Porous materials as effective chemiresistive gas sensors

2530 | Chem. Soc. Rev., 2024, 53, 2530–2577 This journal is © The Royal Society of Chemistry 2024 Cite this: Chem. Soc. Rev., 2024, 53, 2530 Porous materials as effective chemiresistive gas sensors Akashdeep Sharma, a Sunil Babu Eadi, b Hemanth Noothalapati, c Michal Otyepka, de Hi-Deok Lee * bf and Kolleboyina Jayaramulu * a Chemiresistive gas sensors (CGSs) have revolutionized the field of gas sensing by providing a lowpower, 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. 1. Introduction The detrimental impact of growing air pollution on our planet and human well-being is undeniable. The rapid surge in industrialization and globalization has led to the widespread release of harmful gases and volatile compounds into our environment and homes. These emissions have far-reaching consequences, profoundly affecting both human health and the delicate ecological balance of Earth. 1 According to the United Nations Environmental Agency, approximately 7 million premature deaths every year are due to air pollution. The deadliest illnesses linked to PM 2.5 air pollution are stroke, heart disease, lung disease, lower respiratory diseases (such as pneumonia), and cancer. High levels of fine particles also contribute to other illnesses, like diabetes, can hinder cognitive development in children and also cause mental health problems. 2 Furthermore, as emphasized by the United Nations, air pollution has exacted a devastating toll on the biodiversity of our planet’s plant and animal ispecies. The presence of sulfur and nitrogen oxides in our atmosphere has given rise to acid rain and smog, causing extensive harm to plant life and marine ecosystems. To combat these pressing problems of air pollution, innovative solutions are urgently needed. One promising approach involves the use of chemical sensors capable of detecting and alarming us to the presence of harmful pollutants. These sensors can serve as a Hybrid Porous Materials Laboratory, Department of Chemistry, Indian Institute of Technology Jammu, Jammu & Kashmir, 181221, India. E-mail: [email protected], jayaramulu.kolleboyin[email protected] b Department of Electronics Engineering, Chungnam National University, Daejeon, South Korea. E-mail: [email protected] c Faculty of Life and Environmental Sciences, Shimane University, Matsue, 6908504, Japan d Regional Centre of Advanced Technologies and Materials, Czech Advanced Technology and Research Institute (CATRIN), Palacky ´University Olomouc, S ˇlechtitelu ˚27, 783 71 Olomouc, Czech Republic e IT4Innovations, VSB–Technical University of Ostrava, 17. listopadu 2172/15, 708 00 Ostrava-Poruba, Czech Republic f Korea Sensor Lab, Department of Electronics Engineering, Chungnam National University, Daejeon, South Korea Received 19th September 2023 DOI: 10.1039/d2cs00761d rsc.li/chem-soc-rev Chem Soc Rev REVIEW ARTICLE Open Access Article. Published on 01 February 2024. Downloaded on 10/15/2024 9:59:47 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online View Journal | View Issue This journal is © The Royal Society of Chemistry 2024 Chem. Soc. Rev., 2024, 53, 2530–2577 | 2531 early warning systems, allowing us to take proactive measures to reduce pollution and protect our health and the environment. Chemiresistive gas sensors (CGSs) have emerged as a promising technology for the accurate and simple detection of harmful gases and volatile organic compounds (VOCs) in various applications. These sensors are based on the principle that certain gases and VOCs can induce changes in the electrical resistance of a sensing material. 3–5 This property allows CGSs to detect and quantify the presence of specific substances, providing valuable information for assessing air quality, ensuring safety, and monitoring environmental conditions. CGSs have found applications in diverse fields. Advances in nanotechnology, materials science, and fabrication techniques continue to expand the range of sensing materials and improve the overall functionality of these sensors. CGSs play a vital role in environmental monitoring, particularly in detecting and quantifying pollutants. For example, in urban areas with heavy traffic, these sensors are deployed in air quality monitoring stations to measure concentrations of gases like carbon monoxide (CO), nitrogen dioxide (NO 2 ), and volatile organic compounds (VOCs). Such sensors provide real-time data, allowing authorities to assess air quality and implement measures to mitigate the adverse effects of pollution, such as the reduction Akashdeep Sharma Akashdeep Sharma, a doctoral candidate in the Hybrid Porous Materials Laboratory at the Department of Chemistry of the Indian Institute of Technology, Jammu (India) under Prof. Kolleboyina Jayaramulu. He obtained his master’s degree from the University of Jammu specialized in organic chemistry. His current research focuses on developing organic porous materials for sensor based applications. Sunil Babu Eadi Dr Sunil Babu Eadi holds a BS degree in Chemistry from Andhra University, India, and MS degree in Chemistry from the University of Hyderabad, India. He earned his PhD degree in Advanced Materials Engineering from Chungnam National University in 2015. Following his doctoral studies, Dr Eadi served as a postdoctoral researcher at the Kumoh National Institute of Technology in Gumi, Korea, from 2015 to 2018. Subsequently, from 2018 to 2023, he held the position of Research Professor at Chungnam National University, Republic of Korea. Currently, a Scientific Officer at the Indian Institute of Technology Jodhpur, his research focuses on chemical sensor development, Contact Resistance Reduction Technology and Silicide Technology. Hemanth Noothalapati Dr Hemanth Noothalapati, an Assistant Professor at Faculty of Life and Environmental Sciences, Shimane University, Japan, earned his PhD in Applied Chemistry from National Chiao Tung University (NCTU), Taiwan, through a prestigious Taiwan Scholarship in 2013. Following a postdoctoral fellowship at NCTU’s Ultimate Spectroscopy and Imaging laboratory, he started his independent research at Shimane University. Specializing in labelfree molecular spectroscopy and imaging, Dr Noothalapati employs chemometrics, machine learning and artificial intelligence to explore diverse applications in biology, medicine, materials, and the environment. His innovative approach contributes to advancing scientific understanding and technological applications in these fields. Michal Otyepka Michal Otyepka is Head of CATRINRCPTM, a research division at Palacky ´University in Olomouc. He is a member of the Scientific Board of the Czech Grant Agency and the LUMI Supercomputer (Finland). His research interests cover physical– chemical properties and reactivity of graphene derivatives and 2D materials, noncovalent interactions to 2D materials, and photoluminescence properties of carbon dots (CDs). He has been developing the chemistry of fluorographene (2D chemistry) toward graphene derivatives, which can be applied in (bio)sensing, catalysis, and energy storage. He specializes also in modeling of biomolecules, nanomaterials, and complex molecular systems and the development of force fields, multiscale methods, and their applications. Review Article Chem Soc Rev Open Access Article. Published on 01 February 2024. Downloaded on 10/15/2024 9:59:47 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online 2532 | Chem. Soc. Rev., 2024, 53, 2530–2577 This journal is © The Royal Society of Chemistry 2024 of CO emissions from vehicles. 4,6,7 In healthcare, they offer the potential for non-invasive disease diagnosis by detecting specific biomarkers in exhaled breath. They enable the detection of biomarkers associated with various diseases. For instance, in diabetes management, sensors can detect glucose levels in blood or interstitial fluid, providing crucial information for insulin dosing. Additionally, breath analysis using CGSs has shown promise for non-invasive disease diagnosis and monitoring. A prime example is the detection of acetone in the breath of individuals with diabetes as an indicator of their blood glucose levels. 8–10 They also play a crucial role in industrial safety, where the detection of hazardous gases is vital to protect workers and prevent accidents. CGSs are indispensable in industrial settings for process control and safety. They monitor and regulate parameters like gas concentrations, humidity, and solvent levels, ensuring optimal operating conditions. These sensors are vital for leak detection as well, preventing potentially hazardous situations in industries dealing with volatile substances. For example, in the chemical industry, these sensors can detect leaks of toxic or flammable gases, allowing for immediate response and containment. 11,12 The food industry relies on CGSs to assess food quality and safety. They play a crucial role in monitoring food freshness and preventing waste. For example, these sensors can detect spoilage gases, such as ammonia and ethylene, emitted by fruits, and vegetables, enabling timely interventions to maintain food quality. Additionally, CGSs are used in food packaging to ensure integrity, prevent contamination and ensure product safety. 12–14 CGSs are a crucial component of chemical and biological warfare agent detection systems. They can identify toxic substances and provide early warning in defence and security applications. For instance, in military applications, these sensors are used to detect chemical agents and protect personnel from exposure to harmful substances. Additionally, they are employed in explosives detection, enhancing security measures at airports and public spaces by detecting trace amounts of explosive materials. 15–17 Recent advancements have expanded the utility of CGSs to emerging applications. These include the detection of volatile organic compounds (VOCs) in indoor air quality monitoring. For example, in smart buildings, these sensors can detect VOCs released from building materials or cleaning products, ensuring healthy indoor air quality. Wearable devices incorporating CGSs have been developed for personal health monitoring, measuring parameters like sweat electrolyte levels. In the food and perfume industries, electronic noses equipped with these sensors are used for flavour and fragrance analysis, ensuring product consistency and quality. Efforts are ongoing to enhance the performance of CGSs by improving their selectivity, sensitivity, and response time. In the medical field, CGSs are extensively used for diagnostic purposes. Fig. 1 shows the different types of gas detection techniques in general use. The significance of CGSs lies in their potential to address the drawbacks associated with traditional detection technologies, While electrochemical, colorimetric, luminescent, sol–gel, infrared (IR), and paramagnetic IR optical sensors are effective, they often suffer from complexities such as high costs, limited scalability, and energy consumption issues. In contrast, CGSs offer advantages such as simplicity, cost-effectiveness, compactness, and low power requirements, making them attractive candidates for widespread adoption. These sensors can be designed using a variety of sensing materials, including metal oxides, polymers, nanomaterials, and carbon-based materials like graphene and carbon nanotubes. The sensing material’s surface interacts with the target gases or VOCs, causing changes in its electrical conductivity or resistance. These Hi-Deok Lee Prof. Hi-Deok LEE, with BS, MS, and PhD degrees in electrical engineering from the Korea Advanced Institute of Science and Technology (1990, 1992, 1996), joined LG Semicon Company Ltd. in 1993, contributing to CMOS technology development. Since 2001, he is a Professor at Chungnam National University, specializing in nanoscale CMOS technology, reliability physics, and sensor enhancement. A recipient of the Excellent Professor Award (2001, 2003, 2014), Prof. LEE was a Visiting Scholar at the University of Texas at Austin (2006–2008). A member of the Institute of Electronics, he currently heads the Intelligent ICT Education & Research Program for Future Defense Technology. His research focuses on nanoscale CMOS technology, reliability physics, silicide technology, test element group design, and sensor development. Kolleboyina Jayaramulu Kolleboyina Jayaramulu (Ram) is an Assistant Professor in the Department of Chemistry Indian Institute of Technology Jammu, India. He earned a PhD in Materials Chemistry at Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore, India. His scholarly pursuits have been further enriched by international experiences, having been honored with an Alexander von Humboldt Postdoctoral Fellowship in Germany, an ICMS Postdoctoral Fellowship and Sakura Science Exchange Program Japan. Ram is indeed a distinguished member of the prestigious Indian National Young Academy of Sciences (INYAS) (2023–2027). His research expertise is in the design and development of the structure–property relationship of hybrid (2D) porous materials for industrially relevant conditions. Chem Soc Rev Review Article Open Access Article. Published on 01 February 2024. Downloaded on 10/15/2024 9:59:47 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online This journal is © The Royal Society of Chemistry 2024 Chem. Soc. Rev., 2024, 53, 2530–2577 | 2533 changes are then translated into measurable signals that can be analyzed to identify and quantify the concentration of the target substances. Traditional chemiresistive materials and porous materials for gas sensing have distinct characteristics and advantages, and comparing them can help us understand the differences and potential benefits of using porous materials in gas sensing applications. Traditional chemiresistive gas sensors are often based on metal oxides e.g., SnO 2 , ZnO, WO 3 etc. These materials change their electrical resistance in the presence of specific gases due to chemical reactions on their surface. The rapid expansion of metal-oxide-based sensors can be traced back to the pioneering work of Seyama et al., employing a ZnO thin film as the sensing layer, successfully showcasing the feasibility of gas sensing through uncomplicated electrical devices. 18 Since then, there have been tremendous reports on the applications of semiconducting metal oxides as gas sensors such as TiO 2 , SnO 2 ,WO 3 , V 2 O 5 ,Fe 2 O 3 , NiO, CeO 2 , CuO, In 2 O 3 ,Nb 2 O 5 ,etc. Tin dioxide (SnO 2 ) sensors are widely used for detecting gases like methane and carbon monoxide. 19,20 SMO gas sensor devices have several unique advantages such as low cost, small size, measurement simplicity, durability, ease of fabrication, and low detection limits (oppm levels). In addition, most SMO-based sensors tend to be long-lived and somewhat resistant to poisoning. For these reasons, they have rapidly grown in popularity, becoming the most widely used gas sensors available these days using various conducting polymers, carbonaceous materials and various metal/metal oxide nanoparticles. However, the selectivity of specific analytes such as acetone, methanol, ethanol, isoprene etc., by various metal oxide materials is still a big problem. However, porous materials possess certain requisite features that make them potential materials for gas sensing. In order to get detected by the sensing device, adsorption and desorption of gas molecules is the primary requirement. 21–30 Porous materials being custodians of exceptional surface area and bearers of appropriate interactive functional sites fulfil the need of host–analyte interaction. CGSs feature the transduction of chemical interactions into electrical outputs such as conductance or resistance in low-cost, high performance, lessFig. 1 The schematic illustration portrays a range of advanced porous materials such as MOFs, COFs, and POPs, designed for applications in chemiresistive gas sensors across the industrial, environmental, healthcare, and food quality monitoring sectors. Review Article Chem Soc Rev Open Access Article. Published on 01 February 2024. Downloaded on 10/15/2024 9:59:47 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online 2534 | Chem. Soc. Rev., 2024, 53, 2530–2577 This journal is © The Royal Society of Chemistry 2024 energy consumption and portable devices. In order to bring chemiresistivity in the sensing materials, conduction of charges in the material is the foremost requirement. Porous materials provide vast options for selecting such appropriate materials owing to their extended frameworks that work as conduction highways for the movement of charges. Porous materials are an interesting class of materials that have developed a great career in terms of storage, molecular level seiving, catalysis, water treatment, sensing and so on. The use of modern crystalline porous materials such as MOFs, COFs and POPs was well-established as a sensing platform primarily due to their porous nature. 21,23,31–49 An unavoidable feature for an efficient chemiresistive gas sensing material is the electrical response towards the analyte gas which not all porous materials usually offer. Thus, this section will deal with only those materials meticulously which are usually conductive in nature or which show an electrical response towards the target gases based on their band-gap energies. Metal organic frameworks (MOFs) provide exceptionally high surface area, rigid and ordered framework, and functional group versatility that provide optimal host–analyte interaction and high selectivity. MOFs can be 2D or 3D based on the geometry of the organic linker being used. 29,50–57 Although 3D MOFs provide facile host–guest interactions, their nonconductive nature limits their use in chemiresistive sensing. Conversely, 2D MOFs serve as highly conductive materials owing to the planar and conjugated organic monomeric units such as porphyrin, triphenylene, phthalocyanine, etc. and thus offer an excellent option as a chemiresistive gas sensing material. 58 COFs can provide highly tunable structures and vast functionalization opportunities which remain beneficial for efficient guest–host interactions. Although their electrical conductivities are not much higher, their crystalline nature enables them to be excellent chemiresistive sensing materials even with their minute conductivities, though metal and conductive carbon doping based conductivity increments were reported previously. Graphene-based materials are conductive in nature and thus their functionalized counterparts are realized to show chemiresistive nature for gas sensing purposes. Graphene has extensively gone through various modifications and hybridizations to produce materials such as graphene oxide (GO), reduced graphene oxide (rGO), metal/metal oxide decorated graphene nanocomposites, Graphene–polymer composites and so on. 59 These modifications have been used for facilitating host–analyte interactions that are beneficial for sensing various analytes including gases. In this section, MOFs and COFs will be focused extensively as porous materials for chemiresistive gas sensing, while other prominent porous Fig. 2 Publication history of chemiresistive gas sensing (a) traditional material (b) advanced porous MOFs, COFs and POPs over the years. Data are obtained from the web of science by searching the keywords ‘‘chemiresistive gas sensing’’ and ‘‘porous material’’ (up to September 15, 2023). The number of publications on chemiresistive gas sensing materials using traditional materials has almost doubled every year since 2000, and a similar trend has been followed by advanced porous materials since 2014. Data are obtained (c) timeline chart showing the major developments of and progress in advanced porous materials towards chemiresistive gas sensing in terms of synthesis, design pristine, hybrids and derivatives of MOFs, COFs and POPs for various gas sensing applications. Chem Soc Rev Review Article Open Access Article. Published on 01 February 2024. Downloaded on 10/15/2024 9:59:47 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online This journal is © The Royal Society of Chemistry 2024 Chem. Soc. Rev., 2024, 53, 2530–2577 | 2535 materials that have created milestones in this field will also be discussed. The remarkable expansion of chemiresistive gas sensing using advanced porous materials is clearly evident, as reflected in the steadily rising number of publications from 2014 to 2023 (Fig. 2a and b). However, a diverse array of porous materials have found application in chemiresistive gas sensors, with key milestones highlighted in Fig. 2c. In summary, the choice between traditional chemiresistive materials and porous materials for gas sensing depends on the specific requirements of the application. Traditional materials are often preferred for their rapid response, robustness, and cost-effectiveness, while porous materials are gaining attention for their exceptional sensitivity and selectivity, especially in applications where precise gas detection is critical. Researchers continue to explore ways to harness the advantages of both types of materials for enhanced gas sensing capabilities. 2. Definition and operating principle of chemiresistive sensors CGSs are devices that detect and quantify the presence of gases and volatile organic compounds (VOCs) based on changes in electrical resistance. The operating principle of CGSs relies on the interaction between a sensing material and the target analyte. In general, a gas sensor comprises two main elements: a receptor and a transducer, as depicted in Fig. 3 The sensing material (receptor) used in CGSs is carefully chosen to exhibit a change in its electrical conductivity or resistance (transducer) when exposed to specific gases or VOCs (analytes). This material can be a metal oxide, a conducting polymer, a nanomaterial, or a combination of these. The surface of the sensing material is designed to have a high surface-to-volume ratio to maximize the interaction with the target analyte. When the target gas or VOC molecules come into contact with the sensing material’s surface, they adsorb onto it, causing a change in the electrical conductivity or resistance of the material. This change in resistance is proportional to the concentration of the target analyte in the surrounding environment. The electrical resistance of the sensing material is typically measured using a setup that includes electrodes connected to a measurement circuit. The resistance measurement can be performed using various techniques, including fourterminal measurements, two-terminal measurements, or impedance spectroscopy. The measured resistance value is then correlated with the concentration of the target gas or VOCs using calibration curves or mathematical models. The selectivity and sensitivity of CGSs can be enhanced by functionalizing the sensing material’s surface. This involves modifying the surface with specific coatings, catalysts, or receptors that selectively interact with the target analyte, increasing the sensor’s response to the desired gas while minimizing interference from other substances. Overall, the operating principle of CGSs relies on the change in electrical resistance of a sensing material when exposed to target gases or VOCs, allowing for the detection and quantification of these substances as shown in Fig. 4. The simplicity, sensitivity, and selectivity of CGS make them attractive for a wide range of applications, including environmental monitoring, industrial safety, healthcare, and more. 2.1. Device structure Device structure, in the context of gas sensors, refers to the physical architecture and composition of the sensor’s core components. It encompasses the arrangement of materials, electrodes, and other integral elements within the sensor framework. This seemingly technical aspect carries immense significance as it fundamentally shapes the sensor’s performance characteristics. One critical facet influenced by the device structure is sensitivity. The meticulous selection of materials and their spatial organization can determine how responsive the sensor is to the presence of specific gases. For example, semiconductor gas sensors employ carefully designed sensing layers, where the arrangement of semiconductor Fig. 3 Schematic illustration depicts gas sensor parts and typical measurement characteristics of chemiresistive sensors. Review Article Chem Soc Rev Open Access Article. Published on 01 February 2024. Downloaded on 10/15/2024 9:59:47 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online 2536 | Chem. Soc. Rev., 2024, 53, 2530–2577 This journal is © The Royal Society of Chemistry 2024 materials directly impacts the sensor’s ability to detect and respond to particular gases. The design and configuration of these layers are tailored to optimize sensitivity. Selectivity is another vital aspect influenced by the device structure. It determines the sensor’s capacity to distinguish between different gases in complex environments. By engineering the sensor’s structure to interact selectively with specific gas molecules, interference from unrelated gases can be minimized. This selectivity is crucial in applications where accurate identification of target gases is paramount, such as environmental monitoring or safety systems. Fig. 5 shows the different device structures used for gas detection. Response time, a critical metric for gas sensors, is intimately tied to the device structure. The arrangement of sensor components can affect the time it takes for the sensor to detect and register changes in gas concentration. An optimized structure ensures rapid response, enabling timely actions in critical situations like gas leak detection or air quality monitoring. Furthermore, the overall performance and reliability of a gas sensor are heavily influenced by its structural design. A robust structure can withstand environmental variations, ensuring stable and consistent operation over extended periods. This durability is crucial, particularly in industrial settings where sensors may be exposed to harsh conditions. In essence, the device structure in gas sensors is not merely a technical detail but a foundational element that determines the sensor’s ability to fulfil its intended purpose. It is through careful consideration and engineering of this structure that gas Fig. 4 A schematic illustration provides the potential chemiresistive sensing mechanism, including: (a) the step-by-step synthesis of a chemiresistive gas sensor setup, (b) the interaction of the effect of gas molecules on inducing a positive change in resistance. Notably, upon the removal of gas molecules, the system returns to a steady-state baseline, indicating the reversibility of gas molecules interaction. Fig. 5 Chemiresistive gas sensors are commonly built using various structures, including (a) sintered blocks, (b) thin alumina tube coatings, (c) screenprinted thick films, (d) small beads with coil and needle electrodes, (e) small beads with a single coil (heater and electrode), and (f) practical sensor elements assembled with metal caps and filters. Redraw the picture with permission from ref. 60 Copyright 2023 Elsevier. Chem Soc Rev Review Article Open Access Article. Published on 01 February 2024. Downloaded on 10/15/2024 9:59:47 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online This journal is © The Royal Society of Chemistry 2024 Chem. Soc. Rev., 2024, 53, 2530–2577 | 2537 sensors can exhibit enhanced sensitivity, selectivity, response time, and reliability, making them invaluable tools across a spectrum of applications, from ensuring workplace safety to safeguarding the environment and advancing healthcare diagnostics. Looking forward to the future of gas sensor technology, the existing guidelines for device fabrication are poised to serve as a cornerstone for further advancements. These principles are expected to guide the development of next-generation gas sensors with enhanced capabilities and broader applications. Anticipated advancements include the exploration of nanostructured oxide semiconductors with even smaller crystallite sizes, potentially revolutionizing gas sensing by significantly boosting sensitivity. Future technologies may also focus on advanced methods for the precise dispersion of sensitizers within semiconductor materials, leading to sensors with remarkable selectivity and responsiveness. The optimization of sensing layer parameters, enabled by advanced materials engineering and computational modelling, promises to produce sensors that are both highly selective and exceptionally durable. Additionally, thin film-type gas sensors, often overlooked but showing promise, may gain renewed attention with the maturation of advanced fabrication techniques like Sputtering, PECVD, ALD etc. In this vision of the future, gas sensors are poised to play a pivotal role in addressing multifaceted challenges, from environmental monitoring to healthcare diagnostics, setting new standards in sensor performance and utility. 2.2. Electrical and gas sensing characterization measurements The electrical and gas sensing properties of fabricated gas sensors with a comprehensive analysis of their performance conducted through a systematic measurement approach. The evaluation can be carried out using a multi-meter (Keithley 2400) with two conductive electrodes, as illustrated in the schematic diagram and digital photograph presented in Fig. 6. The experimental setup involves placing the sensors within a sealed chamber on a manually controlled heater equipped with one inlet and one outlet. Initial evacuation of the chamber should be performed using a high vacuum pump, achieving a pressure range from 760 torr to 10 3 torr. Subsequently, the carrier gas, either dry air or N 2 gas, can be introduced into the sensing chamber, with precise control over the gas amount facilitated by external mass flow controllers (MFCs). Ensuring the formation of an Ohmic contact between the active layer and electrodes is a crucial step before each measurement. This confirmation can be achieved through current–voltage (I–V) measurements, wherein the applied voltage varies from 5 V to +5 V in increments in applied bias V. To enhance stability, the sensor is preheated before the actual sensing measurements. The gas dynamics of the sensor are assessed by passing the sensing gas (Target), along with the carrier gas, and measuring the resulting current and resistance. Furthermore, the sensing response can be examined at Fig. 6 Schematic illustration of electrical and gas sensing measurements. Review Article Chem Soc Rev Open Access Article. Published on 01 February 2024. Downloaded on 10/15/2024 9:59:47 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online 2538 | Chem. Soc. Rev., 2024, 53, 2530–2577 This journal is © The Royal Society of Chemistry 2024 different temperatures by adjusting the heater temperature. The carrier gas flow rate is maintained in the chamber, while the sensing gas concentration is varied by controlling the flow rate. This systematic and controlled approach allows for a comprehensive examination of the gas sensors’ performance, shedding light on their electrical and gas sensing capabilities under varying conditions. 2.3. Fabrication techniques Various methods have been employed to fabricate nanostructured metal oxide materials for gas sensing applications, each imparting a wide range of sensor characteristics. The properties of these materials heavily rely on their composition and structure, which, in turn, are dictated by the building blocks of nanoparticles. Key factors governing the overall gas sensor performance include particle size, morphology, and crystal structure. 61 Synthesis techniques for nanomaterials can be broadly categorized as either ‘‘top-down’’ or ‘‘bottom-up’’ approaches. 62–77 Top-down approaches typically initiate with a bulk material, from which nanoscale structures are created by subsequent material removal processes. Common top-down methods encompass e-beam lithography, photolithography, milling, and dry or ion/plasma etching. While top-down processes generally offer high manufacturing throughput, they have limited control over surface morphology. Additionally, these methods often involve complex fabrication techniques that are less suitable for cost-effective and large-scale industrial production, particularly in applications like gas sensing. 2.3.1. Top-down approaches. Top-down approaches involve starting with a bulk material and then creating nanoscale structures by removing or etching away material. Common top-down methods include: E-beam lithography. This technique uses a focused electron beam to pattern a surface, allowing for the precise creation of nanostructures. However, it is limited in its scalability and is more suited for research and small-scale fabrication. Photolithography. Photolithography uses light to transfer a pattern onto a substrate coated with a photosensitive material. It is widely used in the semiconductor industry but may not be suitable for gas sensor materials that require specific morphologies. Milling. Mechanical milling involves grinding and reducing bulk material into fine nanoparticles. It is a versatile technique but can lead to agglomeration and limited control over particle size and shape. Dry or ion/plasma etching. These methods involve removing material from a substrate using chemical reactions or ion bombardment. While they offer high throughput, they may not provide precise control over surface morphology and often require sophisticated equipment. Top-down processes are known for their potential for high manufacturing throughput, making them attractive for certain industries. However, they may not offer the level of control over surface morphology and composition needed for gas-sensing materials. Additionally, the complex fabrication techniques involved in some of these methods can be cost-prohibitive for large-scale production. 78,79 2.3.2. Bottom-up approaches. On the other hand, bottomup approaches involve the assembly of nanomaterials atom by atom or block by block. Nanoparticle building blocks are generated on surfaces by depositing vapor molecules (in the gas phase) or ions (in the liquid phase). These atoms/ions are then assembled to form crystal planes or atomic clusters, which can subsequently grow into larger particles and material structures. These crystal planes and clusters ultimately give rise to the nanostructure of the sensing material. Alternatively, one can utilize nanoparticles synthesized in either the gas phase (aerosol) or the liquid phase (colloid) and then further manipulate these building blocks for desired sensor material properties. Vapor phase deposition. In this method, vapor molecules are deposited onto a substrate, where they assemble atom by atom or block by block to form nanostructures. Techniques like chemical vapor deposition (CVD) and metal organic chemical vapor deposition (MOCVD) atomic layer deposition, and Sputtering fall under this category. Solution-based methods: these methods involve creating nanoparticle building blocks in the gas phase (aerosol) or the liquid phase (colloid). These building blocks can then be assembled into desired structures. Solutionbased techniques include sol–gel synthesis and hydrothermal/ solvothermal growth. Bottom-up approaches offer precise control over crystal structure, particle size, and morphology. This level of control is essential for optimizing gas sensing materials to enhance sensitivity and selectivity. These methods are more suitable for research and development geared toward tailoring materials for specific gas sensing applications. 80 In summary, the choice of synthesis method significantly impacts the characteristics of metal oxide nanomaterials for gas sensing applications. While top-down approaches offer high throughput, they often lack surface morphology control and costeffectiveness. Bottom-up methods, on the other hand, enable precise control over material structure and are more suitable for tailoring properties to specific gas sensing requirements. 2.4. Gas sensing characteristics 2.4.1. Response and response transients. The ‘‘response’’ of a gas sensor is a fundamental parameter that describes how the sensor reacts when exposed to a particular gas or a change in gas concentration. It essentially quantifies the sensor’s ability to detect and respond to the presence of a specific gas. This response is typically expressed as a change in an electrical property (e.g., resistance, capacitance, voltage) or another measurable output of the sensor when exposed to the target gas. The response is often characterized by metrics such as sensitivity, which quantifies how much the sensor’s output changes in response to a given change in gas concentration. For instance, in a semiconductor gas sensor, an increase in the concentration of a specific gas like carbon monoxide (CO) may Chem Soc Rev Review Article Open Access Article. Published on 01 February 2024. Downloaded on 10/15/2024 9:59:47 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online This journal is © The Royal Society of Chemistry 2024 Chem. Soc. Rev., 2024, 53, 2530–2577 | 2545 target analytes, improving sensitivity and selectivity. Surface functionalization can involve the deposition of specific coatings, catalysts, or receptors that selectively bind to the target analyte. This process enhances the adsorption and detection of the desired gas, while minimizing interference from other substances, thus optimizing the sensor’s performance. Transduction techniques. CGSs employ various transduction techniques to convert the changes in electrical resistance into measurable signals. The choice of transduction technique depends on the specific application requirements and the desired sensitivity. Common transduction methods include direct resistance measurement, impedance spectroscopy, and frequency-dependent measurements. These techniques allow for the accurate quantification of the target gas or VOC concentration. Signal processing and analysis. Signal processing and analysis techniques play a vital role in extracting meaningful information from the sensor’s response. This involves filtering and amplifying the sensor’s electrical signal, followed by data analysis and interpretation. Signal processing techniques can enhance the sensor’s signal-to-noise ratio and improve the detection limit. Advanced data analysis methods, such as pattern recognition algorithms and machine learning techniques, can be employed to identify and classify different gases or VOCs based on their unique sensor responses. Calibration and validation. Calibration is essential for establishing a relationship between the sensor’s electrical response and the concentration of the target analyte. Calibration curves or mathematical models are developed using known concentrations of the target gas or VOC. Regular calibration and validation ensure the sensor’s accuracy and reliability over time. Stability and drift compensation. Long-term stability of CGSs is crucial for continuous and reliable gas detection. Sensor drift, caused by environmental factors or aging effects, can lead to false readings and reduced accuracy. Drift compensation techniques, such as baseline correction, temperature compensation, and periodic recalibration, help maintain the sensor’s stability and compensate for any drifts, ensuring consistent and accurate measurements. Overall, optimizing the performance of CGSs involves careful selection of sensing materials, surface functionalization, appropriate transduction techniques, signal processing, calibration, and addressing stability and drift issues. By considering these factors and employing suitable techniques, CGSs can achieve high sensitivity, selectivity, and accuracy, enabling reliable gas detection for various applications in healthcare, safety, environmental monitoring, and industrial processes. 4.2. Strategies to improve selectivity and sensitivity Functionalization techniques are used for enhancing the interaction between sensing materials and target analytes. Transduction methods include resistance measurement, impedance spectroscopy, and other signal transduction approaches. This comprehensive review provides valuable insights into the advancements made in chemiresistive sensor technology, highlighting its potential to revolutionize gas detection across various sectors. The knowledge and understanding presented in this review will aid researchers, engineers, and stakeholders in harnessing the full potential of CGS and driving further advancements in this field. 5. Chemiresistive gas sensing of porous materials To highlight the importance and advancement of this field, some very good reviews have already been published on porous materials for sensing purposes. 172 We have also tried to summarise all of the prominent porous materials such as MOFs, COFs, POPs and their hybrid examples which are also tabulated as given in the respective sections based on pristine and hybrid sensing materials. Metal–organic frameworks (MOFs) have emerged as an interesting class of porous materials that possess highly crystalline structures, robust frameworks, first-rate high surface area, high conductivities and tunable pore sizes. These properties of MOFs make them practically useful and highly functional in many arenas such as energy storage, 173–175 gas storage 176–178 and separation, 179–181 sensing, 182–184 biomedical field 185–187 and other scientific areas. Looking into a world where sensing is one of the inevitably required things, MOFs are one of those materials that have proved themselves to work excellently in this field owing to the requisite properties that are required for the purpose of sensing. 188–190 5.1. CGS sensing metal–organic framework based materials 5.1.1. Pristine MOFs. The very first work of using a pristine MOF for chemiresistive gas sensing was reported in 2014, when Zhang et al. 191 studied the chemiresistive nature of the cobaltbased zeolitic imidazole framework (Co-ZIF-67); although gas sensing with ZIFs has already been studied before, 192 the ‘chemiresistive’ sensing technique for gas sensing by using a MOF was never been used before. In this work, the group studied and demonstrated the sensing of various gases such as acetone, formaldehyde, methanol, and triethylamine (Fig. 8a–d). The band Gap of a material is a crucial property of a material that plays a driving role in its chemiresistive nature since the optimum band gap is directly related to the conductive properties of a material. MOFs generally have a high band gap of B4–8 eV which is not suitable for their application involving electrical properties. In this work, Co-ZIF-67 was used which was found to have a low band gap of 1.98 eV which is adequate for the electrical conductivity required for chemiresistive operation. The porous sodalite-like structure of Co-ZIF67 possesses high specific surface area (1832.2 m 2 g 1 ), providing a highly accessible surface for the interaction of gases (Fig. 8a). Among the targeted gases, formaldehyde was found to give the highest sensing response (Fig. 8b). Response and recovery performances (Fig. 8c) of this material were found to be fast but not faster than typical metal-oxide-based sensing materials. This can be due to the high surface area of MOFs Review Article Chem Soc Rev Open Access Article. Published on 01 February 2024. Downloaded on 10/15/2024 9:59:47 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online 2546 | Chem. Soc. Rev., 2024, 53, 2530–2577 This journal is © The Royal Society of Chemistry 2024 that take more time to reach the highest adsorption–desorption equilibrium extent and thus shows the highest resistance value. The selectivity of a gas by a particular sensing material is due to various factors (discussed in Section 4). The same group reported the use of a cobalt imidazolate framework [Co(Im) 2 ] n as a sensor for trimethylamine (TMA) gas (Fig. 8d). The resulting imidazole framework exhibits excellent selectivity, high gas response, and a low detection limit of 2 ppm, primarily due to the weak interaction between the TMA molecules and the framework. The ZIF-based materials were able to sense specific gases with high selectivity and good detection limits, but when it comes to sensing diversity for gases, efficient sensing was shown only for a few analyte gases. Also, the band gap is low for the ZIF-based MOFs but still, higher conductivities are needed for fast chemiresistive sensing operations. Conductivity in chemiresistive gas sensing materials is of utmost importance in order to bring the best performance out of them. Although high conductivities in MOFs were reported previously, 194–201 their applications remain limited and had never been used specifically for chemiresistive gas sensing before the revolutionary work by Campbell et al. 202 (Fig. 9a). High electrical conductivities of two-dimensional structures inspired the fabrication of 2D MOFs using such specific metals that can form 2D geometries as well as 2D organic linkers such as triphenylene, porphyrin and phthalocyanine-based monomers. In this current work, authors synthesized various novel 2D conducting Cu and Ni-MOFs (2D-cMOFs). 202,203 Responserecovery performance of the Cu 3 (HITP) 2 based chemiresistive gas sensor (Fig. 9b) revealed a fast chemiresistive response of the sensing material towards various NH 3 vapour concentrations. The swift response could be due to the 2D morphologically provided highly accessible interaction sites. Since Ni and Cu are in different electronic configurations (d 8 vs. d 9 , respectively), the theoretical studies suggest that the charge transfer between the target gas and the sensing 2D MOF material directly impacts the electronic response because metals with different electron counts result into different Fermi levels and thus tune the band gap values. 204 For example, it is predicted that the use of higher electron count elements in place of Ni, such as Cu, in M 3 HITP 2 type MOFs raises the Fermi level of this conducting material (Fig. 9d and e). 205 This was also found to be legitimate in experimental evidence as polar analytes such as methanol, ethanol, and acetone show high variations in sensing responses whereas not much difference could be seen for non-polar analytes, such as cyclohexane and pentane, which lack free electrons (Fig. 9c). These results provide us with the opportunity to tune the sensing performance of chemiresistive gas sensors by examining the effect of versatile metals in 2D MOFs. To further study the effect of different metals on the Fig. 8 (a) Sodalite topology of Co-ZIF-67. (b) Sensitivity of the Co-ZIF-67 sensor to various gases from 75–200 1C. (c) Chemiresistive sensing performance of ZIF-67 at different gas concentrations. Reproduced with permission from ref. 191 Copyright 2014, American Chemical Society. (d) Sensitivity of the [Co(im) 2 ] n sensor to various gases from 50–175 1C. Reproduced with permission from ref. 193 Copyright 2014, American Chemical Society. Chem Soc Rev Review Article Open Access Article. Published on 01 February 2024. Downloaded on 10/15/2024 9:59:47 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online This journal is © The Royal Society of Chemistry 2024 Chem. Soc. Rev., 2024, 53, 2530–2577 | 2547 sensing of gases in 2D MOFs, Dinca et al. worked out three triphenylene-based 2D MOFs with copper and nickel for metallic nodes. The high intrinsic conductivities of these MOFs come through the planar extended structure that is stacked on one another to form 1D pores. The electrons are free to move more swiftly through the rigid and planar triphenylene cores consisting of conjugated double bonds and electron-rich nitrogen and oxygen atoms, in the case of HITP and HHTP, respectively. Fig. 9 (a) First report on 2D conductive MOFs (with conductivity and pore size) (b) relative responses of a Cu 3 (HITP) 2 sensor to 0.5, 2, 5, and 10 ppm of ammonia. Reproduced with permission from ref. 202 Copyright 2015, Wiley-VCH. (c) PCA analysis for groupings of various VOCs for sensing purposes. Reproduced with permission from ref. 203 Copyright 2015, American Chemical Society. (d) and (e) Theoretical band, DOS, charge density isosurface and Kagome bands and SOC gaps in 2D M 3 (HITP) 2 MOFs (d) Ni 3 (HITP) 2 and (e) Cu 3 (HITP) 2 respectively. Reproduced with permission from ref. 204 Copyright 2015, Royal Society of Chemistry. Review Article Chem Soc Rev Open Access Article. Published on 01 February 2024. Downloaded on 10/15/2024 9:59:47 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online 2548 | Chem. Soc. Rev., 2024, 53, 2530–2577 This journal is © The Royal Society of Chemistry 2024 Carefully looking at the sensing performances of these 2D MOFs with various analytes, the Cu 3 (HITP) 2 ,Co 3 (HHTP) 2 and Ni 3 (HITP) 2 show high differences in sensing responses. The main difference is suggested to come majorly from different metal centres (Cu vs. Ni), although other factors such as different heteroatoms (NH and O) in HATP and HHTP linkers, respectively, and sensing device nature should also have significant effects. Another essential parameter that cannot be overlooked while studying the sensing of an analyte is the limit of detection or LOD values. The LOD of a sensing material can be described by the ability of the material to sense the lowest gas concentration and produce a readable intensity of the chemiresistive signal. This parameter is directly related to the interaction between the analyte and the sensing material. The sensing material would be able to sense the target analyte when the analyte molecules get free access to the materials’ interaction sites and the interaction causes a change in the electronic parameters of the material. MOFs are known to provide record-breaking surface area from a materials perspective 172,206 and accessible functional sites 207,208 for analytes owing to their highly ordered rigid porosity due to excellent crystallinity. Two-dimensional MOFs possess ordered long 1D channels with accessible metal and linker functional sites that induce sub-ppm level detection of analyte gases. For instance, 2D conductive MOFs were able to sense lower gas concentrations. Conductive MOFs provide reliable high cross-reactive detection of a particular analyte gas among various VOCs and high electrical conductivities favour the ease of sensing response for gas analytes. However 2D MOFs with powder or bulk morphology are not much favourable for fast charge transfer and also fast gas diffusion. In this regard, Yao et al. came up with a layer-by-layer (LBL) fabrication technique for synthesizing conductive MOFs, here Cu 3 (HHTP) 2 -xC, which can excellently control the 2D conductive MOF’s thin-film thickness down to an accuracy of 2 nm (Fig. 10a–c). 209 The LBL thickness controlled synthesis of 2DcMOFs was effected by simultaneously spraying metal salt and organic ligand on the –OH functionalized substrate such as quartz, sapphire etc (Fig. 10a). This improved the sensitivity of already existing 2D conductive MOFs that were used in powder or bulk forms, as depicted by the sensing of the ammonia gas (DR avg = 129%) which is many folds as compared to the previous studies (Fig. 10b and c). 210 This thin layer morphology of the MOFs also helped in a fast response time due to the fast accessibility of the available surface area to the target gases. In terms of the layered growth of 2D conductive MOFs, Smith et al., 183 have been working extensively on textile-based materials, came up with a 2D conductive MOF fabrication methodology on textiles for flexible MOFs using Ni 3 (HITP) 2 and Ni 3 (HHTP) 2 MOFs via a bottom-up modular growth approach (Fig. 10d). This methodology led to the expected sub-ppm level detection of gases (NO and H 2 S), high gas responses (49–98%) and room temperature sensing. MOFs can also be equipped with various functionalities that can play as interaction sites for various gases and VOCs to produce chemiresistive modulation in the sensing MOF material. MOFs can be tuned very easily in terms of functionality which comes from the organic linkers. Different gases interact with different functional groups with some particular force of interaction and this feature can be easily induced into MOFs to improve the MOF–gas interaction. One of the such initial studies was demonstrated by Wang et al. 211 where they exploited the free carboxyl functionalities of MOF linkers for their strong H-bonding ability with volatile amine protons. MD Mello et al. played with the interaction sites in the organic linker fragments of UiO-66 MOF where they used BDC, BDC–NH 2 and BDC–OH as organic linkers for the detection of acidic gases such as NO 2 ,SO 2 etc. 212 As expected from common acid–base interactions, in these MOFs, basic functionalities such as NH 2 ,OHetc interacted in a similar way with the acidic gases such as NO 2 ,SO 2 etc. Since there is some electronic charge transfer from basic to acidic moieties in acid– base interactions, the resulting change in the electron density of the basic moiety can be seen reflected as a chemiresistive signal when employing the MOF material in chemiresistive gas sensing. The detection of the gases was carried out at a temperature of 150 1C, but for an ideal gas sensor, it should be operable at ambient temperature to widen the operation window of gas sensing in various fields. At elevated temperatures, the multiple heat cycles can lead to abnormal crystal growth which can break the electronic interconnections 213 and can also reduce the fabricated sensors’ lifetime. Thus, chemiresistive gas sensing is always sought and preferred at room temperature over high-temperature gas sensing. But, high temperature is required for higher sensitivity and reversibility in the case of most of the traditional metal-oxide gas sensors. Since porous materials provide a high degree of sensing material–gas analyte interaction owing to their high surface area and approachable interactive sites due to their highly crystalline nature (especially in the case of MOFs and COFs), thus high-temperature requirements for gas sensing are unneeded. Extensive work on 2D MOFs and the study of their electronic properties to apply for chemiresistive gas sensing has been done. The gas sensing performance gives excellent results at room temperature revealing the structural and functional superiority of 2D porous materials such as 2D MOFs over conventional nonporous materials. Stassen et al. worked up on copper hexaiminobenzene(HIB)-based 2D electronically conductive MOFs for CO 2 sensing. 214 Cu 3 (HIB) 2 consisting of iminosemiquinonate (–NH–) groups which are electron-rich and thus interact with acidic gases such as CO 2 . –NH– moieties are found to be of significant relevance since CO 2 sensing with the hexaoxytriphenylene (HOTP)-based MOF, Cu 3 (HOTP) 2 provided unmeasurable signals indicating the efficient electron donating influence of imino moieties in contrast to oxo moieties. Working in the direction of 2D conductive MOFs, Meng et al. fabricated phthalocyaninebased 2D MOFs containing Ni and Cu as metal centres and as phthalocyanine-cavity metal – thus producing bimetallic 2D MOFs. 215 These 2D MOFs provided three major advances in chemiresistive gas sensing. First, the presence of twoChem Soc Rev Review Article Open Access Article. Published on 01 February 2024. Downloaded on 10/15/2024 9:59:47 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online This journal is © The Royal Society of Chemistry 2024 Chem. Soc. Rev., 2024, 53, 2530–2577 | 2549 dimensional structures of the MOFs due to planar, conjugated phthalocyanine linkers, they provide a high surface area, excellent conductivity, and a highly ordered distribution of active sites in the sensing material. Intrinsic conductivity of metal-phthalocyanine moieties provides ultra sensitivity to the sensing material towards the target gases (H 2 S, NO, NH 3 ). Second, by varying the organic linker (metallophthalocyanine and metallonaphthalocyanine), sensitivity and selectivity can be tuned via isoreticular modulation of the sensing MOF material. Third, due to high intrinsic conductivities (B10 2 Scm 1 ), excellent sensing performance can be seen even at lower sensing voltages (B0.01 V to 1.0 V). Thus, the sensor requires a lesser amount of energy to operate. 2D c-MOFs provide highly electroactive chemiresistive sensing materials but faster sensing performance and fair gas selectivity are challenging task. Improvising this condition, Wang et al. improved these two features in a single 2D c-MOF by tuning its surface-polarity. 216 Usually, 2D c-MOFs are hydrophilic in nature due to the presence of metal nodes and polar Fig. 10 (a) Illustration of the Cu 3 (HHTP) 2 crystal structure (b) the preparation of Cu 3 (HHTP) 2 thin-film gas sensors. (c) Response of Cu 3 (HHTP) 2 towards different reducing gases. (d) Response of Cu 3 (HHTP) 2 towards NH 3 analytes with different concentrations. Reproduced with permission from ref. 209 Copyright 2017, Wiley-VCHGmbH. (e) Customized Teflon container with SOFT-textile-MOF based sequentially stacked sensing material and gas response curves for the I-material (solid line) and II-material (dashed line) for Ni 3 (HHTP) 2 and Ni 3 (HITP) 2 . Reproduced with permission from ref. 183 Copyright 2017, American Chemical Society. Review Article Chem Soc Rev Open Access Article. Published on 01 February 2024. Downloaded on 10/15/2024 9:59:47 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online 2550 | Chem. Soc. Rev., 2024, 53, 2530–2577 This journal is © The Royal Society of Chemistry 2024 atoms such as N, O etc (Fig. 11a). Grafting with long alkyl chains, hydrophobicity can be introduced into the same material. With the infusion of hydrophobicity, the adsorption/ desorption of water vapours becomes more frequent, thus decreasing response/recovery times by a much greater extent. For instance, surface modification studies of 2D c-MOF Ni 2 [MPc(NH) 8 ] with organosilanes such as phenyltrichlorosilane (PTCS), (3-aminopropyl)trimethoxysilane (APTMS), and octadecyltrimethoxysilane (OTMS) (Fig. 11b). Out of these, Ni 2 [MPc(NH) 8 ]-OTMS produced the highest levels of hydrophobicity (water contact angle = 1381) which leads to faster recovery from humidity (Fig. 11c) and thus was used as a VOC sensing material. Since the polarity trend of lower alcohols follows methanol 4ethanol 4isopropanol trend, methanol with the highest polarity showed the fastest diffusion and response towards the surface modified sensing material (Fig. 11d–f). Thus, surface modification of 2D c-MOFs established opportunities to develop these materials in the field of electronics. Discussing the advantages of 2D c-MOFs, they provide a 2D surface to the analytes for the interaction for sensing purposes. The surface underlying area thus remains unapproached by the analyte particles which was highlighted by Lin et al. while proposing an improvement in this scenario by replacing 2D cMOFs with LBL grown 3D c-MOFs. 217 Langmuir–Blodgett deposition of one kind of material on the relevant substrate provides a thickness-controlled and highly oriented material designing approach. This approach was used to fabricate a CuHHTP based 3D sensing material which otherwise previously used to be synthesized as 2D c-MOF materials. LBL over flat surfaces yields only surface-exposed materials whereas over 3D substrates such as a nanowire array, provides higher surface area of layered material with higher exposure to active sites. This resulted in an ultralow detection of ammonia gas (5 ppb) which is about a thousand times lower than reported for 2D CuHHTP MOFs. Since the 3D Cu-HHTP material is grown on TiO 2 - NWAs, which doesn’t show an observable response towards the ammonia gas, the sensing performance of this material can be considered solely due to the 3D-grown Cu-HHTP MOF. Carrying the excellent potential of 2D-based c-MOFs, Meng et al. (Fig. 12a and b) and Aykanat et al. synthesized (Fig. 12c–e) bimetallic phthalocyanine and napthalophthalocyanine based 2D conducting MOFs in two different studies but by the same group to enlighten the sensing and differentiating various gases such as H 2 S, NH 3 and NO in the former studies by Meng et al. and CO gas sensing tunability and intensifying the sensing performance in the latter studies by Aykanat et al. 218 In the former experiments, isoreticular synthesis of robust bimetallic MOFs and their chemiresistive response towards various gases reveals the effective interaction between analyte and sensing material due to the presence of uniformly distributed metallic and linker sites, which are able to effectively differentiate between various types of gases. During the latter experiments, Co and Ni-based MPc-O 8 -Cu MOFs were generated, structured as Co or Ni inserted hydroxy-functionalized phthalocyanine linkers interconnected via Cu nodes. The 2D nature and presence of bi-metals in the structure resulted into the high conductivity, thus providing the advantage of using a low voltage (0.1 V) to power the sensing device and receive optimal results. The bimetallic phthalocyanine pattern of the sensing material helped for two particular reasons – first, the Fig. 11 (a) Chemical structure and Ni 2 [MPc(NH) 8 ] (M = Cu & Ni). (b) Surface modification of Ni 2 [MPc(NH) 8 ] MOF by APTMS, PTCS and OTMS and (c) response curves (DG/G o values vs time) for different H 2 O vapors concentration fed to the sensing material. (d) and (e) Response curves (DG/G o values vs time) for different (d) methanol and (e) ethanol concentrations fed to the sensing material. (f) Response curves (DG/G o values vs. time) of various protic and aprotic analytes interact with Ni 2 [MPc(NH) 8 ]. Reproduced with permission from ref. 216 Copyright 2021, Wiley-VCH. Chem Soc Rev Review Article Open Access Article. Published on 01 February 2024. Downloaded on 10/15/2024 9:59:47 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online This journal is © The Royal Society of Chemistry 2024 Chem. Soc. Rev., 2024, 53, 2530–2577 | 2551 need for active CO host sites and the sites for electronic transduction of the electronic perturbation due to CO interaction with the metal-phthalocyanine MOF. Second, the presence of Cu induces redox active charge hopping through the 2D-cMOF network by acting as a node for metallophthalocyanine network extension. The presence of these features in these MOFs helped in the ultralow sub-ppm level detection of CO. Further insight into the CO sensing performance was achieved by computational studies using DFT calculations. The interaction of CO with Co-based MOFs Fig. 12 (a) Illustration of and naphthalocyanineand phthalocyanine-based MOFs – NiNPc-M and NiPc-M respectively. (b) Chemiresistive gas sensing response for NiPc-M and NiNPc-M MOFs upon exposure to 40 ppm of NH 3 and H 2 S, and 1 ppm of NO in dry nitrogen (solid bar) and drenched in 5000 ppm H 2 O (bar with droplet symbols). PCA analysis for NiPc-Cu (blue), NiPc-Ni (green), NiNPc-Cu (yellow) and NiNPc-Ni (red) sensor arrays. Reproduced with permission from ref. 183 Copyright 2017, American Chemical Society. (c) Bonding optimization of Coand Ni-based Pc-MOFs. (d) Binding energy calculations for CO interaction with Co and Ni-Pc MOFs. (e) Highest sensing performance for Coand Ni-based Pc-MOFs as compared with other triphenylene linker-based 2D c-MOFs. Reproduced with permission from ref. 218 Copyright 2015, Wiley-VCHGmbH. Review Article Chem Soc Rev Open Access Article. Published on 01 February 2024. Downloaded on 10/15/2024 9:59:47 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online 2552 | Chem. Soc. Rev., 2024, 53, 2530–2577 This journal is © The Royal Society of Chemistry 2024 (Fig. 12c) was optimized and some constant CO–Cu and CO–Co bonding distances were realized for the best sensing results. The linearity of these bonds was realized by the metal–CO back bonding. Binding energy optimization studies revealed the incompatibility of the Ni based MOF to bind CO efficiently because of the positive free energy for the Ni–CO binding whereas the negative Co–CO free energy suggested the positive results in the case of the Co-based MOF (Fig. 12d). Also, the high sensitivity of the Co-based MOF than the Ni-based MOF can be explained by the same theoretical reasons. The superior response from phthalocyanine based sensing materials as compared with the triphenylene based 2D sensing materials could be because of the presence of doped Coand Nimetallic entities. Thus, the synthesis of pristine MOFs, their incorporation into the fabricated chemiresistive gas sensing device and tuning of their features such as porosity, functionalization, and morphology results in the tuning of sensing performances of the pristine MOF-based gas sensors (Table 1). 5.1.2. MOF hybrids/derivatives. MOFs can be broadly classified into two types of materials – one type that includes MOFs as templates or precursors for hybrids/composites that contain MOF as a constituent and other types that includes materials that themselves don’t contain MOF entities but are derived from MOFs. Metal-oxide semiconductor (MOS)-based chemiresistive gas sensors have already been developed significantly by improving their sensing performance by hybridizing with heteronanostructures, 219 doping or loading of metal oxides/ mixed metals 220 and making composites with carbon materials such as graphene. 221 MOFs provide an exceptional surface area and a robust framework for granting chemical stability to the materials they get linked with. MOF–MOS composites have been developed with critical insight into the conductivity, stability, selectivity and sensing time of the resultant material in chemiresistive gas sensing. These composites have mostly served as an upgrade to the pristine materials. MOFs can have fixed-size pores that can be used for molecular separation. 222 Drobek et al recognized this trait of MOFs and used it for creating a metal-oxide/MOF composite viz. ZIF-8 cloaked ZnO nanowires (NWs) (ZnO/ZIF-8 NWs) (Fig. 13a) for size-based response selectively to H 2 while being aloof for larger C 6 H 6 and C 7 H 8 molecules (credit to ZIF-8 porosity) while giving excellent sensitivity to H 2 at 300 1C (credit to ZnO NWs) (Fig. 13b). 113 Since MOF synthesis requires a metal source, Tian et al. followed an interesting strategy where they used ZnO nanorods (NRs) as the source for the zinc ions for the synthesis of ZIF-8 MOF shell around themselves. 223 This synthetic technique not only provided the MOF–MOS core–shell heterostructure, the selectivity ratio for formaldehyde sensing was also improved (Fig. 13c) with respect to other interfering VOCs such as ethanol, acetone, ammonia etc. in contrast to the bare ZnO nanorod sensors because of the molecular sieving via ZIF-8 windows. 224 Koo et al using a similar protocol, fabricated H 2 gas sensors by developing a ZIF-8 nanofiltration network on Pd NWs. 225 This work improved the hydrogen gas sensing speeds and selectivity by fastening the adsorption/desorption of H 2 molecules on/from Pd-NWs. Molecular sieving was exploited to restrict the entry of larger molecules of O 2 (0.345 nm) and N 2 (0.364 nm) than H 2 molecules (0.289 nm) with a ZIF-8 micropore size of 0.34 nm. Molecular sieving properties of pristine MOFs can be modulated with the help of metal nanoparticles. As metal NPs have the tendency to be engulfed inside the MOF pores, their spacious occupation further restricts the larger molecules and thus smaller gas molecules can be separated more specifically. Zhou et al., while considering this idea, performed H 2 sensing by using Ag NPs encapsulated ZnO/ZIF-71 which showed higher selectivity towards H 2 and decreased response for acetone owing to their size preferences (Fig. 14a). 226 This work thus supported the enhancement of metal oxide sensing performance which otherwise would have been poor as the normal case. The variation in the sieving process changes from pore to pore due to the different levels of Ag nanoparticles filling inside the pores of ZIF-71. Temperature programmed desorption (TPD) studies revealed the obstruction of acetone desorption while supporting the hydrogen desorption by increasing the silver nanoparticle concentration inside the ZIF-71 pores (Fig. 14b). This revealed the effect of pore size tuning on the molecular sieving by embedding metal nanoparticles inside the cavities of the MOF and thus the subsequent sensing of singled out gases. The sensing response of the ZnO@ZIF-71*Ag NRA towards acetone and hydrogen with different Ag-loading concentrations also support the metal nanoparticle based sieving tuning where with an increase in the Ag-load, H 2 response increases and acetone response decreases (Fig. 14c). Similarly, recently, the molecular sieving property of ZIF-8 was exploited for the H 2 sensing at lower temperatures of 100 1C as compared to the previously studied ZnO-NPs/ZIF-8 composites. In this work, Poschmann et al. pointed out the use of ZnO nanoparticles in polycrystalline powder form and the conductivity hindrance effect of the ZnO–MOF–ZnO sequenced sensing material morphology. 228 Thus, they used the tetrapodal single-crystalline ZnO microparticles to avoid this undesired conductivity reduction. The ZIF-8 layer over the singlecrystalline ZnO microparticles provides excellent selectivity towards the H 2 molecules, even in the presence of optimum methane concentration. This is observed due to the pinholefree morphology of ZnO microparticles plus ZIF-8’s microporosity. Another compounding reason is the increased rigidity of the pore windows due to the composite formation with single unit ZnO particles. Here, due to the single-crystalline nature of the ZnO particles which are surrounded by MOF layers, the change in conductivity cannot possibly occur due to the lack of adsorbed atmospheric oxygen molecules. This might be because of the presence of oxygen defects on the crystal surface which provide the interacting surface for the hydrogen molecules and result into the change in the electrical parameters. One important thing to note here is the optimum thickness of the MOF layers grown over the metal/metal-oxide nanowires. Thin MOF layers are being used here to provide molecular sieving and faster diffusion, whereas thicker MOF layers decrease the traveling speed of gas molecules and thus increase Chem Soc Rev Review Article Open Access Article. Published on 01 February 2024. Downloaded on 10/15/2024 9:59:47 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online This journal is © The Royal Society of Chemistry 2024 Chem. Soc. Rev., 2024, 53, 2530–2577 | 2553 Table 1 MOF and its hybrids/derivatives for chemiresistive gas sensors S. no. Material VOCs Sensing mechanism Response (%) LOD (ppm) t res –t rec (s) Ref. 1 ZIF-67 TMA 14.1 2 193 2 ZIF-67 FA 13.9 5 191 3Cu 3 (HITP) 2 NH 3 0.5 202 4M 3 (HHTP) 2 (M = Cu, Ni) Various VOCs CT 203 5Ni 3 (HHTP) 2 @textile NO CT 49 10 1.4 183 6H 2 S989 0.23 7Ni 3 (HITP) 2 @textile NO CCT 81 6 0.16 8H 2 S972 0.52 9Cu 3 (HHTP) 2 NH 3 DA 129 0.5 81.6–46.6 209 10 Cd(H 2 L) 2 EDA HD-DA o3–10 211 11 [Cd(TMA)(DPP) 0.5 H 2 O] n Humidity Proton hopping 11–56 282 12 UiO-66-NH 2 SO 2 DA 21.6–2.7 1 26.8 5.4 212 NO 2 7.6–0.4 10 CO 2 11.4–2.2 500 35 2 13 Cu 3 HIB 2 CO 2 DA 0.62 420–660 214 14 Cu-TCPP@ Cu-HHTP NH 3 CCT 94 91.8–643.2 283 C 6 H 6 153 15 NiNPc NH 3 CT/Redox action 43–45 0.31 215 H 2 S 64–98 0.019 NO 657–397 0.001 16 Cu-BHT NH 3 ET 7.88 0.23 58–102 284 17 Ni 2 [MPc(NH) 8 ]CH 3 OH Proton transfer 4.7 10 36–13 216 18 Cu-HHTP NH 3 Charge/Mass transport 42 0.005 35–900 217 19 Cu-HITP NH 3 CCT 7.1 0.5 285 20 NiPc@CoTAA NO 2 ET 37.6 300–3600 286 21 HITP@Cu-HHTP NH 3 CCT 80 0.024 o60–600 287 Benzene 75 0.096 22 HIB-Cu Humidity CT 200 21–40 288 23 Zn-BDC-NH 2 H 2 2.93 289 24 CoPc-O 8 -Cu CO 27.4 0.8 0.5-3 218 NiPc-O 8 -Cu 18.9 0.8 25 CopyNDI NH 3 CT 46.7 0.00015 ppb 168–198 290 NipyNDI NH 3 426 ZnpyNDI 32.5 26 Cu 2 O/CuO Ethanol DA 291 27 ZIF-CoZn/ZnO Acetone DA 27 0.0019 43.2–61.2 292 28 ZnO@ZIF-8 N H 2 DA 1.44 113 29 ZnO@ZIF-8 FA DA — 5.6 16–9 223 30 Pd@ZnO (ZIF-8)-WO 3 NFs Toluene DA S= 4.37 0.1 293 31 PEDOT@MIL101(Cr) NO 2 DA 0.9 0.06 30–150 259 32 PdO@ZnO(ZIF-8)-SnO 2 Acetone DA 5.6 0.010 o20–64 294 33 PdO@Co 3 O 4 (ZIF-67) Acetone DA 2.51 0.1 295 34 Pd NWs@ZIF-8 H 2 DA 3. 0.6 7–10 225 35 ZnFe 2 O 4 @MOF-5 Acetone DA 64.4 296 36 PdO@Co 3 O 4 -nSnO 2 Acetone DA 22.8 1 90.8–108.4 297 37 ZnO@ZIF-8 Acetone ethanol DA 298 ZnO@ZIF-67 38 Pd-ZnO@ZnCo 2 O 4 Acetone DA 69 299 39 ZIF8/Pd/ZnO NWs H 2 DA 8.5 0.5 227 40 ZIFCo 3 O 4 rods@ZnO Acetone DA 25 0.005 300 41 ZIF-67@Co 3 O 4 p-Xylene DA 78.6 63–86 301 Toluene 43.8 42 ZIF-67@WS 2 NO 2 DA 48.2 0.1 302 43 ZIF-67@SnO 2 CO 2 DA 48.2 18–25 229 44 POM@ZIF-8@ZnO FA DA 4.4 0.387 15.1–16.2 230 45 TiO 2 /Co 3 O 4 NFs by TBT@ZIF-67 Ethanol DA 16.7 5 303 46 ZnO NSs@ZIF-L CO DA 3.2 0.134 30–1 304 VOCs 1.4 0.02 47 Pt@Cu 3 (HHTP) 2 NO 2 DA 62.11–57.38 828–840 233 48 Ag@ZnO@ZIF-71 H 2 DA 226 49 Au-ZnO@ZIFs Acetone DA 231 0.0001 305 50 In 2 O 3 /MoS 2 MIL-68(In) NO 2 DA 371.9 306 51 MIL-53(Al)/CNT CO 2 Pore transition of MOF 15 o30 249 52 ZIF-8/ZnO NRs H 2 S DA 52.1 0.05 420 307 53 HKUST-1/MoS 2 H 2 O PT 8–14 0.38 182 54 Cu(BTC)@PDMS CO DA 0.46 264 MIL-160@PDMS Humidity 55 Ln(acac) 3 @ZIF-8 NO 2 DA 187.9 0.0002 1050–1230 308 56 CoSnO 3 @MOF H 2 S DA 12.1 10 0.00018 234 Review Article Chem Soc Rev Open Access Article. Published on 01 February 2024. Downloaded on 10/15/2024 9:59:47 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online 2554 | Chem. Soc. Rev., 2024, 53, 2530–2577 This journal is © The Royal Society of Chemistry 2024 the response and recovery times. SnO 2 NPs/ZIF-67, due to the tuneable porosity of the MOF counterpart, was explored as a novel gas sensing material by DMello et al. 229 The exclusive and stable sensing response to CO 2 was delivered because of the suggested synergistic effect between SnO 2 and ZIF-67. Synergistic involves the transfer of electrons at the interface of ZIF-67 and SnO 2 from the organic linker (Imidazole) to the MOS metal (Sn) and parallelly from the oxide to the MOF metal (Co), which stabilizes the carbonate formation from CO 2 after interaction with oxides. Synergistic effects were further explored by Wang et al. as they studied formaldehyde detection via higher photocurrent emergence values for the POM@ZIF-8@ZnO (POM = Polyoxometalate) as compared to ZIF-8/ZnO nanorods. 230 ZIF-8 pores served as an analyte concentrating medium so that formaldehyde molecules can gather densely and produce an equivalently higher photocurrent. These observations further corroborated the advantageous role of synergistic effects of metal-oxide-MOF combinations. Getting some mechanistic insight into these excellent outcomes of metal-oxide NWs/ MOFs hybrids, incorporating metal nanoparticles greatly advances the sensing performance of the MO NWs/MOFs materials. Metal atoms can exist as agglomerated nanoparticles which are found to show a spill-over effect. Gaseous molecules while interacting with the metallic nanoparticles dissociate into smaller fragments and thus can interact more intensely with the surrounding host material. Hydrogen spill-over effect (HSPE), first discovered in 1964, is a peculiar surface phenomenon in which H 2 molecules dissociate into H-atoms after being ‘‘cut’’ by tiny metal nanoparticles and subsequently ‘‘spill’’ over the supported material. 231 Weber et al. fabricated a ZnO NWs/ZIF-8 hybrid embedded with Pd nanoparticles (NPs) for chemiresistive H 2 sensing and increased selectivity excellently towards the H 2 sensing compared to bare ZnO NWs observed because of the molecular filtering out VOCs other than H 2 on the basis of size. 227 The improved results were seen because of the spill-over effect due to Pd-NPs combined with the nanofiltration due to ZIF-8 pores. Although Pd@ZnO NWs/ZIF-8 exhibits a lower response towards H 2 sensing, the porous ZIF-8 cladding helped this material to outperform both Pd@ZnO NWs and ZnO NWs in H 2 selectivity. The comparison of H 2 sensing mechanisms for ZnO NWs, Pd/ZnO NWs and ZIF-8@Pd/ZnO NWs as gas sensing materials is schematically shown in (Fig. 14d). H 2 being a reducing gas leads to reduction of ZnO to Zn metal in ZnO NWs and additional Pd NPs to PdH x in Pd/ZnO NWs and ZIF8@Pd/ZnO NWs, each of which causes conductivity increments. Due to the absence of a spill-over effect for the reducing gases other than H 2 results in no significant enhancement in the sensing performance for these VOCs, thus producing a better comparative study of H 2 sensing. Metal nanoparticles are remarkable catalysts and due to their nano-sized nature, they can be infused into nano, micro and macroporous materials. Smaller size of metal NPs provides them with larger surface area which thus improves the catalytic performance of their hybrids. 232 Based on these properties of metal NPs, their encapsulation in MOFs has also shown an enhancement in their catalytic properties. 2D conductive MOFs incorporated into metal NPs such as Pd, Pt and Au NPs, have been shown to improve both their catalytic as and conducting properties. Koo et al. demonstrated the revamping of Cu-HHTP based c-MOFs by fusing them with Pd and Pt NPs (Fig. 15a and b). 233 The catalytic effect of metal NPs was observed for Pt@Cu 3 (HHTP) 2 and can be supported by the activation energy depletion of NO 2 adsorption by the sensing material which resulted in the higher decrease in the p-type c-MOFs resistance while interacting with strong electron acceptor NO 2 gas (Fig. 15e). In the case of Pd@Cu 3 (HHTP) 2 , the activation energy was only raised up but it still showed a decrease in the resistance of the sensing material. This provided an insight further into the electronic structure change upon NO 2 adsorption on the sensing material. While interacting with the Pd NPs, NO 2 molecules interacted via the formation of a nitrito-complex with the Pd NPs, absorbing electron density from them, and thus reducing the Schottky junction barrier that persists between Pd NPs and c-MOF. This increased the conductivity of the sensing material and thus enhanced its sensing performance via electronic sensitization of NO 2 by Pd NPs (Fig. 15e). This provides the sensing material with a more enhanced response towards the NO 2 gas molecules (Fig. 15c) and thus acts as a state-of-the-art NO 2 gas sensing device (Fig. 15d). The study of reaction kinetics of these eventual reactions was beneficial for understanding the effect of metal nanoparticles’ catalytic properties on the NO 2 gas interaction reactions with the sensing material surface. Adsorption/desorption rates of NO 2 on/from M@Cu 3 (HHTP) 2 were enhanced more prominently during Pt NP doping as compared to Pd NP doping. This trend was observed because Pt NPs decrease the activation energy of NO 2 interaction with the metal Table 1 (continued) S. no. Material VOCs Sensing mechanism Response (%) LOD (ppm) t res –t rec (s) Ref. 57 G@Cu-BTC CHCl 3 DA B0.15 41 309 G@ZIF-8 B0.10 G@UiO-66 B0.04 58 ZnO/CuO from Zn/Cu-BTC H 2 S DA 393.35 0.0003 173–3000 310 59 GA@UiO-66-NH 2 CO 2 DA 8.6 18 221 60 GO@PDDA@Co 3 (HITP) 2 NO 2 IC 9.05 0.0068 24–41 311 61 t-ZnO@ZIF-8 H 2 DA 546 1–2 228 DA = Donor–acceptor; CT = Charge transfer;Charge carrier transport = CCT;Ethylenediamine = EDA;Formaldehyde = FA;Trimethylamine = TMA;Electron transfer = ET;Intrasheet conductivity = IC;Proton transfer = PT;Hydrogen bonding = HB. Chem Soc Rev Review Article Open Access Article. Published on 01 February 2024. Downloaded on 10/15/2024 9:59:47 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online This journal is © The Royal Society of Chemistry 2024 Chem. Soc. Rev., 2024, 53, 2530–2577 | 2561 access to the amine interaction sites ensures faster response and recovery times for CO 2 sensing (t res = 18.5 s, t rec = 18.7 s). Consequently, the introduction of amine functional groups inside the porous pathway of GA@UiO-66-NH 2 provides a high response of 8.6% (Fig. 19b–d) compared to the pristine GA which shows a negligible response to CO 2 under the same sensing conditions. In comparison, graphene has a low electrical resistivity (B10 6 ohms) and high carrier electron mobility. Still, the lack of specific and strong interaction sites for the adsorption of gaseous analytes, renders it unusable for chemiresistive sensing in its pristine form. These results are clarified by temperature-dependent in situ Raman spectroscopy during the exposure of the material to CO 2 . While up to 125 1C, the softening takes place which can be observed further up to 200 1C, suggesting the gradual uptake of CO 2 into the porous GA-UiO-66-NH 2 network while strongly interacting with the amide bonds. A further increase in the temperature causes an increase in the frequencies of both –CO–NH linkages and Fermi modes of CO 2 suggesting a very weak interaction of CO 2 with the sensing material. 5.2. Covalent-organic frameworks as chemiresistive gas sensors Covalent-organic frameworks (COFs) have been employed for sensing purposes due to their highly ordered structure, functional group versatility, and high thermal and chemical stability. They have been primarily utilized for explosive sensing, metal ion sensing, humidity sensing, biosensing, gas sensing, and more. 312 However, covalent-organic frameworks (COFs) are in their nascent stage regarding the application of chemiresistive gas sensing. 313 One major reason for this is the challenging task of bringing intrinsic electronic conductivity in them in addition to the robust framework that is the fundamental necessity for a successful gas sensing material with requisite results that can outshine or at least compete with the already available sensing materials’ performance (Table 2). 314–321 5.2.1. Pristine COFs. Despite their poor conductivity, some researchers have actively pursued using COFs as chemiresistive gas sensors, aiming to compare them directly with other sensing materials due to their promising porous and versatile functionality. Singh et al. reported a Truxene-based COF for humidity sensing. 322 The high surface area and ordered structural porosity along with the boronate ester groups in the COF resulted into an excellent interactive material for humidity. The stability of this COF in a high humid environment is might be due to its structural robustness. The conductivity required for the chemiresistiveness comes from the planar COF sheets and the multilayer formation of water vapours at higher RH% that acts as a proton conduction medium throughout the sensing COF material. Thus, the sensing can be attributed much to the humid medium rather than the intrinsic COF conductivity. COFs most often are very low (B10 3 to 10 10 Scm 1 )or non-conductive materials. To tackle this problem of conductivity requirements, many researchers have worked to produce conductive COFs by modulating their spatial stacking, electronic band gap and tuning framework backbone functionalities. Meng et al. have reported a pristine COF, named COF-DC-8, with an intrinsic bulk electronic conductivity of 2.51  10 3 Sm 1 that was the highest reported for a COF at that time 323 (presently, Huang et al. achieved 12.7 S m 1 for a piperazine-linked COF). 324 The high conductivity can be attributed to full p–pconjugation spread over the entire intrinsic 2D framework and the DFT-suggested out-of-plane charge transferring due to the anisotropic band structure (Fig. 20a and b) Fig. 19 (a) Schematic illustration of the synthesis of GA@UiO-66-NH2 and GA@UiO-66-NH 2 porous network with amide linkage. (b) Gas sensing performance of GA@UiO-66-NH 2 (from 100% to 5% CO 2) at an operating temperature of 200 1C. Gas sensing characteristics (% response, response time and recovery time) of the hybrid at different concentrations of CO 2 ; Temperature effect on the sensing characteristics of GA@UiO-66-NH 2 (c) on the response % and (d) response and recovery times. Reproduced with permission from ref. 221 Copyright 2021 Royal Society of Chemistry. Review Article Chem Soc Rev Open Access Article. Published on 01 February 2024. Downloaded on 10/15/2024 9:59:47 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online 2562 | Chem. Soc. Rev., 2024, 53, 2530–2577 This journal is © The Royal Society of Chemistry 2024 (synthesized by condensation of two planar, rigid and themselves fully conjugated metal-phthalocyanine and pyrene core monomers). This COF showed chemiresistive nature towards various gases with excellent results for NH 3 ,NO 2 ,H 2 S and NO gases (Fig. 20c–f). The metal-analyte interaction was exploited in this case for chemiresistiveness since metal doped phthalocyanine rings have improved electronic conductivities in contrast with the pristine phthalocyanine rings. The charge transfer interaction of oxidizing (NO and NO 2 ) and reducing (H 2 S and NH 3 ) gases with the Ni-centre corresponds to the mechanistic performance of the sensing COF material. Ni containing free valence electrons has the potential to interact weakly via non-covalent interactions as well as stronger covalent interactions too with the analyte gases. This results into a significant change in the electronic parameters supporting the highly sensitive chemiresistive nature of the COF towards the targeted gases which might also be a reason for the low and ultralow ppb level detection limits for this material. COFs are usually formed as a powdered material by traditional solvothermal processes but for convenient practical applications, robust materials play the required role. For an efficient, practical gas sensor, the sensing COF material should have such properties so that the sensor can work for longevity with the same performance. Fathoming this need for the COF-based chemiresistive gas sensors, Mei et al. fabricated a flexible and robust COF-5 film on a polyimide (PI) substrate and employed it for humidity sensing. 325 The synthesis of COF films was approached via a vapourassisted method. This method provides a sophisticated way to control the film thickness of the COF material by changing the monomer concentration in the solution. After achieving the required thickness of the COF-5 film and sensor fabrication, the device was tested from 11–98% RH range which showed a remarkable linear response between log of resistance and Fig. 20 (a) Schematic representation of a phthalocyanine-based 2D conducting COF (COF-DC-8). (b) DFT calculated anisotropic electronic band structure and the corresponding first Brillouin zone. Response curves of COF-DC-8 when interacting with (c) NH 3 ,(d)H 2 S, (e) NO, (f) NO 2 gases. Reproduced with permission from ref. 323 Copyright 2019, American Chemical Society. Table 2 COF and its hybrids for chemiresistive gas sensors S. no. Material VOCs Sensing mechanism Response (%) LOD (ppm) t res /t rec (s) Ref. 1 COF-TXDBA Humidity Lewis acid–base interaction 2.3 37 322 42 2 COF-DC-8 NH 3 CT-DA 39 0.07 323 H 2 S 62 0.204 NO 3939 0.005 NO 2 6338 0.016 3 M-TPCOF NO 2 DA 2713 (Co) 0.0068 318 (Co) 329 2056 (Cu) 510 (Cu) 690 (Cu) 4 COF-5 Humidity 90 26/16 325 5 COF@PANI NH 3 710 5 5 COF@Ppy 6 TiO 2 @COF-316 NO 2 DA 572 (NA) 1.41 8–1.1 330 7 Pd@TpPa-SO 3 H COF H 2 DA 60.1 10 0.2% 5.3–3.1 332 Chem Soc Rev Review Article Open Access Article. Published on 01 February 2024. Downloaded on 10/15/2024 9:59:47 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online This journal is © The Royal Society of Chemistry 2024 Chem. Soc. Rev., 2024, 53, 2530–2577 | 2563 relative humidity. The COF-5 film showed better performance in gas sensing than the COF-5 powder. The uniform morphology and uninterrupted conductive contact due to the 2D film nature proved to be superior to the discrete powder in terms of electrical sensing requirements. 5.2.2. COF hybrids. Synthesizing intrinsic COFs with high electrical conductivities is one tremendous challenge that limits their application as a practically usable chemiresistive material. Polymers, on the other hand, such as polyaniline 326 and polypyrrole, 327 have high intrinsic conductivities – about 600 S cm 1 and 380 S cm 1 , respectively. Conducting polymers are known for their composition into various materials such as MOFs, COFs, Metal oxides, Carbon materials etc. to either induce or increase their electrical conductivities of the latter. Their infusion with these materials has been explored extensively in the field of chemiresistive gas sensing. This strategy for COF-Conductive polymer composites for chemiresistive gas sensing was effected insightfully by Sahiner and group in 2019, by using a mesoporous COF developed from condensation between melamine and dibromoalkane monomeric units, to create in situ formation conditions for conducting polymers such as polypyrrole (PPy) and polyaniline (PANi) which resulted into a semi-interpenetrated network of COF and conductive polymers. 5 This intrinsic growth of conducting polymers inside the COF led to an increase in the electrical conductivity by 3million folds for COF-PANi and 0.5-million folds for COF-PPy networks, respectively. This high conductivity was utilized for gas sensing (NH 3 & HCl vapours) as well as dye sensing (Methyl Orange and Methyl Blue) purpose in a chemiconductive manner. Both pristine COF and COF-Polymer composites were tested for gas and dye sensing. Towards NH 3 and HCl vapours, pristine COF showed an increase in conductance while both COF-Polymer composites showed an opposite effect. This could primarily be due to the protonation of amine groups which resulted into the formation of positively charged ammonium ions, thus causing an increase in the ionic conductivity of the pristine COF material. The opposite effect in the case of composites could be explained by the fact that the reducing gases such as ammonia cause de-doping of PANi 328 by deprotonating the bridging amino groups and over-oxidation of chlorine ions by PPy. Materials in their bulk phase cannot able to offer the entire surface area for interaction with the analyte particles. In contrast, 2D layered materials or bulk materials after exfoliation reveal their maximum surface area and thus can interact with the lower concentrations of the analyte particles. In this regard, Liu et al. developed porphyrin-based 2D COF nanosheets (M-TPCOF, M = Co, Cu) where porphyrin moieties can be used to coordinate metal active sites as a result of their post-metallization (Fig. 21a). 329 The easily formed stable nanosheets after exfoliation are a result of the non-linear linker (2,6-pyridinedicarboxaldehyde) used in synthesizing this COF which hinders the p–pstacking of extended porphyrin layers due to the strong deviation from the planar COF layers. Thus, it produced high surface area nanosheets with exposed metal active sites which increased the extent of NO 2 gas interaction with the metal active sites and thus resulted in the excellent sensitivity. The highly specific affinity towards the NO 2 gas is proposed to the presence of electron-rich metal centres (Co & Cu) due to the highly electron withdrawing nature of acidic NO 2 gas, which eventually produces excellent selectivity performance as can be figured by the adsorption studies of various analytes (Fig. 21b). DRIFT IR spectroscopy has proven to be a useful technique for analysing gas-host interactions by carefully observing the changes in the intensity of interaction signals (Fig. 21c). The broad peak at 1500–1290 cm 1 corresponds to the NQO stretching of the monodentate nitrite or the asymmetric stretching NO 2 vibration of the nitro group. The peak at 1398–1353 cm 1 implies to Co–NO 2 species, indicating Co as the active site for NO 2 .A significantly wider peak observed at 2310–2230 cm 1 might be due to the NQO stretching in the NO + ions. With an ultra-low LOD of 6.8 ppb, the device showed a fast response and strong binding of NO 2 with the metal centre of M-TPCOF nanosheets. This level of sensing performance is a result of few-layered structures of the materials, which provide fast diffusion access pathways to the gases. Thus, distortion-based exfoliation of metalloporphyrin bulk COFs to COF nanosheets provides scope for the exfoliated material-based sensing performance improvements. COFs can attune surface properties effectively due to their versatile nature towards functional modifications. They can be used for the easy tuning of redox properties of the materials based on their electronic interactions with them. This nature of COFs was beautifully explored by Chen et al. while working with the MOS-based chemiresistive gas sensors. The TiO 2 nanowire array was coated with COF-316 (a dioxinlinked COF), which resulted into the reversing of the reductive to oxidative nature of TiO 2 (Fig. 22a). 330 This work represents the benefits of 2D porous materials for enhancing the sensing properties of a pre-existing metal-oxide gas sensor. The high porosity, facile redox interchange and abundant active sites offered by the COF-sheath over TiO 2 nanowires for NO 2 gas sensing can be possible due to these intrinsic properties of porous COFs. Redox reversing being the striking advantage of this COF/TiO 2 heterojunction can be explained by the Z-scheme of charge separation. 331 Since the band gaps in COF-316 and TiO 2 overlap, the electrons excited from the valence band to the conduction band in TiO 2 relax by occupying the valesnce band in COF-316 under the influence of an intrinsically evolved electric field between COF/TiO 2 heterojunction. This event results into the concealment of oxidative sites on TiO 2 and revelation of the reductive sites on the outer COF-sheath for the sensing reversal from the reductive gas sensing such as EtOH towards the oxidizing gas sensing such as NO 2 (Fig. 22b). Due to the intrinsic hydrophobic nature of the TiO 2 /COF-316 hybrid (contact angle = 108.31) in contrast with the hydrophilic nature of the bare TiO 2 (contact angle = 7.11), the sensing device integrated with this hybrid leads to undisturbed sensing performance of the device with consistent results of the NO 2 sensing (Fig. 22c). The NO 2 sensing mechanism of the TiO 2 /COF-316 hybrid reveals the reversal of the sensing of oxidizing NO 2 gas (Fig. 22d). This hybrid promotes the reductive sensing Review Article Chem Soc Rev Open Access Article. Published on 01 February 2024. Downloaded on 10/15/2024 9:59:47 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online 2564 | Chem. Soc. Rev., 2024, 53, 2530–2577 This journal is © The Royal Society of Chemistry 2024 mechanism for NO 2 gas which otherwise produces an oxidative sensing mechanism for pristine TiO 2 -based sensors. Hydrogen gas detection is of crucial concern since it is a highly flammable gas and it provides one of the cleanest fuels, making it a precious entity. Various hydrogen chemiresistors are already in function including metal based (Pd, Pt) materials, transition metal dichalcogenides (TMDs) and graphene-based materials. Although graphene-based materials are good in terms of conductivity and stability, but they have poor interaction with gases due to the lack of any functionality. Thus, inducing functionality or adding other materials such as metal nanoparticles (Pd/ Pt) improves interaction of graphene materials with hydrogen gas molecules. Eventually, Krishanveni et al. synthesized Pddoped imine-COF which offers a stable hybrid structure and highly efficient hydrogen sensing performance. 332 The presence of sulphonic functional groups in the constituent COF monomer effectively stabilizes the Pd-nanoparticles. The interaction of H 2 molecules with the Pd-decorated COF particles was excellently improved as compared to that of bulk COF by acid exfoliating the bulk COF material to COF-nanosheets since individual Pd-doped COF nanosheets provide maximum surface area for hydrogen molecules to get interacted with the exfoliated nanosheets. This provides highly approachable Pd nanoparticles for hydrogen spillover, i.e., dissociation of H 2 particles onto the Pd nanoparticles and the resulting formation of PdH x species. This causes electron enrichment of the material and a subsequent decrease in the resistance which can be termed as chemiresistive change in the material. Fig. 21 (a)–(d) Synthesis, structural characterization and sensing performance of M-TPCOF (M = Co & Cu) towards NO 2 gas. (a) Synthesis and structural features of H 2 -TPCOF before metalation. (b) Adsorption energy DFT based comparison of interaction of various gases with Co-TPCOF. (c) In situ DRIFT spectra of Co-TPCOF during NO 2 adsorption. (d) Schematic demonstration of specific chemiresistive sensing of NO 2 by M-TPCOF. Reproduced with permission from ref. 329 Copyright 2022, Wiley-VCHGmbH. Chem Soc Rev Review Article Open Access Article. Published on 01 February 2024. Downloaded on 10/15/2024 9:59:47 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online This journal is © The Royal Society of Chemistry 2024 Chem. Soc. Rev., 2024, 53, 2530–2577 | 2565 5.3. Porous conjugated polymers as chemiresistive gas sensors Porous conjugated polymers or conjugated porous polymers (CPPs) are a class of porous materials that are realized by the polymerization of monomeric units via a strong bonding between them. 333–335 They are different from other porous materials such as MOFs and COFs as they do not rely on the crystallinity of the materials. Thus, they can be prepared with robust conditions keeping in mind only the stability of the product and no special conditions for achieving crystallinity. This makes CPPs a promising choice for chemiresistive gas sensing materials. Although they are very stable materials and can be potentially used in harsh environments, not much work in the direction of chemiresistive gas sensing is done (Table 3). In this regard, Wisser et al. performed a polymerisation of 4,4diacetyl-2,2-diamino-biphenyl and 1,4-diacetylbenzene resulting into microporous polymers (DUT-92(NH 2 ) and DUT92(NO 2 )). 336 These polymeric materials were mixed with PTFE binder and carbon black to form flexible sensor films. The porous structure of the polymer causes a huge uptake of the target VOCs which causes expansion of the material, resulting Fig. 22 (a) Schematic representation of COF-316 structure and ‘filter amplifier’ and ‘redox reversal’ nature of TiO 2 /COF-316 hybrid. (b) Sensing response of TiO 2 /COF-316 hybrid towards NO 2 compared with other gases. (c) Response analysis of bare TiO 2 and TiO 2 /COF-316 hybrid towards RH%. (d) Chemiresistive response of NO 2 interaction with TiO 2 /COF-316 hybrid in the presence of light and corresponding illustrated sensing mechanism. Reproduced with permission from ref. 330 Copyright 2023, American Chemical Society. Review Article Chem Soc Rev Open Access Article. Published on 01 February 2024. Downloaded on 10/15/2024 9:59:47 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online 2566 | Chem. Soc. Rev., 2024, 53, 2530–2577 This journal is © The Royal Society of Chemistry 2024 into an increase in the tunnelling distance between the embedded particles of conductive carbon black and thus a simultaneous increase in the resistivity of the material. This mechanism is in contrast with the change in the resistivity in non-porous polymeric materials. A high surface area of about 680 m 2 g 1 provides effective interaction with VOCs which leads to the LOD values of o1 ppm. The swelling behaviour is observed to be effective for effective sensing performances of DUT-family polymers towards various VOCs. 5.3.1. Pristine porous organic polymers as chemiresistive gas sensors. Porous organic polymers represent the polymeric porous materials consisting of only organic constituting parts. The class consists of – conjugated microporous polymers (CMPs), porous aromatic frameworks (PAFs), hyper crosslinked polymers (HCPs), polymers of intrinsic-microporosity (PIMs), covalent organic polymers (COPs) and covalent triazine frameworks (CTFs). Covalent organic frameworks (COFs) are also a kind of POP material but their high superiority over the other POPs in terms of crystallinity and potential conductive nature makes them to be discussed critically as a separate section. Owing to their porous nature, we will be interested in POPs to check their potential for applications which involve the interaction of analyte molecules while passing through the porous passage, like gas sensing. Not all POPs have been explored for the chemiresistive gas sensing application because of the need for conductivity in the material, whereas most POPs, except COFs and CTFs, are either non-conductive or become conductive in hybrid forms. Thus, we would be highlighting here some important landmarks that prove the potential of these materials in chemiresistivity based sensing applications. Since CTFs are conductive in nature, they remain as the protagonist in the current scenario. Yang et al. synthesized a 2D organic polymer derived from a covalent triazine framework (CTFs), shortened as T-2DP (Fig. 23a). 337 The presence of a porous 2D pathway provides the facile gas transport and the presence of the high-density imine functional groups provides optimum adsorption/desorption which leads to highly improved sensing performances compared with the CTF-based sensor. This T-2DP-based sensor thus produces superfast response and recovery times for NO 2 sensing. Response/recovery times (35–47 s) are fast due to the presence of uniformly distributed interacting sites (–CQN– sites in this material) (Fig. 23b). The distribution of interacting sites is usually out-of-plane in the case of 2D inorganic materials such as MXenes, Graphene, rGO and so on. In the case of the present material, the interacting sites are in the 2D plane, which contributes to the conduction of electrons in this material. Thus, the interaction of target gas molecules (NO 2 ) with these in-plane interacting sites produces highly sensitive output signals (Fig. 23c) with an excellent sensitivity of 452.6 ppm 1 . The performance comparison with the corresponding bulk CTF based sensing material gives similar sensing behaviour with diminished sensitivity because of the slow diffusion of the analyte gas into the bulky CTF assembly. Due to the excellent recovery property of T-2DP, it produces highly reproducible response results with a mere 4% perturbation in response values over a series of 15 cycles (Fig. 23d). Comparative chemiresistive studies for CTF, RGO and MXene indicate that T-2DP is superior to all of them for faster response/recovery values by several magnitudes (Fig. 23e–h). Non-crystalline POPs usually show poor electronic conductivity due to the absence of an electronic conduction channel in contrast to what is usually found in, say, 2D conductive COFs. Maiti et al. synthesized a trialdehydebenzene and phenyldihydrazine combined hydrazide linkage based COP for the room temperature sensing of H 2 S gas. 338 Amine based macromolecules show conductivity through proton-conduction mechanism. This sensing material also showed fast sensing behaviour due to its high sensitivity towards acidic gases. H 2 S being an acidic gas, when interacts with this sensing material, produces an abrupt decrease in the proton conductivity of the material which is seen as fast response time (9 s) for 200 ppm of gas input. Thus, non-electronic conductive materials can be used for gas sensing by exploiting their proton conductivity, similar to what exists in amine-based covalent organic polymers. Metal-oxide semiconductors give excellent sensitive performance in the case of gas sensing but their selectivity and sensitivity are demolished to a great extent when they are kept under humid conditions because of the active site’s deactivation by the humidity. Organic-based materials provide a superior alternative for chemiresistive gas sensing jobs when they are stable under humid conditions, with accurate and reliable values. Taking forward the concept, Ko et al. fabricated a NO 2 sensing device, powered by covalent organic nanosheets which are two-dimensional semiconductive in nature, abbreviated CON-10. 339 The 2D nanosheet morphology of this material endows it with a hydrophobic nature with a water contact angle of 135.41along with the thermal stability of this material up to 320 1C. The exfoliated nature of CON-10 nanosheets provides a highly exposed surface area for the analyte gas molecules to interact with the p-electrons moving on these nanosheets surface analogous to the graphene electronic conduction behaviour. This can be the reason for the lower concentration detection limit of 2.242 ppb. The hydrophobic behaviour of Table 3 Porous organic polymers based chemiresistive gas sensors S. no. Material VOCs Sensing mechanism Response (%) LOD (ppm) t res –t rec (s) Ref. 1 T-2DP NO 2 CT 452.6 0.0002 35–47, 56–140 337 2 Heptazine-based COP NH 3 DA 70 1 65–9 340 3 Hydrazide-based COP H 2 S Proton conduction 51 9–12 338 4 CONs NO 2 CT 73.7 0.00024 339 5P 2 O 5 -CTF NH 3 CT 8 54–200 341 Chem Soc Rev Review Article Open Access Article. Published on 01 February 2024. Downloaded on 10/15/2024 9:59:47 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online This journal is © The Royal Society of Chemistry 2024 Chem. Soc. Rev., 2024, 53, 2530–2577 | 2567 CON-10 benefits the material to detect NO 2 gas in the ambient atmosphere and weather. Thus, the material is proposed to be suitable for trace-level detection of NO 2 . The inherent hydrophobicity in this material is supposed to be the result of uniformly distributed covalent bonding and the presence of N–S bonds. The hydrophobic nature of the sensing material is an important requirement for the real-time application of a chemiresistive gas sensor since it protects the material from the baseline resistance change when repeated cycles of the gas sensing are performed as is being observed in the present case. Comparing with the SnO 2 NW-based gas sensor, the damping effect of relative humidity on the baseline resistance value of the sensing material can be observed as highly impactful. High selectivity towards the NO 2 gas sensing can be observed in this material because of the hydrophobic nature of the CON nanosheets as they prevent the interaction of other gases with the sensing material, thus causing a negligible change in the conductivity values of the sensing material. CONs display excellent sensing performance at the real-life application value and thus prove the extensive morphological dependence of gas sensing performance. Similar to other porous materials, hydrogen-bonded organic frameworks (HOFs) consist of the polymerization of functional organic moieties through hydrogen bonding. They exhibit high permanent porosity, facile synthesis, and show promising applications such as molecular/gas separation, optical and biomedical applications. 342 Wang et al. synthesized a porphyrin-based hydrogen-bonded organic framework (HOF) for NO 2 sensing, leveraging abundant amine groups that contribute to excellent selectivity and an ultrafast response at room temperature. 343–345 The one-dimensional channels of HOF, the sensor demonstrated response and recovery times of 17.6 s and 15.4 s, respectively. The abundance of amine groups in the sensor results in n-type semiconductor behavior, leading to an ultrafast response to acidic NO2 gas molecules. Similarly, Lee et al. demonstrated the significance of HOFs in the separation of noble gases, such as isolating xenon (Xe) from an Xe/Kr mixture. 346 Xenon (Xe) gas-based devices find diverse applications, ranging from lighting, laser technology, space exploration to medical devices. Nevertheless, monitoring selective Xe gas and achieving sensitive separation using chemiresistive gas sensing will be an interesting study in the field of sensors. 6. Conclusion and future perspective The field of chemiresistive gas sensing has rapidly gained prominence due to its remarkable responsiveness to various Fig. 23 (a) Diagrammatic illustration of a triazine-based 2D polymer. (b) Exclusive chemiresistive response towards NO 2 gas while comparing with other inorganic and organic analytes. (c) Response curve of the T-2DP sensor towards NO 2 sensing at ppb-level. (d) Chemiresistive response of T-2DP towards NO 2 (1 ppm) feed. Dynamic sensing response of (e) pristine CTF to various NO 2 concentrations (5 ppm to 200 ppb), (f) RGO to different NO 2 levels (5 ppm to 500 ppb). (g) MXene to different NO 2 levels (5 ppm to 500 ppb). (h) Response vs. concentration curves for T-2DP compared to CTF, RGO, and MXene. (i) Response of MXene, RGO and T-2DP to various target analytes. Reproduced with permission from ref. 337 Copyright 2020, American Chemical Society. Review Article Chem Soc Rev Open Access Article. Published on 01 February 2024. Downloaded on 10/15/2024 9:59:47 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online 2568 | Chem. Soc. Rev., 2024, 53, 2530–2577 This journal is © The Royal Society of Chemistry 2024 gases, driven by swift electronic changes in the sensing material. Achieving sensitivity in gas sensing demands an ideal interplay between the gas sensing material and target gases, a substantial surface area providing accessible interaction sites, and rapid responses to analytes. Porous materials have emerged as highly promising candidates in the realm of chemiresistive gas sensing, as they effectively meet these essential requirements for modern and efficient gas sensing materials. This review offers an up-to-date overview of advancements in chemiresistive gas sensing, covering fundamental principles, sensing parameters, and the various materials employed. It places particular emphasis on porous materials, including metal–organic frameworks (MOFs) and their hybrids, covalent organic frameworks (COFs) and their hybrids, graphene-based materials, and porous conjugated polymers. The future of chemiresistive gas sensors holds exciting prospects driven by advancements in materials, fabrication techniques, and sensor designs. Advanced materials, such as 2D wonders like graphene and transition metal dichalcogenides, hybrid nanomaterials, and functionalized polymers are at the forefront. 146,347–354 These materials offer exceptional sensitivity and selectivity, promising the detection of a broader spectrum of gases, including elusive volatile organic compounds and industrial hazards. The world of nanotechnology continues to unfold, with nanowires, quantum dots, and hybrid nanomaterials driving the development of highly efficient sensors capable of swift responses and astonishingly low detection thresholds. Additionally, the integration of machine learning and data analytics promises to make gas sensors smarter and more responsive. These sensors will not only detect gases but also interpret data in real-time, recognizing patterns and identifying multiple gases simultaneously. Such intelligent sensors are set to revolutionize environmental monitoring, healthcare, and safety systems across various domains (Fig. 24). Furthermore, we are on the brink of witnessing gas sensors embedded in wearable devices and seamlessly integrated into the Internet of Things (IoT). 355–373 This transformation enables continuous monitoring of air quality, personal health, and safety in diverse environments, from urban landscapes to remote industrial sites. Flexible and printable sensors are also emerging, offering a cost-effective, disposable solution for various applications. These sensors are used in food packaging, healthcare, and beyond. Environmental applications remain a key focus, with gas sensors leading the charge in monitoring pollution, greenhouse gases, and indoor air quality. Healthcare is another area where gas sensors are poised to make significant contributions. From disease diagnosis through breath analysis to non-invasive health assessments via skin emissions, these sensors are poised to enable early disease detection and personalized healthcare. Industrial safety, particularly in the energy sector, is set to benefit as gas sensors play pivotal roles in detecting hazardous gases. Their deployment ensures safer oil and gas exploration and production. As the world turns its gaze towards environmental sustainability, gas sensors will embrace eco-friendly materials and sustainable manufacturing processes. This shift aligns with the growing commitment to environmental responsibility. Finally, global collaborations are catalysing these advancements. Scientists, engineers, and industries worldwide Fig. 24 Schematic illustration of current summary, future perspectives of advanced porous materials based CGS including critical challenges. Chem Soc Rev Review Article Open Access Article. Published on 01 February 2024. Downloaded on 10/15/2024 9:59:47 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online This journal is © The Royal Society of Chemistry 2024 Chem. Soc. Rev., 2024, 53, 2530–2577 | 2569 are uniting, pooling knowledge, and resources to accelerate the development and commercialization of cutting-edge sensor technologies. In conclusion, the future of gas sensors is radiant, holding promise for improving air quality, safety, healthcare, and environmental stewardship on a global scale. In this perspective, porous materials provide almost all the basic requirements as an effective chemiresistive gas sensing material. They provide high surface area, functional group versatility, an extended structural framework with electronic conjugation and many more properties for optimal host–analyte interaction and the aftermath results consisting of reliable and accurate sensing performance and data. Since there is always a scope for improvement in any work, this field can also be taken further towards a more powerful gas sensing level by considering some key future perspective points. 1. MOFs have already proven valuable in chemiresistive gas sensing due to their outstanding surface area and tuneable functionality, facilitating close interaction with target gases. Additionally, certain 2D conductive MOFs like M-HHTP, MHITP, and M-TCPP exhibit conductivity, offering electronic conduction for chemiresistive behaviour. To enhance their performance, there is room for improving MOFs by transitioning toward a more crystalline nature and developing highsurface-morphology frameworks. Controlling the shape of MOF-based nanoparticles can also be optimized to maximize the accessible surface area for analyte gases. MOFs are primarily used in powder form, posing processing challenges. Considering the actual material for gas sensing purposes, thinfilms and membranes like material are of considerable importance. Therefore, it is essential to focus on developing facile synthetic methods for MOF membranes or easily processable solutions. Exploration into MOF combinations with materials like CNTs, metal oxides, and conducting polymers has shown promise. Further investigations into highly conductive materials like MXenes hold potential, although stability improvements, especially under ambient conditions, are necessary. 2. COFs face challenges due to their instability in humid or ambient conditions and limited conductivity. To make them viable for gas sensing, more robust and conductive COFs are needed. COFs offer adaptable frameworks and versatile functionality, crucial for ideal target gas sensing. They are particularly suitable for gas sensing in living organisms, avoiding concerns associated with metal-based materials like MOS and MOFs. While examples using other materials like MOS and MOFs already exist, particularly for VOCs sensing in humans and plants, research involving COFs in this context remains unexplored. Also, the requirement of flexible and robust COF materials is a compelling need for a device level development of this work. Some COF membranes and thin-films are already being explored, 374–376 but no reports on their pristine membranes or films were found to the best of our knowledge. Further, investigation in this direction can be done by developing new universal methods for processing COFs to thin-films and membranes. 3. Graphene, a 2D conductive material, has been successfully employed in chemiresistive gas sensing applications. Graphene, primarily in its 2D monolayer form, serves as an excellent chemiresistive material owing to its superior electronic properties compared to multilayer graphene. Consequently, graphene and its derivatives, as well as hybrids, have emerged as top-notch materials for room-temperature chemiresistive gas sensing. However, despite their physical and chemical robustness, graphene-based materials face challenges such as limited selectivity and sensitivity due to their chemical inertness, which hampers host-gas analyte interactions. To address this issue, efforts have been made to introduce metal/metal oxide sites and related functionalities into pristine graphene, resulting in improved gas sensing performance. Further enhancements can be achieved by developing these hybrids with more intriguing morphologies and additional functionalities, building upon the success of materials like GO, rGO, and G-COOH in gas sensing. Incorporating dopants like B, N, or -SH becomes crucial for fine-tuning the redox properties and electronic structure, including band gaps and Schottky barriers, to enable precise sensing of interacting gases. Synthesis of graphene-based devices is still restricted to monolayers or thin-layer graphene derivatives which are difficult to achieve as it require critical precision and sophisticated tools. Chemical exfoliation methods to develop large monolayers need to be explored further. 4. Porous organic polymers including amorphous porous materials like porous carbon black, ionic liquids, hyper-crosslinked polymers (HCPs), and PIMs offer notable advantages in terms of stability, ease of synthesis, and functionality. Despite these merits, their complex microstructure and lack of longrange order pose challenges, particularly regarding electronic conductivity. To overcome this hurdle, inducing conductivity through space becomes a viable strategy, addressing the need for extended conjugation and through-bond or through-space conduction. This can be accomplished by meticulously controlling the orientation of individual moieties, allowing their empty orbitals or charges to traverse the entire amorphous framework and enhancing the utility of these materials in chemiresistive gas sensing. Powders are usually not processable if they are robust in nature. Thus, solution processable POPs or membranes are the need of the hour. To synthesize these membranes or film methods like doctor blade, Langmuir–Blodgett nanostructured film formation kind of methods are required to be more in use for producing effective chemiresistive films. Conflicts of interest There are no conflicts of interest to declare. Acknowledgements A. S. and S. E. equally contributed to this work. K. J. R. acknowledges support from the Indian Institute of Technology Jammu for providing UGC-DAE CSR project CRS/2022-23/01/708, SERB CRG/2023/004685,CSIR-HRDG 01/3101/23/EMR-II and DST/ INT/DAAD/P-06/2023. We acknowledge the support of the project TECHSCALE (no. CZ.02.01.01/00/22_008/0004587) financed Review Article Chem Soc Rev Open Access Article. Published on 01 February 2024. Downloaded on 10/15/2024 9:59:47 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online 2570 | Chem. Soc. Rev., 2024, 53, 2530–2577 This journal is © The Royal Society of Chemistry 2024 from the ERDF and ESF. This work was supported by the Ministry of Education, Youth and Sports of the Czech Republic through the e-INFRA CZ (ID:90254). This work was supported by the Materials and Components Technology Development Program of Ministry of Trade, Industry and Energy (MOTIE)/ Korea Institute for Industrial Economics and Trade (KIET) (10080527). This work was also supported by the Ministry of Education of the Republic of Korea and the National Research Foundation of Korea (NRF-2022S1A5C2A03093218). The financial support of the European Union under the REFRESH – Research Excellence For Region Sustainability and High-tech Industries project number CZ.10.03.01/00/ 22_003/0000048 via the Operational Programme Just Transition is also acknowledged References 1 H. François, R. Samacoı ¨ts, D. N. Bird, J. Ko ¨berl, F. Prettenthaler and S. Morin, Nat. Clim. Change, 2023, 13, 935–942. 2 I. Manisalidis, E. Stavropoulou, A. Stavropoulos and E. Bezirtzoglou, Front. Public Health, 2020, 8. 3 H. Zhang, Z. Zhang, Z. Li, H. Han, W. Song and J. Yi, Nat. Commun., 2023, 14, 3495. 4 S. Dhall, B. R. Mehta, A. K. Tyagi and K. Sood, Sens. 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