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.

We report on the heterogeneous sensitization of metal-organic framework (MOF)-driven metal-embedded metal oxide (M@MO) complex catalysts onto semiconductor metal oxide (SMO) nanofibers (NFs) via electrospinning for markedly enhanced chemical gas sensing. ZIF-8-derived Pd-loaded ZnO nanocubes (Pd@ZnO) were sensitized on both the interior and the exterior of WO NFs, resulting in the formation of multiheterojunction Pd-ZnO and ZnO-WO interfaces. The Pd@ZnO loaded WO NFs were found to exhibit unparalleled toluene sensitivity (R/R = 4.37 to 100 ppb), fast gas response speed (∼20 s) and superior cross-selectivity against other interfering gases. These results demonstrate that MOF-derived M@MO complex catalysts can be functionalized within an electrospun nanofiber scaffold, thereby creating multiheterojunctions, essential for improving catalytic sensor sensitization.

The increase of surface area and the functionalization of catalyst are crucial to development of high-performance semiconductor metal oxide (SMO) based chemiresistive gas sensors. Herein, nanoscale catalyst loaded CoO hollow nanocages (HNCs) by using metal-organic framework (MOF) templates have been developed as a new sensing platform. Nanoscale Pd nanoparticles (NPs) were easily loaded on the cavity of Co based zeolite imidazole framework (ZIF-67). The porous structure of ZIF-67 can restrict the size of Pd NPs (2-3 nm) and separate Pd NPs from each other. Subsequently, the calcination of Pd loaded ZIF-67 produced the catalytic PdO NPs functionalized CoO HNCs (PdO-CoO HNCs). The ultrasmall PdO NPs (3-4 nm) are well-distributed in the wall of CoO HNCs, the unique structure of which can provide high surface area and high catalytic activity. As a result, the PdO-CoO HNCs exhibited improved acetone sensing response (R/R = 2.51-5 ppm) compared to PdO-CoO powders (R/R = 1.98), CoO HNCs (R/R = 1.96), and CoO powders (R/R = 1.45). In addition, the PdO-CoO HNCs showed high acetone selectivity against other interfering gases. Moreover, the sensor array clearly distinguished simulated exhaled breath of diabetics from healthy people's breath. These results confirmed the novel synthesis of MOF templated nanoscale catalyst loaded SMO HNCs for high performance gas sensors.

The oxygen, O, in air interferes with the detection of H by palladium (Pd)-based H sensors, including Pd nanowires (NWs), depressing the sensitivity and retarding the response/recovery speed in air-relative to N or Ar. Here, we describe the preparation of H sensors in which a nanofiltration layer consisting of a Zn metal-organic framework (MOF) is assembled onto Pd NWs. Polyhedron particles of Zn-based zeolite imidazole framework (ZIF-8) were synthesized on lithographically patterned Pd NWs, leading to the creation of ZIF-8/Pd NW bilayered H sensors. The ZIF-8 filter has many micropores (0.34 nm for gas diffusion) which allows for the predominant penetration of hydrogen molecules with a kinetic diameter of 0.289 nm, whereas relatively larger gas molecules including oxygen (0.345 nm) and nitrogen (0.364 nm) in air are effectively screened, resulting in superior hydrogen sensing properties. Very importantly, the Pd NWs filtered by ZIF-8 membrane (Pd NWs@ZIF-8) reduced the H response amplitude slightly (ΔR/R = 3.5% to 1% of H versus 5.9% for Pd NWs) and showed 20-fold faster recovery (7 s to 1% of H) and response (10 s to 1% of H) speed compared to that of pristine Pd NWs (164 s for response and 229 s for recovery to 1% of H). These outstanding results, which are mainly attributed to the molecular sieving and acceleration effect of ZIF-8 covered on Pd NWs, rank highest in H sensing speed among room-temperature Pd-based H sensors.

Hydrogen (H 2 ) is one of the next-generation energy sources because it is abundant in nature and has a high combustion efficiency that produces environmentally benign products (H 2 O). However, H 2 /air mixtures are explosive at H 2 concentrations above 4%, thus any leakage of H 2 must be rapidly and reliably detected at much lower concentrations to ensure safety. Among the various types of H 2 sensors, chemiresistive sensors are one of the most promising sensing systems due to their simplicity and low cost. This review highlights the advances in H 2 chemiresistors, including metal-, semiconducting metal oxide-, carbon-based materials, and other materials. The underlying sensing mechanisms for different types of materials are discussed, and the correlation of sensing performances with nanostructures, surface chemistry, and electronic properties is presented. In addition, the discussion of each material emphasizes key advances and strategies to develop superior H 2 sensors. Furthermore, recent key advances in other types of H 2 sensors are briefly discussed. Finally, the review concludes with a brief outlook, perspective, and future directions.