Ultrafast In Situ Synthesis of Large-Area Conductive Metal-Organic Frameworks on Substrates for Flexible Chemiresistive Sensing

Bioinspired 2D nanofluidic membranes for energy applications

Bioinspired two-dimensional (2D) nanofluidic membranes have been explored for the creation of high-performance ion transport systems that can mimic the delicate transport functions of living organisms. Advanced energy devices made from these membranes show excellent energy storage and conversion capabilities. Further research and development in this area are essential to unlock the full potential of energy devices and facilitate the development of high-performance equipment toward real-world applications and a sustainable future. However, there has been minimal review and summarization of 2D nanofluidic membranes in recent years. Thus, it is necessary to carry out an extensive review to provide a survey library for researchers in related fields. In this review, the classification and the raw materials that are used to construct 2D nanofluidic membranes are first presented. Second, the top-down and bottom-up methods for constructing 2D membranes are introduced. Next, the applications of bioinspired 2D membranes in osmotic energy, hydraulic energy, mechanical energy, photoelectric conversion, lithium batteries, and flow batteries are discussed in detail. Finally, the opportunities and challenges that 2D nanofluidic membranes are likely to face in the future are envisioned. This review aims to provide a broad knowledge base for constructing high-performance bioinspired 2D nanofluidic membranes for advanced energy applications.

Porous materials as effective chemiresistive gas sensors

Chemiresistive gas sensors (CGSs) have revolutionized the field of gas sensing by providing a low-power, low-cost, and highly sensitive means of detecting harmful gases. This technology works by measuring changes in the conductivity of materials when they interact with a testing gas. While semiconducting metal oxides and two-dimensional (2D) materials have been used for CGSs, they suffer from poor selectivity to specific analytes in the presence of interfering gases and require high operating temperatures, resulting in high signal-to-noise ratios. However, nanoporous materials have emerged as a promising alternative for CGSs due to their high specific surface area, unsaturated metal actives, and density of three-dimensional inter-connected conductive and pendant functional groups. Porous materials have demonstrated excellent response and recovery times, remarkable selectivity, and the ability to detect gases at extremely low concentrations. Herein, our central emphasis is on all aspects of CGSs, with a primary focus on the use of porous materials. Further, we discuss the basic sensing mechanisms and parameters, different types of popular sensing materials, and the critical explanations of various mechanisms involved throughout the sensing process. We have provided examples of remarkable performance demonstrated by sensors using these materials. In addition to this, we compare the performance of porous materials with traditional metal-oxide semiconductors (MOSs) and 2D materials. Finally, we discussed future aspects, shortcomings, and scope for improvement in sensing performance, including the use of metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), and porous organic polymers (POPs), as well as their hybrid counterparts. Overall, CGSs using porous materials have the potential to address a wide range of applications, including monitoring water quality, detecting harmful chemicals, improving surveillance, preventing natural disasters, and improving healthcare.

An ultra-sensitive electrochemical biosensor for the detection of procalcitonin in sepsis patients' serum, using a Cu-BHT-based thin film

Procalcitonin (PCT) is a polypeptide produced by the parafollicular cells of the thyroid gland and serves as a vital marker for the diagnosis and treatment of sepsis and other infectious diseases, as well as multiple organ failure, due to its high expression levels in affected patients. This article reports on a highly sensitive electrochemical biosensor based on MOF composite materials, based on Cu-BHT, for detecting PCT levels. The surface of the glassy carbon electrode may have better charge transfer resistance owing to the nano-composite material made of Cu-BHT, chitosan, and AuNPs. At the same time, the anti-PCT antibody may also be covalently bonded to the composite material and measure PCT concentration using electrochemical impedance spectroscopy (EIS). The results of the investigation demonstrate that the sensor's response has excellent linear conjunction with the logarithm of PCT concentration under optimum circumstances. The detection limit (LOD) is 14.579 × 10-9 μg/mL, and the linear range of detection is 10-7 μg/mL to 10-1 μg/mL. Simultaneously, we successfully applied this method to detect serum PCT before and after treatment in different sepsis patients and compared it with chemiluminescence immunoassay. The findings indicate that the proposed method holds promising potential for timely diagnosis and treatment of sepsis patients.

Fabrication of robust and cost-efficient Hoffmann-type MOF sensors for room temperature ammonia detection

The development of fast-response sensors for detecting NH3 at room temperature remains a formidable challenge. Here, to address this challenge, two highly robust Hoffmann-type metal-organic frameworks are rationally applied as the NH3 sensing materials which possess ultra-high static adsorption capacity for NH3, only lower than the current benchmark material. The adsorption mechanism is in-depth unveiled by dynamic adsorption and simulation studies. The assembled interdigital electrode device exhibits low detection limit (25 ppb) and short response time (5 s) at room temperature, which set a record among all electrical signal sensors. Moreover, the sensor exhibits excellent selectivity towards NH3 in the presence of 13 other potential interfering gases. Prominently, the sensor can stably output signals for more than two months at room temperature and can be recovered by simply purging nitrogen at room temperature without heating. This study opens up a way for reasonably designing gas sensing materials for toxic gases.

Electrocatalytic Microdevice Array Based on Wafer-Scale Conductive Metal-Organic Framework Thin Film for Massive Hydrogen Production

The synthesis of large-scale 2D conductive metal-organic framework films with tunable thickness is highly desirable but challenging. In this study, an Interface Confinement Self-Assembly Pulling (ICSP) method for in situ synthesis of 4-in. Ni-BHT film on the substrate surface is developed. By modulating the thickness of the confined space, the thickness of Ni-BHT films could be easily varied from 4 to 42 nm. To eliminate interference factors and evaluate the effect of film thickness on the catalytic performance of HER, an electrocatalytic microdevice based on the Ni-BHT film is designed. The effective catalytic thickness of the Ni-BHT film is found to be around 32 nm. Finally, to prepare the electrocatalytic microdevice array, over 100 000 microdevices on a 4-in. Ni-BHT film are integrated. The results show that the microdevice array has good stability and a high hydrogen production rate and could be used to produce large amounts of hydrogen. The wafer-scale 2D conductive metal-organic framework's fabrication greatly advances the practical application of microdevices for massive hydrogen production.

Two-Dimensional Electrically Conductive Metal-Organic Frameworks as Chemiresistive Sensors

Metal-organic frameworks (MOFs) have emerged as attractive chemical sensing materials due to their exceptionally high porosity and chemical diversity. Nevertheless, the utilization of MOFs in chemiresistive type sensors has been hindered by their inherent limitation in electrical conductivity. The recent emergence of two-dimensional conductive MOFs (2D c-MOFs) has addressed this limitation by offering enhanced electrical conductivity, while still retaining the advantageous properties of MOFs. In particular, c-MOFs have shown promising advantages for the fabrication of sensors capable of operating at room temperature. Thus, active research on gas sensors utilizing c-MOFs is currently underway, focusing on enhancing sensitivity and selectivity. To comprehend the potential of MOFs as chemiresistive sensors for future applications, it is crucial to understand not only the fundamental properties of conductive MOFs but also the state-of-the-art works that contribute to improving their performance. This comprehensive review delves into the distinctive characteristics of 2D c-MOFs as a new class of chemiresistors, providing in-depth insights into their unique sensing properties. Furthermore, we discuss the proposed sensing mechanisms associated with 2D c-MOFs and provide a concise summary of the strategies employed to enhance the sensing performance of 2D c-MOFs. These strategies encompass a range of approaches, including the design of metal nodes and linkers, morphology control, and the synergistic use of composite materials. In addition, the review thoroughly explores the prospects of 2D c-MOFs as chemiresistors and elucidates their remarkable potential for further advancements. The insights presented in this review shed light on future directions and offer valuable opportunities in the chemical sensing research field.

MOF-Based Chemiresistive Gas Sensors: Toward New Functionalities

The sensing performances of gas sensors must be improved and diversified to enhance quality of life by ensuring health, safety, and convenience. Metal-organic frameworks (MOFs), which exhibit an extremely high surface area, abundant porosity, and unique surface chemistry, provide a promising framework for facilitating gas-sensor innovations. Enhanced understanding of conduction mechanisms of MOFs has facilitated their use as gas-sensing materials, and various types of MOFs have been developed by examining the compositional and morphological dependences and implementing catalyst incorporation and light activation. Owing to their inherent separation and absorption properties and catalytic activity, MOFs are applied as molecular sieves, absorptive filtering layers, and heterogeneous catalysts. In addition, oxide- or carbon-based sensing materials with complex structures or catalytic composites can be derived by the appropriate post-treatment of MOFs. This review discusses the effective techniques to design optimal MOFs, in terms of computational screening and synthesis methods. Moreover, the mechanisms through which the distinctive functionalities of MOFs as sensing materials, heterostructures, and derivatives can be incorporated in gas-sensor applications are presented.

Recent advances on surface mounted metal-organic frameworks for energy storage and conversion applications: Trends, challenges, and opportunities

Establishing green and reliable energy resources is very important to counteract the carbon footprints and negative impact of non-renewable energy resources. Metal-organic frameworks (MOFs) are a class of porous material finding numerous applications due to their exceptional qualities, such as high surface area, low density, superior structural flexibility, and stability. Recently, increased attention has been paid to surface mounted MOFs (SURMOFs), which is nothing but thin film of MOF, as a new category in nanotechnology having unique properties compared to bulk MOFs. With the advancement of material growth and synthesis technologies, the fine tunability of film thickness, consistency, size, and geometry with a wide range of MOF complexes is possible. In this review, we recapitulate various synthesis approaches of SURMOFs including epitaxial synthesis approach, direct solvothermal method, Langmuir-Blodgett LBL deposition, Inkjet printing technique and others and then correlated the synthesis-structure-property relationship in terms of energy storage and conversion applications. Further the critical assessment and current problems of SURMOFs have been briefly discussed to explore the future opportunities in SURMOFs for energy storage and conversion applications.

Chemical Vapor Deposition and High-Resolution Patterning of a Highly Conductive Two-Dimensional Coordination Polymer Film

Crystalline coordination polymers with high electrical conductivities and charge carrier mobilities might open new opportunities for electronic devices. However, current solvent-based synthesis methods hinder compatibility with microfabrication standards. Here, we describe a solvent-free chemical vapor deposition method to prepare high-quality films of the two-dimensional conjugated coordination polymer Cu-BHT (BHT = benzenehexanothiolate). This approach involves the conversion of a metal oxide precursor into Cu-BHT nanofilms with a controllable thickness (20-85 nm) and low roughness (<10 nm) through exposure to the vaporized organic linker. Moreover, the restricted metal ion mobility during the vapor-solid reaction enables high-resolution patterning via both bottom-up lithography, including the fabrication of micron-sized Hall bar and electrode patterns to accurately evaluate the conductivity and mobility values of the Cu-BHT films.

Defect Engineering To Tailor Metal Vacancies in 2D Conductive Metal-Organic Frameworks: An Example in Electrochemical Sensing

Two-dimensional conductive metal-organic frameworks (2D conductive MOFs) with π-d conjugations exhibit high electrical conductivity and diverse coordination structures, making them constitute a desirable platform for new electronic devices. Defects are inevitable in the self-assembly process of 2D conductive MOFs. Arguably, defect engineering that deliberately manipulates defects demonstrates great potential to enhance the electrocatalytic activity of this family of novel materials. Herein, a facile and universal defect engineering strategy is proposed and demonstrated for metal vacancy regulation of metal benzenehexathiolato (BHT) coordination polymer films. Controllable metal vacancies can be produced by simply tuning the proton concentration during the confined self-assembly process at the liquid-liquid interface. This facile but universal defect design strategy has been proven to be effective in a class of materials including Cu-BHT, Ni-BHT, and Ag-BHT for physicochemical regulation. To further demonstrate the feasibility and practicality in electrochemical applications, the elaborately fabricated Cu-BHT films with abundant Cu vacancies deliver competitive performance in electrocatalytic sensing of H₂O₂. Mechanistic analysis revealed that the Cu vacancies act as effective active sites for adsorption and reduction of H₂O₂, and the tuned electronic structure boosts the electrocatalytic reaction. The developed advanced sensing platform confirms the excellent commercial potential of Cu-BHT sensors for H₂O₂. The findings provide insights into the molecular structure design of 2D conducting MOFs by defect engineering and demonstrate the commercial potential of Cu-BHT electrochemical sensors.

Conductive Metal-Organic Frameworks for Supercapacitors

As a class of porous materials with crystal lattices, metal-organic frameworks (MOFs), featuring outstanding specific surface area, tunable functionality, and versatile structures, have attracted huge attention in the past two decades. Since the first conductive MOF is successfully synthesized in 2009, considerable progress has been achieved for the development of conductive MOFs, allowing their use in diverse applications for electrochemical energy storage. Among those applications, supercapacitors have received great interest because of their high power density, fast charging ability, and excellent cycling stability. Here, the efforts hitherto devoted to the synthesis and design of conductive MOFs and their auspicious capacitive performance are summarized. Using conductive MOFs as a unique platform medium, the electronic and molecular aspects of the energy storage mechanism in supercapacitors with MOF electrodes are discussed, highlighting the advantages and limitations to inspire new ideas for the development of conductive MOFs for supercapacitors.

Facile Synthesis of Conductive Metal-Organic Frameworks Nanotubes for Ultrahigh-Performance Flexible NO Sensors

Cu-benzenehexathiol (Cu-BHT) has attracted significant attention due to its record high electrical conductivity and crystal defects Cu_2c . However, the nonporous structure and small specific surface area of Cu-BHT with two-dimensional kagome lattice invariably limit its practical application in sensing and catalysis. In this work, Cu-BHT nanotubes (Cu-BHT-NTs) are designed and prepared via a facile homogeneous reaction to solve these problems. Compared with the traditional nanorod-like structure, the Cu-BHT-NTs not only have a higher specific surface area but also possess a higher proportion of crystal defects (66.6%). The successfully configured DPPTT/Cu-BHT-NTs heterostructure organic field-effect transistor (OFET)-based sensor exhibits excellent sensitivity as high as 13 610%, a minimum detection limits down to 5 ppb, and exceptional selectivity to nitric oxide (NO) toxic gases. Theoretical analysis systematically shows that Cu_2c sites in the Cu-BHT-NTs increase the number of electrons transferred from the heterostructure to NO molecules, confirming that the high sensitivity and selectivity result from the high binding between Cu-BHT-NTs and NO molecules. Furthermore, a fully flexible device based on the heterojunction OFET sensor is prepared to ensure the convenience of wearing and carrying gas sensors, opening up a new avenue for the next generation of wearable intelligent electronics.

Integration of a Metal-Organic Framework Film with a Tubular Whispering-Gallery-Mode Microcavity for Effective CO2 Sensing

Carbon dioxide (CO₂) sensing using an optical technique is of great importance in the environment and industrial emission monitoring. However, limited by the poor specific adsorption of gas molecules as well as insufficient coupling efficiency, there is still a long way to go toward realizing a highly sensitive optical CO₂ gas sensor. Herein, by combining the advantages of a whispering-gallery-mode microcavity and a metal-organic framework (MOF) film, a porous functional microcavity (PF-MC) was fabricated with the assistance of the atomic layer deposition technique and was applied to CO₂ sensing. In this functional composite, the rolled-up microcavity provides the ability to tune the propagation of light waves and the electromagnetic coupling with the surroundings via an evanescent field, while the nanoporous MOF film contributes to the specific adsorption of CO₂. The composite demonstrates a high sensitivity of 188 nm RIU^-1 (7.4 pm/% with respect to the CO₂ concentration) and a low detection limit of ∼5.85 × 10^-5 RIU. Furthermore, the PF-MC exhibits great selectivity to CO₂ and outstanding reproducibility, which is promising for the next-generation optical gas sensing devices.

Metal-Organic-Frameworks: Low Temperature Gas Sensing and Air Quality Monitoring

As an emerging class of hybrid nanoporous materials, metal-organic frameworks (MOFs) have attracted significant attention as promising multifunctional building blocks for the development of highly sensitive and selective gas sensors due to their unique properties, such as large surface area, highly diversified structures, functionalizable sites and specific adsorption affinities. Here, we provide a review of recent advances in the design and fabrication of MOF nanomaterials for the low-temperature detection of different gases for air quality and environmental monitoring applications. The impact of key structural parameters including surface morphologies, metal nodes, organic linkers and functional groups on the sensing performance of state-of-the-art sensing technologies are discussed. This review is concluded by summarising achievements and current challenges, providing a future perspective for the development of the next generation of MOF-based nanostructured materials for low-temperature detection of gas molecules in real-world environments.