Metal-Organic Frameworks for Chemiresistive Sensors

Metal-organic frameworks (MOFs) have attracted much attention in diverse research communities because of their ultrahigh surface areas, high porosity, and tunable structures. In particular, MOFs are considered one of the most ideal sensing materials since chemical sensing properties are mainly influenced by surface reactions. Recently, the use of MOFs in chemiresistive sensors that transduce electrical signals from surface reactions has rapidly emerged. The development of conductive MOFs has fueled the use of pure MOFs as a new class of chemiresistors. MOFs with unique gas adsorption and separation properties also enable their use in gas sensors as selective filtration layers. In addition, as sacrificial templates, MOFs can be converted to various types of gas-sensitive nanomaterials such as carbon composites and metal oxides via controlled pyrolysis or calcination. In this review, we summarize the latest studies on MOF-based chemiresistive sensors and suggest future research directions. Highly sensitive and selective chemical sensors are needed for use in a wide range of applications such as environmental toxic gas monitoring, disease diagnosis, and food quality control. Although some chemiresistive sensors have been commercialized, grand challenges still remain: ppb-level sensitivity, accurate cross-selectivity, and long-term stability. Metal-organic frameworks (MOFs) with record-breaking surface areas and ultrahigh porosity are ideal sensing materials because chemical sensors rely highly on surface reactions. In addition, MOFs can be used as a membrane to utilize their unique gas adsorption and separation characteristics. Furthermore, the use of MOFs as precursors to enable facile production of various nanostructures is further combined with other functional materials. Based on these fascinating features of MOFs, there have been great efforts to elucidate reaction mechanisms and address limitations in MOF-based chemiresistors. In this review, we present a comprehensive overview and recent progress in chemiresistive sensors developed by using pure MOFs, MOF membranes, and MOF derivatives. Highly sensitive and selective chemical sensors are needed for use in a wide range of applications such as environmental toxic gas monitoring, disease diagnosis, and food quality control. Although some chemiresistive sensors have been commercialized, grand challenges still remain: ppb-level sensitivity, accurate cross-selectivity, and long-term stability. Metal-organic frameworks (MOFs) with record-breaking surface areas and ultrahigh porosity are ideal sensing materials because chemical sensors rely highly on surface reactions. In addition, MOFs can be used as a membrane to utilize their unique gas adsorption and separation characteristics. Furthermore, the use of MOFs as precursors to enable facile production of various nanostructures is further combined with other functional materials. Based on these fascinating features of MOFs, there have been great efforts to elucidate reaction mechanisms and address limitations in MOF-based chemiresistors. In this review, we present a comprehensive overview and recent progress in chemiresistive sensors developed by using pure MOFs, MOF membranes, and MOF derivatives. Superior chemical sensors are needed to monitor environmental conditions, to realize an early diagnosis of disease, and to control food quality.1Kim S.J. Choi S.J. Jang J.S. Cho H.J. Kim I.D. Innovative nanosensor for disease diagnosis.Acc. Chem. Res. 2017; 50: 1587-1596Crossref PubMed Scopus (49) Google Scholar, 2Loutfi A. Coradeschi S. Mani G.K. Shankar P. Rayappan J.B.B. Electronic noses for food quality: a review.J. Food Eng. 2015; 144: 103-111Crossref Scopus (161) Google Scholar, 3Brown S.K. Sim M.R. Abramson M.J. 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Thus far, carbon-based nanomaterials and semiconducting metal oxides (SMOs) have been extensively investigated for the detection of target gases. However, there are still grand challenges in the performance of chemical sensors in terms of sensitivity, selectivity, responding speed, and stability. For example, metal oxides are typically operated at an elevated temperature (200°C‒400°C),6Kim I.-D. Rothschild A. Tuller H.L. Advances and new directions in gas-sensing devices.Acta Mater. 2013; 61: 974-1000Crossref Scopus (163) Google Scholar which often lead to poor selectivity and low stability due to the high reactivity of metal oxides with interfering gases at high temperatures. In general, the operation of chemical sensors relies on the transduction of signals such as optical, mechanical, or electrical changes induced by the surface reaction of the analytes. Thus, in order to achieve superior sensing performances, an accelerated surface reaction is essential, transducing the sensing signals more efficiently. Metal-organic frameworks (MOFs), consisting of metal nodes and organic linkers, have exceptionally high surface area, ultrahigh porosity, and diverse structures.7Furukawa H. Cordova K.E. O’Keeffe M. Yaghi O.M. The chemistry and applications of metal-organic frameworks.Science. 2013; 341: 1230444Crossref PubMed Scopus (3753) Google Scholar Inspired by these fascinating features, MOFs have been widely explored in various fields, such as gas storage and separation,8Li J.R. Kuppler R.J. Zhou H.C. Selective gas adsorption and separation in metal–organic frameworks.Chem. Soc. Rev. 2009; 38: 1477-1504Crossref PubMed Scopus (0) Google Scholar, 9Morris R.E. Wheatley P.S. Gas storage in nanoporous materials.Angew. Chem. Int. Ed. Engl. 2008; 47: 4966-4981Crossref PubMed Scopus (1129) Google Scholar energy applications,10Wang H. Zhu Q.-L. Zou R. 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Mater. 2006; 18: 1337-1346Crossref Scopus (399) Google Scholar MOF sensors have been introduced to transduce gas-sensing signals in the last several years. These MOF-based chemical sensors showed interesting and promising sensing properties. However, their sensing systems are rather complicated, and further enhancement in the transduction values of sensing signals is required. For instance, MOFs with a low band gap or high electrical conductivity can effectively transfer the electrical signals. In addition, unique gas storage and separation properties of MOFs enable selective chemical sensors. Moreover, the extensive post-synthetic methods of MOFs have opened up a variety of possibilities,18Dang S. Zhu Q.-L. Xu Q. Nanomaterials derived from metal–organic frameworks.Nat. Rev. Mater. 2017; 3: 17075Crossref Scopus (163) Google Scholar thus diverse MOF structures with controlled compositions can be easily obtained via MOF-templating routes. Inspired by these innovative characteristics of MOFs, the researches on MOF-based chemiresistive sensors, which are operated by a change in electrical resistance following the surface reactions or adsorptions of target gases, have rapidly grown in recent years. However, there have been no focused review papers for introducing a comprehensive understanding on the current status, relevant challenges, and limitations of pure MOFs, MOF membranes, and MOFs derivatives for chemiresistive sensors, although important review papers related to MOF-based chemiresistors have been reported recently.19Stassen I. Burtch N. Talin A. Falcaro P. Allendorf M. Ameloot R. An updated roadmap for the integration of metal–organic frameworks with electronic devices and chemical sensors.Chem. Soc. Rev. 2017; 46: 3185-3241Crossref PubMed Google Scholar, 20Campbell M.G. Dincă M. Metal–organic frameworks as active materials in electronic sensor devices.Sensors. 2017; 17: 1108Crossref Scopus (52) Google Scholar, 21Zhao W. Peng J. Wang W. Liu S. Zhao Q. Huang W. Ultrathin two-dimensional metal-organic framework nanosheets for functional electronic devices.Coord. Chem. Rev. 2018; 377: 44-63Crossref Scopus (8) Google Scholar, 22Chidambaram A. Stylianou K.C. Electronic metal–organic framework sensors.Inorg. Chem. Front. 2018; 5: 979-998Crossref Google Scholar, 23Fang X. Zong B. Mao S. Metal–organic framework-based sensors for environmental contaminant sensing.Nanomicro Lett. 2018; 10: 64PubMed Google Scholar In this review, we present recent progress on various MOFs, particularly optimized for applications in chemiresistive sensors. First, we summarize the basic principles and key parameters of chemiresistive sensors and describe the challenges of conventional gas-sensing materials and the opportunity of MOF-based chemiresistors. Then, we highlight the representative articles focused on potential uses of MOFs for applications in chemiresistors: (1) pure MOFs, including three-dimensional (3D) MOFs, and two-dimensional (2D) conductive MOFs as a new class of sensing materials; (2) MOF membranes, as selective gas filtration layers, on conventional sensing materials; and (3) MOF derivatives, such as carbon composites and metal oxides, for highly sensitive and selective chemiresistors (Figure 1). The various synthetic routes and sensing characteristics of 3D MOFs, 2D conductive MOFs, MOF-metal composites, MOF-metal oxides composites, MOF-derived carbon composites, and MOF-derived metal oxides are discussed. Moreover, we emphasize the important correlation between MOFs-driven material properties and sensing characteristics and discuss the prevailing challenges as well as the future perspective. We hope that this review inspires scientists in various fields and guides the development of next-generation chemical sensors. The sensing mechanism of chemiresistive sensors is mainly attributed to the transfer of electrons or holes caused by the surface reactions or adsorptions of gas molecules on the sensing materials.6Kim I.-D. Rothschild A. Tuller H.L. Advances and new directions in gas-sensing devices.Acta Mater. 2013; 61: 974-1000Crossref Scopus (163) Google Scholar Therefore, when sensing layers react with the adsorbed target gases, the resistance (or conductance) change of the chemiresistive sensors occurs. Apart from the simple sensing mechanism, chemiresistive sensors offer several benefits, including low-cost fabrication, facile integration with various electronic devices, and ease of miniaturization. The origin of the change in the electrical resistance in chemiresistive sensors depends on the type of sensing material. Because of the diversity and high compatibility of MOFs and MOF derivatives with other materials, multiple principles can be incorporated for MOF-based chemiresistors. For instance, the adsorbed analytes on the surface of pure MOFs donate electrons to pure MOFs or deprive them of their electrons, causing the change in the resistance of the sensors.24Campbell M.G. Sheberla D. Liu S.F. Swager T.M. Dincă M. Cu3 (hexaiminotriphenylene)2: an electrically conductive 2D metal–organic framework for chemiresistive sensing.Angew. Chem. Int. Ed. 2015; 54: 4349-4352Crossref PubMed Scopus (0) Google Scholar The metal nodes and active functional organic groups in MOFs can act as effective adsorption sites that are beneficial to chemical sensors. The redox reaction of transition metals in MOFs can also affect the conductivity of pure MOFs.25Aubrey M.L. Wiers B.M. Andrews S.C. Sakurai T. Reyes-Lillo S.E. Hamed S.M. Yu C.J. Darago L.E. Mason J.A. Baeg J.O. et al.Electron delocalization and charge mobility as a function of reduction in a metal–organic framework.Nat. Mater. 2018; 17: 625-632Crossref PubMed Scopus (17) Google Scholar In addition, the volume change of MOFs upon gas adsorption modulates the number of electrons hopping between MOFs, causing resistance changes.26Rubio-Giménez V. Almora-Barrios N. Escorcia-Ariza G. Galbiati M. Sessolo M. Tatay S. Martí-Gastaldo C. Almora-Barrios N. Escorcia-Ariza G. Galbiati M. Sessolo M. Tatay S. Martí-Gastaldo C. Origin of the chemiresistive response of ultrathin films of conductive metal–organic frameworks.Angew. Chem., Int. Ed. 2018; 57: 15086-15090Crossref PubMed Scopus (2) Google Scholar In general, the sensing mechanisms of composite layers are predominantly governed by majority phases. Thus, sensing operations of MOF derivatives and MOF composites are also determined by major components, which can be selected from metal oxides, carbon composites, nitrides, sulfides, and so on. Gas sensors using metal oxides, which are the well-known sensing materials, are highly dependent on the surface reactions between target gas molecules and chemisorbed oxygen species (O2−, O−, and O2−).6Kim I.-D. Rothschild A. Tuller H.L. Advances and new directions in gas-sensing devices.Acta Mater. 2013; 61: 974-1000Crossref Scopus (163) Google Scholar At an elevated temperature, oxygen molecules in air are chemisorbed on the surfaces of metal oxides by trapping electrons in metal oxides, generating either electron depletion regions for the case of n-type SMOs or hole accumulation layers for p-type SMOs. Reducing or oxidizing gas molecules react with the chemisorbed oxygen species adsorbed on SMOs, modulating the thickness of electron depletion regions or hole accumulation layers, resulting in the resistance change of the sensors. On the other hand, the resistance changes of carbon-based materials, such as carbon nanotubes (CNTs), graphene, graphene oxides, carbonized MOFs, and so on, are mainly caused by the adsorption of target analytes on their surfaces.27Schroeder V. Savagatrup S. He M. Lin S. Swager T.M. Carbon nanotube chemical sensors.Chem. Rev. 2019; 119: 99-663Crossref Scopus (5) Google Scholar, 28Liu Y. Dong X. Chen P. Biological and chemical sensors based on graphene materials.Chem. Soc. Rev. 2012; 41: 2283-2307Crossref PubMed Google Scholar Depending on the position of adsorption sites, the adsorbed gas molecules (1) directly interact with carbon-based materials by donating electrons or depriving electrons (intra-carbon) or (2) change the electron hopping currents between the sensing materials (inter-carbon) by swelling. In addition, the sensing behaviors of 2D transitional metal dichalcogenides (MoS2, WS2, and SnS2) are mainly attributed to the direct interaction between target analytes and sensing materials,29Anichini C. Czepa W. Pakulski D. Aliprandi A. Ciesielski A. Samorì P. Chemical sensing with 2D materials.Chem. Soc. Rev. 2018; 47: 4860-4908Crossref PubMed Google Scholar similar to the intra-carbon-sensing mechanism. Based on these mechanisms, we briefly summarize the important parameters of sensors in terms of response (or sensitivity), selectivity, speed (response and recovery times), and stability. We will further discuss the key points for developing highly sensitive and selective MOF-based sensors. The response (R) of a sensor is defined as a ratio of resistance (or conductance) change to baseline resistance (or conductance).27Schroeder V. Savagatrup S. He M. Lin S. Swager T.M. Carbon nanotube chemical sensors.Chem. Rev. 2019; 119: 99-663Crossref Scopus (5) Google ScholarR (for resistance change) = (Rgas ‒ R0)/R0 = ΔR/R0(Equation 1) R (for conductance change) = (Ggas ‒ G0)/G0 = ΔG/G0,(Equation 2) where R0 or Go is the baseline resistance or conductance, and Rgas or Ggas is the resistance or conductance of a sensor when exposed to target analytes. In addition, the sensitivity (S), which is defined as the slope of the resistance change (dy) of sensors versus the concentration change (dx) of gas molecules,30Franke M.E. Koplin T.J. Simon Metal and metal oxide nanoparticles in chemiresistors: does the nanoscale matter?.Small. 2006; 2: 36-50Crossref PubMed Scopus (835) Google Scholar can be also used as a parameter for evaluating gas-sensing capability:S = dy/dx.(Equation 3) Since the response of chemiresistors highly relies on the surface reaction or adsorption of a target gas on the sensors, large surface areas and high reactivity to the target gas are needed to achieve a higher response. The high response can also lead to a low detection limit, because the limit of detection (LOD) is determined as the lowest concentration at which the response is 3-fold higher than the standard deviation (σnoise) of the baseline resistance (R0)31Ammu S. Dua V. Agnihotra S.R. Surwade S.P. Phulgirkar A. Patel S. Manohar S.K. Flexible, all-organic chemiresistor for detecting chemically aggressive vapors.J. Am. Chem. Soc. 2012; 134: 4553-4556Crossref PubMed Scopus (80) Google Scholar:LOD = 3 × rmsnoise/S,(Equation 4) where rmsnoise is the root-mean-square noise of the sensors and S is the sensitivity (dy/dx). The response and recovery times are used to quantify the sensing speed of sensors. In general, the response time is defined as the time taken for the resistance to increase from the baseline resistance (R0) to 90% of the maximum resistance change (R0 + 0.9ΔRmax) at a given level of a target gas.30Franke M.E. Koplin T.J. Simon Metal and metal oxide nanoparticles in chemiresistors: does the nanoscale matter?.Small. 2006; 2: 36-50Crossref PubMed Scopus (835) Google Scholar On the other hand, the recovery time is defined as the time required for the resistance to decrease from the maximum resistance change (R0 + ΔRmax) to 10% of the maximum resistance change (R0 + 0.1ΔRmax).30Franke M.E. Koplin T.J. Simon Metal and metal oxide nanoparticles in chemiresistors: does the nanoscale matter?.Small. 2006; 2: 36-50Crossref PubMed Scopus (835) Google Scholar The response and recovery times of conductance-based chemiresistors are also defined in the same way, except that the baseline conductance (G0) and the maximum conductance change (ΔGmax) are used. To develop fast responding sensors, it is necessary to lower the activation energy of surface reactions and accelerate adsorptions of analytes by using catalysts. Selectivity is one of the most important sensing parameters because there is a variety of interfering gas molecules, which is detrimental to accurate target gas detection. Selectivity of sensors is usually investigated by comparing the cross-sensitivity toward various analytes at given concentrations.32Torsi L. Magliulo M. Manoli K. Palazzo G. Organic field-effect transistor sensors: a tutorial review.Chem. Soc. Rev. 2013; 42: 8612-8628Crossref PubMed Scopus (341) Google Scholar If there is a unique surface reaction with specific analytes, the sensors can have superior selectivity. The selectivity is hugely affected by various factors such as operating temperatures, composition of the sensors, humidity, and reactivity between target molecules and sensing materials at a given operating temperature. The decoration of selectors or sensitizers (catalysts) on the sensing materials can promote specific surface reactions with the target gas, leading to improved selectivity. In addition, sensor stability is an important parameter. The long-term operation of sensors with high reliability is critical for their practical application. Typically, when the sensing materials are exposed to water vapors in air, water vapors are adsorbed on the active sites of sensing layers, leading to the deterioration of the sensing properties.6Kim I.-D. Rothschild A. Tuller H.L. Advances and new directions in gas-sensing devices.Acta Mater. 2013; 61: 974-1000Crossref Scopus (163) Google Scholar Several challenges of present chemiresistive sensors include low sensitivity (or response), poor selectivity, and instability. Although metal oxides have relatively higher response than other materials, they have drawbacks such as low selectivity and baseline drift caused by high temperature operation.33Lee J.-H. Gas sensors using hierarchical and hollow oxide nanostructures: overview.Sens. Actuators B. 2009; 140: 319-336Crossref Scopus (0) Google Scholar Room temperature operating carbon-based materials, even though they possess relatively high surface area, suffer from low response, poor selectivity, and low reproducibility.34Swager T.M. Sensor technologies empowered by materials and molecular innovations.Angew. Chem. Int. Ed. Engl. 2018; 57: 4248-4257Crossref PubMed Scopus (11) Google Scholar In addition, 2D transition metal dichalcogenides (TMDs) have critical issues; they are easily oxidized in air.29Anichini C. Czepa W. Pakulski D. Aliprandi A. Ciesielski A. Samorì P. Chemical sensing with 2D materials.Chem. Soc. Rev. 2018; 47: 4860-4908Crossref PubMed Google Scholar To date, a number of researchers have attempted to address several critical issues of above-mentioned conventional sensing materials, but further significant improvements are still needed. MOFs, as new emerging materials, have various fascinating features for use in chemiresistive gas sensors (Figure 1). First, MOFs have giant surface areas with high gas accessibility compared with conventional porous materials.7Furukawa H. Cordova K.E. O’Keeffe M. Yaghi O.M. The chemistry and applications of metal-organic frameworks.Science. 2013; 341: 1230444Crossref PubMed Scopus (3753) Google Scholar If the entire surface of a MOF is fully utilized for transduction of the sensing signals (resistance changes), one can expect a dramatic enhancement of the gas response. Second, the pristine MOFs with high structural tunability have been considerably studied as efficient membrane layers for gas separation or gas storage.8Li J.R. Kuppler R.J. Zhou H.C. Selective gas adsorption and separation in metal–organic frameworks.Chem. Soc. Rev. 2009; 38: 1477-1504Crossref PubMed Scopus (0) Google Scholar These MOFs with selective gas penetration and adsorption properties offer a potential solution to address the selectivity issue in chemiresistive sensors. Last, MOF derivatives, which are easily obtained from the post-synthetic processes of pure MOFs, still exhibit large surface area and high porosity.18Dang S. Zhu Q.-L. Xu Q. Nanomaterials derived from metal–organic frameworks.Nat. Rev. Mater. 2017; 3: 17075Crossref Scopus (163) Google Scholar Considering the diverse structures of pure MOFs, high tunability on structures and compositions of MOF derivatives allow the creation of various nanomaterials, which possess heterogeneous sensing structures or new catalytic function. MOFs have record-breaking large surface areas (up to ∼8,000 m2/g)7Furukawa H. Cordova K.E. O’Keeffe M. Yaghi O.M. The chemistry and applications of metal-organic frameworks.Science. 2013; 341: 1230444Crossref PubMed Scopus (3753) Google Scholar; thus, they have attracted much attention in broad applications particularly relying on the surface reactions.35Czaja A.U. Trukhan N. Müller U. Industrial applications of metal–organic frameworks.Chem. Soc. Rev. 2009; 38: 1284-1293Crossref PubMed Scopus (0) Google Scholar In this regard, MOFs are emerging as next-generation gas-sensing materials. The chemiresistive sensing properties of pure MOFs were first reported by Chen et al.36Chen E.X. Yang H. Zhang J. Zeolitic imidazolate framework as formaldehyde gas sensor.Inorg. Chem. 2014; 53: 5411-5413Crossref PubMed Scopus (97) Google Scholar They synthesized a Co-based zeolite imidazole framework (ZIF-67) that consists of Co ions and methylimidazole linkers and used it as a formaldehyde sensor (Figure 2A). Note that low ppm levels of formaldehyde can cause sick building syndrome.37Wargocki P. Wyon D.P. 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The same group further investigated the sensing properties of cobalt-imidazole frameworks (Co[(im)2]n) synthesized by the assembly of imidazole and cobalt (II) acetate (Figure 2C).39Chen E.X. Fu H.R. Lin R. Tan Y.X. Zhang J. Highly selective and sensitive trimethylamine gas sensor based on cobalt imidazolate framework material.ACS Appl. Mater. Interfaces. 2014; 6: 22871-22875Crossref PubMed Scopus (44) Google Scholar The Co[(im)2]n exhibited selective sensing properties toward trimethylamine (Rgas/Rair = 2 to 2 ppm) at an operating temperature of 75°C (Figure 2D). The sensors exhibited a stable response in various humidity ranges, similar to ZIF-67-based formaldehyde sensors. Amine functionalized Zr-based MOFs [Zr6(O)4(OH)4(1,4-benzenedicarboxylate-NH2)6, NH2-UiO-66] also exhibited chemiresistive sensing characteristics to sulfur dioxide (SO2) at 150°C in Ar atmosphere (Figure 2E).40DMello M.E. Sundaram N.G. Singh A. Singh A.K. Kalidindi S.B. An amine functionalized zirconium metal–organic framework as an effective chemiresistive sensor for acidic gases.Chem. Commun. 2019; 55: 349-352Crossref PubMed Scopus (0) Google Scholar The high acidity of SO2 enables a charge transfer coupling with amine groups in the organic ligands of MOFs, thus leading to the resistance decrease (|ΔR/R0| = 21.6% to 10 ppm of SO2) of NH2-UiO-66 (band gap = 2.75 eV)41Long J. Wang S. Ding Z. Wang S. Zhou Y. Huang L. Wang X. Amine-functionalized zirconium metal–organic framework as efficient visible-light photocatalyst for aerobic organic transformations.Chem. Commun. 2012; 48: 11656-11658Crossref PubMed Scopus (183) Google Scholar upon SO2 adsorption (Figure 2F). Although these works demonstrated the feasibility of MOF-based chemiresistive sensors, there were some limitations: (1) the origin of selectivity remains unclear, (2) the need for a heating system complicates the sensing devices, and (3) much enhanced sensitivity is needed to detect sub-ppm levels of analytes. In addition, there are bigger challenges in pure MOFs for applications in chemiresistive sensors: poor electrical conductivity and chemical stability. Most MOFs are not conductive at ambient atmospheres because of the hard metal ions in MOFs and the poor orbital overlap that restricts the facile transport or flow of electrons.42Sun L. Campbell M.G. Dincă M. Electrically conductive porous metal–organic frameworks.Angew. Chem. Int. Ed. Engl. 2016; 55: 3566-3579Crossref PubMed Scopus (0) Google Scholar Therefore, the ultrahigh surface area and high porosity of MOFs are not fully involved in the electrical signal transductions for chemiresistive detection of gas molecules. In addition, the poor chemical stability of most MOFs hinders practical applications in MOF-based sensors under harsh conditions.7Furukawa H. Cordova K.E. O’Keeffe M. Yaghi O.M. The chemistry and applications of metal-organic frameworks.Science. 2013; 341: 1230444Crossref PubMed Scopus (3753) Google Scholar Although the integration of MOFs with conductive materials having high chemical stability enabled the detection of NH3 ga