is made of Sr@SnO2 as the gas-sensing membrane and ZSM-5 as the catalytic membrane by the microelectro mechanical system. The results indicate that in temperature modulation mode, the Sr@SnO2/ZSM-5-laminated sensor has good resistance gas-sensing response to most different types of gases but only shows a characteristic peak on the time-resistance and temperature-resistance curves of ethanol gas response. By defining and calculating this characteristic peak, the selectivity of ethanol gas response signal is improved. The Sr@SnO2/ZSM-5 sensor also exhibits high sensitivity to ethanol gas at the parts per billion level, fast response/recovery time in seconds, excellent anti-interference, and stability, indicating the reliability and practicality of this highly selective scheme. This scheme is of great significance for the study of high selectivity of a metal oxide gas sensor and promotes its wide application.

Cobalt-based metal-organic framework-functionalized graphene oxide modified electrode as a new electrochemical sensing platform for detection of free chlorine in aqueous solution

This work reports the profit of using a MOF compound for developing a sensitive electrochemical sensor to free chlorine detection in an aqueous solution. Co-MOF and FGO composites were synthesized and combined with the carbon paste (CP) to prepare an efficient electrochemical sensor with high sensing ability. The fabricated Co-MOF and FGO composites were characterized by SEM, EDX, FT-IR, and XRD techniques. Meanwhile, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were utilized to assess the electrochemical performance of the Co-MOF-FGO/CP modified electrode. Under the optimized condition, the amperometric detection showed that the reduction current of free chlorine increased linearly with a coefficient determination of 0.995 during its wide concentration range of 0.1-700 ppm. Also the detection limit (LOD) (S/N = 3) was 0.01 ppm. The selectivity of the sensor was tested with possible interferences, and satisfactory results were obtained. The proposed sensor was successfully used to determine the free chlorine in tap water and swimming pool water real samples. The results suggested that this proposed sensor could pave the way for developing the electrochemical sensor of free chlorine in aqueous media with MOFs.

Reticular Chemistry for Optical Sensing of Anions

In the last few decades, reticular chemistry has grown significantly as a field of porous crystalline molecular materials. Scientists have attempted to create the ideal platform for analyzing distinct anions based on optical sensing techniques (chromogenic and fluorogenic) by assembling different metal-containing units with suitable organic linking molecules and different organic molecules to produce crystalline porous materials. This study presents novel platforms for anion recognition based on reticular chemistry with high selectivity, sensitivity, electronic tunability, structural recognition, strong emission, and thermal and chemical stability. The key materials for reticular chemistry, Metal-Organic Frameworks (MOFs), Zeolitic Imidazolate Frameworks (ZIFs), and Covalent-Organic Frameworks (COFs), and the pre- and post-synthetic modification of the linkers and the metal oxide clusters for the selective detection of the anions, have been discussed. The mechanisms involved in sensing are also discussed.

Metal-Organic Framework Coated Devices for Gas Sensing

The demand for monitoring chemical and physical information surrounding, air quality, and disease diagnosis has propelled the development of devices for gas sensing that are capable of translating external stimuli into detectable signals. Metal-organic frameworks (MOFs), possessing particular physiochemical properties with designability in topology, specific surface area, pore size and/or geometry, potential functionalization, and host-guest interactions, reveal excellent development promises for manufacturing a variety of MOF-coated sensing devices for multitudinous applications including gas sensing. The past years have witnessed tremendous progress on the preparation of MOF-coated gas sensors with superior sensing performance, especially high sensitivity and selectivity. Although limited reviews have summarized different transduction mechanisms and applications of MOF-coated sensors, reviews summarizing the latest progress of MOF-coated devices under different working principles would be a good complement. Herein, we summarize the latest advances of several classes of MOF-based devices for gas sensing, i.e., chemiresistive sensors, capacitors, field-effect transistors (FETs) or Kelvin probes (KPs), electrochemical, and quartz crystal microbalance (QCM)-based sensors. The surface chemistry and structural characteristics were carefully associated with the sensing behaviors of relevant MOF-coated sensors. Finally, challenges and future prospects for long-term development and potentially practical application of MOF-coated sensing devices are pointed out.

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.

Materials for Chemical Sensing: A Comprehensive Review on the Recent Advances and Outlook Using Ionic Liquids, Metal–Organic Frameworks (MOFs), and MOF-Based Composites

The ability to measure and monitor the concentration of specific chemical and/or gaseous species (i.e., “analytes”) is the main requirement in many fields, including industrial processes, medical applications, and workplace safety management. As a consequence, several kinds of sensors have been developed in the modern era according to some practical guidelines that regard the characteristics of the active (sensing) materials on which the sensor devices are based. These characteristics include the cost-effectiveness of the materials’ manufacturing, the sensitivity to analytes, the material stability, and the possibility of exploiting them for low-cost and portable devices. Consequently, many gas sensors employ well-defined transduction methods, the most popular being the oxidation (or reduction) of the analyte in an electrochemical reactor, optical techniques, and chemiresistive responses to gas adsorption. In recent years, many of the efforts devoted to improving these methods have been directed towards the use of certain classes of specific materials. In particular, ionic liquids have been employed as electrolytes of exceptional properties for the preparation of amperometric gas sensors, while metal–organic frameworks (MOFs) are used as highly porous and reactive materials which can be employed, in pure form or as a component of MOF-based functional composites, as active materials of chemiresistive or optical sensors. Here, we report on the most recent developments relative to the use of these classes of materials in chemical sensing. We discuss the main features of these materials and the reasons why they are considered interesting in the field of chemical sensors. Subsequently, we review some of the technological and scientific results published in the span of the last six years that we consider among the most interesting and useful ones for expanding the awareness on future trends in chemical sensing. Finally, we discuss the prospects for the use of these materials and the factors involved in their possible use for new generations of sensor devices.

Metal-Organic Frameworks for Wastewater Decontamination: Discovering Intellectual Structure and Research Trends

Due to their simple synthesis method and excellent properties, such as superior adsorption and regeneration capabilities, with a large surface area and tunable pores, metal-organic frameworks (MOFs) have emerged as a suitable option for wastewater treatment. Although an exponential growth in MOF literature has been observed in recent years, conducting a quantitative literature analysis of MOF application in wastewater treatment is a novelty. To fill this gap, a total of 1187 relevant publications were extracted from the Web of Science, published during the last 50 years, and analyzed using bibliometric and content analysis techniques. A bibliometric analysis was conducted to reveal growing publication trends, leading journals, prolific countries, and organizations; whereas, a content analysis was used to highlight key research themes and hot topics in this field. The analyses revealed that there is a strong international collaboration among authors, countries, and organizations. Chemical Engineering Journal, Journal of Hazardous Materials, and Journal of Environmental Chemical Engineering are the most prolific journals in this field. Furthermore, the use of MOFs for removing antibiotics from wastewater was identified as a recent hot topic. In addition, performance enhancements of MOFs, in terms of a higher adsorption capacity and water stability, were identified as topics of great interest. To cater to these issues, the application of graphene, graphene oxides, nanoparticles, and quantum dots was also observed in the research fronts in this field.

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 Status Update on the Development of Polymer and Metal-Based Graphene Electrochemical Sensors for Detection and Quantitation of Bisphenol A

The detection and quantitation of bisphenol A (BPA) in the environment and food products has been a subject of considerable interest. BPA, a diphenylmethane derivative is a well-known industrial raw material with wide range of applications. It is a well-known endocrine disruptor and acts as an estrogen mimic. BPA is an environmental health concern and its accumulation in hydro-geological cycles is a matter of serious ecological peril. This review basically assesses various chemically modified electrodes composed of diverse components that have been employed to recognize BPA in different matrices. Electrochemical sensors prepared using graphene materials in combination with metals and polymers for selective detection of BPA have been discussed extensively. The emphasis is on detection of BPA in various samples encountered in routine use such as plastic bottles, receipts, baby feed bottles, milk samples, mineralized water, tissue paper, DVDs, and others. Although research in this field is in the exploratory stage, deeper insights into fundamental studies of sensing systems, fast analysis of real samples and validation of sensors are some of the factors that need major impetus. It is expected that chemically modified electrode-based sensing systems will soon take over as a viable option for monitoring diverse pollutants.

Gas sensing based on metal-organic frameworks: Concepts, functions, and developments

Effective detection of pollutant gases is vital for protection of natural environment and human health. There is an increasing demand for sensing devices that are equipped with high sensitivity, fast response/recovery speed, and remarkable selectivity. Particularly, attention is given to the designability of sensing materials with porous structures. Among diverse kinds of porous materials, metal-organic frameworks (MOFs) exhibit high porosity, high degree of crystallinity and exceptional chemical activity. Their strong host-guest interactions with guest molecules facilitate the application of MOFs in adsorption, catalysis and sensing systems. In particular, the tailorable framework/composition and potential for post-synthetic modification of MOFs endow them with widely promising application in gas sensing devices. In this review, we outlined the fundamental aspects and applications of MOFs for gas sensors, and discussed various techniques of monitoring gases based on MOFs as functional materials. Insights and perspectives for further challenges faced by MOFs are discussed in the end.

Understanding the endocrine disruptor and determination of bisphenol A by functional Cu-BTABB-MOF/rGO composite as facile rapid electrochemical sensor: an experimental and DFT investigation

A pioneering CuBTABB-MOF/rGO composite customized electrode is fabricated and utilized as a sensor towards identifying bisphenol A (BPA) in a phosphate buffer solution of pH 7.0. The composite is characterized by FTIR, Raman spectroscopy, XRD, SEM, EDX, HRTEM, and XPS to study its structural and morphological properties. Compared with Cu-BTABB-MOF and Cu-BTABB-MOF@GO, the Cu-BTABB-MOF@rGO modified electrode is more sensitive and selective to BPA due to a strong interaction between them. The developed Cu-BTABB-MOF@rGO modified electrode exhibits good sensitivity (6.95 × 10^-5 A mol^-1 L^-1) for BPA having a wide linear range of 0-100 μmol L^-1 with the LOD of 2.08 × 10^-5 mol L^-1, reproducibility of 4.35%, and relative standard deviation (RSD) and stability of 90% for thirty days. In addition, the developed electrocatalyst remained unoccupied from interfering substances and consequently provided an encouraging platform for swift detection of BPA in real samples such as pond water and packed water bottles. Additionally, we utilized DFT (density functional theory) to model GO and Cu-BTABB-MOF structures for detecting BPA molecules.

MOF-derived electrochemical catalyst Cu-N/C for the enhancement of amperometric oxygen detection

Electrochemical sensors using ionic liquids as electrolytes for oxygen detection are now getting more and more attention. Recently, an ionic liquid combined with an electrochemically active catalyst system has become popular for boosting the sensing performance of oxygen sensors. In this work, the imidazolyl-based ionic liquid 1-butyl-2,3-dimethylimidazole bis((trifluoromethyl)sulfonyl)imide [Bmmim][TFSI] is first prepared by a facile two-step method. Subsequently, a transition metal and N-codoped porous carbon oxygen reduction electrochemical catalyst Cu-N/C is synthesized by calcining the Cu-doped ZIF-8 precursor and then blending it in different ratios with the ionic liquid [Bmmim][TFSI] as composite electrolytes for oxygen detection. The composite electrolyte Cu-N/C/[Bmmim][TFSI] exhibits increased responses in cyclic voltammetry (CV) and chronoamperometry (CA) relative to that of the pure ionic liquid. Furthermore, the CV and CA data show that 6% Cu-N/C/[Bmmim][TFSI] has the optimum oxygen sensing response with an enhanced reduction peak current, a sensitivity of 0.1678 μA/[% O₂] and a good linear fitting coefficient of 0.9991. In conclusion, the results confirm the success of using Cu-N/C as an electrochemical catalyst composed of the Cu-N/C/[Bmmim][TFSI] electrolyte for improving the responsivity, stability and sensitivity towards a wide range of oxygen concentrations.

Hetero-metallic metal-organic frameworks for room-temperature NO2 sensing

Metal-organic frameworks (MOFs) with exceptional features such as high structural diversity and surface area as well as controlled pore size has been considered a promising candidate for developing room temperature highly-sensitive gas sensors. In comparison, the hetero-metallic MOFs with redox-active open-metal sites and mixed metal nodes may create peculiar surface properties and synergetic effects for enhanced gas sensing performances. In this work, the Fe atoms in the Fe₃ (Porous coordination network) PCN-250 MOFs are partially replaced by transition metal Co, Mn, and Zn through a facile hydrothermal approach, leading to the formation of hetero-metallic MOFs (Fe₂^IIIM^II, M = Co, Mn, and Zn). While the PCN-250 framework is maintained, the morphological and electronic band structural properties are manipulated upon the partial metal replacement of Fe. More importantly, the room temperature NO₂ sensing performances are significantly varied, in which Fe₂Mn PCN-250 demonstrates the largest response magnitude for ppb-level NO₂ gas compared to those of pure Fe₃ PCN-250 and other hetero-metallic MOF structures mainly attributed to the highest binding energy of NO₂ gas. This work demonstrates the strong potential of hetero-metallic MOFs with carefully engineered substituted metal clusters for power-saving and high-performance gas sensing applications.

Iridium Oxide Enabled Sensors Applications

There have been numerous studies applying iridium oxides in different applications to explore their proton-change-based reactions since the 1980s. Iridium oxide can be fabricated directly by applying electrodeposition, sputter-coating method, or oxidation of iridium wire. Generally, there have been currently two approaches in applying iridium oxide to enable its sensing applications. One was to improve or create different electrolytes with (non-)electrodeposition method for better performance of Nernst Constant with the temperature-related system. The mechanism behind the scenes were summarized herein. The other was to change the structure of iridium oxide through different kinds of templates such as photolithography patterns, or template-assisted direct growth methods, etc. to improve the sensing performance. The detection targets varied widely from intracellular cell pH, glucose in an artificial sample or actual urine sample, and the hydrogen peroxide, glutamate or organophosphate pesticides, metal-ions, etc. This review paper has focused on the mechanism of electrodeposition of iridium oxide in aqueous conditions and the sensing applications towards different biomolecules compounds. Finally, we summarize future trends on Iridium oxide based sensing and predict future work that could be further explored.

Metal-organic Framework Materials Coupled to Optical Fibers for Chemical Sensing: A Review

Metal-organic frameworks (MOFs), a newer class of crystalline nanoporous materials, have been in the limelight owing to their exceptional tunability for structures and physicochemical properties, and have found successful applications in gas storage, gas separation, and catalysis. The mesmerizing properties of MOFs, especially the extensive and tunable porosity and chemical selectivity, also make them an excellent candidate class as chemo-sensory materials. Moreover, MOF-based sensors have attracted considerable attention in the past decade. Recent literature reviews focused on the progress of MOF-based electronic sensors and luminescent MOF sensors, while sensors exploiting the dielectric properties (refractive index) of MOFs were also demonstrated and discussed very recently. The motivation of this report is to provide, for the first time, a general review on such MOF sensors with a particular focus on miniature optical fiber (OF) based MOF sensors and to demonstrate the promising potential of MOFs as dielectric coatings on OF for highly sensitive chemical sensing. The fundamental principle of OF-MOF sensors relies on the tunability of the refractive index of a MOF, which is dependent on the amount and type of adsorbed guest molecules in the MOF pores. MOF sensors based on different optical sensing principles are reviewed; challenges and perspectives on further research into the field of OF-MOF sensors are also discussed.

Metal-organic frameworks for chemical sensing devices

Metal-organic frameworks (MOFs) are exceptionally large surface area materials with organized porous cages that have been investigated for nearly three decades. Due to the flexibility in their design and predisposition toward functionalization, they have shown promise in many areas of application, including chemical sensing. Consequently, they are identified as advanced materials with potential for deployment in analytical devices for chemical and biochemical sensing applications, where high sensitivity is desirable, for example, in environmental monitoring and to advance personal diagnostics. To keep abreast of new research, which signposts the future directions in the development of MOF-based chemical sensors, this review examines studies since 2015 that focus on the applications of MOF films and devices in chemical sensing. Various examples that use MOF films in solid-state sensing applications were drawn from recent studies based on electronic, electrochemical, electromechanical and optical sensing methods. These examples underscore the readiness of MOFs to be integrated in optical and electronic analytical devices. Also, preliminary demonstrations of future sensors are indicated in the performances of MOF-based wearables and smartphone sensors. This review will inspire collaborative efforts between scientists and engineers working within the field of MOFs, leading to greater innovations and accelerating the development of MOF-based analytical devices for chemical and biochemical sensing applications.

Nanofibers of Polyaniline and Cu(II)-l-Aspartic Acid for a Room-Temperature Carbon Monoxide Gas Sensor

In the present study, the carbon monoxide (CO) sensing property of Cu(II)-l-aspartic acid nanofibers/polyaniline (PANI) nanofibers composite was investigated at room temperature. The nanofiber composite was formed through the ultrasound mixing of emeraldine salt PANI nanofibers and Cu(II)-l-aspartic acid nanofibers, which were synthesized by using a polymerization process and simple self-assembly method, respectively. The nanofibers composite demonstrated a branched structure in which the Cu(II)-l-aspartic acid nanofiber framework is similar to the trunk of a tree and the polyaniline nanofibers is like its branches. It seems that this special structure and one-dimension/one-dimension interface are suitable for gas adsorption and sensing. The performance of the prepared sensor toward CO gas was investigated at room temperature in a wide concentration range (200-8000 ppm). The experimental results indicate that the incorporation of amino acid-based copper metal-biomolecule framework nanofibers to PANI nanofibers enhances the response value (12.41% to 4000 ppm), yielding good selectivity and acceptable response and recovery characteristics (220 s/240 s) at room temperature. The detection limit of Cu(II)-l-aspartic acid nanofibers/PANI nanofibers sensor for carbon monoxide is obtained at 120 ppm.

Petkov P, Heine T, Mannsfeld SCB, Feng X, Dong R. Surface-Modified Phthalocyanine-Based Two-Dimensional Conjugated Metal-Organic Framework Films for Polarity-Selective Chemiresistive Sensing

2D conjugated metal-organic frameworks (2D c-MOFs) are emerging as electroactive materials for chemiresistive sensors, but selective sensing with fast response/recovery is a challenge. Phthalocyanine-based Ni2 [MPc(NH)8 ] 2D c-MOF films are presented as active layers for polarity-selective chemiresisitors toward water and volatile organic compounds (VOCs). Surface-hydrophobic modification by grafting aliphatic alkyl chains on 2D c-MOF films decreases diffused analytes into the MOF backbone, resulting in a considerably accelerated recovery progress (from ca. 50 to ca. 10 s) during humidity sensing. Toward VOCs, the sensors deliver a polarity-selective response among alcohols but no signal for low-polarity aprotic hydrocarbons. The octadecyltrimethoxysilane-modified Ni2 [MPc(NH)8 ] based sensor displays high-performance methanol sensing with fast response (36 s)/recovery (13 s) and a detection limit as low as 10 ppm, surpassing reported room-temperature chemiresistors.

Synthesis of High-Quality Mg-MOF-74 Thin Films via Vapor-Assisted Crystallization

The unique features of metal-organic frameworks (MOFs), such as their large surface areas and diversity of structures, make them suitable for a broad range of applications including storage, separation, and sensing of gases. Among all the MOFs, Mg-MOF-74 with the highest CO₂ uptake at 1 bar and 25 °C would be particularly beneficial for CO₂-related applications. One of the most critical enabling technologies for implementing Mg-MOF-74 is the preparation of dense and continuous films that would maximize the sorption behaviors. However, Mg-MOF-74 thin films present significant challenges in demonstrating large-scale coatings. Herein, we demonstrate for the first time high-quality Mg-MOF-74 films synthesized via a vapor-assisted crystallization (VAC) process. The VAC process described herein provides dense and highly crystalline layers of the Mg-MOF-74 thin film with a low coefficient of variation of film thickness below 7%. By minimizing the solvent use, the VAC process is also more environmentally friendly than conventional techniques. In this work, we first optimized a precursor solution for the VAC process and then investigated the effects of synthesis temperature, time, and droplet volume on the growth, crystallinity, and thickness of VAC Mg-MOF-74 films. The porosity of the MOF film was assessed by measuring the CO₂ uptake at room temperature and 1 bar. The obtained VAC Mg-MOF-74 films possess a well-defined microporosity, as deduced from CO₂ adsorption studies via quartz crystal microbalance (QCM) and comparison with bulk Mg-MOF-74 reference data. Furthermore, CO₂ cyclic adsorption-desorption experiments on the VAC Mg-MOF-74 films showed scaled uptakes to a wide range of CO₂ concentration without showing significant variations in the baseline. We specifically demonstrate how the film's quality of the MOF affects adsorption behavior of CO₂ on VAC Mg-MOF-74 and drop-cast Mg-MOF-74 films.

In-situ development of metal organic frameworks assisted ZnMgAl layered triple hydroxide 2D/2D hybrid as an efficient photocatalyst for organic dye degradation

Metal organic framework (MOF) supported layered triple hydroxide (LTH) 2D/2D hybrid material was prepared by a simple hydrothermal method. The photophysical properties of the prepared samples were investigated through a set of analytical methods such as X-ray diffraction, Fourier-transform infrared spectroscopy, field emission scanning electron microscope, high-resolution transmission electron microscopy, energy-dispersive X-ray spectroscopy and mapping. The photocatalytic degradation activity of as prepared 2D/2D MOF-5/LTH hybrid sample was investigated against methylene blue (MB) dye under the UV-visible light irradiation. The degradation efficiency of the MOF-5/LTH hybrid sample was twice a time greater than that of pristine MOF-5, particularly degradation efficiency of the MOF-5, LTH and MOF-5/LTH hybrid samples are 43.3, 57.7 and 98.1% respectively. The Pseudo first order rate and the reusing investigation was further used to study the catalytic activity and stability of the as-synthesized 2D/2D photocatalyst. The observed improvement in the photocatalytic activity of the hybrid samples were owed to enhance visible light absorption, efficient separation and transportation of photoinduced electrons and holes.

Centimeter-Scale Pillared-Layer Metal-Organic Framework Thin Films Mediated by Hydroxy Double Salt Intermediates for CO2 Sensor Applications

Fabrication of metal-organic framework (MOF) thin films over macroscopic surface areas is a subject of great interest for gas sensor application platforms such as optics and microelectronics. However, a direct synthesis of MOF films at ambient conditions, in particular pillared-layer MOF films due to their anisotropic structures, remains a significant challenge. Herein, we demonstrate for the first time a facile construction of dense and continuous pillared-layer MOF thin films on a centimeter scale via an aluminum-doped zinc oxide template and hydroxy double salt (HDS) intermediates at room temperature. A series of Cu(II)-based pillared MOFs with different 1,4-benzenedicarboxylic acid (bdc) ligands were explored for optimizing MOF film formation for CO₂ sensor applications. Nonpolar ligands with lower water solubility preferentially formed crystalline pillared MOF structures from HDS intermediates. A Cu₂(ndc)₂(dabco) (ndc = 1,4-naphthalene-bdc; dabco = 1,4-diazabicyclo[2.2.2]octane) MOF demonstrated the most dense and uniform film growth with micrometer thickness over one square centimeter area. This synthetic approach for growing Cu₂(ndc)₂(dabco) MOF thin films was successfully translated toward two sensing platforms: a quartz crystal microbalance and an optical fiber sensor. These Cu₂(ndc)₂(dabco) MOF-coated sensors displayed sensitivity toward CO₂ and response/recovery time on the scale of seconds, even at moderate humidity levels. This work provides a road map for producing continuous and anisotropic crystalline MOF thin films over a centimeter scale area on various substrates, which will greatly facilitate their utilization in MOF-based sensor devices, among other applications.

Recent Advances in the Design and Sensing Applications of Hemin/Coordination Polymer-Based Nanocomposites

The fabrication of biomimetic catalysts as substituents for enzymes is of critical interest in the field due to the problems associated with the extraction, purification, and storage of enzymes in sensing applications. Of these mimetics, hemin/coordination polymer-based nanocomposites, mainly hemin/metal-organic frameworks (MOF), have been developed for various biosensing applications because of the unique properties of each component, while trying to mimic the normal biological functions of heme within the protein milieu of enzymes. This critical review first discusses the different catalytic functions of heme in the body in the form of enzyme/protein structures. The properties of hemin dimerization are then elucidated with the supposed models of hemin oxidation. After that, the progress in the fabrication of hemin/MOF nanocomposites for the sensing of diverse biological molecules is discussed. Finally, the challenges in developing this type of composites are examined as well as possible proposals for future directions to enhance the sensing performance in this field further.

Petkov P, Evans JD, Krylova S, Krylov A, Kaskel S. Tailoring adsorption induced switchability of a pillared layer MOF by crystal size engineering

The main factors affecting switchability are identified for DUT-8(Zn): energetics of the host, particle size, and desolvation stress. They influence the flexible behaviour to the same order of magnitude and should be always considered collectively.

Evaluating the Fitness of Combinations of Adsorbents for Quantitative Gas Sensor Arrays

Robust, high-performance gas-sensing technology has applications in industrial process monitoring and control, air quality monitoring, food quality assessment, medical diagnosis, and security threat detection. Nanoporous materials (NPMs) could be utilized as recognition elements in a gas sensor because they selectively adsorb gas. Imitating mammalian olfaction, sensor arrays of NPMs use measurements of the adsorbed mass of gas in a set of distinct NPMs to infer the gas composition. Modular and adjustable NPMs, such as metal-organic frameworks (MOFs), offer a vast material space to sample for combinations to comprise a sensor array that produces a response pattern rich with information about the gas composition. Herein, we frame quantitative gas sensing, using arrays of NPMs, as an inverse problem, which equips us with a method to evaluate the fitness of a proposed combination of NPMs in a sensor array. While the (routine) forward problem is to use an adsorption model to predict the mass of gas adsorbed in each NPM when immersed in a gas mixture of a given composition, the inverse problem is to predict the gas composition from the observed masses of adsorbed gas in the NPMs of the array. The fitness of a given combination of NPMs for gas sensing is then determined by the conditioning of its inverse problem: the prediction of the gas composition provided by a fit (unfit) combination of NPMs is insensitive (sensitive) to inevitable errors in the measurements of the mass of gas adsorbed in the NPMs. For illustration, we use experimentally measured adsorption data to analyze the conditioning of the inverse problem associated with a (IRMOF-1 and HKUST-1) CH₄/CO₂ sensor array.

Analytical validation using a gas mixing system for the determination of gaseous formaldehyde

Formaldehyde levels in the atmosphere are a concern in the indoor and outdoor air and many methods for determining this compound have been developed. The use of 2,4-dinitrophenylhydrazine (DNPH) for reaction with formaldehyde, catalyzed by acid, forming a hydrazone derivative in cartridges is considered the standard method for analyzing formaldehyde compounds in the air. However, formaldehyde is quantified using an analytical curve, created by diluting liquid standards of the formaldehyde-DNPH product. The analysis aims to quantify the gas phase formaldehyde, and it may be subject to experimental biases from the differences in the matrix of the sample (gas) and calibration standard (liquid). The objective of this work was to build an analytical curve in the gaseous phase using a synthetic air/formaldehyde mixing system (SFMS) and sampling with SPE-DNPH-tubes, comparing with the analytical curve in the liquid phase adopted by the Environmental Protection Agency (EPA). Parameters of linearity, sensitivity, limit of detection (LOD), limit of quantification (LOQ), precision and accuracy (recovery) were determined from the analytical curve in the gaseous phase. The best recovery in DNPH-tubes was obtained using the range of 400-1600 mL min^-1 of flow rates in the gaseous phase. The sampling and reaction/elution of formaldehyde using DNPH-tubes presented adequate linearity and a similar sensitivity in the liquid analytical curve. Considering the LOD and LOQ in the gaseous phase, the values in nanograms are higher than those in the liquid phase. This study suggests that the quantification of formaldehyde in ambient air may be subject to bias due to differences in derivatization reaction efficiency. However, the results prove the efficiency of formaldehyde recovery from the atmosphere and the validity of the use of this DNPH-tube method.

Conversion from Heterometallic to Homometallic Metal-Organic Frameworks

Two new heterometallic metal-organic frameworks (MOFs), LnZnTPO 1 and 2, and two homometallic MOFs, LnTPO 3 and 4 (Ln=Eu for 1 and 3, and Tb for 2 and 4; H₃ TPO=tris(4-carboxyphenyl)phosphine oxide) were synthesized, and their structures and properties were analyzed. They were prepared by solvothermal reaction of the C₃ -symmetric ligand H₃ TPO with the corresponding metal ion(s) (a mixture of Ln³⁺ and Zn²⁺ for 1 and 2, and Ln³⁺ alone for 3 and 4). Single-crystal XRD (SXRD) analysis revealed that 1 and 3 are isostructural to 2 and 4, respectively. TGA showed that the framework is thermally stable up to about 400 °C for 1 and 2, and about 450 °C for 3 and 4. PXRD analysis showed their pore-structure distortions without noticeable framework-structure changes during drying processes. The shapes of gas sorption isotherms for 1 and 3 are almost identical to those for 2 and 4, respectively. Solvothermal immersion of 1 and 2 in Tb³⁺ and Eu³⁺ solutions resulted in the framework metal-ion exchange affording 4 and 3, respectively, as confirmed by photoluminescence (PL), PXRD, IR, inductively coupled plasma atomic emission spectroscopy (ICP-AES), and energy-dispersive X-ray (EDX) analyses.

Co-Fe-layered double hydroxide decorated amino-functionalized zirconium terephthalate metal-organic framework for removal of organic dyes from water samples

In this study, a new efficient adsorbent of Co-Fe-layered double hydroxides@metal-organic framework (Co-Fe-LDH@UiO-66-NH₂) was synthesized and used for extraction of methylene blue (MB) and methylene red (MR) from water samples prior to their determination by UV-Vis spectrophotometer. The adsorbent was characterized by Fourier Transform Infrared Spectroscopy (FT-IR), Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray (EDX), X-ray Diffraction (XRD), and Brunauer-Emmett-Teller (BET) analyses. The impact of various parameters such as pH of the aqueous phase, extraction time, amount of adsorbent, type and volume of eluent solvent, desorption time, and sample volume were studied. The maximum extraction recovery was obtained at an optimized pH 8.0 and extraction time 10.0 min. The adsorption process was fitted by the Langmuir model with a maximum adsorption capacity of 555.62 mg/g and 588.2 mg/g, respectively, for MB and MR. Under optimum conditions, the limit of detection (LOD) for MB was 0.7 μgL^-1 and 0.9 μgL^-1 for MR. Furthermore, the Co-Fe-LDH@UiO-66-NH₂ composite showed high efficiency for the removal of the analytes from environmental water samples.

Metal-Organic Framework Nanoparticles Induce Pyroptosis in Cells Controlled by the Extracellular pH

Ion homeostasis is essential for cellular survival, and elevated concentrations of specific ions are used to start distinct forms of programmed cell death. However, investigating the influence of certain ions on cells in a controlled way has been hampered due to the tight regulation of ion import by cells. Here, it is shown that lipid-coated iron-based metal-organic framework nanoparticles are able to deliver and release high amounts of iron ions into cells. While high concentrations of iron often trigger ferroptosis, here, the released iron induces pyroptosis, a form of cell death involving the immune system. The iron release occurs only in slightly acidic extracellular environments restricting cell death to cells in acidic microenvironments and allowing for external control. The release mechanism is based on endocytosis facilitated by the lipid-coating followed by degradation of the nanoparticle in the lysosome via cysteine-mediated reduction, which is enhanced in slightly acidic extracellular environment. Thus, a new functionality of hybrid nanoparticles is demonstrated, which uses their nanoarchitecture to facilitate controlled ion delivery into cells. Based on the selectivity for acidic microenvironments, the described nanoparticles may also be used for immunotherapy: the nanoparticles may directly affect the primary tumor and the induced pyroptosis activates the immune system.

A Reusable Polymeric Film for the Alternating Colorimetric Detection of a Nerve Agent Mimic and Ammonia Vapor with Sub-Parts-per-Million Sensitivity

Thin polymeric films were developed for the vapor-phase sequential colorimetric detection of a nerve agent mimic and ammonia with high sensitivity. N-(4-Benzoylphenyl)acrylamide (BPAm), N,N-dimethylacrylamide (DMA), and (E)-2-(methyl(4-(pyridine-4yldiazenyl)phenyl)amino)ethyl acrylate (MPDEA, M1) were copolymerized via free radical polymerization (FRP) to yield p(BPAm-co-DMA-co-MPDEA), hereafter referred to as P1. P1 exhibits selective sensing properties toward diethyl chlorophosphate (DCP), a nerve agent mimic, in pure aqueous media. Upon the addition of DCP, the pyridine groups of P1 were quaternized with DCP, accompanied by a color change from yellow to pink due to the enhancement of the intramolecular charge transfer (ICT) effect. In situ generated quaternized P1, hereafter referred to as P2, after DCP sensing was used to selectively detect ammonia via dequaternization in an aqueous medium. Ammonia detection was indicated by a color change in the solution from pink back to yellow. A surface-immobilized P1 film was prepared and employed for the vapor-phase detection of DCP, demonstrating that an amount of as low as 2 ppm was detectable. Ammonia vapor was also successfully detected by the P2 film via the ammonia-triggered removal of the quaternized phosphates. Alternating exposure of the film to DCP and ammonia resulted in the corresponding color changes, thereby demonstrating the reversibility of the system. The reusability of the polymeric film for detecting DCP and ammonia in the vapor phase was confirmed by performing four sequential colorimetric detection cycles.

Fabrication of Biobased Hydrophobic Hybrid Cotton Fabrics Using Molecular Self-Assembly: Applications in the Development of Gas Sensor Fabrics

Inadvertent inhalation of various volatile organic compounds during industrial processes, such as coal and metal mining, metal manufacturing, paper and pulp industry, food processing, petroleum refining, and concrete and chemical industries, has caused an adverse effect on human health. In particular, exposure to trimethylamine (TMA), a fishy odor poisonous gas, resulted in numerous health hazards such as neurotoxicity, irritation in eyes, nose, skin, and throat, blurred vision, and many more. According to the environmental protection agency, TMA in the level of 0.10 ppm is generally considered as safe, and excess dose results in "trimethylaminuria" or "fish odor syndrome." In order to avoid the health hazards associated with the inhalation of TMA, there is an urge to design a sensor for TMA detection even at low levels for use in food-processing industries, medical diagnosis, and environment. In this report, for the first time, we have developed a TMA sensor fabric using a sequential self-assembly process from silver-incorporated glycolipids. Formation of self-assembled supramolecular architecture, interaction of the assembled structure with the cotton fabric, and sensing mechanism were completely investigated with the help of various instrumental methods. To our surprise, the developed fabric displayed a transient response for 1-500 ppm of TMA and a stable response toward 100 ppm of TMA for 15 days. We believe that the reported flexible TMA sensor fabrics developed via the sequential self-assembly process hold great promise for various innovative applications in environment, healthcare, medicine, and biology.

Exchange coupled Co(ii) based layered and porous metal-organic frameworks: structural diversity, gas adsorption, and magnetic properties

Four new Co(ii) based metal-organic frameworks (MOFs) ({[Co3(L)(TDCA)3(DMF)2]n·2nCH3CN}) (1), ({[Co3(L)2(BDCA)3]n·2nCH3CN}) (2), {[Co2(L)2(CA)2]n·4nCH3CN} (3) and {[Co2(L)(OBBA)2]n·3nCH3CN} (4) are synthesized, where L is [4'-(4-methoxyphenyl)-4,2':6',4''-terpyridine], a V-shaped flexible neutral spacer, and the four dicarboxylates are TDCA = thiophene 2,5-dicarboxylic acid, BDCA = benzene 1,4-dicarboxylic acid, CA = (1R,3S)-(+)-camphoric acid and OBBA = 4,4'-oxybisbenzoic acid. Structural analysis reveals that 1 and 2 are two dimensional (2D) layered structures having interesting sql and hxl topologies respectively with trinuclear SBUs (secondary building units). Compound 3 has a 3D structure, whereas 4 has a 2-fold interpenetrated 3D packing structure with a paddlewheel dinuclear SBU and both have pcu topology. Magnetic investigation revealed that 1, 3 and 4 show dominant antiferromagnetic behavior, while 2 shows ferromagnetic interaction at very low temperature. Interestingly 4 shows a sharp decrease in the χMT value from room temperature and this may be because of the direct Co(ii)Co(ii) interaction. Gas sorption studies reveal that 1, 2 and 3 show surface areas of 11.8 m2 g-1, 8.3 m2 g-1 and 28.5 m2 g-1 respectively and better adsorption behavior for CO2 over CH4, whereas 4 is nonporous in nature due to its 2-fold interpenetrated structure.

Emergence of electrical conductivity in a flexible coordination polymer by using chemical reduction

Postsynthetic chemical reduction enhanced the electrical conductivity of a new flexible 1D coordination network with a naphthalenediimide (NDI)-based ligand.

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

This review summarizes the recent advances of metal organic framework (MOF) based sensing of gases and volatile compounds.

Controlling the morphology of metal–organic frameworks and porous carbon materials: metal oxides as primary architecture-directing agents

We give a comprehensive overview of how the morphology control is an effective and versatile way to control the physicochemical properties of metal oxides that can be transferred to metal–organic frameworks and porous carbon materials.